Nanoscale Science, Engineering and Technology

Research Directions

**ABSTRACT**

This report describes important future research directions in nanoscale
science, engineering and technology. It was prepared in connection with
an anticipated national research initiative on nanotechnology for the
twenty--first century. The research directions described are not
expected to be inclusive but illustrate the wide range of research
opportunities and challenges that could be undertaken through the
national laboratories and their major national scientific user
facilities with the support of universities and industry.

####### TABLE OF CONTENTS

# Basic Energy Sciences Nanoscience/Nanotechnology Group [iv](#basic-energy-sciences-nanosciencenanotechnology-group) {#basic-energy-sciences-nanosciencenanotechnology-group-iv .TOC-1}

# Section Editors [v](#section-editors) {#section-editors-v .TOC-1}

# EXECUTIVE SUMMARY [vi](#executive-summary) {#executive-summary-vi .TOC-1}

# 1. INTRODUCTION [1](#introduction) {#introduction-1 .TOC-1}

# 2. EMERGENCE OF PROPERTIES  {#emergence-of-properties .TOC-1}

#  AT MULTIPLE LENGTH AND TIME SCALES [4](#emergence-of-properties-at-multiple-length-and-time-scales) {#at-multiple-length-and-time-scales-4 .TOC-1}

# 3. MANIPULATION AND COUPLING OF PROPERTIES  {#manipulation-and-coupling-of-properties .TOC-1}

#  AT THE NANOSCALE [10](#manipulation-and-coupling-of-properties-at-the-nanoscale) {#at-the-nanoscale-10 .TOC-1}

# 4. CONTROLLED SYNTHESIS AND PROCESSING AT THE NANOSCALE (PRIMARILY ORGANIC) [23](#controlled-synthesis-and-processing-at-the-nanoscale-primarily-organic) {#controlled-synthesis-and-processing-at-the-nanoscale-primarily-organic-23 .TOC-1}

# 5. CONTROLLED SYNTHESIS AND PROCESSING AT THE NANOSCALE (PRIMARILY INORGANIC) [26](#controlled-synthesis-and-processing-at-the-nanoscale-primarily-inorganic) {#controlled-synthesis-and-processing-at-the-nanoscale-primarily-inorganic-26 .TOC-1}

# 6. NANOSCALE PRECURSORS AND ASSEMBLY {#nanoscale-precursors-and-assembly .TOC-1}

#  FOR MACROSTRUCTURES AND DEVICES [35](#for-macrostructures-and-devices) {#for-macrostructures-and-devices-35 .TOC-1}

# 7. UNDERSTANDING AND MIMICKING BIOLOGICAL FUNCTIONS:  {#understanding-and-mimicking-biological-functions .TOC-1}

#  "DRAWING INSPIRATION FROM NATURE" [46](#understanding-and-mimicking-biological-functions-drawing-inspiration-from-nature) {#drawing-inspiration-from-nature-46 .TOC-1}

# 8. HYBRID SYSTEMS: INTEGRATING FUNCTIONALITY [52](#hybrid-systems-integrating-functionality) {#hybrid-systems-integrating-functionality-52 .TOC-1}

#  {#section .TOC-1}

# 9. NANOSCALE INSTRUMENTATION [57](#nanoscale-instrumentation)  {#nanoscale-instrumentation-57 .TOC-1}

# 10. INFRASTRUCTURE AND FACILITIES  {#infrastructure-and-facilities .TOC-1}

#  FOR NANOSCALE SCIENCE AND TECHNOLOGY [67](#infrastructure-and-facilities-for-nanoscale-science-and-technology) {#for-nanoscale-science-and-technology-67 .TOC-1}

# APPENDIX: NANOMATERIALS RESEARCH CENTERS [70](#appendix-nanomaterials-research-centers) {#appendix-nanomaterials-research-centers-70 .TOC-1}

#  {#section-1 .TOC-1}

#   {#section-2 .TOC-1}

# 

# Basic Energy Sciences Nanoscience/Nanotechnology Group

> Chair: Douglas H. Lowndes (ORNL)
>
> A. Paul Alivisatos, Lawrence Berkeley National Laboratory (LBNL)
>
> Mark Alper, Lawrence Berkeley National Laboratory (LBNL)
>
> Robert S. Averback, University of Illinois--Urbana Champaign (UI--UC)
>
> Jacob Barhen, Oak Ridge National Laboratory (ORNL)
>
> Jeffrey A. Eastman, Argonne National Laboratory (ANL)
>
> Dan Imre, Brookhaven National Laboratory (BNL)
>
> Douglas H. Lowndes, Oak Ridge National Laboratory (ORNL)
>
> Ian McNulty, Argonne National Laboratory (ANL)
>
> Terry A. Michalske, Sandia National Laboratories (SNL)
>
> Kai--Ming Ho, Ames Laboratory (AL)
>
> Arthur J. Nozik, National Renewable Energy Laboratory (NREL)
>
> Thomas P. Russell, University of Massachusetts (UM)
>
> Richard A. Valentin, Argonne National Laboratory (ANL)
>
> David O. Welch, Brookhaven National Laboratory (BNL)
>
> NOTE: Information also was provided by university professors and
> national laboratory research staff. Contributors are listed on the
> inside back cover page.

#  {#section-4 .TOC-1}

**General Editor**

# 

> D. H. Lowndes (ORNL)

# Section Editors

> 1\. Introduction: D. H. Lowndes (ORNL)
>
> 2\. Emergence of Properties: D. O. Welch (BNL)
>
> 3\. Manipulation and Coupling of Properties: I. McNulty (ANL)
>
> 4\. Controlled Synthesis and Processing (Organic): A. P. Alivisatos
> (LBNL)
>
> 5\. Controlled Synthesis and Processing (Inorganic): R. S. Averback
> (UI--UC)
>
> 6\. Nanoscale Precursors and Assembly: A. J. Nozik (NREL)
>
> 7\. Understanding and Mimicking Biological Functions: M. Alper (LBNL)
>
> 8\. Hybrid Systems: T. A. Michalske (SNL)
>
> 9\. Nanoscale Instrumentation: J. A. Eastman (ANL)
>
> 10\. Infrastructure and Facilities: T. P. Russell (UM)
>
> Appendix. Nanomaterials Centers: D. H. Lowndes (ORNL)
>
> NOTE: Jacob Barhen (ORNL) edited contributions from the Engineering
> Research Program.

# EXECUTIVE SUMMARY

The principal missions of the Department of Energy (DOE) in Energy,
Defense, and Environment will benefit greatly from future developments
in nanoscale science, engineering and technology. For example, nanoscale
synthesis and assembly methods will result in significant improvements
in solar energy conversion; more energy-efficient lighting; stronger,
lighter materials that will improve efficiency in transportation;
greatly improved chemical and biological sensing; use of low-energy
chemical pathways to break down toxic substances for environmental
remediation and restoration; and better sensors and controls to increase
efficiency in manufacturing.

The DOE's Office of Science has a strong focus on nanoscience discovery,
the development of fundamental scientific understanding, and the
conversion of these into useful technological solutions. A key challenge
in nanoscience is to understand how deliberate tailoring of materials on
the nanoscale can lead to novel and enhanced functionalities. The DOE
National Laboratories are already making a broad range of contributions
in this area. The enhanced properties of nanocrystals for novel
catalysts, tailored light emission and propagation, and supercapacitors
are being explored, as are hierachical nanocomposite structures for
chemical separations, adaptive/responsive behavior and impurity
gettering. Nanocrystals and layered structures offer unique
opportunities for tailoring the optical, magnetic, electronic,
mechanical and chemical properties of materials. The Laboratories are
currently synthesizing layered structures for electronics/photonics,
novel magnets and surfaces with tailored hardness. This report supplies
numerous other examples of new properties and functionalities that can
be achieved through nanoscale materials control. These include:

-   Nanoscale layered materials that can yield a four-fold increase in
    the performance of permanent magnets

-   Addition of aluminum oxide nanoparticles that converts aluminum
    metal into a material with wear resistance equal to that of the best
    bearing steel

-   New optical properties achieved by fabricating photonic band gap
    superlattices to guide and switch optical signals with nearly 100%
    transmission, in very compact architectures

-   Layered quantum well structures to produce highly efficient,
    low-power light sources and photovoltaic cells

-   Novel optical properties of semiconducting nanocrystals that are
    used to label and track molecular processes in living cells

-   Novel chemical properties of nanocrystals that show promise as
    photocatalysts to speed the breakdown of toxic wastes

-   Meso-porous inorganic hosts with self-assembled organic monolayers
    that are used to trap and remove heavy metals from the environment

-   Meso-porous structures integrated with micromachined components that
    are used to produce high-sensitivity and highly selective chip-based
    detectors of chemical warfare agents

These and other nanostructures are already recognized as likely key
components of 21^st^ century optical communications, printing,
computing, chemical sensing and energy conversion technologies.

The DOE is well prepared to make major contributions to developing
nanoscale scientific understanding, and ultimately nanotechnology,
through its materials characterization, synthesis, in situ diagnostic
and computing capabilities. The DOE and its National Laboratories
maintain a large array of major national user facilities that are
ideally suited to nanoscience discovery and to developing a fundamental
understanding of nanoscale processes. Synchrotron and neutron sources
provide exquisite energy control of radiation sources that are able to
probe structure and properties on length scales ranging from Ångstroms
to millimeters. Scanning Probe Microscope (SPM) and Electron Microscopy
facilities provide unique capabilities for characterizing nanoscale
materials and diagnosing processes. DOE also maintains synthesis and
prototype manufacturing centers where fundamental and applied research,
technology development and prototype fabrication can be pursued
simultaneously. Finally, the large computational facilities at the DOE
National Laboratories can be key contributors in nanoscience discovery,
modeling and understanding.

In order to increase the impact of major DOE facilities on the national
nanoscience and technology initiative, it is proposed to establish
several new Nanomaterials Research Centers. These Centers are intended
to exploit and be associated with existing radiation sources and
materials characterization and diagnostic facilities at DOE National
Laboratories. Each Center would focus on a different area of nanoscale
research, such as materials derived from or inspired by nature; hard and
crystalline materials, including the structure of macromolecules;
magnetic and soft materials, including polymers and ordered structures
in fluids; and nanotechnology integration. The Nanomaterials Research
Centers will facilitate interdisciplinary research and provide an
environment where students, faculty, industrial researchers and national
laboratory staff can work together to rapidly advance nanoscience
discovery and its application to nanotechnology. Establishment of these
Centers will permit focusing DOE resources on the most important
nanoscale science questions and technology needs, and will ensure strong
coupling with the national nanoscience initiative. The synergy of these
DOE assets in partnership with universities and industry will provide
the best opportunity for nanoscience discoveries to be converted rapidly
into technological advances that will meet a variety of national needs
and enable the United States to reap the benefits of a technological
revolution.

# 1. INTRODUCTION

This report describes important future research directions in nanoscale
science, engineering and technology. It was prepared by members of the
Office of Basic Energy Sciences (BES) Nanoscience/Nanotechnology (N/N)
Group \[1\] in connection with an anticipated interagency national
research initiative titled "Nanotechnology for the Twenty--First
Century: Toward a New Industrial Revolution" \[2\].

The national initiative, described in the report of the Interagency
Working Group on Nano Science, Engineering and Technology (IWGN) \[2\],
identifies five "high priority" research areas for additional funding
beginning in FY2001. The first and largest of these is "long--term
science and engineering research leading to new fundamental
understanding and discoveries of phenomena, processes and tools for
nanotechnology". The IWGN report identifies the major DOE interest in
nanotechnology as basic energy science and engineering. It also mentions
several modes of R&D support that will be needed by DOE/BES to ensure
the success of a nano science and technology initiative. Among these are
appropriate capital resources for the national labs; secondary funding
of universities for collaborations with DOE labs; funding for the
national labs to work with other government agencies, and later with
industry; and several new national user facilities focused on
nanomaterials. The national nanotechnology initiative is considered
highly collaborative and interdisciplinary. It is expected to have
substantial long--term impact, with the stated intention of facilitating
a technological revolution and ensuring that the United States reaps the
benefits.

The reason that nanoscale materials and structures are so interesting is
that size constraints often produce qualitatively new behavior. We now
understand, in a general way, that when the sample size, grain size, or
domain size becomes comparable with a specific physical length scale
such as the mean free path, the domain size in ferromagnets or
ferroelectrics, the coherence length of phonons, or the correlation
length of a collective ground state like superconductivity, then the
corresponding physical phenomenon will be strongly affected. Although
such changes in behavior can be the dominant effects in nanoscale
structures, we still have remarkably little experience or intuition for
the expected phenomena and their practical implications, except for
electronic systems. The physics, chemistry and biology of phenomena
occurring in nanoscale systems is effectively a new subject with its own
set of physical principles, theoretical descriptions, and experimental
techniques, which we are only in the process of discovering. Thus, there
is an urgent need for broadly based investigations of the physical
phenomena associated with confined systems, especially in materials and
structural contexts where the implications are not at all well
understood.

Nanotechnology arises from the exploitation of new properties,
phenomena, processes, and functionalities that matter exhibits at
intermediate sizes between isolated atoms or molecules (\~ 1 nm) and
bulk materials (over 100 nm). As opposed to the microscale, the
nanoscale is not just another step towards miniaturization, but is a
*qualitatively new* scale. Here quantum and size phenomena are allowed
to manifest themselves either at a purely quantum level or in a certain
"admixture" of quantum and classical components. At the foundation of
nanosystems lie the quantum manifestations of matter that become
relevant and measurable. Consequently, instead of being a limitation or
an elusive frontier, quantum phenomena have become the crucial enabling
tool for nanotechnology \[3\].

This report consists of 10 sections plus an Appendix. Each section deals
with a particular nanoscale research theme. The themes were selected by
the BES N/N Group as an effective means of organizing and explaining a
complex and highly interdisciplinary subject. Thematic organization also
permits focusing attention on fundamental ideas with broad implications
for nanoscale science, and illuminating the most important
opportunities, challenges and barriers to progress, as we seek to
understand phenomena and processes on the nanoscale and use this
knowledge as the basis for a new nanotechnology.

Most sections of this report follow a similar format. First, a brief
summary is given of selected BES and national laboratory capabilities
and recent research accomplishments, in order to indicate the current
status of research on each scientific theme. Next comes a discussion of
the important scientific issues and opportunities that can be addressed
through the national laboratories and universities. The principal
problems and barriers to progress are identified wherever possible.
Finally, future research directions and corresponding research needs are
outlined, with emphasis on the promise and potential of certain choices.
Thus, this report mainly presents *ideas* regarding questions to be
answered and the *kinds* of things that could be done by BES in order to
take a leadership position in nanoscale science, engineering and
technology.

In order to unequivocally identify and characterize nanoscale phenomena
it is necessary to directly probe the small samples in which they occur.
This is problematic, because sub--micron samples produce tiny signals in
most relevant physical measurements. Possible ways to address this
problem, by developing new, high sensitivity nanoscale instrumentation,
are discussed in section 9.

BES is especially well prepared for materials, chemical, biological and
engineering \[4\] research on the nanoscale because of its substantial
investment in major scientific user facilities, among which are several
synchrotron x--ray and high--flux neutron sources. The wavelength or
energy range of the radiation provided by these sources is ideally
suited for the reciprocal or real space characterization of nanoscopic
structures. Their high flux makes possible probing either static
nanoscale structure or real--time changes. From many other perspectives
the match of synchrotron x--ray and neutron sources to nanoscale science
is also ideal, as described in section 10. However, the potential exists
for much more effective use of these sources by consolidating and better
focusing the nanomaterials research effort. This requires an investment
in synthesis and characterization facilities that would be focused on
specific nanomaterials research areas and associated with existing major
photon and neutron sources, which can be accomplished by establishing
several Nanomaterials Research Centers at national laboratories.
Establishing such Centers also will facilitate a necessary change in the
current infrastructure and scientific mode of operation. The research
efforts of scientists who are currently trained in different academic
disciplines will be focused into interdisciplinary research groups. The
Centers will provide the infrastructure support necessary for these
scientists, for large numbers of external users, and for the
interdisciplinary training of students and postdocs to perform nanoscale
research effectively and efficiently. The rationale for establishing
Nanomaterials Research Centers is presented in more detail in section
10, and an outline description of possible focal research areas for the
Centers is given in the Appendix to this report. These Centers, in
association with the principal BES x--ray, neutron and nanofabrication
facilities, are expected to be among the most important tools for the
national nanotechnology initiative.

**References**

1\. The members of the BES Nanoscience/Nanotechnology Group are listed
in the Preface, and the contributions of some members as section editors
for this report also are acknowledged.

2\. [Nanotechnology for the Twenty--First Century: Toward a New
Industrial Revolution]{.underline}, report of the Interagency Working
Group on Nano Science, Engineering and Technology (IWGN, chaired by M.
Roco, National Science Foundation) to be issued September, 1999.

3\. D. N. Noid, R. F. Tuzun and B. G. Sumpter, "On the Importance of
Quantum Mechanics for Nanotechnology," *Nanotechnology* **8**, 119
(1997).

4\. The relationship of the BES Engineering Research Program (ERP) to
nanotechnology follows a path similar to the program's involvement with
engineering research for larger--scale systems. It is envisioned that
eleven topical areas will fall within the programmatic scope of ERP.
These areas, along with understanding the quantum relationships and any
thermodynamic, chaotic or nonlinear issues that could influence the
underlying nanosystems, will capture the important engineering paradigms
to be studied and developed under ERP. No programmatic priority is
implied in the following list.

-   Manufacturing procedures to build tomorrow's nano--machines beyond
    the current and immediate future silicon--based fabrication
    technology.

-   Property characterization (thermal, mechanical, optical, chemical,
    biological) at the nanoscale.

-   Modeling of the complex behavior of extremely tiny devices at the
    fundamental molecular--microscopic-- components and assembly levels.

-   Innovative control methods for nanosystems.

-   Nanoelectronic computing devices.

-   Using quantum phenomenology for new communications paradigms.

-   Reliable packaging of nano--devices.

-   Innovative solutions for energy redistribution, powering and
    actuation of the nano--machines.

-   Micro--instrumentation for on--line diagnostics and control.

-   Infrastructure for testing procedures suitable for nanodevices (in a
    manner analogous to what is now done for IC devices).

-   Diagnostics and manipulation of nano--systems for usage,
    environmental consideration and ability to construct larger systems
    from the nano--components.

-   Friction, lubrication, cracking, failure analysis and nondestructive
    testing in a nanometer world.

# 2. EMERGENCE OF PROPERTIES AT MULTIPLE LENGTH AND TIME SCALES

The era of materials technology that we are now entering presents an
unprecedented opportunity to understand the emergence of new materials
properties on the nanometer scale \[1\] and to exploit this knowledge
for our benefit. The combination of tools responsible for this
opportunity includes the ongoing development of synthesis methods that
permit atomic and electronic structure to be controlled down to near the
atomic scale, in one through three dimensions; the actual or expected
availability of instrumental probes that are capable of characterizing
nanoscale materials and structures, as well as their properties, on a
wide range of length and time scales; and, the continuing explosive
growth of computational power which permits theoretical exploration of
structure--properties relationships on scales from the atomic to the
macroscopic.

Thus far, the most concentrated effort to apply the tools of synthesis,
characterization and modeling, in order to understand the consequences
of controlling nanoscale structure, has been in studies of the
electronic properties of semiconductors and related materials, driven
primarily by their vital importance to information technology \[2\].
Nevertheless, it is quite clear from this experience that many other
novel combinations of properties can be expected to emerge, for a wide
range of materials, upon entering the world of the nanoscale.

*Effects of Size Constraints*

Size constraints alone often produce *qualitatively* new behavior. For
example, if the size of a nanoscale structure becomes less than the
characteristic length scale for scattering of electrons or phonons (the
mean free path) this can lead to qualitatively new modes of transport
for electrical current and/or heat. This effect explains the discovery
in the early 1980s of persistent normal (non--superconducting) currents
in mesoscale metallic rings \[3\], and more recently of the ballistic
transport of current in carbon nanotubes \[4\]. Other examples are the
remarkably rich set of new phenomena that arise when the de Broglie
wavelength of carriers is constrained by the dimensions of quantum dots
\[1, 2\].

Thermodynamic properties, including collective phenomena and phase
transitions such as ferromagnetism, ferroelectricity, and
superconductivity, have long been expected to demonstrate substantial
changes when structures contain a small number of the relevant particles
or when the system size is comparable to the particle size or the
coherence length for collective behavior \[5\]. Systems with component
sizes ranging from a few tenths to about ten nanometers lie at the fuzzy
boundary between the quantum and classical domains. Such systems are
also in the size range where thermal energy fluctuations and Brownian
motion can have significant effects.

Contemporary needs in data storage and information technology have
stimulated considerable research activity in nanoscale magnetism \[1\].
One area presenting great opportunities is the magnetism of complex
molecules containing tens to hundreds of atoms with localized magnetic
moments. Such "molecular magnets" exhibit many unusual properties
including tunneling, quantum coherence, and thermo--induced spin
crossover transitions. Possible technological applications include the
use of magnetic molecules in medicine \[6\]. Exchange--coupled nanoscale
structures also provide a path toward the next generation of powerful
permanent magnets \[7\]. An approach to electronics based on spin,
rather than the charge of electron and hole carriers, in nanostructured
magnetic devices shows great promise \[8\]. Ultrathin films (a few
atomic layers) yield quasi--two--dimensional magnetic materials, reveal
novel magnetic domain physics, and provide a fascinating arena for
studying the gradual onset of magnetism \[9\].

The mechanical properties of materials also change dramatically as the
grain size in polycrystalline materials or the concentration of strain
fields approaches the nanometer scale \[10\]. This was demonstrated
recently by nanoindentation experiments \[11\] which showed that
atomic--height steps on Au single--crystal surfaces have dramatic
effects on the yield strength. Computer simulations suggest
significantly higher mechanical strength of grain boundaries in
nanocrystalline diamond than those in coarse--grained diamond films
\[12\].

Changes in the strength of nanoscale structural elements, changes in the
nature of friction, and changing modes of fluid flow (hence, of
lubrication), all will require new design strategies for micromachines.
The impact of such changes becomes dramatic as the scale of such
machines approaches the nanoscale \[13\]. Modes of failure also will
change, as the scale of devices and machines decreases toward the
nanoscale. The causes include different mechanical properties that will
modify fracture characteristics; the increased importance of surface
tension; and, the enhanced role of diffusion and corrosion at the large
surface--to--volume ratios that will occur. Even the notion of the
thermodynamic efficiency of a nanoscaled machine or device will differ
from its macroscopic, or even microscopic, counterpart because of
thermal fluctuations, quantum noise \[13\] and changes in the efficiency
of nanoscaled engines \[14\]. This is all largely unknown territory.

*Shift of Characteristic Time Scales*

This discussion has focussed so far on the emergence of new properties
as the characteristic length scale decreases into the nanoscale regime.
However, such a length scale change will be accompanied by concomitant
changes in the characteristic time scale of physical phenomena. In part,
this is no more than the increase in characteristic frequencies that
follows from the decreased time required to transit shorter distances at
a fixed propagation velocity (for phonons, photons, electrons, etc.).
However, genuine changes of regime also will occur. These include an
increased importance of surface phonons compared with volume phonons,
and ballistic electron transport rather than the scattering--limited
transport described by the Boltzmann equation.

Another time--scale phenomenon that will emerge is the increased rate of
kinetic processes because of the increased fluctuation rate as the
system size decreases and as the reduced dimensionality of important
structural features (e.g. surface--to--volume ratio) becomes important
or dominant at the nanoscale. The latter effect of course also leads to
the increased effectiveness of sensor elements in biological systems.
Very large surface--to--volume ratios must at least be taken into
account, and possibly exploited, in man--made nanosystems \[15\]. The
shift of characteristic time scales from this variety of mechanisms
provides opportunities *but will also require the use of instrumentation
with improved time resolution and of ultrafast probes*.

*Modeling and Simulation*

The emergence of genuinely new phenomena at the nanoscale creates a
great need for theory, modeling and large--scale computer simulation in
order to understand the new nanoscale phenomena and regime. The links
between the electronic, optical, mechanical, and magnetic properties of
nanostructures and their size, shape, topology, and composition are not
well understood, although for the simplest semiconductor systems, carbon
nanotubes, and similar "elementary" systems there has been considerable
progress. However, for more complex materials and hybrid structures even
the basic outlines of a theory describing these connections remains to
be made.

In nanoscale systems, thermal energy fluctuations and quantum
fluctuations are comparable to the activation energy scale of the
materials and devices, so that statistical and thermodynamic methods
must include these effects adequately. Stochastic simulation methods, as
well as computational models incorporating quantum and semiclassical
methods, are required to evaluate the performance of nanoscale devices.
Consequently, computer simulations, both electronic--structure--based
and atomistic, will play a major role in understanding materials at the
nanometer scale and in the development "by design" of new nanoscale
materials and devices. The vastly increased computational capabilities
that will become available through the Scientific Simulation Initiative
(SSI) will play an important role and will provide many opportunities
for substantial progress in this area. Moreover, by exploiting quantum
phenomena that have no classical analogues, it is expected that certain
computational tasks can be performed much more efficiently than can be
done by any classical computer (see the box on the following page).

In addition to such *ab--initio* and large--scale computational methods,
there also is a continuing need for phenomenological methods such as
Ginzburg--Landau models and mean--field methods to aid in understanding
and to develop intuition. There are great opportunities for all scales
of theory and modeling in understanding behavior at the nanoscale, but
perhaps the greatest challenge and opportunity will be in those
transition regions where nanoscale phenomena are just beginning to
emerge from the macroscopic and microscale, regimes which are
describable by bulk properties plus the effects of interfaces and
lattice defects.

**Quantum Computing and Communications at the Nanoscale**

The possibility of transferring quantum information between arbitrarily
remote locations (i.e., *quantum teleportation* \[17\]) is one of the
cornerstones of the emerging field of quantum communications \[18,19\].
Spectacular refinements of the experimental techniques and equipment
make it possible for rather modest laboratories to perform "purely
quantum" experiments \[20\] with far reaching epistemological,
scientific, and practical consequences \[21\].

![](media/image1.png)

**Figure 2.1:** Femtosecond laser system used in teleportation
experiments

*Entanglement* is the key to a new realm of quantum phenomena that,
until very recently, could not be observed, analyzed, or utilized
\[22--24\]. It is an *inherently quantum* (i.e. non--classical) property
that offers reasonable and consistent explanations to the violation of
Bell's inequalities or Schrödinger's famous cat paradox.
Quantum--mechanical analysis yields predictions of non--classical
behaviors such as amplitude--phase squeezing or "apparent violation of
causality" in experiments involving entangled states. Moreover
entanglement offers solid prospects for implementing secure (quantum)
cryptography, quantum--computing algorithms, and quantum teleportation.

Measurements performed on such quantum systems would unequivocally
demonstrate non--local behavior with unprecedented practical relevance
\[25\]. For instance, if a signal beam (entangled part of the wave
function to the 'target') is sent to one person and the idler beam
(entangled part to the 'detector'), one person can modulate their own
locally received beam and the other can observe the results of the
modulation via the visibility parameter. A message sent in this fashion
is secure from interception because an eavesdropper lacks the
unmodulated beam, which is the 'receiver' beam. If an eavesdropper
managed to intercept the unmodulated beam, the person intending to
perform the modulation would know it by performing a simple measurement
before modulating.

In other applications, intervention on one side cannot be dismissed as a
causal influence on the other. In some sense, this implies
"*faster--than--light*" (FTL) communication, where *no* energy or
particles are travelling faster than light, but the wave function *is*
\[26\]. The dramatic miniaturization in computer technology over the
past few decades is rapidly reaching the point at which we will be
forced to use quantum physics to describe elementary computational
operations \[27\]. Thus, at the nanoscale the very theory describing
what computers can do must be revised.

Initially, researchers in quantum computing tried to understand how the
basic operations of a conventional computer might be accomplished using
quantum mechanical interactions. However, it was soon realized that
quantum physics offered something genuinely new \[28\]. By exploiting
delicate quantum phenomena that have no classical analogues, it is
possible to do certain computational tasks much more efficiently
\[29,30\] than can be done by any classical computer. Moreover, these
same quantum phenomena allow unprecedented tasks to be performed such as
breaking supposedly unbreakable codes, generating true random numbers,
and communicating with messages that betray the presence of
eavesdropping. In summary, quantum computing and communications are
expanding the foundations of information processing in a way that is
consistent with quantum physics -- the most accurate model of reality
that is currently known.

####### References

1\. G. Timp, editor, [Nanotechnology]{.underline}, AIP Press,
Springer--Verlag (New York, 1999).

2\. Special Issue on Quantum Dot Structures, Japanese Journal of Applied
Physics, Vol. 38, No. 1B (1999).

3\. L. L. Sohn, L. P. Kouenhoven, and G. Schoen, editors; [Mesoscopic
Electron Transport]{.underline}, Kluwer (Dordrecht, 1996).

4\. S. Frank, P. Poncharal, Z. L. Wang, and W. A. de Heer, "Carbon
Nanotube Quantum Resistors," *Science* **280**, 1744 (1998).

5\. Terrell L. Hill, [Thermodynamics of Small Systems]{.underline},
Dover (New York, 1994).

6\. E. Coronado, P. Delhaes, D. Gatteschi, and J. S. Miller, editors:
[Molecular Magentism: From Molecular Assemblies to Devices]{.underline},
NATO AISI Series E. Vol. 321, Kluwer (Dordrecht, 1996).

7\. D. Givord and M. F. Rossingol, "Coercivity," p. 210, in [Rare Earth
Permanent Magnets]{.underline}, J. M. D. Coey, editor, Oxford University
Press (Oxford, 1997).

8\. G. A. Prinz, "Magnetoelectronics," *Science* **282**, 1660 (1998).

9\. J. Harris and D. Awschalom, "Thin films squeeze out domains,"
*Physics World* **19**, (1999).

10\. Special Issue on "Mechanical Behavior of Nanostructured Materials,"
MRS Bulletin, Vol. 24, No. 2, February 1999.

11\. T. A. Michalske and J. E. Houston, "Dislocation Nucleation at
Nano--Mechanical Contacts," *Acta Mater.* **46**, 391 (1998).

12\. P. Keblinski et al., "Amorphous Structure of Grain Boundaries and
Grain Junctions in Nanocrystalline Silicon by Molecular--Dynamical
Simulation," *Acta Mater.* **45**, 987 (1997); D. Wolf et al.,
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13\. J. Knight, "The Dust Mite's Dillema," New Scientist, April 1999, p.
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14\. J. Travis, "Making Light Work of Brownian Motion," Research News,
*Science* **267**, 1593, 1995; B. G. Levi, "Measured steps advance the
understanding of molecular motors," Search and Discovery, *Physics
Today*, **19**, (1995).

15\. R. McGraw and R. LaViolette, "Fluctuations, temperature, and
detailed balance in classical nucleation theory," *J. Chem. Phys*.
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16\. G. Adam and M. Delbrück, "Reduction of Dimensionality in Biological
Diffusion Processes," p. 198, in [Structural Chemistry and Molecular
Biology]{.underline}, A. Rich and N. Davidson, editors, W. H. Freeman
(San Francisco, 1968).

17\. D. Bouwmeester, J.--W. Pan, K. Mattle, M. Eibi, H. Weinfurter, and
A. Zeilinger, "Experimental Quantum Teleportation," *Nature* **390**,
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18\. Special Issue: Quantum Communications, *J. Mod. Optics* **41**
(1994).

19\. O. Hirota, and C.M. Caves, Quantum Communication, Computing, and
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21\. A. Watson, "Entangled Trio to Put Nonlocality to the Test,"
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30\. G. Berman, et al*.,* "Introduction to Quantum Computers," *World
Scientific* (1998).

# 3. MANIPULATION AND COUPLING OF PROPERTIES AT THE NANOSCALE

**A. Current Status**

######### Semiconductor Properties

Research on the optical, electronic, and transport properties of
semiconductors at the nanoscale is experiencing astonishing growth. The
fundamental properties of nanoscale semiconductor structures can be
dramatically altered by controlling their size and shape without
changing their composition. When the electrons and holes in
semiconductors are confined to dimensions less than their de Bröglie
wavelength (typically 1--30 nm) quantum mechanical size effects appear.
The carrier confinement can be in one dimension (quantum films or
quantum wells), two dimensions (quantum wires), or three dimensions
(quantum dots, QDs, or nanocrystals). The most dramatically affected
electronic properties include: (a) a large blue--shift of the absorption
edge with decreasing size, which can be several electron--volts for
semiconductor nanocrystals, resulting in absorption or photoluminescence
that can be tuned to range from near--UV through the visible to the near
IR; (b) conversion of optical spectra from continuous bands to discrete
lines; (c) concentration of oscillator strengths in fewer allowed
transitions, resulting in greatly enhanced non--linear optical
properties; (d) enhanced photoredox and photocatalytic properties due to
higher redox potentials of photogenerated electrons and holes; (e)
conversion of direct bandgap semiconductors to indirect bandgap
semiconductors and vice versa; (f) slowed relaxation and cooling of
photogenerated hot carriers, resulting in reduced electron--phonon
interactions; (g) greatly enhanced excitonic transitions at room
temperature; (h) an optical absorption coefficient that increases with
the inverse--cube of the nanocrystal radius.

In the newly--coined field of "nano--optics" the wavefunction of a QD is
tailored for a specific response. Electronic and optical properties of
QDs have been observed that closely resemble those of simple atomic
systems, including sharp line spectra and relatively simple nonlinear
optical signatures \[1\]. In recent experiments employing wavefunction
engineering techniques, the excitonic wave function in a single QD was
manipulated and monitored on a time scale short compared to the loss of
quantum coherence, by controlling the phase and polarization of
two--picosecond optical pulses \[2\]. These experiments have extended
the concept of coherent control in semiconductor nanostructures to the
limit of a single quantum system in a zero--dimensional QD, and provide
the basic tools to create more complex wavefunctions in future such
"solid state atoms".

######### Ballistic Electron Devices

One of the challenges of electron physics is to create devices that
allow controlled charge and energy transport at nanometer length scales.
Such nanoscale quantum devices may be components in electrical circuits
or they could provide sources of stimulated coherent radiation. They
might be fabricated from semiconductor wafers, clusters of atoms, carbon
fibers, or even from biological molecules. Ballistic electron devices
comprise one class of quantum devices that hold great promise for future
communications applications. The size and energy at which they operate
directly determines their characteristic switching times. For example,
present--day nanoscale electron waveguides are typically fabricated from
semiconductor materials, operate at very low temperatures, and typically
have picosecond switching times. The recent discovery of conduction
quantization in carbon nanotubes \[3,4\] provides hope that such devices
might eventually be fabricated to operate with far higher switching
speeds at room temperature.

The dynamics of ballistic electron propagation can be studied using the
two--dimensional electron gas (2DEG) that can be created at the
interface of two semiconductor materials such as GaAs and AlGaAs.
Electron channels and cavities of variable shape can be formed at such
an interface by means of gates placed on the outer surface of the
semiconductor wafer. The observation \[5\] that electron conductance is
quantized for electron currents passing between point contacts separated
by approximately 100 nm showed conclusively that the wave nature of
electrons plays a critical role at the nanoscale. At temperatures of 1K
or lower, the electron--phonon interaction can be neglected and the
electron motion becomes ballistic. The shape of the confining walls in
the two--dimensional plane can be controlled and strongly influences the
waveguide conductance \[6\].

Much of the research effort on electron waveguides until now has focused
on the effect of quantum chaos on conductance, both in the presence and
absence of external magnetic fields. Waveguide cavities used in 2DEG
experiments typically have widths and lengths of order 100--1000 nm. In
this regime, enough propagating modes are present that semiclassical
methods (such as periodic orbit theory) can be used to make predictions
about the electron conductance. Numerical and some laboratory work \[6\]
has shown that the electron conductance has qualitatively different
behavior depending on whether the waveguide cavities give rise to
integrable or chaotic behavior. One of the important discoveries in
quantum chaos research is that if a quantum chaotic system has a
classical counterpart that is fully chaotic, then it exhibits universal
statistical behavior in qualitative agreement with the predictions of
random matrix theory \[7\]. Recently, a possible connection has been
made between these predictions for the effects of chaos and disorder in
electron waveguides. However, as the size of electron waveguides becomes
closer to the nanoscale we enter the quantum regime, where far less is
known about the electron dynamics.

One reason for the need to understand the quantum influence on
conductance is the possibility of developing more energy--efficient and
much higher density electronic circuitry at the nanoscale. It has been
shown that integrable waveguides may have better electron--trapping
ability than chaotic waveguides \[8\]. It appears that the conductance
in integrable systems can be shut down and electrons stored for longer
periods. Waveguides formed at semiconductor interfaces (by means of
gates on the outer surface) have leads and cavities which are soft, and
the dynamics of these systems is more integrable than chaotic. Indeed,
recent work on the conductance of softwall waveguides with mixed
dynamics has shown fractal fluctuations in the conductance as a weak
magnetic field is varied \[9\]. Resonances give rise to waveguide
capacitance in the presence of high--frequency electromagnetic driving
fields, allowing enhanced power absorption \[10\]. An important problem
about which little is currently known is the effect that Coulomb
interactions between electrons have on the electron conductance in the
fully quantum regime. A related development, based on the nonlinear
charge distribution in capacitively coupled conductors, is the
possibility of quantum scanning capacitance microscopy at the nanoscale
\[11\].

######### Magnetic Properties

Dramatic quantization and other effects also occur in magnetism and
magnetic materials at the nanoscale. The ability to control thin--film
growth at the near--atomic level to form epitaxial and heteroepitaxial
structures has recently been extended to magnetic nanostructures,
including metallic, oxide, and semiconducting phases \[12,13\]. This
progress is driven both by the intriguing physics of these materials and
by the \$50B per year magnetic storage industry (\$150B per year if
magnetic recording tape, video, etc. are included). Materials advances
have opened up entirely new avenues of study. To illustrate, theoretical
limits suggest that a near doubling of the energy product of hard
magnets may be possible with an optimum nanoscale configuration of
Fe--Co, Nd--Fe--B, or Nd--Fe--Al bulk metallic glass \[14,15\]. However,
putting this on firm footing requires a better understanding of
coercivity development and domain wall pinning in nanocomposite hard
magnets, how coercivity is developed in bulk metallic glasses, and
extensive microstructural evaluation to determine the makeup and
locations of atomic clusters that contribute to the magnetic remanence.

######### Thermal Properties

Thermal transport properties of nanostructured materials have received
relatively little attention in the past decade. It is well known that
polycrystalline materials exhibit lower thermal conductivity than
low--defect single crystals of the same material. Several investigators
realized recently that this could result in significantly reduced
thermal conductivities in nanostructured materials such as
yttria--stabilized zirconia (YSZ), which could lead to improvements for
applications such as thermal barrier coatings \[16,17\]. Reduced thermal
conductivities are expected because of a reduction in the phonon mean
free path due to grain boundary scattering. Indeed, recent research has
determined that the thermal conductivity is reduced approximately
two--fold at room temperature in 10--nm grain--sized YSZ, compared to
coarse--grained or single crystal YSZ \[18\]. While several groups are
now investigating the thermal properties of nanostructured materials,
there is currently no detailed understanding of nanostructure--thermal
properties relationships. Unexplored at present, but perhaps even more
interesting, is the effect of grain size on the thermal properties of
high--conductivity phonon--based conductors such as diamond.

In contrast to the reduced thermal conductivity expected for
nanostructured thin films or coatings, opportunities exist for
increasing thermal transport rates in fluids by suspending
nanocrystalline particles in them. These "nanofluids" have recently been
shown to exhibit substantially increased thermal conductivities and heat
transfer rates compared to fluids that do not contain suspended
particles \[19\]. However, there is again no real understanding of the
mechanisms by which nanoparticles alter thermal transport in liquids.
Multibillion--dollar industries, including transportation, energy,
electronics, textiles, and paper, employ heat exchangers that require
fluids for efficient heat transfer. If researchers can improve these
fluids, then manufacturers can make heat exchange systems smaller and
lighter, and they can reduce the amount of pumping energy and
heat--transfer fluid required.

######### Energy Conversion and Transport

Energy conversion and transport in nanostructures impacts a variety of
fields and applications. Although energy conversion and transport at
macroscales is relatively well understood, it is not clear how it occurs
at the nanoscale. For example, it is well known that thermoelectric
refrigerators and engines are not as efficient as other energy
conversion devices because heat conduction by phonons is too high in
thermoelectric materials. There is partial and currently controversial
evidence that nanostructuring can both improve electron transport
\[21--23\] and reduce phonon transport \[24--27\]. However, it is not
clear how the phonon transport is actually reduced. Similarly, the
mechanisms for friction, and for the control of friction, at the
nanoscale are not understood (see the box following "Future Research
Directions" below).

######### Photonic Crystals

Photonic crystals have emerged as unique structures with the capability
to manipulate the flow of light energy as well as its coupling with
excited electronic states. Many novel applications of these structures
have been proposed in the areas of optoelectronics and photocatalysis
\[28--31\]. To create photonic crystals operating at optical wavelengths
the smallest feature sizes must be of the order of 100 nm, clearly in
the realm of nanotechnology. State--of--the--art semiconductor
technology \[32,33\] has recently been used to fabricate
three--dimensional photonic--bandgap crystals at 1.5 µm and 10 µm
wavelengths, using stacks of microfabricated polysilicon bars. Further
development of lithography techniques (such as two--photon methods) at
smaller length scales will be needed to extend this technology to
visible wavelengths.

There has been rapid progress recently in the development of alternative
approaches \[34\] to fabricate photonic crystals by using
self--assembled systems that order spontaneously \[35\] at nanoscale
dimensions. Organic or inorganic spheres or block--polymers have been
used as the starting material. The photonic crystals fabricated by such
techniques consist of porous air cavities in an interconnected
dielectric background. The wavelength of the photonic band gap is
controlled by both the spatial periodicity and properties of the
dielectric crystal. Further development in self--assembly techniques
should allow assembling these structures into integrated device systems
at low cost \[36\].

Great advantages are expected from these structures, including the
control of radiative properties and lifetimes that in turn can control
chemical reactions and catalysis. One novel phenomenon is the
localization of light in a disordered photonic crystal, in analogy to
the localization of electrons in disordered systems \[37\]. Such
photonic--crystal structures have immense potential for a large variety
of optoelectronics devices. Another remarkable recent development with
interesting potential applications is the demonstration of organic
photonic--crystal lasers \[38\].

######### Optical Matter

Developments in the nascent field of "optical matter" have demonstrated
the production of highly organized, multidimensional states of combined
matter and light fields \[39\]. By using crossed laser beams to create
carefully controlled standing wave patterns, microscopic structures such
as polystyrene spheres or more recently, various alkali atoms, can be
trapped in the resulting arrays of potential wells to form "optical
crystals" \[40--42\]. The methods employed are an outgrowth of the
optical molasses techniques developed for ultrahigh precision atomic
spectroscopy at micro--Kelvin temperatures \[43\]. Optical crystals have
demonstrated unusual binding, coupling, and energy--quantization
properties, and they similarly offer possibilities for precision
spectroscopy.

Optical matter has been considered by T. Hänsch and others to be an
entirely new state with inherently different properties than
conventional states of matter \[40--42\]. To date, optical--matter
experiments have utilized only visible--light wavelengths. However, the
groundwork has been laid for extension of the methods and theory to the
nanometer length scale. The principal means to achieve this would be by
preparation of intense coherent x--ray standing--wave fields using
perfect--crystal optics. The precursors for this technology have already
been used for surface structure determination \[44\] and have been
proposed for photo--assisted materials growth at nanometer length scales
\[45\]. The anomalous propagation of on--resonance standing--wave fields
in crystals (Borrmann effect) may also find application. \[46\].

######### Role of Theory and Simulation

The optical, electronic, magnetic and mechanical properties of
nanostructures depend sensitively on their size, shape, composition and
outer--shell structure. The links between these properties and degrees
of freedom are incompletely understood but are crucial to their eventual
technological application. Establishing this link is the main challenge
for theory in the field of nanostructures.

Quantum confinement provides a means to control the optical properties
of nanostructures via their size, but different shapes can dramatically
alter nanostructure properties even at constant size. Composition (e.g.,
InxGa1--xAs) changes the confinement potential and the interfacial
strain, thus also altering the properties. The outer--shell structure
determines how much of the wave function is contained in the
nanostructure and how much "spills out", which also can affect the
electronic properties. Size, shape, composition, and outer--shell
structure are controlled during growth or subsequent processing of the
nanostructure. For example, one can grow spherical or pyramidal InAs
dots via colloidal and Stranski--Krastanov methods, and the outer shell
can be organic (e.g., TOPO) or inorganic (GaAs). Sizes can be controlled
via colloidal chemistry. Similar variability in size, shape, composition
and outer--shell structure is afforded by many other growth methods.

The current status of nanostructure theory is as follows:

\(a\) The theory of electronic and optical properties includes the
effective--mass theory and its [k]{.underline}·[p]{.underline}
generalization, tight--binding, and full--pseudopotential methods. These
approaches are able to treat the link between size and shape and the
nanostructure optical and electronic properties with various levels of
precision and accuracy. Recent applications include InP, CdSe, and InAs
free--standing and embedded QDs.

\(b\) The theory of magnetism and spin in nanostructures is in its
infancy. Most theories do not yet treat the spin variable and the
magnetic moments.

\(c\) The theory of thermodynamics and stability in nanostructures is
also incomplete. Methods are available for small nanostructures (less
than 100 atoms), for example the local density approximation, but the
methods known for larger systems are of lower quality.

**B. Key Scientifc Questions**

Many questions remain unanswered concerning the coupling of properties
to our advantage at the nanoscale. For instance, we are pressed to
inquire:

\(a\) How do QDs couple electronically when formed into arrays and how
does order versus disorder affect the coupling? Does the electronic
structure change once the QDs are spatially organized, e.g. vertically
versus laterally? What is the maximum possible inter--dot conductivity?
Can "minibands" be formed analogous to those that are formed in
quantum--film superlattices?

\(b\) How does the ability of a nanostructure to accept carriers affect
its properties ("Coulomb blockade")? Is carrier transport between QDs
characterized by hopping, and can good carrier transport (e.g.
photoconductivity and photovoltaic effects) be obtained?

\(c\) Can magnetism be instilled in a non--magnetic material via quantum
size effects, and what controls the spin lifetime?

\(d\) How does size quantization affect the kinetics of photoinduced
electron transfer between semiconductors and molecules?

\(e\) Can a QD of an indirect--gap material (Si, Ge, GaP) become
optically direct?

\(f\) Can useful non--linear optical devices for optical computing,
information storage and transport be achieved with QDs or other
nanostructured materials?

\(g\) Can phonon transport be *reduced* by using nanostructured
superlattice materials, and can nanostructuring improve the
thermoelectric figure of merit by *improving* electron transport?

Closely related materials questions that must be addressed include:

\(a\) How does the shape of the nanostructure affect its properties?

\(b\) How are the photocatalytic properties affected?

\(c\) Are there surface states in nanostructures, and what is the effect
of the surface structure on their properties?

\(d\) Can one alter the optical response of a nanostructure by changing
its outer--shell structure (e.g. by passivation)?

\(e\) Is the solubility of dopants in nanostructures reduced or enhanced
relative to the bulk? Can QDs be doped?

Many of the above questions can be addressed convincingly using
theoretical tools that are available now or under construction. This
will establish a direct link with experiments and open the way for
nanostructured design of targeted properties. One immediate hurdle is to
develop theoretical tools to describe accurately the electronic
structure of large QDs (104 -- 106 atoms).

Ballistic electron devices, in particular, test the limits of
theoretical knowledge because they involve small systems where boundary
conditions may play a dominant role and the wave nature of the electron
is of fundamental importance. In these systems the interplay of electric
and magnetic interactions is not yet fully understood. The devices
themselves generally are open systems involving electron--decay
processes and tunneling phenomena. Multi--photon or multi--phonon
interactions also may be important. Their dynamics may be chaotic or
regular, or may involve a mixture of chaotic and resonant behavior. The
mechanisms governing electron transport in 2DEGs still are not
completely understood. At temperatures of the order of 1K the
interaction of electrons with phonons is negligible. However, the
electrons, whose motion is largely phase--coherent, still experience
long--range Coulomb interactions, random electric and magnetic fields
due to impurities, and may exhibit manifestations of chaos due to
boundary effects. The electrons are thus sensitive to their local
environment, which depends critically on the fabrication methods and
materials used. Electronic conduction at low temperatures is also
affected by resonances determined by the shape of the waveguide channels
and cavities, as well as the materials used. Ballistic electron devices
operate at the interface between quantum mechanics and thermodynamics,
two disciplines that have not yet been reconciled when dealing with
small numbers of particles. An understanding of all of these effects in
the fully quantum regime is essential, if we are to understand
electrical circuits that function at the nanometer scale.

Since many properties of nanodevices are related to quasibound states of
electrons, it is also important to understand their dynamics. A case in
point is the decaying states of electron waveguides. For waveguides with
long straight leads and a rectangular cavity, the states with a
continuous spectrum in the leads correspond to the continuous field
states, while the quasibound states of the cavity correspond to unstable
states. The case where a discrete energy of the bare states in the
cavity is close to the minimum energy of the continuous spectrum of the
leads results in interesting nonlocal phenomena associated with
"super--luminality." Since nonlocality is a quantum characteristic, this
situation also can provide an interesting experimental test of
fundamental problems in quantum mechanics. Moreover, electron waveguides
can provide an experimental measurement of the magnitude of the
"dressing" of the excited unstable state. This has a bearing on the
decoherence problem as it relates to nanoscale quantum computers because
quantum computation is reliable only for time scales during which one
can neglect decoherence.

**C. Future Research Directions**

The future applications of nanoscale devices are both exciting and
boundless. The greatest near--term payoff is likely to be the
development of ultrahigh--speed and efficient electronic, optical, and
magnetic devices for communications, switching, computing, and storage,
and their

**Friction and Control of Friction at the Nanoscale**

Nano--devices and the new *ultra small* technology are expected to
significantly improve the performance of robots, computers,
communication, and other electro/optical/mechanical devices. However,
friction imposes a significant limitation in usage of these tiny
devices. As a manifestation of the *nano in the macro*, hundreds of
millions of dollars will be lost as a result of wear, friction,
breakdowns, and wasted energy. Research to control friction at the
nanoscale will result in highly improved performance in the macro world
and can produce significant economic savings \[47\].

Traditional lubrication involves the use of organic substances whose
functional groups can adsorb onto polar surfaces to form a
closed--packed arrangement of almost perpendicularly oriented lubricant
chains. However, nano--machine lubricant selection is complicated by new
considerations. Due to the built--in--place nature of nano--mechanics,
lubrication by the conventional means of hidden and contacting surfaces
is prohibitive. Fluid lubricants may also introduce capillary and
viscous shear mechanisms, which result in energy dissipation. Research
will span the following topics:

***Mechanisms for Friction and Lubrication at the Nano--Scale***.
Despite great progress made through the past half century, many basic
issues in fundamental tribology such as origin of friction and failure
of lubrication have remained unsolved. Moreover, current reliable
knowledge related to friction and lubrication is mainly applicable to
macroscopic systems and machinery and, most likely, will be of only
limited use (if at all) in nano--systems. When the thickness of the
lubrication film is of the same order as the molecular or atomic size,
the behavior of the lubricant becomes significantly different \[48\].
Understanding the mechanisms of friction, lubrication, and other
interfacial phenomena at atomic and molecular scales will provide
designers and engineers the required tools and capabilities to control
and monitor friction, reduce unnecessary wear, and predict mechanical
faults and failure of lubrication in nano--devices \[49,50\].

***Electromechanical and Optical Control of Friction***. Friction can be
manipulated by applying small adjustments (perturbations) to accessible
elements and parameters of the sliding system \[51\]. This requires
*apriori* knowledge of the strength and timing of the perturbations,
similar in concept to the "residual statics" perturbation problem in
seismic imaging, where global optimization techniques have shown great
promise \[52\]. In addition, surface roughness and thermal noise are
significant factors in making decisions about control on the micro and
nano--scales. Research to study the response of the sliding system and
lubricating thin films to small mechanical, electrical, and optical
excitations will result in identifying better ways to control friction
and to create a novel type of electromechanical lubricant at the
nano--scale.

***Control of Chaos in Friction***. In many situations, the behavior of
the lubricant and the sliding surfaces is chaotic. Thus control of chaos
and targeting it toward desired behaviors is an important issue for
lubrication and friction. Techniques for controlling chaos have been
under systematic investigation since 1990 \[53\]. The main issue is to
develop efficient and robust methods to selectively drive low and high
dimensional chaotic dynamics by using tiny external perturbations
\[54\]. Both feedback and non--feedback techniques have to be adjusted
and applied to control chaos in friction \[55\]. Control of chaos in
spatially extended systems has not yet reached a high applicability
level. Thus, research in this direction is very significant.

***Quantum Control of Friction***. When the size of sliding surfaces
decreases to the nano--scale, quantum effects play an important role.
Quantum control of friction is significantly different in concept and
methods from classical control, and is much less understood. The main
difficulties associated with developing efficient quantum control
techniques are: (i) insufficient understanding of many--particle
systems; (ii) lack of exact solutions for large many--body systems;
(iii) inaccessibilty/collapse under measurements of the state function;
and (iv) lack of robust algorithms for two--point boundary value
problems. A focused effort to develop algorithms for quantum control of
friction is required.

***Computational Aspects***. Research on friction and lubrication is
highly interdisciplinary and probes the very foundations of physics,
chemistry, mathematics, mechanical, electrical and chemical engineering,
and computer science. Due to the high cost of experiments, computational
modeling and simulations \[56\] to support and predict experimental
findings will play a significant role. Developing new, accurate, fast,
and stable methods to solve large systems of nonlinear differential
equations will be one of the crucial issues in research on friction at
the nanoscale.

incorporation into highly integrated and massively parallel systems.
Current trends toward ever--increasing circuit complexity,
miniaturization, and reduced power consumption in the information
technology arena can be expected to accelerate, and the drive to develop
and commercialize these nanotechnologies will become paramount. But
related developments that are further afield at present, such as quantum
computing and cryptography, will also profit from a comprehensive and
sustained nanoscience research program.

Many other examples can be found to illustrate the close link between
fundamental studies of nanoscale phenomena and their technological
applications. Consider interlayer magnetic coupling in multilayer
structures. Although initial studies of magnetic coupling in multilayers
were motivated primarily by scientific curiosity, as giant
magnetoresistance (GMR) and the concomitant phenomenon of oscillatory
interlayer magnetic coupling were discovered, the technological
relevance of these discoveries resulted in products within an
extraordinarily short ten--year period. Future developments that can be
anticipated include nonlinear magneto--optical effects in magnetic
multilayers and nanostructures that may lead to a new generation of
magnetic sensors, recording media, and imaging methods.

Another area which can be expected to benefit significantly from the
development of nanostructures exhibiting novel geometries and optical
coupling is inexpensive mass--production of high--resolution optical
components. "Optical matter" methods may prove instrumental to organize
and assemble them on the nanoscale. These devices will be capable of
focusing and guiding not only microwave and visible light, but shorter
wavelengths from the ultraviolet down to the x--ray region.
Diffraction--limited performance at the nanoscale requires nanometer
wavelengths, i.e. x--rays. Such x--ray optical elements operating at
unprecedented (\< 10 nm) spatial resolution can in turn be used to
fabricate other nanostructures, quantum devices, and integrated devices,
and to form intense x--ray nanoprobes for microscopy, diffraction, and
scattering experiments. Large arrays of nanofocusing x--ray optics will
quickly find applications in nanolithography, combinatorial materials
and combinatorial biology analysis systems.

Nanoscale coupling of properties clearly has significant energy--related
implications as well. For example, nanocrystalline semiconductors in the
form of fractal films of particles, isolated colloidal QDs, ordered and
disordered arrays of close--packed colloidal QDs, and two--and
three--dimensional arrays of self--organized epitaxially grown QDs have
many potential and existing applications in renewable energy systems.
These include very inexpensive and color tunable (from clear to colored
to black) photovoltaic solar cells based on the dye--sensitization of
nanocrystalline wide bandgap oxides (like TiO~2~) operating in a
photoelectrochemical cell, and novel solar cells with extremely high
conversion efficiency. If slow cooling of photogenerated hot--carriers
and hot carrier transport in nanostructured electrodes can be achieved,
it may be possible to develop highly efficient hot--carrier solar cells
based on QD arrays. Nanocrystals could also be used as efficient
photocatalysts for photodetoxification of polluted or toxic water and
air streams. Semiconductor nanocrystals and nanostructures may be used
as efficient photoactive materials for solar--photon conversion of
simple molecules to fuels and chemicals, for instance photolytic water
splitting to produce hydrogen, photoreduction of carbon dioxide to
alcohol and hydrocarbon fuels, and photoreduction of molecular nitrogen
to ammonia for fertilizer production.

From another standpoint, a deeper understanding of the physics of phonon
transport in nanostructured materials may facilitate production of
practical all--solid--state and environmentally clean thermoelectric
energy--conversion devices with performances far superior to current
vapor--based refrigerators and combustion--based engines. The pervasive
role of hard and soft magnets in electric--power production and
utilization is another arena in which new nanoscale magnetic materials
may yield substantial energy savings by reducing losses and conserving
natural resources consumed in the generation and use of electricity.

Lastly, carbon--based nanotubes have the potential to act as a hydrogen
storage medium that could exhibit very high storage density per unit
weight, which is critical for hydrogen--based transportation systems. A
crucial issue is whether or not the hydrogen could be extracted
efficiently from such a storage medium at relatively low temperatures.

**References**

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7\. H. U. Baranger and P. A. Mello, "Mesoscopic transport through
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8\. G. B. Akguc and L. E. Reichl, "Statistical properties of the
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27\. G. Chen, "Thermal conductivity and ballistic--phonon transport in
the cross--plane direction of superlattices," *Phys. Rev.* **B57**,
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physics and electronics," *Phys. Rev. Lett.* **58**, 2059 (1987).

29\. S. John, "Strong localization of photons in certain disordered
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30\. C. M. Soukoulis, ed., "Photonic band gap materials," *Proceedings
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31\. J. D. Joannopoulos, R. D. Meade, and J. N. Winn, "Photonic
Crystals: Molding the flow of light," Princeton University Press (1995);
J. D. Joannopoulos, P. R. Villeneuve, and S. H. Fan, "Photonic crystals:
putting a new twist on light," *Nature* **386**, 143 (1997).

32\. "Photonic crystals," *Physics Today ,* p. 17 (January 1999).

33\. J. G. Fleming and S.--Y. Lin, "A three--dimensional photonic
crystal with a stop band between 1.35 and 1.95 µm," *Optics Lett.*
**24**, 49 (1999); S.--Y. Lin, J. G. Fleming, D. L. Hetherington, B. K.
Smith, R. Biswas, K. M. Ho, M. M. Sigalas, W. Zubrzycki, S. R. Kurtz, J.
Bur, "A three--dimensional photonic crystal operating at infrared
wavelengths," *Nature* **394**, 251 (1998).

34\. "Photonic crystals: whole lotta holes," *Chem. Engr. News*, p. 38,
(23 November 1998).

35\. J. E. G.J. Wijnhoven and W. L. Vos, "Preparation of photonic
crystals made of air spheres in titania," *Science* **281**, 802 (1998);
A. A. Zakhidov, R. H. Baughman, Z. Iqbal, C. X. Cui, I. Khayrullin, S.
O. Dantas, I. Marti, and V. G. Ralchenko, "Carbon structures with
three--dimensional periodicity at optical wavelengths," *Science*
**282**, 897 (1998); A. Imhof and D. J. Pine, "Ordered macroporous
materials by emulsion templating," *Nature* **389**, 948 (1997); O. D.
Velev, T. A. Jede, R. F. Lobo, and A. M. Lenhoff, "Porous silica via
colloidal crystallization," *Nature* **389**, 448 (1997); B. T. Holland,
C. F. Blanford, and A. Stein, "Synthesis of macroporous minerals with
highly ordered three--dimensional arrays of spheroidal voids," *Science*
**281**, 538 (1998).

36\. Y. N. Xia and G. M. Whitesides, "Soft lithography," *Angewandte
Chemie* **37**, 551 (1998).

37\. See, for example: K. N. Vlasov, "Different regimes of light
localization in a disordered photonic crystal," abstract **SC21.09**,
*presented at Am. Phys. Soc. Centennial Meeting* (Atlanta GA, March
1999).

38\. M. Meier, A. Mekis, A. Dodabalapur, A. Timko, R. E. Slusher, J. D.
Joannopoulos, and O. Nalamasu, "Laser action from two--dimensional
distributed feedback in photonic crystals," *Appl. Phys. Lett.* **74**,
7 (1999); M. Berggren, A. Dodabalapur, R. E. Slusher, Z. Bao, A. Timko,
and O. Nalamasu, "Organic lasers based on lithographically defined
photonic--bandgap resonators," *Eletron. Lett.* **34**, 90 (1998).

39\. M. M. Burns, J.--M. Fournier, and J. A. Golovchenko, "Optical
matter: crystallization and binding in intense optical fields,"
*Science* **249**, 749 (1990).

40\. A. Hemmerich and T. W. Hänsch, "Two--dimesional atomic crystal
bound by light," *Phys. Rev. Lett.* **70**, 410 (1993).

41\. A. Hemmerich, C. Zimmerman, and T. Hänsch, "Sub--kHz Rayleigh
resonance in a cubic atomic crystal," *Europhys. Lett.* **22**, 89
(1993).

42\. "Three--dimensional optical molasses binds a new type of crystal,"
p. 17 in *Physics Today* (June 1993).

43\. P. Verkerk, B. Lounis, C. Salomon, C. Cohen--Tannoudji, J.--Y.
Courtois, and G. Grynberg, "Dynamics and spatial order of cold cesium
atoms in a periodic optical potential," *Phys. Rev. Lett.* **68**, 3861
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Rolston, R. J. C. Spreeuw, and C. I. Westbrook, "Observation of
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44\. P. Fenter, F. Schreiber, L. Berman, G. Scoles, P. Eisenberger, and
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45\. J. Li, A. Mirabedini, L. J. Mawst, D. E. Savage, R. J. Matyi, and
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47\. Y. Braiman, V. Protopopescu, and J. Barhen, "Friction and Control
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48\. Yuan--Zhong Hu, S. Granick, "Microscopic Study of Thin Film
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55\. K. Pyragas, "Continuos Control of Chaos by Self--controlling
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607 (1995).

# 4. CONTROLLED SYNTHESIS AND PROCESSING AT THE NANOSCALE (PRIMARILY ORGANIC)

**A. Current Status**

An important new field of chemistry and materials science is emerging,
dedicated to the systematic and parallel patterning of matter on the
nanometer scale \[1\]. The controlled positioning of atoms within small
molecules is of course routinely achieved by chemical synthesis of moles
of identical molecules. Nanometer--size objects are much larger
entities, with thousands or even millions of atoms in them. There are
many powerful new approaches to patterning on the nanoscale that are
fundamentally serial in nature, for instance atom manipulation using
scanning probe tips, or electron beam lithography. These are
tremendously important techniques. However, there is also a pressing
need to develop methods to prepare macroscopic quantities of nanoscale
components in complex, designed patterns, using techniques of self
assembly. The success of this endeavor will allow the fundamental
discoveries of nanoscale science to find wide application in many
fields, and especially in energy related technologies. This field is
developing rapidly, and this section includes just a brief survey of
recent accomplishments.

*1. Accomplishments in the Synthesis of Individual Building Blocks*

[Nanocrystals]{.underline}. In the last decade there have been
significant advances in the preparation of nanocrystals \[2,3\]. Many
common materials, such as metals, semiconductors and magnets can be
prepared as nanocrystals, using colloidal chemistry techniques. The
concepts of ligand exchange and surface derivitization have been well
developed, and these methods permit nanocrystals with narrow size
distribution (typically 5--15% variation in diameter) to be isolated and
then used further as chemical reagents. This field has been aided
greatly by the improved understanding of size--dependent scaling laws,
which have emerged from fundamental studies in chemical physics and
condensed matter physics. The fact that a simple property like light
emission depends so strongly upon the particle size in semiconductors
has greatly facilitated the development of reliable preparations. The
same size dependence has also led to a wide range of applications in
unexpected areas, such as in biological tagging \[4,5\].

[Nanotubes and Rods]{.underline}. The exciting discovery of the
fullerenes was followed closely by the discovery of nanotubes of carbon
\[6\]. Nanotubes show tremendous promise as building blocks for new
materials. Because of their topology, nanotubes have no dangling bonds,
and so despite being very small, they do not exhibit "surface effects."
As a consequence, individual nanotubes exhibit nearly ideal electrical,
optical, and mechanical properties. Nanorods are also under extensive
development and investigation.

[Polymeric Electronic Materials: Dendrimers and Block
Copolymers]{.underline}. The last decade has seen tremendous advances in
the preparation of organic building blocks of considerable complexity
\[7--9\]. The discovery of a new topology for polymers, dendrimers, has
led to an exciting new class of nanoscale components with interesting
optical and mechanical properties. There have also been steady advances
in engineering new phases using block copolymers; for instance, the
recent development of tri--component blocks is noteworthy in this
regard. While it is not, strictly speaking, a nanotechnology the
tremendous advances in organic electronics, such as organic light
emitting diodes, must be noted, since that field is more than likely to
benefit by advances in organic nanoscale synthesis in the future.

*2. Templated Growth and Assembly*

[Mesoporous Inorganic Solids]{.underline}. In the last decade, great
advances have occurred in the preparation of mesoporous inorganic solids
\[10\]. The initial work showed that it is possible to use organic
surfactant molecules to prepare a complex pattern. That pattern can
serve as the template for the formation of an inorganic phase. This has
led to many exciting discoveries in chemical synthesis, and to immediate
practical advances in catalysis.

In the design of complex materials, such as electrical devices, we
currently rely on the ability to lithographically create designed
patterns. In the last decade, nanoscale objects, such as nanocrystals,
have been assembled into periodic arrays, or supercrystals. Such arrays
exhibit novel optical and electrical characteristics. Several proposals
have been put forward for how to pattern nanocrystals and nanotubes
using biological molecules \[11--13\].

*3. Relation to Energy Science*

Many fundamental phenomena in energy science, such as electron transfer
and exciton diffusion, occur on the nanometer length scale. Thus, the
ability to inexpensively pattern and process matter on this length scale
can be an important asset in designing new--generation photovoltaic
cells and batteries. The potential power of this idea is clearly
illustrated by the Graetzel photovoltaic cell, which is almost randomly
assembled from nanoscale components, and yet very cheaply achieves 10%
power efficiency. Isolated nanoscale components also exhibit nearly
ideal mechanical properties, and the consequences of this for new
materials design have yet to be fully exploited.

####### B. Key Questions

> Many fundamental scientific issues are unanswered in this field. Among
> them are:

-   What is the largest inorganic or organic cluster that can be
    prepared in the laboratory as a molecule, with a precise number of
    atoms in it?

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-   What are the surface reconstructions and rearrangements of atoms in
    nanocrystals and nanorods? Is it possible to prepare epitaxial
    core--shell systems in nanocrystals?

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```
-   Can single wall carbon (or other) nanotubes of a single length and
    helicity be purified as isolated molecular species? Is it possible
    to reproducibly prepare heterojunctions from one--dimensional
    nanostructures?

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```
-   Is it possible to use parallel self--assembly techniques to control
    the relative arrangements of nanoscale components according to a
    complex, designed sequence?

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```
-   Can chemical synthesis be used to generate materials with complex,
    macroscopic shapes and desirable mechanical properties?

####### C. Future Research Directions

The conventional distinctions between soft and hard materials, or
between inorganic and organic materials are rapidly disappearing.
Nanometer--size organic and inorganic building blocks are being
developed rapidly. The ability to assemble these building blocks into
complex, hybrid materials with designed properties is opening the door
to a new generation of composite materials. Over the next twenty years,
these new classes of materials can transform energy--related
technologies, including catalysis, photovoltaics and batteries.

####### References

1\. A. P. Alivisatos, P. F. Barbara, A. W. Castleman, J. Chang, D. A.
Dixon, M. L. Klein, G. L. McLendon, J. S. Miller, M. A. Ratner, P. J.
Rossky, S. I. Stupp, M. E. Thompson, "From molecules to materials:
Current trends and future directions," *Advanced Materials* **10**,
1297--1336 (1998).

2\. L. Brus, "Semiconductor colloids: Individual nanocrystals, opals and
porous silicon," *Current Opinion in Colloid & Interface Science*
**273**, 199--241 (1996**)**.

3\. T. P. Martin, "Shells of atoms," *Physics Reports--Review Section of
Physics Letters* **273**, 199--241 (1996**)**.

4\. W. C. W. Chan, S. N. Nie, "Quantum dot bioconjugates for
ultrasensitive nonisotopic detection," *Science* **281**, 2016--2018
(1998).

5\. M. Bruchez, M. Moronne, P. Gin, S. Weiss, A. P. Alivisatos,
"Semiconductor nanocrystals as fluorescent biological labels," *Science*
**281**, 2013--2016 (1998).

6\. M. Terrones, W. K. Hsu, H. W. Kroto, D. R. M. Walton, *Nanotubes: A
revolution in materials science and electronics*, Vol. 199, pp 189--234
(1999).

7\. O. A. Matthews, A. N. Shipway, J. F. Stoddart, "Dendrimers --
Branching out from curiosities into new technologies," *Progress in
Polymer Science* **23**, 1--56 (1998).

8\. S. I. Stupp, V. LeBonheur, K. Walker, L. S. Li, K. E. Huggins, M.
Keser, A. Amstutz, "Supramolecular materials: Self--organized
nanostructures," *Science* **276**, 384--389 (1997).

9\. D. A. Tomalia, "Starburst Cascade Dendrimers -- Fundamental
Building--Blocks for a New Nanoscopic Chemistry Set," *Advanced
Materials* **6**, 529--539 (1994).

10\. D. M. Antonelli, J. Y. Ying, "Mesoporous materials," *Current
Opinion in Colloid & Interface Science* **1**, 523--529 (1996).

11\. R. C. Mucic, J. J. Storhoff, C. A. Mirkin, R. L. Letsinger,
"DNA--directed synthesis of vinary nanoparticle network materials,"
*Journal of the American Chemical Society* **120**, 12674--12675 (1998).

12\. A. P. Alivisatos, K. P. Johnsson, X. G. Peng, T. E. Wilson, C. J.
Loweth, M. P. Bruchez, P. G. Schultz, "Organization of Nanocrystal
Molecules Using DNA," *Nature* **382**, 609--611 (1996).

13\. E. Braun, Y. Eichen, U. Sivan, G. BenYoseph, "DNA--templated
assembly and electrode attachment of a conducting silver wire," *Nature*
**391**, 775--778 (1998).

# 5. CONTROLLED SYNTHESIS AND PROCESSING AT THE NANOSCALE (PRIMARILY INORGANIC)

####### A. Current Status and Accomplishments

Scientific and technological breakthroughs in materials research and
development are often directly coupled to progress in synthesis and
processing. Nanoscience and nanotechnology currently are following this
path. Three basic strategies are presently widely employed for producing
nanostructured materials: the production of nanoparticles and their
delivery onto surfaces; the growth of epitaxial films and surface
layers; and the fabrication of ultra--fine--grain monolithic and
composite bulk materials.

The growth of epitaxial thin films, in combination with lithography and
ion implantation, is currently the most advanced and productive route in
this field. Such films are employed for a variety of applications,
including Josephson junctions, quantum dots, optical devices, etc. They
provide model structures for testing ideas about the properties of
nanostructured materials. Although much effort is already focused on
epitaxial growth, this is a huge area and many new breakthroughs can be
anticipated. In contrast, the processing of bulk nanostructured
materials has developed far more slowly, despite their promise for use
in a wide range of structural and magnetic applications. The main
processing schemes include the direct consolidation of nanoparticles,
the sintering of powders that consist of micron--sized particles which
are themselves rendered nanocrystalline by mechanical working, and phase
transformations, mostly of amorphous alloys. These processes, however,
are subject to problems associated with porosity and grain growth \[1\],
which will require fundamental studies on the kinetic behaviors and
mechanical properties of these systems to resolve.

The synthesis of nanoparticles also is a large and mature area of
research, which has been driven in the past largely by the need for
improved catalysts. However, the development of *in situ* diagnostics
and characterization for studying the synthesis of nanoparticles in real
time offers new opportunities to *tailor the properties* of
nanoparticles for a wide variety of other specific functions.

####### B. Key Questions, Research Opportunities and Barriers to Progress

####### *Science of Epitaxial Growth*

The science of epitaxial growth is the foundation for a major part of
future nanotechnology. At present, investigations of growth are well
funded only for a few specific materials where technical applications
are clearly identified. The examples of silicon--germanium research and
the growth of high temperature superconductors illustrate an interesting
contrast for the future development of this field. Superconducting
materials are sufficiently complicated that development of a science of
epitaxial growth is a slow process, often characterized by specific
prescriptions for each material. Si--Ge research deals with a much
simpler system and in contrast has been wonderfully productive of new
concepts relating to fundamental step edge behavior, surface
morphological structures and strain--induced metastable topologies, for
example. Most importantly, it has revealed the effect of these
structures on the growth of perfect and nanostructured thin films \[2\].
What requires recognition is that different types of materials exhibit
different classes of behavior, so that knowledge of Si--Ge fails to
illuminate the science that determines growth processes for other
classes of materials. To understand the different behaviors will require
a greatly increased commitment to basic research on epitaxy and surface
topology for a wide variety of prototype materials (such as simple ionic
materials, model metals and organic deposits on inorganic substrates),
as well as studies of growth--enhancement methods such as ion and
laser--beam assisted deposition.

An area of film growth deserving special attention is the deposition of
nanoparticles and clusters, which are useful for creating a variety of
surface features. Most important is the ability to control the size,
size distribution and organization of the nanoparticles. For example,
post--processing of nanoparticles deposited on patterned
near--nanostructured materials either by heating or by using directed
beams of photons or ions, can lead to patterned nanostructures, as
recently illustrated by the alignment of quantum dots \[3\]. Another
potential processing method to reduce the average cluster size and the
size distribution, and provide for particle alignment, employs Coulomb
repulsion in the growth process \[4\]. A first test of this concept
utilizes buffer layer assisted growth in which atoms are deposited on
top of an inert--gas buffer layer condensed on a cold substrate, in
order to make the clusters highly mobile and allow them to
self--assemble into an ordered array, but without coalescence. In this
test, a significant reduction in the average size and size distribution
of Fe clusters deposited on Cu(111) was observed \[5\], as described in
the box on the next page.

It is evident that studies of growth mechanisms and surface topology at
the atomic level can provide a scientific base of understanding from
which the synthesis and nanostructuring of a wide variety of technically
important materials (many of which remain to be identified) can be
effectively undertaken. This will require parallel funding for
independent research teams focused on different materials systems, and a
commitment to provide state--of--the--art instrumentation by which
epitaxial materials can be created and explored at the atomic level.

Progress in this field will require that both synthesis equipment and
advanced surface characterization systems such as STM, AFM, LEEM, etc.
be made much more widely available to research teams. Support for *in
situ* investigations using central facilities such as UHV transmission
electron microscopy and synchrotron radiation, with deposition
capabilities included, will also be necessary. Existing electron
microscopy centers and synchrotron laboratories provide a convenient
means to make these facilities available to a larger number of users.

![](media/image2.wmf){width="2.748031496062992in"
height="2.747244094488189in"}![](media/image3.wmf){width="2.748031496062992in"
height="2.747244094488189in"}

*Driven Systems*

A very promising area for processing three--dimensional bulk nanoscale
structures originates with the recent discovery that mesoscopic
structures can be obtained by nonequilibrium processing, such as ion
irradiation, implantation, and mechanical working (ball milling, etc.)
\[6--8\]. Contrary to equilibrium situations where precipitates should
always coarsen to reduce interfacial energy, competition between
dynamical processes operating during processing may result in the
stabilization of multiphase (composite) materials, where the different
phases coexist at a mesoscopic, tunable length scale. An example is
illustrated in Fig. 5.1, where results of computer simulation on the
ball milling process are shown. The system represents two immiscible
metals subjected to shear deformation at elevated temperatures where
limited diffusion takes place. Control of the length scale by varying
the two control parameters, shearing rate and temperature, is clearly
observed. Interesting new properties have been found and others
predicted when this coexistence of phases takes place at a nanometric
scale. Nanocomposites of hard and soft magnetic phases, for example,
make it possible to fabricate materials with both large magnetic
anisotropy and large saturation magnetization \[9\]; nanocomposites of
elastically hard and soft metals provide great strength to materials;
and metal nanoparticles in glass offer non--linear optical properties
\[10\]. ![](media/image5.wmf){width="1.9506944444444445in"
height="4.145138888888889in"}![](media/image6.wmf){width="1.801388888888889in"
height="4.113194444444445in"}![](media/image7.wmf){width="2.0006944444444446in"
height="4.145138888888889in"}

**Figure 5.1**: Typical steady--state microstructures as observed in a
(111) plane at different shearing rates,γ~sh~: (a) γ~sh~ = 1 s--1 ; (b)
γ~sh~ = 103 s--1; (c) γ~sh~ = 105 s--1. The initial state is a A/B
bilayer, and the temperature is held constant at T=400 K; only B atoms
are shown \[6\].

Before non--equilibrium processing such as this can be employed to full
advantage in creating mesoscopic structures, it will be necessary to
understand the effect of processing parameters on the final
microstructures in a number of situations. This is a challenging task
because the theory of driven systems is not well developed in real
materials. This calls for combined efforts in modeling, computer
simulation, and critical testing. Ion irradiation can play an important
role in this context, since the elemental processes occurring during
irradiation are very well known. Indeed some work along these lines has
been carried out. Roughening of superlattices for improved GMR
properties \[11\], ion mixing and implantation of metal layers in SiO~2~
for optical devices \[12\], and ripple formation during high temperature
sputtering illustrate these ideas \[13\]. The information obtained from
irradiation studies will be of great benefit for understanding the more
complex situations, such as mechanical working, since the underlying
science for driving systems from equilibrium shares many similarities
from one method to the next. However, controlled experimentation on many
of these systems is very difficult. Of great benefit to this effort
would be placing an on--line accelerator at a synchrotron beam line to
provide for *in situ* kinetic investigations. Such studies are needed
because the number of control parameters and kinetic pathways is far too
large to attempt examining specimens by *ex situ* methods.

It is worth mentioning that similar processing schemes occur and perform
very important functions in biological systems. Self--assembly is one of
the tools needed to make ordered nanostructures. Only the dynamics
underlying the self--assembly process in this context are now well
understood \[14\]. A material system embedding enormous complexity
dynamically transits a variety of states, squeezing out free energy
along the way, to arrive at a functional (and hopefully desirable)
configuration. The spontaneous organization of a vesicle and the folding
of a protein are notable examples of this phenomenon. Self--assembly,
however, is not a completely enabling and sufficient tool. Biological
systems make extensive use of both self--assembly and dissipative
processes to make important structures or effect adaptive changes
\[15\]. The double helix assembles, but complex processes involving
substantial energy inputs from metabolism are needed to construct (or
read, transcribe, etc.) the chromosome \[16\]. This central ability of
the living system is far more complex than the driven systems described
above; our science can neither imitate it broadly nor understand it in
terms of detailed theory at this time. For example, how should one think
about a system of objects not at equilibrium which arise via dynamically
transforming interactions---a characteristic one expects for a mesoscale
object obtained via assembly and dissipative processing? A substantial,
and potentially high impact, materials research challenge suggested here
is to devise ways for developing both complex structure and functions
that mimic this powerful ability of the biological world \[17\].

######### Phase Transformations

Many processing schemes involve transformations between different phases
of a material. Although thermodynamics ultimately determines the
equilibrium phase for a given set of conditions, the answers as to
whether and how this phase is produced from the metastable precursor
phase depend on kinetics. Since most phase transitions involving
nanostructured materials occur under conditions far from equilibrium,
the kinetic pathways available to these systems are numerous and not
well understood. This problem is compounded by the large contributions
to the free energy of nanoscale materials from interfaces, which
substantially shift phase boundaries. It will be important for making
progress in the areas of synthesis and processing, therefore, to develop
our understanding of nanochemistry and the broader general issues of
nucleation and growth.

One example of how phase transformations are used in processing
nanostructured materials has already been noted, namely the formation of
metallic nanoparticles in silicate glasses for non--linear optical
devices. Here, metal ions are implanted into glass and subsequently heat
treated to precipitate out the metallic nanoparticles \[10\]. The
processing of bulk nanostructured materials from bulk metallic glasses
provides another important example of using phase transformations in
nanotechnology. This processing scheme is becoming more attractive as
the ability to form bulk metallic glasses broadens to a wider range of
systems. Soft magnetic materials comprised of nanocrystalline bcc
Fe--M--B (M = Zr, Hf, Nb), for example, have been processed from the
bulk glass, and they have excellent (soft) magnetic properties \[18\].
These materials are now commercially available. Despite this potential
for processing of nanocrystalline bulk alloys, a general scheme for
crystallization of amorphous alloys has not been developed beyond this
single area of soft magnets. This is largely due to the difficulty of
producing bulk glasses. Further research is needed in developing a far
more extensive list of bulk glasses that can be synthesized. This will
require a more fundamental understanding of the kinetics of nucleation
and growth of crystalline phases in glasses, and a deeper understanding
of the nature of glassy systems themselves.

A different class of phase transformations important to nanoscience and
nanotechnology involves aerosol methods for synthesizing nanoparticles,
such as inert gas condensation, flame pyrolysis, etc. These particles
can be employed for a variety of applications: sensors, catalysts,
sintered bodies, nanotubes and the like. While much work has been
performed on controlling size and structure of elemental nanoparticles
\[19\], an enormous potential for fabricating tailored, more complex
particles, including alloys, compounds, and organics, currently exists.
Presently, no general understanding is available for predicting the
nanochemistry involved and surprisingly few fundamental *in situ*
studies of the growth environment have been performed. Studies using *in
situ* processing diagnostics will be necessary, however, to develop
processes for truly controlled nanostructure growth.

*Time--Resolved, In Situ Diagnostics of Nanoparticle Formation*

A variety of time--resolved spectroscopic tools are currently being
developed to understand gas--phase nanomaterial synthesis and function.
These include photoluminescence from nanoparticles; optical absorption
spectroscopy and laser--induced fluorescence of ground--state molecules
and precursors; Rayleigh scattering for nanoparticle sizing; plasma
temperature measurements; and many more \[20,21\]. These techniques
should be implemented to determine the precursor species responsible for
nanomaterial growth, the vapor and heat transport processes involved in
their growth, plasma density and temperature, nucleation temperatures
and times, and nanomaterial growth rates.

Another example of a non--invasive, *in--situ* diagnostic of the
quality, quantity and properties of nanoparticles is the use of small
angle neutron scattering (SANS) to study the characteristics of
nanodroplet aerosols formed by expanding a condensable vapor in a
supersonic nozzle. The importance of aerosol SANS stems from its
potential to probe directly the microscopic structure of
nanometer--sized droplets \[22\]. Particles in this size range are the
first condensed--phase fragments to appear during aerosol formation, and
aerosol SANS affords us the first detailed, *in--situ* look at them.
Aerosol SANS measurements also allow the direct measurement of the
particle size distribution of the aerosol and the speed of the particles
in the expanding flow. All of these data were previously unavailable or
had to be inferred from gas--dynamic flow measurements using unrealistic
assumptions and simplified models. This information now provides an
opportunity to perform the most stringent testing to date of theories of
nucleation and droplet growth under the technologically important
conditions of very high supersaturation and cooling rate found in
turbine expansions, jet engine and rocket exhausts, and nozzle--based
particle production techniques.

######### Directed Synthesis of Nanoparticles, Nanotubes and Nanostructured Materials

We note there is vast potential for developing directed synthetic
chemistries that lead to very precisely defined nanoparticles
appropriate for specific applications in technology. It has been shown,
for example, that carbon--supported Pt--Ru alloy nanoparticles can be
prepared from molecular cluster precursors in a form appropriate for use
as an electrocatalyst in Direct Oxidation Fuel Cells (DOFC). Perhaps
most intriguing, the nanoscale alloy clusters made in this way show
essentially no distribution in either the cluster size or composition, a
feature which powerfully enables their detailed structural
characterization by STEM and XAFS methods \[23\].

Nanostructured materials also promise greatly improved structural
properties in comparison with conventional metal alloys. For example,
small--diameter bundles of single--walled carbon nanotubes (SWNT) are
predicted and observed \[24,25\] to have the largest
strength--to--weight ratio of any known material with an elastic modulus
\~ 1 TPa, which is approximately *one hundred times that of steel but
with only one--sixth its weight*. Such materials offer almost
unimaginable economic benefits and product opportunities, if they can be
cross--linked to overcome a low shear modulus \[24\] and synthesized by
an economically competitive process that is compatible with other
manufacturing technologies. It is clear this will require the
development of new approaches for high--volume directed deposition of
nanostructured materials. SWNT, for example, are not useful for
structural applications unless they can be grown *directly* into strong
composite structures 1000 times faster than current CVD methods. In
particular, the carbon nanotube synthesis techniques under development
for molecular electronics (see section 6) are not economically feasible
for high--volume structural applications of SWNT composites.
Consequently, theory, computer modeling and experimental diagnostics of
transport and growth phenomena are needed in order to directly fabricate
SWNT and other nanostructured construction materials at
near--theoretical growth rates.

####### References

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# 6. NANOSCALE PRECURSORS AND ASSEMBLY 

# FOR MACROSTRUCTURES AND DEVICES

####### 

####### A. Introduction

Nanocrystals and nanostructures will be the building blocks for future
materials that will exhibit enhanced or entirely new combinations of
properties for use in energy conversion and related technologies.
However, there are two quite differently motivated categories for the
assembly of macroscopic structures and devices from nanoscale materials.
In the first category, the nanoscale materials are used to form a
precursor state of the macroscopic system or device, and upon formation
of the final state the nanoscale nature of the materials is severely
altered or destroyed. In this approach the unique properties of the
nanoscale materials are used to make the final synthesis of the
macroscopic material easier, better, and/or more efficient. In the
second category, the macroscopic system is composed of functional
nanoscopic materials whose integrity is preserved. In this case, the
properties of the ultimate macroscopic material, device, or system are
derived from those of the nanoscale components, perhaps modified by how
they couple when assembled into a macrostructure. These two categories
are discussed further below.

**B. Nanoscale Precursors to Films for Electronic and Photonic Devices**

*Current Status*

Semiconductor nanocrystals 1 to 20 nm in diameter, synthesized via
colloidal chemistry, have been used as precursor materials to produce
thin films of electronic materials including photovoltaic cells
\[1--3\]. The approach is to first spray a colloidal solution of
semiconductor quantum dots onto a heated substrate to form a
nanostructured semiconductor film. The film is then heat--treated at
high temperature to melt the quantum dots and form macroscopic
semiconductor crystallites which can be used in photovoltaic or other
electronic devices. The advantage of this approach is that the melting
points of semiconductor nanocrystals are markedly depressed compared to
the bulk materials and the final semiconductor film can therefore be
formed at much lower temperature. In addition, the electronic and
optical properties of polycrystalline semiconductor films formed this
way are often superior to films made by conventional deposition
techniques. This approach has been successfully applied to photovoltaic
films of CdTe \[1,2\] and Cu(In,Ga)Se~2~ (CIGS) \[3\] and used to form
contacts to Si \[4\] and CdTe \[4,5\].

Another application using the same principle is the use of semiconductor
nanocrystal precursors to make photonic bandgap materials \[6\]. In this
application, colloidal semiconductor quantum dots 3--4 nm in diameter
are deposited in the interstitial spaces of synthetic opals consisting
of ordered arrays of silica spheres 250 nm in diameter. The quantum dots
self--assemble into a quantum dot solid inside the silica opal. The
silica is then dissolved away leaving a macroporous solid consisting of
densely packed semiconductor nanocrystals. The nanocrystals are then
sintered to form a macroporous bulk semiconductor that has a periodic
modulation of the refractive index on the length scale of 250 nm, which
makes it an ideal candidate for photonic--bandgap applications.
Nanostructured films and materials are potentially very useful in other
applications. Metal oxide nanoparticles have been used to fabricate
nanostructured cathodes for lithium battery applications using the same
spray deposition strategy outlined above. Metal and metal oxide
nanoparticle catalysts are also being examined.

*Scientific Issues*

The main challenge in using nanocrystal precursors to fabricate
macroscopic films is in developing the processing technique that
converts the precursor film into a high--quality large--grained film. In
the case of CdTe, treatment of the precursor film with CdCl~2~ prior to
annealing facilitates the formation of large--grained, dense material.
An analogous approach with ternary or quaternary semiconductor
materials, such as CIGS, has not been as successful. New processing
strategies must be found for these materials.

The synthesis of photonic bandgap materials also requires much better
understanding and control of the mechanism for creating a bulk
semiconductor material from quantum dot precursors upon melting. For
photonic bandgap applications the conversion process must produce
periodic semiconductor structures with the high refractive index of the
bulk material.

*Future Directions*

Nanoparticle precursors are likely to be used to fabricate all of the
components in future photovoltaic device structures. They will be
especially useful for depositing films on top of thermally sensitive
underlayers and on new substrates such as plastics. Deposition of
semiconductor films will be extended to include materials other than
CdTe and CIGS and nanocrystal precursors will be developed for
depositing transparent conducting oxides, metals and other electrical
contact materials. Metal nanoparticle inks will be developed that will
allow deposition of metal lines using inkjet or other printing
techniques. Spray deposition of nanoparticle precursors can also have an
impact in other areas, including the application of inorganic coatings.
The ability of quantum dot precursors to make new photonic bandgap
materials with excellent properties will lead to new and/or better
optical elements that may enable several opto--electronic applications,
including non--linear optical devices, information storage and
transmission, and optical computing systems.

**C. Macroscopic Structures Comprised of Functional Nanoscopic
Materials**

C.1 Nanoscale Computing , Interconnection, and Input/Output (I/O)

*Current Status*

The electronic industry has, for the last 35 years, increased the logic
density of transistors on a chip with a doubling time of approximately
18 months (Moore's Law). This has been possible because of the
continuous shrinking of the minimum feature size that can be generated
with lithography tools. The trend has been to use shorter--wavelength
photons to generate the necessary images. Current standard technology
employs UV (pulsed excimer laser) light with a wavelength of 248 nm to
generate 0.25--micrometer features. The state--of the--art achieves
features of about 0.18 micrometers by using a shorter (ArF, 193 nm)
laser wavelength. However, the absence of optical materials to transmit
and focus still shorter--wavelength photons has resulted in electron
beam and X--ray lithography now being considered for even smaller
feature size. The overwhelming growth in the complexity and cost of chip
manufacturing provides clear evidence that economic limits to
continually shrinking feature size also are rapidly being approached.
However, additional miniaturization is driven inexorably by the need for
higher speed, reduced power consumption, reduced size, and increased
capacity and logic density in future--generation computer systems. Thus,
three promising alternatives, *quantum dots,* *molecular electronics*,
and *ultra--small Josephson junctions* are being considered as
alternatives to the conventional silicon integrated circuit (IC)
paradigm.

Metal, semiconductor or superconductor islands, a few nanometers
diameter, surrounded by an insulating layer can operate as
single--electron devices \[7\]. This property can be used for a variety
of applications such as computation, detection, and remote sensing. In
addition, coupling single devices into arrays opens new technological
horizons, enabling collective effects to be used for coherent
high--intensity radiation, for communications, and for computation.

*Quantum dots* (QD) can be configured to operate as transistors or
associative memory elements through appropriate fabrication processes.
Unlike conventional transistors that operate by moving ensembles of
electrons, QD devices utilize quantum mechanical phenomena that emerge
only as dimensions are reduced to the nanometer scale \[8\]. In
particular, electron movement becomes a discrete phenomenon involving
one electron at a time. It is suggested that quantum dots, fabricated in
two--dimensional arrays, will lend themselves to nanoelectronic
operation of advanced computer architectures such as neuromorphic
networks. *Josephson junctions* \[9\] are natural voltage--to frequency
transducers. These devices have a broad range of applications, the most
prominent one being SQUIDs (superconducting quantum interference devices
\[10\]), which are used to measure extremely small magnetic fields. In
addition to today\'s well--established biomedical and research
applications, SQUIDs are making inroads in nondestructive evaluation,
explosive detection, and geophysics \[11\]. Small and ultra--small
Josephson junctions are promising for nanodevice applications because
(a) these are intrinsically quantum devices (even on the macroscopic
length \[12\]); (b) their very high operating frequencies are capable of
probing other nano--devices; and (c) they are able to emit and detect
coherent radiation in the terahertz range.

Low--capacitance Josephson junction arrays, in the parameter range where
single charges can be controlled, have the potential to provide a
physical realization of quantum bits and, therefore, can be considered
as a component for *quantum computation*. The hope is that they can
provide, for instance, a basis for a controlled quantum logic gate
\[13\], which has a natural implementation in terms of Cooper pair
transport. Future research is required to establish connections between
arrays of small Josephson junctions and quantum computation and quantum
state engineering. In addition to the manipulation of qubits, the
resulting quantum state has to be read out. The possibility of coupling
Josephson junctions to other electronic devices (such as, for example,
single--electron transistor and quantum dots) needs to be better
understood \[14\].

######### Molecular Electronics. The vision of building dense, small, and fast computing elements composed of more than 10^15^ operational molecular--or--atomic scale components is predicated on having the ability to self--assemble and interconnect these nanodevices with an acceptable yield. In fact, even mainstream silicon IC technology may be limited by the interconnect problem before being limited by lithographic constraints \[15\], and the interconnect problem is even more severe for very dense molecular--scale devices. It is difficult to envision supercomputers the size of a grain of sand \[16\] unless breakthroughs are made in the interconnect and I/O areas.

The underlying concept of nanofabrication emerged as one of the
important uses of scanning probe microscopes (SPM) \[17\] which, in a
relatively short time have become powerful tools both to image and to
manipulate matter at the atomic scale. Localized chemical changes are in
most cases an essential component of SPM nanofabrication procedures. The
change results from a molecular species being modified at localized
sites by a pulsed electrical potential transmitted by the SPM tip.
Representative examples include the production of metal--oxide--metal
junctions by anodic oxidation of a previously evaporated titanium film,
or the fabrication of nanowires of a *TaIr* alloy after etching an
over--layer of amorphous silicon patterned in an oxidation step.
Conversely, it also has been possible to create features on silicon and
gallium arsenide with the tip of the microscope *submerged* in a dilute
acid solution, which etched immediately the oxide generated by the tip
potential. Alternatives to nanofabrication by SPM are the deposition of
metal dot arrays through self--assembly, or the evaporation and
selective etching of metal films. These metal dot arrays lend themselves
to operation as resonant tunneling diodes \[18\].

The two principal materials and approaches being considered for
molecular wiring at this time are organic molecular wires \[19\] and
carbon nanotubes (CNTs) \[20\]. Molecular wires have the advantage of
high volume ("beaker--scale") fabrication and can be easily chemically
modified to allow chemical self--assembly \[21\]. However, recent
research has shown that the low power dissipation, remarkable strength
and very small (\~nm) diameter of CNTs may be uniquely suited to both
molecular wiring and I/O. For molecular wiring CNTs can be cut,
separated by length, chemically functionalized, and bonded to gold
substrates or nanoparticles \[22\]. In addition, ordered arrays of CNTs
have been grown on substrates \[23\]. Arrays of CNT field emitters, for
example, may provide a vital link in molecular--scale I/O.

*Scientific Issues and Challenges*

The central scientific issue and problem can be broken naturally into
two parts:

\(1\) the interconnection of molecular devices to each other (molecular
wiring); and

\(2\) the connection of molecular devices to bulk substrates and
structures that allow the introduction of signals (I/O).

Significant challenges for both of these areas include:

\(1\) the need for directed self--assembly of interconnects into
coherent circuits with acceptable yield;

\(2\) understanding and controlling charge transport across dissipative
device/wire and wire/substrate interfaces;

\(3\) understanding and controlling the electrical properties of
molecular wires at different length scales (nm to μm);

\(4\) characterizing the electrical properties of nanoscale devices in
order to extract models for computer simulation of complex molecular
circuits; and

\(5\) making sufficiently rugged interconnect devices that can operate
outside of controlled environments.

*Future Directions and Promise*

*Quantum Dots*. Research is needed to further our understanding of
nanosystem fabrication, and to develop mathematical models and
algorithms to study the electronic properties and computational
capabilities of single--electron device arrays. From an application
perspective, early consideration should be given to neuromorphic
architectures for associative memory. The following research program
elements are envisioned for quantum dots:

*(a) Generation of metal dots*. Note here that the considerable
difference in conductivity between metal and surrounding oxide may
provide a new means to image features.

*(b) Minimum feature size achievable.* Optimization processes must be
developed to establish the smallest feature that can be fabricated.
Variables to be studied may include the effect of film thickness,
substrate thermal characteristics, film adherence and carbon doping and
annealing treatments.

*(c) Device architecture*. Physical effects in nanoscale structures play
a major role in achieving logic functionality not attainable with
current technology. To optimize future nanoelectronic architectures,
quantum transport models and simulations must be developed that include
these physics effects*.*

*(d) Neuromorphic computational architectures and algorithms.* The
dynamics of nanoelectronic networks should be studied with the objective
of deriving a common formalism to describe these networks and configure
neuromorphic systems. The practical goal would be to discover a
mechanism by which neural networks of increasing dynamical complexity
could be embedded in an assembly of nanoscale devices. Computational
models for the study of effects of random dot dispersion should also be
developed and evaluated.

*Josephson junctions*. Recently it was reported \[24\] that a
two--dimensional array of Nb/Al/AlO~x~/Nb Josephson junctions (junction
area *16 µm ^2^*) arranged in square geometry (spacing between junctions
*13* *µm*) emits coherently in a novel synchronized state. Above the
threshold, the power scales as N^2^ which suggest that the array is in
an in--phase synchronized state consistent with the stimulated emission
causing coherence and suggesting population inversion. Theoretical
background predicting analogies between junctions and coherent atomic
systems was developed in the 1970s, but the experimental confirmation
came only very recently. This result opens a new way of looking at an
array of Josephson junctions as a *laser* with the frequency range of
hundreds of gigahertz to terahertz. The next step in this research would
be to establish whether arrays of much smaller, nano--size junctions,
could emit coherent radiation, and estimate the resulting total power.
In particular, the question arises whether Josephson arrays can be used
as emitting devices for quantum teleportation. Both numerical
simulations and theoretical and experimental research are desired to
advance the research in this field. Because these arrays are
intrinsically nonlinear and possess quantum (and perhaps, classical)
dynamics, a deeper understanding in basic quantum and classical
nonlinear dynamics and chaos is required \[25--29\].

*Molecular Electronics*. Individual covalently bonded molecules provide
a unique ultra--dense, low--power framework for purely nanoelectronic
circuitry, including switching devices and wires. A primary advantage of
molecular electronics is that molecules are natural nanometer--scale
structures that can be made absolutely identical in vast (e.g., 10^23^)
quantities \[30\]. Research is needed to understand and harness the
electronic properties of organic molecules, and to develop the molecular
analogs of solid state nanoelectronic devices. An example of a potential
benefit from achieving switching devices of 1 -- 2 nm feature size and
densities in excess of 10^12^ devices per cm^2^ would be a terabit
memory on a chip. Factors determining controlled self--assembly,
interconnectivity (wiring), and corresponding modeling based on first
principles are high priority research efforts.

The challenge and importance of molecular interconnection and I/O,
coupled with the recent advances in CNT fabrication and manipulation,
constitute a major research opportunity. Teams of DOE researchers
including electrical engineers, chemists, solid--state physicists and
computer scientists could make substantial progress in intermolecular
connectivity and macroscale--to--nanoscale communications. Such
breakthroughs are essential to full--scale practical implementation of
nanoscale devices and will provide key enabling technology for the
entire nanoscience and nanotechnology initiative.

C.2 Macroscopic Structures Containing Ion Implanted Nanostructures

######### Current Status

Ion implantation has been used to create nanocrystals and quantum dots
that are encapsulated near the surface of a variety of host materials
\[31\]. This work was motivated by the unique optical and electronic
properties of nanocrystals which result from quantum confinement and the
fact that ion implantation can be utilized to create high densities of
an enormous variety of elemental and compound nanocrystals (metals,
elemental semiconductors, compound semiconductors, and alloys) embedded
in a wide range of host matrices that include SiO~2~, Al~2~O~3~, Si, and
MgO. In some cases, it has been possible to control both the structure
and orientation of the nanocrystals \[32\]. Ion implantation actually
may be the simplest way to form many nanocrystals due to their
sensitivity to high temperatures or oxidizing conditions. Three new
areas of nanotechnology R&D are discussed below.

*Scientific Issues and Future Directions*

[Ordered Arrays of Uniform--Size Nanoparticles Formed by Finely Focused
Ion Beam (FFIB) Implantation.]{.underline} The nanoparticle size
distribution is critically important for many applications but is
difficult to control using large--area ion implantation. However, novel
optical materials could be formed by using FFIB (down to 5 nm beam
diameter) to "write" individual colloidal semiconductor particles of
constant size arranged in a 2--dimensional lattice. The advantages of
this approach would stem principally from the uniform size of the
particles, the accurate spacing of the lattice, and the ability to
control precisely both the size and the spacing. Such materials would
have numerous applications as photonic crystals in lasers, mirrors, and
waveguides and, because of the flexibility of the focused ion beam,
could be tailored to suit individual applications. In this method, a
FFIB will be used to implant an ordered array of spots. Subsequent
annealing leads to nucleation and growth of nanoparticles with the
composite relaxing toward a single particle size as suggested by kinetic
Monte Carlo simulations similar to those carried out for large--area
implants \[33\]. In the FFIB approach, particle size is controlled by
the local dose, which should allow nanocrystal size distributions to be
controlled within a few atoms, the narrowest ever produced for
nanocrystal ensembles. The same techniques would be applied to form
nanowires.

[Dual--Beam Irradiations for Manipulation and Control of Nanostructured
Material Formed by Implantation.]{.underline} Properties of
nanoparticles depend critically upon their size, size dispersion,
spatial distribution, and crystallographic orientation. Consequently, it
is imperative to develop methods to control these characteristics. It is
proposed that a secondary "energizer" beam be used simultaneously with
the ion source beam during formation of nanoparticles by ion
implantation. Supplemental energy deposited by an energizer beam
generates point and extended defects as well as electronic and
vibrational excitations that substantially change the kinetics of
nanoparticle nucleation and growth, thereby altering their size and
orientation \[34\]. It is anticipated that use of an appropriate
energizer beam during implantation will yield uniformly sized particles
that are crystallographically oriented within the host. This will
require that the energizer beam be matched to the primary beam to ensure
that it "activates" the desired property. This technique could be
coupled with the "Smart Cut" method \[35\] for thin--film transfer by
using hydrogen ions as the energizer. In this way, the energizer would
not only manipulate the nanoparticle distribution but allow the
fabricated devices to be separated and readied for assembly in sensors
or other devices.

[Integration of "Ion Slicing" and 3D Wafer Bonding for Assembly of
Nanoscale Devices]{.underline}*.*

Many components of nanoscale devices are likely to be grown either
attached to or embedded near the surface of solid substrates. In order
to fabricate three--dimensional (3D) nanoscale devices from such
components, it will be necessary to separate them from the substrates
and attach them to the rest of the device in the desired configuration,
particularly in cases where means for self--assembly have not been
devised. *Integration of methods such as "ion slicing" for separation
and wafer bonding between 3D nanostructures provides a potential path to
assembly of nanoscale devices.* The term "ion slicing" refers
generically to a class of ion--related processes used to separate or cut
thin films from bulk wafers, and includes the "Smart Cut" process \[35\]
used in silicon microelectronics. Contact bonding of the thin film to
another substrate prior to separation will result in the transfer of the
film to this substrate. While such processes have been demonstrated for
a number of materials, including diamond, silicon, and various
ferroelectric oxides \[36\], their applicability to a wider range of
materials needs to be explored. General issues of dimensional control
and uniformity also must be resolved to advance this technology to the
nanoscale. Critical issues include bonding materials that have 3D or
surface relief structure, and the alignment or registry of the
nanostructures prior to bonding. This will require integration of
standard patterning and lithographic techniques with the new transfer
methods. The result will be to open a path to efficient fabrication of
nano--devices by utilizing large--area substrates to facilitate their
handling and to enable parallel simultaneous fabrication of large
numbers of nanoscale devices.

**D. Nanomechanics and Nano--to--Micro Assembly**

*Current Status*

The future of nanoscale devices depends upon the scientific community's
capacity to physically manipulate nano--sized parts and to integrate
larger micro--to--mini scaled devices \[37\]. However, the relative
importance of fundamental physical forces changes as parts are reduced
in size.

Figure 6.1 shows the variation of forces with respect to scale. When
handling miniature parts with features on the order of a few
millimeters, classical mechanical phenomenon such as mass and friction
dominate the mechanics of manipulation. However, when part sizes are
reduced below the millimeter threshold, surface effects such as adhesion
due to surface condensation and the electrostatic potential between
parts dominate over the classical inertial effects present in
large--scale moving structures. Research currently focuses on the
assembly of millimeter--to--micron--sized parts \[38\]. Figure 6.2 shows
a series of gears with features below a millimeter and with clearance
fits between the parts of less than a micron. At this level surface
forces become relevant but it is still possible to use many classical
techniques for successful manipulation and assembly. However, we expect
paradigm shifts in the methods of assembly as the parts become even
smaller \[39\]. Examples include new methods of "visualizing" the parts
as well as for manipulation and handling \[40\].

*Scientific Issues and Future Directions*

Nanomechanics may be as important as nanoelectronics. Cantilever beams
can measure stress, heat, mass, and NMR. A linear array of cantilevers,
when treated with selectively adsorbing coatings, can distinguish among
various chemical species, and can act as an artificial nose.
Microcantilevers can function as RF switches. A long--term goal for
information storage is to have the capacity of 1000 CDs in a device the
size of a wrist watch. Large arrays of microcantilevers can be
envisioned as high--density storage devices (40 Gbits/cm) with present--
day technology. With further research, it may be possible to get to 400
Gbits/cm using carbon fullerenes as the cantilever elements in large
arrays \[41\].

![](media/image10.wmf){width="5.879861111111111in" height="3.9in"}

**Figure 6.2:** Assembly of micro gears

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# 7. UNDERSTANDING AND MIMICKING BIOLOGICAL FUNCTIONS: "DRAWING INSPIRATION FROM NATURE"

It is unlikely that anyone thinks of a whale while considering
nanoscience or nanotechnology. Even bacteria inhabit the micron regime.
But living organisms are simply collections of atoms, molecules, and
structures that are very much a part of the nanometer world.
Tenth--nanometer ions such as potassium and sodium generate our nerve
impulses. Sugars, amino acids, hormones and DNA are about one nanometer
in size. Membranes that separate one cell from another or one
subcellular organelle from another are about five--fold bigger. Proteins
can be tens of nanometers across. The organism can do what it does, and
cannot do what it does not, because of the presence, absence,
concentration, location and interaction of these "nano--structures".

Dimensions notwithstanding, the study of nanoscience and nanotechnology
does not, at least at this point, involve the use of intact living
systems, either alone, as building blocks of structures, or as devices.
It does, however, look to biological molecules for tools and components,
and to biology broadly, for inspiration about what can be done. The uses
of biological molecules and biology fall into four categories:

• Biological molecules can be used in conjunction with other structures
to perform just as they do in organisms.

• Biological molecules can be used in conjunction with other structures
to perform in a novel manner, quite distinct from their natural
function.

• Biology can provide examples of approaches and solutions to specific
problems.

• Biology can provide inspiration by demonstrating what types of
problems have been solved by Nature (however poorly we might understand
that solution) and therefore what problems might well be solved in our
laboratories (with a lot more creativity and hard work).

**A. Current Status**

Examples have already been reported of successful efforts in each of
these four categories. But whatever has been achieved pales in the face
of the far greater opportunities that remain for exploitation,
especially in the latter categories.

Functional nanostructures can incorporate individual biological
molecules. For example, biosensors can use natural sugars or proteins as
target recognition groups \[1\]. Modified biological structures can be
used to act in devices---for example the use of modified photosynthetic
membranes to split water to hydrogen and oxygen \[2\].

Functional multi--component structures can be fabricated that use
molecules in unusual ways. The filament protein actin, found for example
in muscle, can be attached to the enzyme ATP synthase, which is involved
in the production of most of the cell's ATP, its "energy currency". The
entire structure can then be immobilized on a glass slide. The natural
rotation of the ATP synthase, powered by ATP hydrolysis, can be used to
whip the actin filaments around in a circle, much as one would swing a
lariat. The turning is precise: 120° for each ATP molecule hydrolyzed
\[3\].

Biological approaches to problems guide us in designing non--biological
systems. The concept of combinatorial chemistry---the simultaneous
synthesis on a chip of huge numbers of variants of a given structure,
followed by the rapid screening of those variants for the few with
optimal properties---draws its inspiration from the mechanism of
antibody production by the immune system. In higher organisms, the
10^10^ different antibodies present in the body are synthesized by, in
effect, linking the individual sections of the molecule. Each individual
section is selected from a library of variants of that section \[4\]. In
combinatorial chemistry, the variants of the inorganic material to be
made are created by using the vast number of possible amounts of its
four, five, six or more constituent elements \[5\]. In both cases, the
population produced is rapidly screened to allow identification and then
mass production of the variant that best serves the desired function and
then it is mass--produced.

**B. Key Questions**

Many specific tasks performed by living systems use nanometer--size
structures in particularly intriguing ways---performing functions that
we would very much like to emulate.

• Organisms sense their molecular surroundings, having developed
exquisitely sensitive nanometer--size sensors on their outer surfaces.
Using the principle of molecular recognition, only the specific desired
target can bind to the surface--mounted "receptor" molecules. Upon
binding, these receptors change shape in a manner that alerts other
components of the system, inside the cell, to the presence of the
target.

• Living systems use nanometer--size structures to act as highly
selective pumps. The electron transport chain, which is central to the
trapping of the energy content of nutrients, pumps protons from one side
of the mitochondrial membrane to the other, against a gradient \[6\].
Neurons pump sodium ions out and potassium ions into the cell to ready
it for the next impulse \[7\].

• Living systems use nanometer--size structures as switches. Within 200
msec of the binding of a repellant molecule to a receptor, a phosphoryl
group is transferred to a protein and the rotation of the bacterial
"tail" is switched from counterclockwise to clockwise. This turns the
organism around and allows it to swim away from the repellant \[8\].

• Living systems use nanometer--size structures to perform catalysis,
with specificity, selectivity and rate enhancements that are rarely if
ever achieved artificially. Enzymes can be selective enough to catalyze
a reaction with only one particular molecule, even if it is present in a
mix of many, ignoring even its mirror image. Enzymes can selectively
catalyze only one of many chemically allowed reactions with that
molecule, with rate enhancements up to 10^16^ \[9\].

More broadly, and with more general potential impact, living systems
exhibit a number of characteristics that can serve as models for
nanoscience study. In some cases, our understanding of this behavior
simply alerts us to a concept we may not have considered. In all cases,
these are extraordinarily sophisticated but also very desirable
properties that we would wish to incorporate into structures and devices
to be used in non--biological settings. The very fact that Nature has
succeeded in developing systems with these characteristics (albeit over
hundreds of millions of years of evolution) says that they can be
achieved---they are not unattainable goals. On the other hand, it must
be accepted that in some cases, these goals may not be achievable except
by living systems.

• Living systems exert precise control of the atomic composition and
architecture of large and complex functional structures. Protein
complexes as large as 10,000 cubic nanometers can be rendered
nonfunctional by the change of a single atom.

• Living systems sense their surroundings and report that information to
control systems for the appropriate response.

• Living systems can sense damage and repair it to its original
functional state.

• Living systems can switch among several states in response to short
term and reversible changes in their environment.

• Living systems can evolve over time, adopting new forms in response to
long term, permanent changes in their environment.

• Living systems can self--assemble thousands of individual parts in a
precisely defined functional structure.

• Living systems can build complex structures hierarchically. Collagen,
a fiber that rivals steel in strength, is built through successive
aggregation of single amino acid strands into triple helices, triple
helices into microfibrils, microfibrils into fibrils, fibrils into
fibers.

• Living systems use templates, such as DNA, as blueprints for
structures where there is no thermodynamically or kinetically available
selection for that structure.

• Living systems can create a low entropy structure of importance by
increasing the entropy of the entire system. Protein folding, creating a
highly ordered arrangement of the constituent amino acids, is driven by
the maximization of the entropy of the surrounding water.

• Living systems can reproduce themselves. Once one is made, no
additional fabrication is required

Nanobioengineering

*Nano--engineering* of a biological system deals with the assembly of
nanomaterials to create or enable a specific biological function and/or
the subsequent characterization of that function. This can be viewed as
separate from the study of the nano--biomaterials themselves, which is
defined by their isolation and characterization or their synthesis from
basic building blocks \[10--12\].

The extension of the field of nano--engineering to biology includes the
patterning of 2D templates to direct cell or tissue response for
biocompatability or biosensor applications. Applications in medical
science include implants, prosthesis, drug delivery and diagnostics.
Other examples include creating a biological input/output device using
nano--fabrication techniques to enable communication with individual
cells for information technology applications. The field of computer
science is struggling with the problem of how to integrate information
science with biology. One approach has been through bioinformatics,
where traditional computer science methodology and hardware is applied
to manage the enormous amount of information now available from
biotechnology. Another approach envisions the *integration* of the
biology with computer science to create new hardware to enable direct
communication with the biology. If successful, this will provide the
platform to allow the treatment of biology as just another peripheral
for functions such as sensing, data storage and information security.
Other applications of this hybrid system could be communication with
individual cells to switch on and off biomanufacturing of drugs *in
vivo*, or the construction of biological--nonbiological hybrids for
robotics study and application. Several basic research directions seem
promising to enable fabrication of such devices:

## 

## The primary objective should be the Nanoengineering of the Biological Interface \[13\]. The success of two--way communication strategies between electrically active cells and microelectronics depends on the proper registration of cells with the microelectrodes and their close association or "seal" to those microelectrodes \[14\]. The "seal resistance," R~seal~, is measured between the electrode and the grounded recording media. Low values of R~seal~ are associated with poor electrode--to--cell adhesion resulting in an attenuated and distorted bioelectrical signal, a problem inherent in most extra--cellular recording systems. In contrast, R~seal~ values in the giga--ohm range, which is typically achieved using glass micropippettes during electrophysiology measurements, permit clearer resolution of bioelectric signals. In tissue this seal is accomplished by the interactions of proteins and other biological macromolecules with the glycocalyx, or outside surface, of the cells. The first step in recreating this interface is nanoengineering of the non--biological surface with biologically compatible materials such as proteins, peptides and biologically active functional groups. Surface modification allows a large variety of biomolecules to be attached to a microelectrode surface which will facilitate cell--microelectrode adhesion, increase R~seal~, and allow for geometric placement of cells \[15\]. For example, it is possible to control the nutrients fed to cells so that they produce desirable surface groups that allow binding of those cells to specific nonbiological surfaces \[16\]. The immediate benefit is that if a cell is associated with a single microelectrode, it becomes an individual sensor permitting multiple independent assays that then enable statistical analysis. Extracellular recording of these individual sensor elements would then allow long--term, multi--site measurements of electrically--active cell bodies and processes for building information technology devices.

Another major focus area is the creation of ***Hybrid
Cellular/Electronic Control Systems***. The goal is to develop the
electronic/neuronal interface and then use this to control a biological
transducer to study robotics applications. Developing this system will
demonstrate a new strategy for controlling a simple
cellular/mechanical/electrical hybrid robotic system. Simple model
systems could provide information as to what hardware and software
enable truly adaptable, coordinated, and graceful systems for movement
and problem solving. One example of a hybrid model system is the spinal
stretch reflex arc, one of the most fundamental neuronal feedback
circuits in the nervous system. Duplication of just this system would
increase robotic capability enormously.

• Living systems can be manipulated. We can specifically alter the
genetic blueprint, chemically modify the individual molecules, and in
fact now even alter the genetic code itself.

• Living systems can be components of hybrid structures---part living,
part synthetic, incorporating selected properties of both as required.

• Living systems are, in the absence of immunological rejection,
biocompatible.

**C. Future Directions**

Each of these characteristics or concepts can be regarded as a target
property of nanostructured materials and devices. The effort to achieve
this must be multi--disciplinary. The goals of the biologist are to
identify desirable functions performed by living systems and to work to
understand how they are performed. That knowledge, used by chemists,
physicists and engineers would then lead to the development of nanoscale
systems that use those techniques to achieve similar required functions.

The nature of the materials selected to achieve this, and their
organization and architecture is left to the imagination of scientists
who will be drawn to these challenges. They will use their particular
background, coupled with that of their collaborators from other fields,
as the basis for proposing novel approaches to these problems.
Recreating these properties is in many cases a difficult challenge, but
the rewards are incalculable. It is a multi--decade program of a broad
range of research, but it is one that will provide breakthroughs of
great significance almost from the start.

#### References

1\. D. Charych, Q. Cheng, A. Reichert, G. Kuziemko, N. M. Stroh, J. O.
Nagy, W. Spevak, and R. Stevens, "A \`Litmus Test' for Molecular
Recognition Using Artificial Membranes," *Chemistry and Biology* **3**,
113 (1996).

2\. E. Greenbaum, "Platinized Chloroplasts: A Novel Photocatalytic
Material," *Science* **230**, 1373 (1985).

3\. H. Noji, R. Yasuda, M. Yoshida, K. Kinosita Jr., "Direct Observation
of the Rotation of F~1~--ATPase," *Nature* **381**, 623 (1996).

4\. T. Honjo, and S. Habu, "Origin of Immune Diversity: Genetic
Variation and Selection," *Annual Review of Biochemistry* **54**, 803
(1985).

5\. X--D. Xiang, and P. G. Schultz, "The Combinatorial Synthesis and
Evaluation of Functional Materials," *Physics C* **282**, 428
(1997)**.**

6\. D. G. Nicholls, and S. J. Ferguson, "Bioenergetics 2," Academic
Press (1992).

7\. B. Sakmann, "Elementary Steps in Synaptic Transmission Revealed by
Currents Through Single Ion Channels," *Science* **256**, 503 (1992).

8\. D. E. Koshland Jr., "Chemotaxis as a Model Second Messenger System,"
*Biochemistry* **27**, 5829 (1988).

9\. C. Walsh, W. H. Freeman, [Enzymatic Reaction Mechanisms]{.underline}
p. 33, (1979).

10\. V. Stupp, LeBonheur, K. Walker, L. S. Li, K. E Huggins, M. Keser,
and A. Amstutz, "Supramolecular Materials: Self--Organized
Nanostructures," *Science* **276**, 384 (1997).

11\. S. I. Stupp, and P. V. Braun, "Molecular Manipulation of
Microstructures: Biomaterials, Ceramics, and Semiconductors," *Science*
**277**, 1242 (1997).

12\. D. A. Hammer and M. Tirrell, "Biological Adhesion at Interfaces,"
*Annu. Rev. Mater.* *Sci.* **26**, 651 (1996).

13\. J. J. Hickman and D. A. Stenger, "Interactions of Cultured Neurons
with Defined Surfaces," in: *Enabling Technologies for Cultured Neural
Networks* (D.A. Stenger and T.M. McKenna, eds.), 1st edition. San Diego:
Academic Press, Inc. 51 (1994).

14\. D. S. Jung, J. J. Cuttino, P. Pancrazio, T. Manos, R. S. Custer, L.
E. Sathanoori, M.G., Coulombe, M. A. Czarnaski, D. A. Borkholder, G. T.
Kovacs, P. Bey, D. A. Stenger, and J. J. Hickman, "Cell--Based Sensor
Microelectrode Array Characterized by Imaging XPS, SEM, Impedance
Spectroscopy, and Cellular Recordings," *J. Vac. Sci. Tech. A* **16**,
1183 (1998).

15\. M. S. Ravenscroft, K. E. Bateman, K. M. Shaffer, H. M. Schessler,
D. R. Jung, T. W. Schneider, C. B. Montgomery, T. L. Custer, A. E.
Schaffner, Q. Y. Liu, Y. X. Li, J. L. Barker, J. J. Hickman,
"Developmental Neurobiology Implications from Fabrication and Analysis
of Hippocampal Neuronal Networks on Patterned Silane--Modified
Surfaces," *J. Amer. Chem. Soc*. **120**, 12169 (1998).

16\. K. J. Yarema, L. K. Mahal, R. Bruehl, E. C. Rodriguez, and C. R.
Bertozzi, "Metabolic Delivery of Ketone Groups to Sialic Acid Residues:
Application to Cell Surface Glycoform Engineering," *J. Biol. Chem.,*
**273**, 31168 (1998).

# 8. HYBRID SYSTEMS: INTEGRATING FUNCTIONALITY

Nanotechnology exploits the new properties, phenomena, processes, and
functionalities that matter exhibits at sizes intermediate between
isolated atoms or molecules (\~ 1 nm) and bulk materials (over 100 nm).
Nature abounds with examples of nanoscale structures that are used to
provide critical functions including mechanical actuation (muscles),
energy conversion (photosynthesis), and sensing and molecular
recognition (olfactory centers).

The organization of artificial nanoscale structures has been achieved by
taking advantage of inherent molecular interactions including van der
Waals, hydrogen bonding, electrostatic, or covalent bonding forces that
promote spontaneous self--assembly. Using this approach, nanoscale
assemblies that demonstrate extraordinary optical \[1\], mechanical
\[2\], and sensing \[3\] functions have been developed. Unfortunately
though, the key requirement of *addressability,* the ability to direct
the movement of energy or signals within a useful architecture, is a
more difficult challenge than can be accomplished using the tools of
self--organization alone.

Biological analogues suggest that the ideal architectures for exploiting
nanoscale structures will be hierarchical, spanning from the nanoscale
to the microscale and perhaps even human dimensions. By combining the
best features of both "top--down" and "bottom--up" strategies it is
possible to exploit the unique capabilities provided by nanotechnology
in useful and addressable machine architectures. Since the top--down
approaches, based on various forms of lithography and replication, now
extend to approximately 100 nm, we have the opportunity to form a
seamless transition between nano-- and microtechnology.

####### A. Current Status and Recent Accomplishments

Researchers are currently exploring approaches that integrate unique
functions achieved through nanotechnology into microscale devices. As an
example, consider miniaturized instruments for chemical and biomolecular
analysis. This will be an area of intense research with strong impact
likely on health, environment, and national security.

Microfabricated chips for DNA analysis \[4,5\] and for polymerase chain
reaction \[6\] have already been demonstrated. These are perhaps the
initial steps towards a full--fledged technology of biomedical
microdevices, which will not only study and analyze nucleic acids but
other biological molecules such as proteins, carbohydrates etc.

The development of chips for rapid detection of biological pathogens is
a critical area with applications in the food handling/processing
industry, in biological/chemical warfare, and in early warning for
exposure to air and water--borne bacteria, viruses, and other antigens.
The µChemLab project \[7\] is learning to incorporate similar structures
into a fully autonomous chemical analysis system that is integrated into
on--chip architectures. For example, organically functionalized
meso--porous structures have been integrated on a micromachined heating
and flow stage to provide 1000--fold chemical pre--concentration for
on--chip analysis of chemical warfare agents.

Thundat et al. \[8\] recently demonstrated that the interaction of
antigen molecules with their corresponding antibody, attached to the
surface of a cantilever, can provide sufficient surface stress to bend
the cantilever beam (see Fig. 8.1). Such a device is an example of
hybridization and integration at several levels, because it has
nanocomponents (antibodies bound to a cantilever surface) and
microcomponents (cantilever beams) which can be delivered in a chip
(millimeter scale components) that together integrate biology and
biochemistry with engineering.

Boxer \[9\] recently demonstrated the deposition of lipid membranes into
lithographically defined corrals. Early results suggest that this
approach may allow *electrochemical addressing* of photo--defined
membrane cells. Consequently, this is a first step toward integrating
the functional nanotechnology demonstrated by living cells into robust
machine architectures.

The design and realization of specific structures at near--atomic scales
requires controlling materials according to prescribed macro- and
meso-scopic specifications, which are actually decided at the quantum
level. Despite recent advances, a number of outstanding problems remain
to be solved (see box on "Control Strategies at the Nanoscale").

####### B. Key Questions and Barriers

1\. The processing conditions for "top--down" (lithographic) and
"bottom--up" (nanoscale) manufacturing can be quite different and may
create thermal and chemical incompatibilities. For them to seamlessly
mesh we will need to better understand how to improve the stability of
nanostructures and to modify or find new patterning approaches.

2\. The unique properties associated with nanoscale structures and
materials can be dramatically influenced by the *dominance of
interfaces* on the nanoscale. We will need to understand how to tailor
and control interfaces that maintain *and potentially enhance* the
performance of nanoscale functions and properties.

# Control Strategies at the Nanoscale

Experimental and technical advances now make it possible to design and
realize specific structures at near--atomic scales. This process
requires *controlling* materials and structures according to prescribed
macro/mesoscopic specifications, which are actually decided at the
quantum level over \~nanometer spatial scales \[10--19\]. Despite
notable advances, outstanding problems remain to be solved, namely (i)
deeper theoretical understanding of new emerging properties from the
purely quantum description; (ii) reproducible control techniques at the
nanoscale; and (iii) high throughput, reliable assembly at industrial
scale. Research on efficient control techniques at the nanoscale will
span the following directions.

***Optimal Control**.* Within the optimal--control formalism, the
\"goal\" (the desired effect) and \"cost\" (the effort needed to achieve
it) are combined into an objective functional. This functional is
maximized under the dynamical constraint that the Schrodinger equation
describing the underlying system be satisfied. The quantum control
problem differs from the traditional engineering control algorithm in
that the wave function is not available to observation. Therefore, one
must construct the control functional in terms of those functionals of
the wave function that can be measured, and which will be used in the
control algorithm as constraints. From these global quantities, one
tries to obtain local quantities, such as the potential as a function of
position along the mesoscopic structure. Since several combinations of
adjustable parameters may yield similar results, the determination of
the \"cloning\" potential may not be unique. However, for practical
applications the main issue is not *uniqueness*, but *easy*
*feasibility* and *robust reproducibility* \[13,19\]*.*

## 

## Coherent Control. The coherence of laser light has been used to guide quantum systems into desired target states through interfering pathways. The measured shape of the wave function is used as feedback into the laser, which reprograms the optical field. The process is iterated until the shape of the wave function matches the target wave packet. If successful, the ability to control the shape and motion of the wave function may lead to efficient methods for bond--selective chemistry and quantum computing. Laser technology has advanced in regimes of frequency, pulse duration and shape, and peak intensity so that concepts of coherent control may be applied to individual atoms. The current state--of--the art in solid--state lasers operating at wavelengths near 1000 nm has yielded pulses as short as 5 fs and intensities of 10^22^ W/cm^2^, well in excess of the atomic unit of intensity. Under these extreme conditions, new physical phenomena appear, as substantiated by the steady flow of results on multiphoton ionization and high--harmonic generation. In such phenomena, the external field is not a small perturbation, but significantly shapes or distorts the quantum structure and dynamics. Individual states of the atom in the field experience large dynamical energy shifts, and may even cross energy thresholds, disappearing from the spectrum, only to be replaced by other \"light--induced\" states (LIS). Applying coherent control to the quantum evolution of the atom--laser system, unique phenomena, such as the stabilization of atoms against ionization, may be exploited. (Atomic stabilization is the \"counter-- intuitive\" trend by which, at some high intensity, the ionization rate decreases with increasing intensity.) The sustained, controlled exposure of atoms to super--high--intensity radiation will extend the application of current high--harmonic generation techniques. The question of what happens to harmonic emission of radiation in the stabilization regime of ionization or the role of the LIS, are open questions that require more research \[10,14,16\].

## Statistical Control. To produce, verify, and diversify nanomaterials and nanosystems at industrial scale, for specific requirements and customers, while maintaining high throughput and high economic efficiency, will require implementing a robust and efficient form of statistical control.

## 

## New Control Paradigms. It will be profitable to shift the current \"chisel down\" approach to nanostructures to a \"grow up\" approach based on molecular self--assembly and/or biological assembly, self--replication, self--aligned quantum dots structures, and bioarray technologies. The challenges of this approach are related to: combinatorial searches, kinetics in self--assembly, evolvable hardware, autonomous reprogramming, bioassembly, and related topics.

3\. The integration of nanoscale and microscale functionalities will
provide new opportunities that are not reflected in simple
miniaturization of macroscopic device analogues. We will need to step
away from conventional thinking about device architectures in order to
better exploit the real benefits of nanotechnology.

**C. Future Research Directions**

The technology for microelectronic, optoelectronic, and magnetic devices
is well established. They interact with their environment in either one
or two ways, i.e. either purely electrically or optoelectronically or
else magneto--electronically etc.

One can imagine a wide variety of other devices that will interact
differently with their environment. These interaction mechanisms include
but are not restricted to: chemically (sensors), biologically (sensors,
analyzers, drug delivery), thermally (engines, refrigerators),
acoustically (sensors and actuators), and mechanically (MEMS devices).
In the future, it is expected that a technology for *truly hybrid
devices* will be developed that will be not only multi--functional but
inexpensive. Hybridization will occur in scale as well as interactions.
Such devices will contain components ranging in size from
nanometer--scale (biomolecules, nanocrystals etc.) to micrometer--scale
(cantilever beams, electronic interconnects) and to millimeter--scale
(packaging/handling).

In addition to hybrid devices that provide new functions and
capabilities, the integration of nanoscale and microscale
functionalities will provide new means to directly study nanoscale
phenomena. For example, micromachined scanning--probe instruments are
creating new means to image and measure a wide range of properties of
nanoscale materials and structures. The addition of nanoscale structures
such as carbon nanotubes, to the probe itself, holds promise for even
greater resolution and functional capability. (See "Nanoscale
Instrumentation" in this report.)

**References**

1\. T. J. Marks, M. A. Ratner, *Angew. Chem. Int. Ed. Engl.* **34**, 155
(1995).

2\. H. Noji, R. Yasuda, M. Yoshida, K. Kinosita, *Nature* **386**, 299
(1997).

3\. X. Song, J. Nolan, B. I. Swanson, *J. Am. Chem. Soc.* **120**, 11514
(1998).

4\. B. Lemieux, A. Aharoni, M. Schena, "Overview of DNA chip
technology," *Molecular Breeding* **4**, 277--289 (1998).

5\. K. M. Kurian, C. J. Watson, A. H. Wyllie, "DNA chip technology," *J.
Pathology* **187**, 267--271 (1999).

6\. M. U. Kopp, A. J. deMello, A. Manz, "Chemical amplification:
Continuous--flow PCR on a chip," *Science* **280**, 1046--1048 (1998).

> 7\. Frye et al., **477** in "Micro Total Analysis Systems \'98" Ed.
> Harrison and van den Berg Kluwer Aced. Press (1999).
>
> 8\. T. Thundat, P. I. Oden, R. J. Warmack, "Microcantilever sensors,"
> *Microscale Thermophysical Engr*. **1**, 185--199 (1997).
>
> 9\. J. Groves, N. Ulman, S. G. Boxer, *Science* **275**, 651 (1997).

10\. W.S. Warren et al., "Coherent control of quantum dynamics: the
dream is alive," *Science* **259**, 1581 (1993).

11\. D. Bessis et al., "Electron wave filters from inverse scattering
theory," *Europhys*. *Lett*. **37**, 151 (1997).

12\. T. Seideman, "Molecular optics in an intense laser field: a route
to nanoscale material design," *Phys. Rev. A.* **56**, R17 (1997).

13\. A. N. Zakhariev and V. M. Chabanov, "New situation in quantum
mechanics (wonderful potentials from the inverse problem)," *Inverse
Problems* **13**, R47 (1997).

14\. D. Meshulach and Y. Silberberg, "Coherent control of two--photon
transitions by femtosecond laser pulse," *Nature* **396**, 239 (1998).

15\. A. Bulatov et al., "Nonadiabatic cooling and optimal control in
off--resonance dipole optical potentials," *Phys. Rev***. A** **58**,
1346 (1998).

16\. N. Bonadeo et al., "Coherent optical control of the quantum state
of a single quantum dot," Science **283**, 1473 (1998).

17\. R. B. Perez, V. Protopopescu, and L. Munoz--Cobo, "Control of
quantum systems," *Math*. *Models and Math. in Appl. Sci*. **9**, 305
(1999).

18\. T.C. Weihnacht, J. Ahn, P.H. Bucksbaum, "Controlling the shape of a
quantum wavefunction," *Nature* **397**, 233 (1999).

19\. G. Harel and V. M. Akuli, "Complete control of Hamiltonian systems:
engineering of Floquet evolution," *Phys. Rev. Lett*. **82**, 1 (1999).

# 9. NANOSCALE INSTRUMENTATION

Progress in nanotechnology requires having the tools to observe,
characterize, and control phenomena at the nanometer scale. A whole new
generation of analytical instrumentation and nanoscale devices, capable
of providing information about nanoscale physical, chemical and
mechanical phenomena must be developed. Nanotechnology is already
benefiting from novel instrumentation developed during the past two
decades, for example scanning probe microscopes and a new generation of
synchrotron x--ray sources capable of studying materials at the
nanoscale. On the other hand, discoveries in nanoscale science and
engineering can provide the basis for the development of unprecedented
new tools. Thus, the next decade will provide both opportunities and
challenges in developing instruments for synthesizing nanostructures as
well as for characterizing nanostructures and measuring properties at
the nanoscale.

**1. Recent Advances and Current Status**

Several recent advances in the national laboratories are contributing to
the development of new nanoscale instrumentation. Rapid progress in
micro--electromechanical systems (MEMS) is opening new directions in
materials research at small length scales. For example, researchers have
developed a MEMS strain gauge from nanocrystalline diamond as seen in
Figure 9.1 \[1\].

![](media/image13.jpeg){width="6.840277777777778in" height="2.08125in"}

**Figure 9.1:** SEM micrograph of the vernier indicator of a
nanocrystalline diamond strain--indication structure. The four anchor
pads are on the Si substrate, and the vernier scales and arms are
deposited on SiO~2~. The 2--μm--thick diamond indicators and support
arms are detached from the substrate by a gas--phase CF~4~ etch (Ref.
1).

Understanding an atom\'s travels over a surface at different
temperatures, as well as its ultimate incorporation into the surface, is
crucial to making smaller, faster, smarter devices. A new instrument
called the "Atom Tracker" \[2\] now can observe an atom in motion. The
Atom Tracker supplements the STM\'s vertical feedback with lateral
electronic feedback to keep the probe hovering over the atom of
interest, and can track atomic motions 1000 times faster than a
conventional STM. By continuously monitoring the motion of an individual
atom as it binds to various sites on a surface the diffusing atom itself
becomes a probe of the surface structure and properties.

An electron--beam tool based on reflection electron holography performed
at ultra--low beam energies is being developed for critical dimension
(CD) and defect review metrology \[3\]. The target resolution limit is
0.5nm at 100eV energy, permitting the system to perform CD metrology for
devices built to design rules well below 10nm (i.e. the device
generation currently projected for 2009). The *molecular* level of
resolution and the low beam damage resulting from the choice of beam
energy make this tool ideally suited for emergent nano--technologies
including purly biological or hybrid device structures.

In order to understand processes used to produce nanometer--sized
particles deposited on surfaces, new aerosol instrumentation is needed.
Significant progress has been made recently, with instruments now
available for detecting particles as small as several nanometers in
diameter \[4,5\]. Mobility classifiers that enable high--resolution
classification according to particle size also have recently been
reported \[6,7\]. These instruments operate at gas pressures in the
range of one atmosphere, while measurements of nanoparticle size
distributions at pressures down to 0.1 Torr are possible with a
particle--beam mass spectrometer \[8\].

The inventions of the scanning tunneling microscope (STM) \[9\] and the
atomic force microscope (AFM) \[10\] have spawned the development of a
variety of new scanning probe microscopes (SPMs) \[11\]. Whereas the STM
and AFM have found applications mainly in topographical imaging, the
other SPMs add more functionality. Examples include the near--field
scanning optical microscope (NSOM) \[12\], scanning thermal microscope
(SThM) \[13,14\], scanning capacitance microscope (SCM) \[15\], magnetic
force and resonance microscopes (MFM) (see Figure 9.2) \[16,17\], and
the scanning electrochemical microscope (SECM) \[18\]. By providing
access to and enabling observation of physical, chemical, and biological
phenomena at nanometer scales, these SPMs have changed the landscape of
nanoscale scientific research.

A sensor that can measure the atomic--level forces that develop between
two surfaces as they approach each other and come into contact also has
been developed \[20\]. The Interfacial Force Microscope (IFM) utilizes a
feedback force sensor that eliminates the snap--to--contact event that
is inherent in other scanning force microscope designs. The IFM allows
measurement of the full range of adhesive interactions and can be used
to image surfaces by controllably hovering out of range of contact.

Just as scanning probes have revolutionized our ability to image and
characterize nanoscale structures, it is clear that the ability to
explore multiple functions with a single probe (multi--tasking scanning
probes) will bring about a new revolution. For example, the combination
of a force microscope with a scanning near--field optical microscope was
recently demonstrated \[21\]. This new capability allows one to directly
image and correlate changes in molecular conformation, as indicated by
specific optical transitions, with the mechanical compliance of
molecules.

![](media/image14.png){width="2.975in" height="2.7in"}

**Figure 9.2:** AFM (left) and MFM (right) images of a Sm--Fe film grown
epitaxially by sputter deposition. The MFM image shows stripe domain
patterns characteristic of magnetic films with strong perpendicular
magnetic anisotropy. The epitaxial Sm--Fe film lacks domain wall pinning
and is an ideal candidate base material for creating artificial pinning
sites. The information provided by studies of domain wall--defect
interaction would greatly benefit research on permanent magnet
materials. (Ref. 19).

Recent advances in laser desorption mass spectroscopy have made possible
real--time detection, sizing and chemical analysis of individual
aerosols larger than 200nm \[22,23\]. It is possible to develop a
versatile system making use of various interfaces which deliver intense
beams of size--selected particles from ambient conditions into a vacuum
chamber for analysis. The combination of differential mobility analyzers
with such a system will make possible sizing and chemically
characterizing individual aerosols in real--time.

Current x--ray capabilities at third--generation synchrotron sources
such as the Advanced Photon Source, as well as neutron--scattering
capabilities being developed at The Spallation Neutron Source,
facilitate new types of *in situ* studies of the synthesis and
processing of nanostructured materials, and will clearly play an
important role in establishing structure--property relationships for
nanostructured materials. Direct radiation from synchrotron sources
already provides x--ray beams with intensities capable of real--time
(spatial average) monitoring of the formation, evolution, coating, and
encapsulation of nanoparticles on substrates using techniques such as
small--angle scattering, large--angle diffuse scattering, and direct
Bragg diffraction measurements \[24\]. The application of significantly
submicron focused beams can provide *in situ,* spatially--resolved
structure and property measurements during synthesis, processing, and
physical testing. Real--time x--ray diffraction measurement of
solution--injection nucleation, growth, and evolution of thin films and
precipitates is presently possible. For example, real--time x--ray
scattering techniques have been used recently to investigate the growth
modes and surface structure of GaN thin films grown by metal--organic
chemical vapor deposition \[25,26\]. Using microscale (i.e. laboratory
on a chip) techniques \[27\] in combination with micro and nano x--ray
beams, high--resolution time--resolved measurements can be accomplished
in steady state by controlling the stream velocity and varying the
distance between the injection point and the measuring position. Scaling
measurements to smaller sizes, to more complex geometries, and the use
of combinatorial analysis techniques are obvious extensions of microbeam
x--ray techniques to the nanoscale.

**2. Key Questions and Barriers**

A number of questions need to be addressed in developing the next
generation of instruments capable of probing structure and properties at
the nanoscale. Among these are:

1\. *Can new techniques be developed to determine local but non--surface
structure and properties in composite materials?* In such materials the
local region of interest is of nanometer--size but is only a small
fraction of the total sample volume. While techniques such as x--ray
diffraction can provide global--averaged information, and techniques
such as AFM can provide nearly atomic--scale information about surface
structures and properties, currently only electron microscopy techniques
are useful in probing local (nm--scale) non--surface structures, and
there are no techniques at all for investigating properties of
nanometer--sized non--surface regions.

2\. *What materials and materials research are needed to develop the
next generation of fine--scale instrumentation?* MEMS devices currently
are almost exclusively Si--based. Very short operating lifetimes due to
poor friction and wear properties of Si severely limit their potential
usefulness. Devices manufactured from alternative materials are clearly
needed. One possibility is to develop new MEMS devices based on
nanocrystalline diamond \[1\].

3\. *How small is too small?* The push for miniaturization eventually
will reach a quantum--mechanical limit where, for example, Newtonian
mechanics will not be able to adequately predict behavior. Future
instruments that manipulate individual atoms will need, for example, to
take the Heisenberg uncertainty principle into account.

**3. Future Directions**

Properties of nanomaterials are exquisitely sensitive to details of
their atomic structure, 3D shape, surface termination, surface and
\'bulk\' defects, impurity atoms and locations. New tools are required
to map structure, chemistry and functionality, ideally at the atomic
level. We need single--atom sensitivity to impurities; 3D shape
determination with atomic accuracy; and the ability to find functional
sites and determine the origin of that function. It is desirable to
measure optical absorption and emission from individual elements,
preferably with femtosecond time resolution. These tools will come from
the further development of current microscopies including scanning
tunneling microscopy, confocal and near--field optical microscopy,
atomic resolution transmission and scanning transmission electron
microscopy, electron energy loss spectroscopy, cathodoluminescence and
electron--beam--induced--current imaging.

Traditionally, improved or new characterization tools have paved the way
for increased understanding in materials science, e.g. electron
microscopy. The same will be true for nanoscale materials science.
Scanning probes already are making a huge impact. But new tools are
needed to image and characterize buried interfaces with nanoscale
resolution, and these must operate in a wide range of temperature and
environments to be of greatest use.

While various forms of microscopy (TEM, SEM, AFM, STM, etc.) can furnish
detailed information about morphology, size, and on occasion the
composition of nanoparticles, none are capable of providing real--time,
on--line information. There is a clear and pressing need for new
techniques and instruments that allow for rapid detection and for
chemical and physical characterization of nanoparticles, as well as for
automated or quasi--automated systems that require a minimum of operator
attention.

With the STM atoms can be manipulated \[28\], but only at low
temperatures. We can manipulate molecules at room temperature but only
two examples have been shown to date \[29,30\]. With scanning probes, we
can fabricate nanoclusters containing under 10,000 atoms and nanowires
less than 4 nm wide with great control, but at an unacceptable cost in
time. The metallicity of a cluster can be determined with scanning
tunneling spectroscopy, but we cannot determine its chemical identity or
its magnetic properties. To address these shortcomings will require a
broad program of technique and instrumentation development.

A major effort also must be devoted to invent communication and
networking devices that operate at the nano--scale. The underlying
methods and technologies may be targeted at non--invasive exploitation
of indigenous media, and should include capabilities for interfacing
with operators and other macro--scale devices in the outside world.

Scanning probe microscopes (SPM) are limited to analyzing surface or
near--surface properties of solids with nanometer--scale spatial
resolution. Sub--surface imaging and truly 3D microscopy with
nanometer--scale resolution is not currently available but is extremely
important for future development of nanotechnology. For example, most
biological nanostructures are 3D and are currently imaged by x--ray
crystallography, which is expensive and time consuming. Even in
micro/nano--electronics, which is progressing toward 3D multi--layer
structures, 3D imaging would be very useful. Possible approaches for
sub--surface imaging include ultrasonic echo imaging, non--linear
(multi--photon) optical microscopy, or thermal spectroscopic imaging. It
is unclear at present which technique will be most suitable, but this
area of investigation needs to be emphasized.

Nano--sensors have a high potential for deployment in areas and media
that are not readily amenable to probing by traditional devices. An
example is medical operations requiring minimally invasive surgery. A
collection of specialized devices may be needed to provide all of the
necessary functionalities. One can imagine an ensemble of micro or
nano--robots that cooperatively explore, assess, and perhaps operate on
various locations in an organ. This poses formidable challenges in
signal processing, control, and interconnectivity. (See box on "Networks
of Nano--Sensors.")

The realization of new technologies based on nanostructured materials is
limited in part by our ability to develop and control materials
processing to achieve desired properties. However, revolutionary
developments in x--ray focusing optics, in connection with ultra--high
brilliance third--generation synchrotron x--ray sources, should produce
rapid advances in materials probe capabilities that will complement TEM
and SPM capabilities. Fresnel zone plates have been used to demonstrate
intense, focused beams of hard x--rays with resolution below 100 nm at
the Advanced Photon Source \[31,32\] and hard x--ray beams down to the
20 nm range now appear within reach. These x--ray probes are
complementary to the many powerful scanning probe and scanning electron
microcopies in that they provide non--contact, nondestructive access to
the depth dimension in harsh environments, and both physical properties
and atomic structure information is available. (See box on "Synchrotron
X-rays for Process Discovery and Control in Gaseous Environments").
Nanobeam x--ray applications in the 10^2^ -- 10^3^ nm resolution range
are

**Networks of Nano--Sensors**

Conventional networking methods are inadequate for ensembles of
nano--sensors and micro-- or nano--robots because of limitations of size
as well as the medium in which the devices may be deployed. Optical
communication is not possible in opaque media nor is electromagnetic
propagation possible in electrically conducting liquids. New methods are
needed to exploit the native medium to communicate between sensors with
minimal invasion. One example is to use tiny sonic bursts to communicate
between sensors deployed in fluid or other media. Nanocommunication
devices themselves have to be developed to meet limitations of size and
power. Interconnectivity among nanosystems and I/O with the macro world
also are primary research challenges. Clearly, a major effort will be
required to invent nanoscale communication and networking devices.
Several research topics are illustrated here.

***Transmitting and receiving devices**.* Seeking suitable transmitting
and receiving device candidates is expected to be a major part of future
research. Both electronic and optical devices have significant merits
for communication. Optical devices have an advantage in speed and larger
bandwidth and in higher radiated power. Combining nonlinear elements
into arrays \[38\] may significantly enrich the performance of the
communicating device. Single nonlinear devices (transmitters and
receivers) produce only temporal waveform characteristics of the
transmitted/received signal. Arrays of coupled devices will introduce an
important additional characteristic to the communicating signal, namely
a spatial waveform. Coupling the spatial with the temporal dimension
will enhance the ability to decode and encode information for
communication and position location.

![](media/image16.png)

Schematic representation of a nanosensors / nanorobots network

***Chaotic Communication.*** This new communication paradigm, in which
the transmitted signal is chaotic, promises greater efficiency, greater
number of channels, higher security, and potentially low cost \[39,
40\]. The outstanding issue in chaotic communication is synchronization
between the transmitter and the receiver \[41--44\]. Better ways must be
identified for robust synchronization between devices and for
synchronization of the oscillations in the same device. In addition to
already established directions to synchronize chaotic systems \[45\] new
and counter--intuitive approaches such as using thermal noise \[46\] and
quenched disorder \[47,48\] to synchronize and control chaotic systems
should be explored. Using noisy and disordered arrays should lead to
large savings in fabrication costs.

***Biomimetic Communication**.* Here the goal is to adapt information
processing techniques used in biological systems (such as the heart
and/or brain) to enhance the capacity of sensing. The key issues
requiring study are the ability to communicate by structures and to
synchronize oscillations \[49,50\]. Communication paradigms exhibited by
biological oscillators (both experimental and theoretical) may lead to
pioneering ways of sensing, detection and transmission of information,
and may contribute insights as to how to mimic the communications and
interconnectivity of biological systems in man--made nanosystems.

***Computational components**.* Research on communication at the
nano--scale should be highly interdisciplinary, since it probes the
foundations of physics, chemistry, biology, mathematics, electrical and
optical engineering, and computer science. Computational modeling and
simulations to interpret and predict experimental findings are
essential. Verifying the validity of computational models describing
nonlinear nanoscale elements, as well as the efficiency and speed of
performing the computations, also will be crucial issues.

Synchrotron X--rays for Process Discovery and Control in Gaseous
Environments

![](media/image17.png)

Schematic of the MOCVD real--time processing facility at the Advanced
Photon Source. While currently used in studies of epitaxial growth of
GaN, such a system is well--suited for future studies of MOCVD
processing of oxide and nitride nanometer--scale thin--film
heterostructures. The flow patterns and temperatures shown at the right
of the figure are the results of unpublished calculations by R.
Venkataramani and K.F. Jensen \[MIT\]. (Ref. 25)

Nanoscale heterostructures are being produced for increasingly complex
multicomponent and multilayered oxide films. These oxide multilayers
have great potential for device applications. But a major challenge to
realize their potential lies in developing in situ diagnostics to
control film thickness and nanostructure, including grain size, phase,
composition, defect density, and homo-- and hetero--phase interfacial
properties. Because oxide--film synthesis usually takes place in
non--vacuum environments, process monitoring using traditional
electron--beam methods is difficult or impossible. In order to provide
needed atomic--scale structural and chemical information in oxygen and
other gaseous environments, real--time synchrotron x--ray diffraction
(SXRD) techniques are currently being developed at the Advanced Photon
Source by two national laboratory groups \[25,26,33,34\]

One group's research seeks to provide scientific understanding of the
many coupled phenomena occurring during MOCVD processing of thin films.
Current work is focused on GaN thin film growth behavior and surface
structure. Future plans include studies aimed at understanding the
physics of ferroelectric domains, particularly in ferroelectric
capacitors of size comparable to the domain order parameter, i.e.,
[\<]{.underline} 500 nm. Their use of SXRD provides sophisticated
measurements of atomic--scale structure, interface morphology,
composition, phase content, and chemical states during MOCVD growth and
processing (see figure) \[25,26\]. The other group's research exploits
the intermittent nature of pulsed--laser deposition, together with the
pulsed temporal structure of synchrotron x--ray beams, to experimentally
separate the deposition and surface diffusion/aggregation steps in film
growth. This makes possible real--time investigations of the
non--equilibrium processes associated with the pulsed--deposition and
surface--crystallization and relaxation steps of film growth, on time
scales ranging from microseconds to seconds \[34\].

Support for the continuing development of facilities for *in situ,*
real--time structure and property measurements is needed to speed
scientific understanding and to develop advanced growth and processing
methods for the production of nanoscale heterostructures.

under development \[33\] and transfer of these techniques to still
smaller length scales can be anticipated. Consequently, as
non--destructive, real--time measurements of the physical properties of
nanoparticles and nanostructured materials evolve, and as their use to
monitor nanomaterials processing is developed, the field of nanobeam
x--ray investigations will provide exceptional opportunities for
fundamental and applied investigations.

The challenge of efficient production of nanocluster arrays is most
reasonably addressed through parallel processing or self--assembly
approaches such as buffer--layer assisted growth \[35\] or
strained--layer growth \[36\]. While current efforts are promising, they
will still require a great deal of developmental effort.

The technology for fabricating thermal, optical, magnetic, or molecular
probe structures has been demonstrated \[37\]. However, probes currently
are made for an individual function. An important future goal is to make
multi--functional probes for "laboratory on a tip," or "nanoscale total
analysis." This will require integration of knowledge from engineering,
chemistry, physics, and biology.

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# 10. INFRASTRUCTURE AND FACILITIES FOR NANOSCALE SCIENCE AND TECHNOLOGY

**1. Accomplishments and Current Status**

The national laboratories are uniquely poised to impact research on
nanoscopic materials, due to their large--scale and multi--user neutron
and x--ray facilities. Argonne National Laboratory is the site of the
Advanced Photon Source and the Intense Pulsed Neutron Source; Brookhaven
National Laboratory is the home of the National Synchrotron Light Source
and the High Flux Brookhaven Reactor; Oak Ridge National Laboratory
houses the High Flux Isotope Reactor and will be the location of the
Spallation Neutron Source; the Los Alamos Neutron Scattering Center,
LANSCE, is located at the Los Alamos National Laboratory; the Lawrence
Berkeley National Laboratory is the home of the Advanced Light Source;
and the Stanford Synchrotron Radiation Laboratory is situated at the
Stanford Linear Accelerator Center. Stewardship of these facilities is a
specific mission of the Office of Science of the Department of Energy.

The wavelength or energy range of the radiation provided by these
facilities is ideally suited for the reciprocal or real space
characterization of nanoscopic structures, which have spatial dimensions
from tenths to thousands of nanometers. X--rays in the 7--10 keV range
are a perfect match for nanomaterials structural characterization. The
high flux of these sources makes possible probing either the static
structure or real--time changes in nanoscale structure (as described in
the highlight box on "Synchrotron X-rays" in section 9). While softer
x--rays can be used to this same end, soft x--rays also can be used for
real--space imaging through x--ray microscopy. Although at present the
spatial resolution is not sufficient, progress is being made to improve
it.

The ability to change x--ray energies continuously and precisely also
affords other opportunities. Anomalous scattering methods, for example,
are particularly well suited to the characterization of multicomponent
inorganic materials, where the scattering from nanoscopic particles
embedded in a host can be determined. Materials also can be
characterized spectroscopically, which leads immediately to determining
the electronic structure of materials, a property that is known to
undergo substantial changes as the size scale is reduced. Both hard and
soft x--rays can be used for lithography, where the short wavelengths of
x--rays extend the spatial resolution (minimum feature size) to the
nanometer level. Thus, from many perspectives, the match between
existing synchrotron sources and nanoscale science is ideal.

Reactor and spallation neutron sources, providing neutrons with
wavelengths from one--tenth to several nanometers in size, are similarly
well--matched to the nanoscale. While neutrons can be used to study
nanoparticles themselves, they also can be used to study protective
coatings or dispersants that are placed on the nanoparticles to prevent
oxidation or to disperse the particles in a solvent or host medium.
These coatings are typically organic layers anchored to the surface of
the particles. Alternatively, nanoscopic particles can be grown with a
micellar structure or even in dendrimers, hyperbranched organic
molecules. By selective labeling of the organic with deuterium, or by
the use of deuterated solvents, the contrast between nanoparticle and
coating can be optimized. This provides an unparalleled means of
investigating the organic material surrounding the nanoparticle. The use
of organic materials, self--assembled organic structures and organic
hybrid materials is emerging as a simple yet robust route to the
production of functional nanoscopic structures. The potential impact of
neutron sources for the elucidation of these structures is tremendous.

The Department of Energy also supports microcharacterization and
specialized single--purpose centers, including electron microscopy
centers, microanalysis centers, and surface modification and
characterization centers. Electron microscopy and electron diffraction
have been indispensable for the visualization of nanoparticles and for
quantitatively determining the crystal structures of nanoparticles.
Ordered arrays of nanostructures are well suited for investigation by
electron microscopic techniques.

**2. Barriers to Using National Facilities for Nanoscale Research**

The mere existence of superb facilities is not sufficient to ensure a
productive research output. It is necessary to ensure that user
facilities operate in a highly reliable manner, i.e. maximum
availability and beam delivery. Reliability for an external "on--site"
user is absolutely essential to ensure a viable research effort. For a
facility to have an acceptable level of reliability, it is necessary to
invest significantly in the maintenance and upgrading of the facilities.
In addition, ancillary equipment at user facilities must be supported to
permit full use of the facilities.

While the national facilities described above are important tools, even
when flawlessly operated they cannot be used effectively without a
dedicated team of scientists actively pursuing nanoscale research.
Currently several national laboratories are conducting research in
nanoscopic structures and are, to a greater or lesser extent, using some
of the facilities available. However, the potential exists to much more
effectively use these facilities by better focusing and consolidating
the nanomaterials research effort. This requires an investment in
nanomaterials characterization and synthesis facilities that are
associated with existing major national photon and neutron sources and
nanointegration facilities, and the concentration of the efforts of
scientists at these laboratories into a cohesive unit. This can be
accomplished by establishing Centers for Nanomaterials Research in the
national laboratories in order to take full advantage of the most
intense sources and capabilities, as described in the Appendix. This
also will permit using facilities at these Centers to promote
inter--laboratory collaborations and to build on already established
expertise.

**3. Future Directions**

Establishing Centers for Nanomaterials Research will require a change in
the infrastructure of national laboratories that deviates somewhat from
their current mode of operation. At present the scientific effort of
national laboratories tends to be broken down according to academic
discipline, as opposed to a focused research thrust. However, by
establishing Nanomaterials Centers (similar to the NSF Materials
Research Science and Engineering Centers) the expertise from a broad
range of backgrounds will be tapped. Research on nanoscopic structures
involves chemistry, physics, materials science, chemical engineering,
electronics, computation and biology. By focusing the research efforts
of scientists in these different disciplines into interdisciplinary
research groups that take advantage of the facilities, synergistic
interactions will develop both within the research group and with other
scientists at the facilities. While Centers will focus the research
effort, they also will provide the infrastructure support (support
personnel, technical support, laboratory support, common instrumentation
needs, and close association with the major photon, neutron and
nanointegration facilities) in order to perform research effectively and
efficiently.

The question arises: "Should the research efforts of the Centers be
focused only on utilizing the synchrotron, neutron or electron
microscopy resources?" Absolutely not. The Centers must be driven by the
need to carry out nanoscale research and technology development of the
highest quality, be this chemistry, physics, biology or engineering, and
should not be dominated by one or two techniques, in support of a
nanotechnology initiative that promises great economic benefits.
Nonetheless, the inherently unique capabilities for nanoscale research
of these major facilities are resources that must be efficiently
organized and tapped.

# APPENDIX: NANOMATERIALS RESEARCH CENTERS

The Department of Energy's multi--user synchrotron x--ray and neutron
facilities are unique tools that can have an unmatched impact on our
understanding of nanoscale materials and processes and on their
application for nanotechnology. The wavelength or energy range of the
synchrotron x--ray and neutron beams provided by the Advanced Light
Source (ALS), the Advanced Photon Source (APS) and the Spallation
Neutron Source (SNS) are ideally suited for both reciprocal and real
space characterization of nanoscopic materials and structures, as
described in section 10. The high flux of these sources makes possible
probing both static structure or time--dependent changes in structure
with nanoscale resolution.

The different characteristics and interaction mechanisms of x--ray
photons or neutrons with condensed matter confer unique strengths on
each for investigations of particular types of materials and phenomena.
For example, synchrotron x--rays are well--suited to study complex,
multicomponent inorganic materials, e.g. nanoscopic particles embedded
in a host matrix. They also permit direct spectroscopic characterization
of materials, leading to the determination of nanoscale changes in
electronic structure. Furthermore, x--rays currently provide the leading
method for lithographic pattern transfer with nanometer (or slightly
better) resolution. Neutrons, on the other hand, can be used to study
nanoparticles themselves, as well as the protective coatings or
dispersants that are placed on them, with very high contrast. Because of
its sensitivity to atomic motions (vibrational modes) and to low--Z
atoms, neutron scattering has unparalleled strength for investigating
biological molecules, as well as self--assembled organic molecules or
hybrid structures that provide a promising route to the production of
functional nanoscopic structures. Thus, the potential impact of intense
x--ray and neutron sources on nanotechnology is enormous.

The impact of nanoscience discoveries will depend on hierarchical
linking across multiple length and complexity scales. This linking from
molecular interactions to nanostructures to functional systems is a
fundamental challenge of the first order, both scientifically and
technologically. Emerging approaches of lithography and replication
extend to 100 nm and lower, providing the opportunity to form a seamless
integration of nano- and microtechnology. Bringing together the broad
range of processing tools, characterization equipment, and technical
expertise needed to span these length scales is a significant challenge.
National laboratories provide the ideal setting and necessary facilities
to accomplish this important task.

In order to fully exploit the nation's suite of synchrotron x--ray and
neutron facilities for the development of nanotechnology, it is proposed
to establish several new Nanomaterials Research Centers. These Centers
would be closely associated with the principal existing x--ray and
neutron sources and facilities for nanotechnology integration. The
purpose of these Centers is to enable and optimize the use of national
x--ray and neutron user facilities for the characterization of
nanomaterials, the fabrication of nanostructures, and to provide for the
seamless integration of nanotechnologies.

Nanomaterials Research Centers will provide a unique environment to
explore new nanoscience developments and allow students, faculty,
industrial researchers, and national laboratory scientists to work
together to propose, design, and assemble these materials into useful
devices. The need for Nanomaterials Research Centers has recently become
widely recognized. The IWGN report calls for several new national user
facilities focused on nanomaterials \[1\], while a recent report of the
National Research Council calls for the DOE to "strengthen its support
for state--of--the--art instrumentation and fabrication capabilities,
including Centers for instrumentation R&D, nanofabrication, and
materials synthesis and processing at national laboratories" \[2\].

Each new Nanomaterials Research Center will be clearly distinguished by
a materials--related "primary focus" that is based both on the type of
radiation available and on existing national laboratory research
strengths and characterization facilities. For example, focal research
areas suitable for four Nanomaterials Research Centers could be:
materials derived from or inspired by Nature; hard and crystalline
materials, including the structure of macromolecules; magnetic and soft
materials, including polymers and ordered structures in fluids (e.g.
micelles, liquid crystals, etc.); and nanotechnology integration.

The principal barriers to applying our national synchrotron x--ray and
neutron sources for nanotechnology are discussed in section 10. The
overriding need is to provide ancillary facilities and an organizational
infrastructure that will focus a truly national nanomaterials research
effort, thereby enabling breakthrough opportunities. The importance of
Centers is that they permit bringing together the research mission of
national laboratories with the educational role of universities and the
problem--defining capabilities of industry \[3\]. The resulting
partnerships across disciplines and among national laboratories,
universities and industry are essential to leverage resources, to
strengthen the interdisciplinary approach, and to ensure a connection to
technology. Centers also provide a long--term commitment to applying
intellectual excellence to significant research problems and to
developing the expertise of a new generation of researchers \[3\]. Thus,
four critical needs will be addressed by establishing Nanomaterials
Research Centers in conjunction with the DOE's leading national x--ray
and neutron sources and nanointegration facilities:

1\. [Facilities]{.underline}. Investment in a suite of ancillary
materials characterization and synthesis facilities is needed to permit
full and efficient use of national x--ray and neutron sources for
investigations of nanoscale materials and structures, by both external
users and resident scientists. Close proximity of these specialized
nanomaterials research facilities to the radiation sources and
integration capabilities is essential in order to maximize collaborative
interactions between external users and resident scientists (see below)
and to fully exploit the high--flux sources.

2\. [Research Organization]{.underline}. The nanomaterials research
effort of scientists at the host laboratories (and probably elsewhere)
requires some reorganization and consolidation, so that they can
function as a more cohesive and interactive unit. Research on nanoscopic
materials and structures involves chemistry, physics, materials science,
biology, chemical engineering, electrical engineering and computation.
By establishing Nanomaterials Research Centers, the research efforts of
scientists at the host laboratories can be focused more effectively into
highly interdisciplinary Center--based research groups that will be able
best to take advantage of shared facilities and synergistic
interactions, both within the Center and with external users. As part of
this reorganization the Nanomaterials Research Centers will also
establish connections with existing microscopy centers and other x--ray
and neutron user facilities at each of the national laboratories, so
that these also can be fully utilized for nanotechnology.

3\. [Support for External Users]{.underline}. Nanomaterials Research
Centers will provide the infrastructure support (scientific
collaboration, technical support personnel, laboratory support, and
common instrumentation) that visiting scientists require in order to
efficiently conduct high quality nanomaterials research.

4\. [Education]{.underline}. The existence of Nanomaterials Centers
equipped with facilities and interdisciplinary research teams will
enable educating a new generation of young scientists, who will be the
first to receive the truly interdisciplinary training that is needed for
nanotechnology. Such intellectual capital is probably the most important
long--term investment for science and technology \[4\]. The Centers as
described here will be well--organized for training graduate students
and postdoctoral researchers and for strong, productive collaborations
with university research groups, who will be among the principal
scientific users.

####### References

1\. This report, p. 1.

2\. [Condensed Matter and Materials Physics: Basic Research for
Tomorrow's Technology,]{.underline} p. 3, National Research Council,
National Academy Press, Washington, D.C., 1999.

3\. Ibid, p. 25.

4\. Ibid, p. 28.