![](media/image1.jpeg){width="6.839583333333334in"
height="9.119444444444444in"}

Revised: February 1, 2006

**Nano and BioTechnology Research at NASA Ames**

M. Meyyappan and Harry Partridge

NASA Ames Research Center

Moffett Field, CA 94035

**Abstract**

This article provides an overview of nanotechnology and biotechnology
research at NASA Ames Research Center and covers current results in the
areas of carbon nanotube (CNT) growth and characterization and
functionalization, nanotubes in scanning probe microscopy, inorganic
nanowires, biosensors, chemical sensors, nanoelectronics
optoelectronics, computational nanotechnology, quantum device
simulation, and computational optoelectronics.

**Introduction**

Advanced miniaturization is a key thrust area to enable new science and
exploration missions for which ultrasmall sensors, power sources,
communication, navigation, and propulsion systems with very low mass,
volume and power consumption are needed. Revolutions in electronics and
computing will allow reconfigurable, autonomous, \"thinking\"
spacecraft. Nanotechnology presents a whole new spectrum of
opportunities to build device components and systems for entirely new,
bold space architectures such as networks of ultrasmall probes on
planetary surfaces, micro-rovers that drive, hop, fly and burrow, and
collection of microspacecraft making a variety of measurements.

NASA Ames started an Integrated Product Team (IPT) on Devices and
Nanotechnology in FY 97 to conduct basic research in the emerging field
of nanotechnology as well as in semiconductor device physics,
computational electronics and optoelectronics, and computational
chemistry in materials processing. This effort was renamed as NASA Ames
Center for Nanotechnology (NACNT) couple of years later. The research
focus, summarized in Table 1, covers a wide range of subjects: carbon
nanotube and inorganic nanowire (CNT) synthesis, characterization,
functionalization, nanoelectrode fabrication for sensor development,
chemical and biosensor development, nanoelectronics,
nano-optoelectronics including detectors and lasers, thermoelectric
devices, application of CNT in atomic force microscopy (AFM), thermal,
radiation and impact protection shields using nanomaterials, development
of quantum device simulator, computational optoelectronics and
computational nanotechnology. This article provides selected results in
the above areas. A complete current publication list and reprints from
the Ames group as well as descriptive project summaries can be found in
the NACNT website
[http://www.ipt.arc.nasa.gov](http://www.ipt.arc.nasa.gov/). This
website also features a nanotechnology gallery containing videos and
images \[1\].

**Table 1.**

+---------------------------------+------------------------------------+
| **Research Focus**              |                                    |
+---------------------------------+------------------------------------+
| **\* Carbon Nanotubes**         | **\* Nanoelectronics**             |
|                                 |                                    |
| • Growth (CVD, PECVD)           | • Synthesis of organic molecules   |
|                                 |                                    |
| • Characterization              | • Characterization                 |
|                                 |                                    |
| • Chemical functionalization    | • Device fabrication               |
|                                 |                                    |
| • AFM tips                      | **\* Nanomaterials**               |
|                                 |                                    |
| \- Metrology                    | • Thermal, Radiation, Impact       |
|                                 | protection                         |
| \- Imaging of Mars Analog       |                                    |
|                                 | **\* Computational                 |
| \- Imaging Bio samples          | Nanotechnology**                   |
|                                 |                                    |
| • Electrode development         | • CNT - Mechanical, thermal        |
|                                 | properties                         |
| • Biosensor                     |                                    |
|                                 | • CNT - Electronic properties      |
| • Chemical sensor               |                                    |
|                                 | • CNT based devices: physics,      |
| • Interconnects                 | design                             |
|                                 |                                    |
| • Thermal Interface Material    | • CNT based composites             |
|                                 |                                    |
| • Gas Adsorption                | • CNT based sensors                |
|                                 |                                    |
| • Device Fabrication            | • DNA transport                    |
|                                 |                                    |
| **\* Inorganic Nanowires**      | • Nanowires: transport,            |
|                                 | thermoelectric effect              |
| • Growth (VLS, Sol-gel)         |                                    |
|                                 | • Transport: molecular electronics |
| • Characterization              |                                    |
|                                 | **\* Quantum Computing**           |
| • Nanoelectronic devices        |                                    |
|                                 | **\* Computational Quantum         |
| • Detectors, lasers             | Electronics**                      |
|                                 |                                    |
| • Thermoelectric devices        | • Noneq. Green's Function based    |
|                                 | Device Simulator                   |
|                                 |                                    |
|                                 | **\* Computational                 |
|                                 | Optoelectronics**                  |
+---------------------------------+------------------------------------+

**Carbon Nanotubes**

[Growth and Characterization]{.underline}

In the early days of nanotechnology research, CNTs were primarily grown
by laser ablation and carbon arc techniques by various research groups
across the world. Both approaches produce single wall CNT in small
quantities scraped off the cooler walls of the reactor. In the last few
years, chemical vapor deposition has emerged as an alternative approach.
CVD, a workhorse in silicon microelectronics, is ideally suited to grow
nanotubes on patterned substrates if one is interested in investigating
nanoelectronic devices or sensors. Ames operates two CVD reactors to
grow nanotubes on substrates. The feed gas may be CO or some hydrocarbon
gas, and typical growth temperatures for multiwall carbon nanotubes is
500-800° and 900° for single-walled nanotubes.

The CNT growth is catalyzed by transition metals such as nickel, iron or
cobalt. The catalyst mixture can be applied to the substrate by solution
chemistry followed by calcinations and few other steps. Parameters
controlling growth appear to be numerous: nature of feedgas and
composition, flow rate, temperature, type of catalyst, catalyst
preparation technique, and substrate material. To date, no research
group has been able to precisely control CNT diameter or chirality.
Since the number of variables involved is very large, a combinatorial
chemistry approach has been used, as pioneered by Alan Cassell at Ames,
for catalyst optimization in CNT synthesis \[2\]. Figure 1 shows a
bundle of multiwall CNTs grown by this approach. Figure 2 shows a
multilayer assembly of multiwall carbon nanotubes grown using CVD
approach. The densities of the upper and lower layers are different and
a controlled assembly would be useful in the preparation of separation
membranes and composites \[3\].

![](media/image2.png){width="3.6354166666666665in"
height="2.8208333333333333in"}

Fig. 1. Multiwall carbon nanotubes grown by CVD

We have also investigated catalyst preparation through direct ion beam
sputtering which allows easy confinement of the catalyst within small
patterns. It was found that adding an underlayer of Al allows increased
nucleation of the nanoparticles needed for CNT growth; this metallic
layer also allows tuning of the conductivity of the substrate \[4, 5\].

![](media/image3.jpeg){width="3.0277777777777777in"
height="2.214583333333333in"}

Fig. 2. Multilayer assembly of nanotubes

Figure 3 shows single wall carbon nanotubes prepared by CVD on a
patterned grid using the above approach \[4\]. Both thermal CVD \[4, 5,
6\] and plasma CVD \[7-10\] are routinely used at Ames to grow SWNTs and
MWNTs. Thermal CVD has allowed SWNT and MWNT growth on patterned
substrates. In the case of MWNTs, continued growth over 10 minutes
yields nice towers of MWNTs. These towers consist of millions of MWNTs
supporting each other by van der Waals force; individual nanotubes, in
high magnification, appear to grow like vines. In contrast, it is
possible to obtain individual, free-standing, vertically-aligned
structures with plasma CVD as seen in Fig. 4 \[7-9\]. But it is
important to recognize that these structures are not MWNTs according to
conventional definition, where each wall should be perfectly parallel to
the central axis. The nanostructures in Fig. 4 have interior walls
exhibiting a small angle with respect to the central axis. This
stacked-cone shape material is preferably called multiwalled carbon
nanofibers (MWNFs) to distinguish them from MWNTs \[7\]. These are also
known as vertically aligned carbon nanofibers (VACNFs). The plasma
reactor commonly used at Ames to grow these MWNTs and MWNFs is a dc glow
discharge reactor; in addition, an inductively coupled reactor with an
independent rf bias at the bottom electrode holding the substrate is
also used \[10\].

The nanotube characterization at Ames is done using scanning electron
microscopy (SEM), high resolution transmission electron microscopy
(HRTEM), Fourier transform infrared spectroscopy (FTIR), Raman
spectroscopy and UV-Vis spectroscopy.

![](media/image4.jpeg){width="3.0145833333333334in"
height="2.1930555555555555in"}

Fig. 3. Single Wall Carbon Nanotubes by CVD

![](media/image5.png){width="3.234722222222222in" height="2.3625in"}

Fig. 4. Plasma CVD of MWNFs

## 

## Functionalization of Nanotubes

Various applications call for the need to attach chemical functional
groups to the sidewall of CNTs. For example, it is believed that
chemical functionalization may provide the "velcro effect" needed for
incorporating CNTs in a host matrix for developing composites. Attaching
fluorine along to the sidewall makes CNTs nearly insulating, changing
the conductivity orders of magnitude. Regardless of the application, a
common approach to functionalizationn is wet chemistry. While a wet
chemistry approach is simple, it is not scalable, deals with large
quantities of chemicals which need to be discarded and suffers from poor
efficiency and a host of other issues which were faced by the
semiconductor community 20 years ago when dealing with wet chemistry for
etching semiconductors, oxides and metals. The innovation then was the
cold plasma based chemistry for etching. Cold plasmas or glow discharges
can be used for functionalization of CNTs as well. Khare and coworkers
\[11-14\] demonstrated this approach by using a simple microwave cavity
to generate a plasma that contains the required species for
functionalizations. They have demonstrated functionalization of CNTs
with atomic hydrogen using a H~2~ plasma, atomic fluorine with a CF~4~
plasma and atomic nitrogen with N~2~ or NH~3~ plasma. In all cases, a
very small quantity of source gas is all what is required.
Functionalization also appears to be rapid. Evidence of attaching the
chemical groups was obtained from appropriate FTIR, Raman and UV
signals.

[Nanoelectrode Fabrication]{.underline}

The MWNFs are ideal for developing nanoscale electrodes. They range in

20-100 nm diameter. Their density and hence, the average spacing between
individual MWNFs can be varied by varying the thickness of the catalyst
films. Of course, if one wishes a precise control on the spacing, then
catalyst patterning by e-beam lithography would be ideal. In
applications as electrodes, particularly with electrochemical approaches
involving electrolytes and indicators, it is important to maintain the
rigidity of the nanostructures. For this reason, gap-filling techniques
were developed to fill the spacing between MWNFs with a dielectric such
as SiO~2~ or spin-on-glass. TEOS CVD was used to deposit SiO~2­~ followed
by chemical mechanical polishing to planarize the top surface (see Fig.
5). Now, only the very tip of the MWNFs are exposed while the rest are
buried inside the SiO~2~. Electrical characterization of these tips
indicate that they function as electrodes as desired \[15\] and hence,
are suitable for attaching DNA or other chemical groups. These
nanoelectrode arrays are useful for biosensor development as described
below.

![](media/image6.png){width="3.2784722222222222in"
height="2.407638888888889in"}

Fig. 5. SEM image of MWNFs after encapsulated in SiO~2~

### Biosensor for Various Applications

Jun Li and coworkers at Ames prepared the above electrode ensemble and
attached DNA to the MWNF tips. The attachment is due to covalent bonding
and they have also demonstrated hybridization by bringing in a
complementary strand for the probe DNA molecule. In addition to the
evidence from fluorescence microscopy for these events, they have also
used an electrochemical indicator and obtained signals that are
proportional to the amount of guanine in the target sequence \[15-18\].

A major area of focus in CNT applications at Ames is development of
biosensors for various applications using the above demonstration. To
date, a 30 sensor die on a 4\" wafer has been fabricated. The
application focus ranges from water quality and bacteria monitoring (for
NASA\'s crew exploration vehicle and other human habitats) to biothreat
monitoring for homeland security and cancer diagnostics. The latter
work, conducted through funding from National Cancer Institute, involves
development of a prototype biosensor catheter that permits detection of
specific oligonucleotide sequences that serve as molecular signatures of
cancer cells. A critical element of this technology involves the ability
to functionalize the tip of a nanotube array with (a monolayer of) probe
molecules as described in the previous paragraph.

The nanoelectrode array is also being used for deep brain stimulation
(DBS) to treat disorders such as Parkinson\'s disease and epilepsy. This
NIH funded work focuses on developing a 3-d interface between the neural
tissues and solid state electronics using the nanoelectrode array
\[19\].

[Chemical Sensors]{.underline}

There is a tremendous need for chemical sensors with high sensitivity in
the parts per million (ppm) to parts per billion (ppb) level. NASA\'s
needs included sensors for cosmochemistry (identification of atmospheric
constituents of various planets), earth observation (greenhouse gases),
and leak detection in the shuttle and future crew exploration vehicle
(CEV) that involves sensing of hydrazine and N~2~O~4~. Chemical sensors
are sorely needed for the detection of toxic industrial chemicals and
nerve agents for homeland security applications. NACNT is developing
SWNT based sensors for the above purpose.

Nanomaterials in general have a very large surface area and we have
characterized that SWNTs possess a surface area of about 1600 m^2^/g
\[20\]. This large surface area translates into large adsorption rate of
gases and vapors. This adsorption can lead to a change in some specific
properties of the SWNTs, for example, conductance, capacitance etc.
Indeed SWNT field effect transistors have been fabricated wherein a
single SWNT or a film of multiple SWNTs forms the conducting channel.
The conductivity of this device has been shown to change reproducibly
upon adsorption of gases like NH~3~ and NO~2~, which forms the basis for
chemical sensing. However, three terminal transistors are expensive and
complex to fabricate and therefore, Jing Li and coworkers at NACNT came
up with an innovative alternative involving an interdigitated electrode
shown in Fig. 6. This device is fabricated using single microfabrication
steps and purified SWNTs are solution-cast across the electrode to form
the conducting channel.

Figure 7 shows the sensor signal for repeated doses of NO~2~ wherein the
response is fairly rapid. The projected sensitivity is about \~40 ppb.
The sensor recovery time (the time to recover to baseline after the
source is removed) needs further improvement; shining a UV source
accelerates the desorption of the vapor from the nanotubes thus
accelerating sensor recovery. This is an area that is currently being
investigated for further improvement. To date, the sensor has been
tested for NO­~2~, NH~3~, CH~4~, Cl~2~, HCl, toluene, benzene, acetone,
formaldehyde and nitrotoulene \[21-23\].

![](media/image7.jpeg){width="2.8326771653543306in"
height="2.8318897637795275in"}![](media/image8.jpeg){width="2.8326771653543306in"
height="2.8318897637795275in"}

Fig. 6. Interdigitated electrode structure for chemical sensors.

![](media/image9.jpeg){width="4.625in" height="3.1458333333333335in"}

Fig. 7. Sensor response to NO~2~

[Nanotubes in Catalysis]{.underline}

CNTs have been postulated to have a high surface area which would be
useful in catalysis, gas adsorption and related applications. Though
theoretical estimates put the surface area in the 3000 m^2^/g range,
experimental results have been well under

1000 m^2^/g. Jing Li and coworkers demonstrated a surface area of 1580
m^2^/g for purified HiPCo SWNTs \[20\]. They used a two step
purification process with the first step involving a debundling
procedure followed by a step to clean up the metal impurities and the
small amount of unwanted amorphous carbon. The large surface area based
on N~2~- adsorption isotherm studies have led them to investigate both
physisorption and chemisorption aspects of adsorbing CH~4~, NO~2~, NO,
CO~2~, etc. onto HiPCo samples. The goal of their study is to
investigate the suitability of SWNTs as catalysts or support material
for waste remediation in long voyage crew cabins \[24\].

[CNT in Microscopy]{.underline}

CNT is an ideal tip for use in AFM \[25, 26\] since it is robust and can
be just a few nanometers in diameter. However, manually attaching a CNT
to the tip of a cantilever can be arduous. Our group has been able to
use CVD to directly grow single wall nanotubes on the AFM cantilever.
Figure 8 shows an example of the effort. We have also developed an
electrochemical approach to easily attach a multiwall nanotube to the
AFM cantilever \[26\].

![](media/image10.png){width="5.965277777777778in"
height="3.3333333333333335in"}

+---------------------------------------+------------------------------+
| TEM Micrograph of Electro-deposited   | TEM Micrograph of Shortened  |
|                                       | NT Synthesized by CVD        |
| Catalyst on Tip of AFM Cantilever     |                              |
+---------------------------------------+------------------------------+
| Fig. 8. Nanotube probe directly grown |                              |
| on AFM cantilever.                    |                              |
+---------------------------------------+------------------------------+

The AFM with nanotube tip is currently used to image and study simulated
Mars dust. Figure 9 shows an image of Red Dune Sand (from Western
Australia) which is a Mars analog. The CNT tip - only a few nanometers
in diameter - is able to offer extraordinary resolution in imaging the
dust particles. In addition, the robustness of CNT provides long lasting
tips in contrast to the quick wearing of silicon tips. Our other
accomplishments with CNT probes include metrology applications (for
example, a profilometer) and nanoscale imaging of semiconductor
surfaces. Figure 10 shows an AFM image of a thin iridium film obtained
using a single-walled nanotube tip. The resolution here is much better
than conventional silicon tips \[25\]. These CNT probes have also been
used to image biological samples both in the dry form and in their
native aqueous environment \[27\]. The latter is accomplished by coating
the nanotube probe with ethylene diamine to render the probe hydrophilic
in order to be able to image liquid samples.

![](media/image11.jpeg){width="5.995138888888889in"
height="2.8402777777777777in"}

Fig. 9. AFM Image of Red Dune Sand (Mars Analog) obtained using a
nanotube tip (left) compared to an image using optical microscope
(right).

![](media/image12.jpeg){width="3.6930555555555555in"
height="2.876388888888889in"}

Fig. 10. AFM image of iridium film using a SWNT tip

[CNT Field Emission for Instrumentation]{.underline}

Carbon nanotubes are well known for their field emission characteristics
as they exhibit large aspect ratio and a small tip radius of curvature
in addition to good electron affinity values. While much of the work in
the literature focuses on field emission for flat panel displays, NACNT
has been working on exploiting this for developing science instruments
for planetary exploration. Optimized emission conditions ideal for
generating X-rays have helped to develop an X-ray tube shown in Fig. 11
that can power an X-ray diffraction spectrometer \[28\]. In addition,
current work focuses on electron source for miniaturized mass
spectrometer and miniaturized SEM.

![](media/image13.png){width="2.3368055555555554in"
height="1.7534722222222223in"}

Fig. 11. CNT based X-ray tube

[CNTs as Interconnects and in Thermal Management]{.underline}

Carbon nanotubes exhibit very high electrical and thermal
conductivities. Their current carrying capacity is also extremely high,
for example up to 10^9^ A/cm^2^ where copper begins to suffer from
electromigration at about 10^6^ A/cm^2^. Jun Li, Alan Cassell and
coworkers have developed a bottom up process \[29\] to integrate PECVD
CNFs as interconnects for DRAM applications. The as-grown CNFs are
gap-filled with SiO~2~ followed by CMP and ready for top metallization.
In a series of publications \[29, 30\], progressive improvement in the
resistance of the interconnects has been reported. A CNF-Cu composite
also has been developed as a thermal interface material for chip cooling
applications \[31\]. This technology has been licensed to a Bay Area
company specializing in chip cooling solutions.

**Inorganic Nanowires**

Whereas the carbon nanotube research continues to address the challenge
of controlling the chirality (thus creating the ability of producing
metallic or semiconducting of nanotubes of specified bandgap at will),
quietly a new technology is emerging based on inorganic nanowires.
Materials such as Si, GaAs, InP, ZnO and other oxides have long been
prepared as very thin films of 1-20 nm which propelled advances in
lasers, transmitters, receivers and sensors, taking advantage of the
quantum properties of these confined layers. Shrinking this by another
dimension from 2-d to 1-d, it is now possible to grow most of these
structures as 1-d nanowires. Researchers at Ames have made tremendous
progress in growing nanowires of ZnO, SnO~2~, InSb, silicon, germanium
and others \[32-38\]. Fig. 12 shows very nicely aligned ZnO nanowires
grown on a sapphire substrate. The choice of sapphire substrate is
dictated by the minimum mismatch with ZnO. The bandgap of these ZnO
nanowires makes them useful in developing blue lasers and blue LEDs.

Though nanowires have been grown by various techniques such as
templating and evaporation, our approach involves the vapor-liquid-solid
mechanism. Here, a thin catalyst metal is deposited on the growth
substrate which melts at the growth temperature and forms tiny beads.
The source material, either sublimated from the solid source (such as
zinc oxide powder) or reacted with graphite powder in carbothermal
reduction (ZnO powder + graphite powder) or generation through chemical
reactions (silane + H~2~ reaction for silicon wires), dissolves into the
tiny droplets and when supersaturation is reached, the material
precipitates as nanowires.

The nanowires are characterized using SEM, TEM, Raman spectroscopy,
FTIR, photoluminescence and other techniques.

[Nanowire Based Electronic Devices]{.underline}

Bin Yu and coworkers have been working on germanium based devices for
memory and logic applications. For this they have come up with a
nanowire-on-insulator (NOI) concept similar to the well known silicon on
insulator (SOI concept). Figure 13 shows a germanium wire bridging two
contacts grown on an insulating layer. The I-V characteristics of this
device are shown on Fig. 14 which exhibit an ambipolar behavior due to
the Schottky contacts. Novel device configuration such as vertical
devices with either a top gate or surround-gate have also been
demonstrated \[39, 40\] using wide band gap materials with future work
focusing on silicon and germanium nanowires.

![](media/image14.png){width="3.05625in" height="2.23125in"}

Fig. 12. ZnO Nanowires

While most of the work in the literature focuses on using gold as a
catalyst, Bin Yu and colleagues have avoided it since gold is thought to
be a contaminant in device processing. They have come up with
alternatives of low melting metals such as Ga or In which, not only
avoid contamination issue, but also allow growth at much lower
temperatures (350° C vs. 800° C).

![](media/image15.png){width="2.25in" height="1.8881944444444445in"}

Fig. 13. Germanium nanowire on insulator

![](media/image16.png){width="3.125in" height="2.4895833333333335in"}

> Fig. 14. I-V characteristics of Ge-NOI device

**Computational Nanotechnology**

Extensive investigations using computational simulations on the
electronics, mechanical and other properties of nanotubes have been
undertaken by the Ames nanotechnology group. SWNTs exhibit remarkable
mechanical properties, for example, a Young\'s modulus of over 1 TPa and
tensile strength of about 200 GPa. They also have been unique electronic
properties in that a CNT, based on its diameter and helicity, can either
metallic or semiconducting. It is interesting to explore the coupling
between the mechanical and electronic properties. Figure 15 shows sample
results from a combination of molecular mechanics, dynamics, and tight
binding simulations. The bandgap, normalized by the hopping parameter
(3.1 eV) and a dimensionless radius R/Ro, is plotted against strain for
various chiral tubes. For reference, the bandgap of a (10, 0) tube at 0%
strain is 1 eV whereas the bandgap of silicon is 1.11 eV. The metallic
(5, 5) tube shows no variation in bandgap under tension or compression
whereas other tubes show varying degree of change. The slope itself
depends on the modulus of (n-m, 3) where n and m are used to define the
chirality. Within each color coded group in Fig. 15, the magnitude of
the change depends on the chiral angle. For example, the bandgap changes
more rapidly for a (10, 0) tube compared to a (6, 5) tube. In general,
there are three transitions seen in Fig. 15. The first is
metal-semiconductor transition, for example, the (9, 0) tube at 1%
strain. Next, the change in bandgap with strain (slope) changes sign due
to quantum number change, for example, the (10, 0) tube at 10% strain.
Finally, another transition is seen when the slope changes sign again
due to mechanical relaxation, for example, the (10, 0) tube at 18%
strain. The critical strain, defined at the transition point due to
quantum number change, varies inversely with tube diameter (not
![](media/image17.pct){width="1.3888888888888888e-2in"
height="1.3888888888888888e-2in"}shown here) which can be verified
experimentally. Further details can be found in ref. 41 which also
describes the effects of torsional strain on the bandgap. Reference 42
describes the effect of bending on the electronic properties.

+-----------------------------------+-----------------------------------+
| ![](media/image18.j               | ![](med                           |
| peg){width="2.6798611111111112in" | ia/image19.pct){width="2.85625in" |
| height="2.535416666666667in"}     | height="2.797222222222222in"}     |
+-----------------------------------+-----------------------------------+
| Fig. 15. Effect of strain on      | Fig. 16. Effect of disorder on    |
| bandgap.                          | CNT\                              |
|                                   | conductance.                      |
| > Legend: n - m = 3q + 1,         |                                   |
| >                                 |                                   |
| > n - 3 = 3q, n - 3 = 3q - 1,     |                                   |
| >                                 |                                   |
| > where n, m and q are integers.  |                                   |
+-----------------------------------+-----------------------------------+

A CNT is expected to be an ideal quantum wire. Ballistic transport
through a nanotube would yield a low bias resistance of 6KΩ. The best
measurements to date for single wall nanotubes have been shown to be in
the range of 20-50 KΩ. The main reasons for the observed low conductance
are defects and Bragg reflection. Theoretical work studying the effect
of disorders and reflection on conductance has been carried out. Figure
16 shows computed conductance for nanotubes with uniform disorders and
vacancies. Uniform disorder in CNT does not significantly affect
conductance. In contrast, vacancy-type defects cause significant
backscattering resulting in a conductance degradation. Further details
can be found in ref. 43. Another important aspect in CNT-based
electronics is the role of contacts, and theoretical investigations have
been carried out to study how a carbon nanotube couples to simple
metals. For good coupling, the K~f~ for metals must be greater than
4π/3a~0~ (0.17 nm^-1^ for graphite). The value of K~f~ for Al and Au are
1.75 and 1.21 respectively and graphite does not couple to these metals.
For armchair nanotubes, this value is only 0.85Å^-1^ and coupling to
simple metals is very good. The armchair tube also couples better than
zigzag tubes to metal. The computations show an increase in transmission
with the length of the contact, as seen in experiments. Details of this
study can be found in ref. 44.

The unique electronic properties of CNT have led to the fabrication of
the first CNT-based field effect transistor by research groups at IBM
and Delft University. The CNT-FET consisted of a SWNT in contact with
source and drain contacts and modulated by a gate on the backside. The
device operated at room temperature. In ref. \[45\], Yamada provides a
theoretical analysis of the experimental data by incorporating
one-dimensional quantum effects in the nanotube channel. He concludes
that the lack of saturation in

![](media/image20.png){width="3.9243055555555557in"
height="2.8673611111111112in"}

Fig. 17. Power dissipation per gate as a function of delay time

drain current as a function of drain voltage is an indication of channel
carrier transport dominated by weak-localization and the electrode
metal-nanotube contact influences subthreshold channel conductance vs.
gate voltage. Yamada recommends a reduction in gate oxide thickness to
increase the transistor gain.

While attempts to fabricate a CNT-FET are necessary first steps and
allow exploration of fundamental issues and ultimate possibilities, it
is critical, at this early stage, to pay attention to nanoelectronic
circuit and architectures. Simple miniaturization of a CMOS-like device
may not be appropriate for future nanoelectronics. Figure 17 shows
switching time vs. power consumption per gate for a CMOS architecture.
As device feature size and switching delay time decrease, the power
consumption per gate goes up significantly and a CNT-based CMOS-like
architecture is likely to face serious problems. It is interesting to
note in the same plot that the brain, admittedly orders of magnitude
slower, consumes significantly less power. It is possible to develop
novel architectural concepts based on the unique properties of CNT,
particularly metal-semiconductor, semiconductor-semiconductor, and
heterojunctions \[46\]. Figure 18 shows nanoscale tunnel junctions for
transistors which were constructed by introducing topological defects
such as five (pentagon) and seven (heptagon) member rings in an
otherwise all six (hexagon) based CNT \[46\]. Since the Y-junction
proposal by Srivastava in 1997, two groups at Brown University and IISC,
India independently have created Y-junctions in CVD reactors and made
electrical measurements. Heterojunctions based on partial chemical
functionalization and/or substitutional doping may also be possible. It
is also possible to conceive of a neural tree consisting of a multiple
of these Y-junctions (see Fig. 19). Deepak Srivastava envisions that
these neural trees can be trained to perform complex switching and other
operations just as in biological systems.

The remarkable mechanical properties of CNT are by now well known.
Applications to high strength composites require extensive investigation
to understand the behavior of CNT under various conditions.
Nanoplasticity of SWNTs under uniaxial compression

![](media/image21.png){width="3.3319444444444444in"
height="1.5347222222222223in"}

Fig. 18. Carbon nanotube \"T\" and \"Y\" junctions

was studied using generalized tight-binding molecular dynamics, and ab
initio electronic structure method \[47\]. The bonding geometry
collapses from a graphitic (sp^2^) to a localized diamond like (sp^3^)
reconstruction under axial compression. Videoclips of this can be seen
on the web under ref. \[1\]. The computed critical stress of about 153
GPa and the shape of the resulting plastic deformation agree well with
experimental observations. Based on measurements of electrical
conductivity, the thermal conductivity of SWNT has been speculated to be
in the range of 1750-5800 W/m.K. which would put it in a class with CVD
grown diamond. Molecular dynamics simulations at Ames has shown \[48\] a
thermal conductivity in the range of 1000-2500 W/m.K for a (10, 10)
nanotube at temperatures 100-500 deg. K. Note that this high thermal
conductivity is only in the axial direction with K values in the radial
direction being small. Further work is in progress to compute thermal
conductivity of multiwall tubes.

![](media/image22.jpeg){width="2.6666666666666665in"
height="2.004166666666667in"}

Fig. 19. Neural Tree

Several contemplated applications for CNT require functionalization of
the nanotubes (both the tip and the sidewall) as mentioned earlier.
Theoretical work at Ames predicts an enhanced chemical reactivity at
regions of local conformational strain on the nanotubes \[49\].
Nanotubes which are bent or twisted show enhanced reactivity for
specific sites near the distortions, as shown in Figure 20 which plots
the binding energy, cohesive energy, and electronic energy for several
highlighted atoms in a bent-tube. Preliminary verification of this
prediction was provided by Rod Ruoff of Washington University where
nanotubes laid over a V-ridge substrate were selectively attacked by
nitric acid only at the sites distorted by the ridge \[49\].

![](media/image23.jpeg){width="5.104166666666667in"
height="2.4451388888888888in"}

The unique properties of CNT make it an attractive candidate for several
nanotechnology innovations. The Ames computational nanotechnology
researchers have designed a CNT-based nanogear \[50, 51\] shown in Fig.
21. Benzyne molecules bonded to the side of the nanotube form teeth
while the nanotube forms the body about which the gear rotates. Computer
simulations show that stable rotations of the driven gear are possible
with the forced rotations of the powered gear. Videoclips of the
nanogear rotation can be found in the website in ref. 1. The use of CNT
tips in an AFM-based lithography and other applications were mentioned
earlier in this report. The possibility of nanoscale etching using CNT
tips has been investigated through molecular dynamic simulations.
Selective atomic scale etching as well as indentation of silicon
surfaces by CNT tips (mounted in an AFM) have been shown to be possible
\[52\]. Parallelization of an array of tips has the potential to
revolutionize future generation lithography. The website in ref. 1.
contains videoclips of CNT etching and indentation. The possibility of
storing data using H and F atoms to signify 0 and 1 bits has also been
theoretically investigated \[53, 54\]. These atoms are sufficiently
small that the interaction between adjacent data atoms on a silicon
surface is small. Reference 53 speculates on how such a memory device
might be constructed. A method then must be devised to differentiate

+-----------------------------------+-----------------------------------+
| ![](media/image24.                | ![](media/image25.                |
| jpeg){width="2.951388888888889in" | pct){width="2.5729166666666665in" |
| height="1.8388888888888888in"}    | height="2.4166666666666665in"}    |
+-----------------------------------+-----------------------------------+
| Fig. 21. A CNT-based nanogear     | Fig. 22. Chemical data storage    |
|                                   | with H                            |
| > with benzyne molecules          |                                   |
|                                   | and F atoms as 0 and 1 to be      |
| bonded as teeth                   |                                   |
|                                   | read by a functionalized CNT\     |
|                                   | tip.                              |
+-----------------------------------+-----------------------------------+

between H and F atoms unambiguously. A suggested mechanism is to have a
probe that is attractive toward one atom and repulsive to the other and
ref. 54 discusses the suitability of a Sc atom and electron-rich
pyridine molecule as probes (to be attached to the tip of a CNT). This
type of chemical storage of data is capable of 10^15^ bytes/cm^2^
storage density. Parallelization of tips again can overcome
speed-related problems. . In addition to CNT, the Ames group has also
been investigating boron nitride nanotubes \[55\], for electronics and
structural applications.

**Computational Electronics**

The main trends in device miniaturization are relentless downscaling of
CMOS technology and exploration of molecular devices. Future generation
smaller and faster devices are of critical importance to powerful
onboard computing, autonomous \"thinking\" spacecraft, and petaflop
computing initiative. Modeling and simulation not only provides an
understanding of how these devices work, but also can serve as a design
tool in developing new generation devices. In submicron devices under
consideration, the electron wavelength is comparable to device
dimensions and the transit time becomes comparable to scattering time.
Under these conditions, classical propagators fail. The Ames
Computational Electronics group has developed a multidimensional quantum
device simulator based on a Nonequilibrium Green\'s Function (NEGF)
approach. Figure 23 shows a 25 nm-MOSFET studied using this simulator
along with contours of electron density.

**Computational Optoelectronics**

Optoelectronics is a major enabling technology for the tera-era
information technology. Information transmission, processing, and
storage are key areas of active research in optoelectronics. Ames has a
significant computational optoelectronics activity in progress. The goal
is to develop comprehensive modeling and large scale simulation
capability for studying and design of quantum optoelectronics devices.
Vertical cavity surface-emitting laser (VCSEL) is one of the most
advanced and smallest semiconductor laser with light coming vertically
out of the semiconductor wafer surface (see Fig. 24). It can be
integrated with transistors in peta-flop computing with VCSEL-based
optical interconnects, interprocessor communication, multi-gigabit
Ethernet, high throughput image processing and virtual reality, and
biological and chemical detection of molecules of interest in
astrobiology and other space applications. Ames has developed a
comprehensive model and simulation of VCSELs in 2-D space and time
domain thus allowing detailed investigation of transverse modes. Figure
21 shows snapshots of computed laser intensity for a VCSEL. Other
investigations include ultrafast modulation of semiconductor lasers
\[56\] for high speed communication and compact and coherent THz sources
\[57\].

  ----------------------------------------------------- ----------------------------------------------------
  ![](media/image26.png){width="2.4298611111111112in"   ![](media/image27.png){width="2.236111111111111in"
  height="1.6159722222222221in"}                        height="1.7027777777777777in"}

  Fig. 23. Electron density contours for a 25 nm CMOS   
  computed using a NEGF approach.                       
  ----------------------------------------------------- ----------------------------------------------------

  ------------------------------------------ ----------------------------------------------------
  ![](media/image28.png){width="2.06875in"   ![](media/image29.png){width="2.486111111111111in"
  height="2.4472222222222224in"}             height="2.0395833333333333in"}

  Fig. 24. 2 Dimensional VCSEL simulation    
  showing laser intensity.                   
  ------------------------------------------ ----------------------------------------------------

**Concluding Remarks**

Nanotechnology - in its various forms such as nanoelectronics,
nanoelectromechanical systems, ultrasmall and highly sensitive sensors,
multifunctional materials, biologically inspired materials, systems and
architectures, and possibly many others scientists have not yet thought
of - is expected to play a strong and critical role in future space
transportation and exploration. Also, the intersection of nano, bio, and
information technologies provides rich possibilities for exploring
useful concepts and breakthroughs. NASA Ames Center for Nanotechnology
has been conducting innovative research in these areas to meet Agency\'s
future needs.

A strong computational program complements all experimental activities.
Excellence in computational sciences has long been a tradition at Ames,
and Ames organizations such as Numerical Aerodynamic Simulation (NAS)
Division and Computational Chemistry Branch have led the way in numerous
subjects of interest to the Agency. In that tradition, a strong program
in computational nanotechnology, computational quantum electronics, and
computational quantum optoelectronics is being pursued. The vision for
this program is to develop highly integrated and intelligent simulation
environment that facilitates the rapid development and validation of
future generation of electronic, photonic, and other devices, and
sensors as well as materials and processes through virtual prototyping
at multiple levels of fidelity.

**Acknowledgement**

The authors acknowledge the excellent research contributions from the
NACNT staff and the web work by Amara de Keczer. The support for the
nanotechnology and biotechnology work at Ames from Scott Hubbard, Jack
Boyd, Steve Zornetzer, Eugene Tu, and David Blake is gratefully
acknowledged.

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