# Fabrication of One-Dimensional Organic Nanostructures Using Anodic Aluminum Oxide Templates

Authors: Rabih O.  Al-Kaysi; Tarek H. Ghaddar; Gonzalo Guirado
Journal: Journal of Nanomaterials
 (2009)

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## Abstract

Organic nanostructures are new comers to the fields of nanoscience and nanotechnology. In recent years novel methods for controlling the growth and uniformity of one-dimensional (1D) organic nanostructures (nanowires and nanotubes) have been developing. The use of hard templates as molds for the formation of organic nanowires or nanotubes seems to be a reliable and convenient method. In this review we will discuss the use of anodic aluminum oxide (AAO) templates as the inorganic hard template of choice. We will briefly survey advances in the fabrication of 1D polymer nanostructures using AAO templates, while the bulk of the review will focus on the synthesis of small molecule nanowires, nanotubes, and nanorods. We will also discuss unique properties of some highly crystalline small molecule nanorods fabricated using AAO templates.


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## Body

## 1. Introduction

Nanomaterial
fabrication and nanotechnology have mushroomed in the last four decades. Ever
since the famous lecture by Richard Feynman at the 1959 meeting of the American
Physical Society at Caltech “There's plenty of room at the Bottom,” scientists
have taken interest in exploring the nanodimensions of matter. Nanostructures offer
properties different from their constituent atoms or the bulk material [1, 2].It
is hard to visualize 21st century science without nanotechnology.
Everything, from medicine [3] to the
clothes we wear, has some sort of nanotechnology weaved into their fabric. The
prefix nano has become synonymous with advancement and modern society. This
term is being overused in many products for promotional purposes, such as nanoenhanced
toothpaste or some other product with “nano” attached to it. Imagine making
cigarettes safer by introducing nanoenhanced filters to remove toxic materials
from cigarette smoke!  It is important to
acknowledge that developing novel materials, from which nanostructures are manufactured,
is one of the most important tasks of nanotechnology. Thus, chemical synthesis and
basic chemistry are still the backbone of nanotechnology.Materials
are generally categorized into organic, inorganic, metallic or composite. Much
of the nanotechnology developed so far focused on the fabrication of nanostrucutres
composed of metallic or inorganic components.  Until recently, people have started
investigating the potential use of organic nanostructures. Why did inorganic and
metallic, materials take the lead as the materials of choice in nanotechnology?
The answer might lie in the nature of the chemical bond that holds organic
molecules or inorganic ions together. In an inorganic nanostructure strong
ionic bonds hold the structure together. Thus uniform inorganic nanowires, with
controllable dimensions, can be grown via chemical vapor deposition method (CVD) on
a surface containing inorganic nanoparticle seeds. This method is generally
used on most inorganic materials that can be vaporized or synthesized under
reduced pressure. Growing inorganic nanostructures, such as quantum dots, can
also be size controlled by solution-based chemical capping methods [4]. Hence, fabricating inorganic nanostructures is guided
by the strong ionic or metallic bond which glues the building blocks of the
nanostructure together. However, in the case of organic nanostructures, weak van
der Waals or hydrogen bonding forces hold organic molecules together. Thus,
many attempts to grow 1D nanostructure via vapor deposition technique did not
yield homogeneous nanowires. Instead, platelets, whiskers, or any other shape
that is predetermined by the nature of the crystal packing of the organic
material is sometimes obtained. It is not always applicable to fabricate
organic nanostructures using techniques developed for making inorganic
nanostrucutres. There are some exceptions where organic nanostructures can be
grown using vapor deposition or by high temperature solid-phase reaction. For
example, 1D anthracene and perylene nanorods (average diameter 40 nm and
several microns long) were synthesized by a solid-phase reaction at a
temperature close to 340C° [5].1D
organic nanostructures have been grown using the principles of supramolecular
chemistry and noncovalent interactions. An organic aromatic molecule designed
to have a hydrophobic and hydrophilic end can be precipitated from water to
form very long self-assembled nanowires [6].
Self-assembled nanowires can also be  achieved via π-π interaction and stacking of small molecule
semiconductors and conducting polymers [7]. The structure of these
nanowires is different than the crystal structure of the bulk material. Another
example of supramolecular self-assembly is a molecule containing both donor and
acceptor parts. They can be reprecipitated from aqueous solution to form
nanoribons, nanotubes, or even nanowires [8]. This method is limited to
some molecules and the slightest variation in the chemical structure of the
molecule yields different unpredictable supramolecular shapes.Methods
based on reprecipitation (organic nanoparticles) [9–12], vapor deposition [5] (organic nanowires),
or self-assembly from aqueous solutions [6] are successful for the synthesis of organic nanostructures
made from specific molecules. There are many other techniques for making organic nanostructures, as
far as we are aware none of them are high yielding or general methods. It has
been accepted for a long time that fabricating organic nanostrucutres was done
in clean rooms using expensive Atomic Force Microscopes and nano-FIB machines
in well funded research centers (top-down method, through which nanostructures
are constructed from the bulk). A general cost effective bench top method of
generating large quantities of clone organic nanostrucutres is favored.This
review will focus on the use of anodic aluminum oxide (AAO) templates for the
synthesis of 1D nanostructures made from organic molecules [13]. Nanowires, nanotubes, or
nanorods made from small molecule or polymer materials will be surveyed. So far
using AAO templates is a general and simple method for fabricating 1D organic
nanostructures.


## 2. Anodic Aluminum Oxide (AAO) Templates

Some
metals such as titanium, zirconium, and aluminum form ordered and evenly spaced
metal oxide nanopores (in the case of Al) or nanotubes (in the case of Ti) on
the surface of the metal when anodized in an aqueous acidic solution [14]. Anodic aluminum oxide (AAO)
membranes are made by a two-step anodization of pure aluminum (99.99%) in an
acidic solution [15–17]. The aluminum is
first annealed in a vacuum oven at ~600C° then thoroughly degreased and electropolished.
Several factors will determine the size,
separation, and length of these honeycomb-structured nanopores. Time of
anodization will determine the thickness of the membrane, while voltage and
type of the electrolyte determine the pore size and separation. Pore size can
be custom fabricated from less than 7 nm to more than 300 nm and all sizes in
between (Figure 1). Varying the potential during the anodization process results
in novel nanostructured AAO templates with regular periodic hierarchal branching
[18]. AAO templates with cone shaped
nanopores were successfully fabricated on small substrates [19]. It has become a common
practice to prepare custom AAO templates in the lab. However, Whatman Inc.
provides commercial AAO membrane filters (Anodisc-13) with an average pore
diameter of 200 nm, a 60 μm thick membrane, and a pore density
close to 1011 pores/cm2. These membranes can withstand
high temperatures (~600C°) without collapsing. Templates with quoted
pore diameter of 100 and 20 nm are only so for the first 2 μm
of the top layer of the membrane. The rest of the membrane pores are mostly 200 nm in diameter. Those templates are good for producing 1D nanostructures with a
relatively narrow diameter distribution (±30 nm). The templates are usually ultrasonicated
with solvents of varying polarity (water, acetone, ethanol, THF, chloroform) to
remove industrial contaminants and to unclog the pores. Then the alumina
surface of the nanopores is activated by drying the template in a vacuum oven
at a temperature close to 240C°. Washing the template with a high purity
solvent such as THF, by placing it over a glass frit and applying suction,
should be sufficient to remove industrial contaminants.(a) Scanning electron microscope (SEM) image of a cross-section of an AAO template with 200 nm pore diameter. (b) SEM of a 200 nm AAO template
surface collected using a Philip XL30 FEG.
(a)(b)AAO
membranes with smaller and more uniform pore diameters and template thickness can
be ordered from specialized companies such as Synkera (http://www.synkera.com/). These
templates are more expensive. Unlike the Whatman templates, these templates are
transparent and more rigid.Using
AAO templates as molds for the synthesis of organic 1D nanostructures is
reminiscent to building sand castles at the beach using a bucket to tightly
pack a load of wet sand then slowly removing the mold to form a column. In the
case of AAO templates the mold is not gently taped off; instead, it is
dissolved away with either an acid or a base. Since aluminum oxide is an amphoteric
substance, it could easily be dissolved in aqueous NaOH, KOH, or an acid such asH3PO4 (Scheme 1). This made it possible to liberate
organic nanostructures that are acid or base sensitive without sacrificing the end
product.Scheme 1
Hydrolysis
and amphoteric reactions of Alumina (Al2O3).


## 3. One-Dimensional (1D) Organic Nanostructures

Before
expanding on this subject matter, it is desirable to put certain terms in
perspective. The term 1D nanostructure is used to describe hair like structure
with a large aspect ratio. Aspect ratio is defined as the ratio of the length
of the nanowire to its diameter (1):(1)AspectRatio(AR)=Length/Diameter.Throughout this review we will toggle between two similar terms “nanowires and
nanorods”. By definition, a wire is considered a slender rod. There is no clear
cut distinction between a nanorod and a nanowire. Roughly, the aspect ratio of
a nanowire is more than 1000 while that of a nanorod is between 3 and 5. A nanowire could be broken down to smaller segments
or nanorods. Unfortunately many authors, including the authors of this review,
did not make a clear distinction between a nanorod and a nanowire. These two
terms were used with disregard to the “aspect ratio” criteria (Figure2).(a) 200 nm
diameter PMMA nanotubes with their characteristic fluted end [39], (b) 200 nm diameter polymer nanorods or nanowires [40].
(a)(b)When
is a nanowire small enough to be worthy of being categorized as a nanostructure? According
to the National Nanotechnology Initiative (NNI), a nanowire is a term applied to
a wire with diameter between 1–100 nm, regardless of its length. Nonetheless,
throughout the literature surveyed 1D nanostructures with a diameter close to
300 nm were categorized as nanowires or nanorods. They should be more
adequately labeled as “submicron structures.” For the sake of this review, let
us consider all these 1D structures as nanowires or nanorods regardless of the
diameter.


## 4. Template Assisted Synthesis of Polymer Nanowires, Nanotubes, and Nanorods

One
of the most convenient, cheapest, and general methods of synthesizing 1D
nanostructures is to use AAO templates as a mold to be filled with the organic
material then dissolving away the inorganic template and releasing the
nanostructure. The nanostructure will have the same diameter and length of the
AAO pores. The chemical literature has several reviews on the use of AAO
template to synthesize polymer nanowires or nanotubes [20, 21].It
is “easy” to make 1D polymer nanostructures, which are flexible and can
withstand the rigor of fabrication. Although the polymer chains are held by
weak van der Waals forces, unlike small molecules, these forces are on many
sights of the polymer chain thus gripping those chains together and
strengthening the polymer.1D
polymer nanostructures were synthesized using template wetting technique. A
solution of the polymer was passed through the AAO membrane. Capillary action
forced the polymer solution to spread evenly on the inside pores of the AAO
template. As the solvent evaporates, a layer of polymer was deposited on the
inside walls of the AAO pores. The thickness of the nanotube depended on the concentration
of the polymer solution. The more concentrated the solution, the thicker the nanotube wall. This method is limited to polymers that are soluble in
organic solvents, and to polymer solutions that have a low viscosity that can
allow them to flow through the AAO nanopores.The
first attempt to fabricate 1D polymer nanostructures was achieved by Martin and
coworkers, who synthesized polymer nanotubes by wetting AAO templates with a
polymer solution of polystyrene [22–24].
A similar procedure was adopted to make polymer nanotubes out of,
poly(2,6-dimethyl-1,4-phenylene oxide), poly(vinylidene fluoride), poly(methyl
methacrylate) [22] poly(bisphenol A
carbonate), and poly(lactic acid). Formation of hollow nanotubes instead of
well-packed nanowires was an unexpected outcome. It was hypothesized that the
capillary action inside the AAO nanopores was very strong, forcing the polymer
to stick and wet the walls. After the solvent evaporates, the AAO was etched out
using phosphoric acid. The polymer nanotube external diameter was equal to that
of the AAO nanopore diameter with a length equal the thickness of the template.
The nanotubular morphology was confirmed using electron microscopy. Polymer
nanowires were also synthesized using 30 nm pore size AAO template [25, 26]. Similar
techniques based on wetting template methods were used to synthesize arrays of polymer
nanotubes made from conductive polymers [27–29], ferroelectric polymers [30], elastic polymers [31], semiconducting polymers [32], nonpolar polymers [33], poly(vinyl pyrrolidone) [34], and magnetic molecularly
imprinted polymers [35]. Insulating polymers such as
polyamide 66 were used to make nanotubes to coat Pt nanowires, thus forming
nanocables [36–38],So
far it seems that wetting an AAO template with a solution of a polymer
generates a nanotube after the solvent has evaporated. Nafion-115 nanowires
were exclusively formed by wetting AAO template (85 nm) with the internal walls
treated with 1% sodium dodecylsulfonate (SDS) solution. SDS increased the
hydrophobicity of the nanopores forcing the Naflion-115 polymer chains to aggregate
together and form a nanowire. Performing a similar procedure on untreated AAO
template yields Naflion-115 nanotubes [41].
A similar result was observed when modifying AAO pores with octatrichlorosilane
to produce Al-OSiC8O17 functionalized alumina surface [34].Solution-based fabrication of polymer nanotubes was used to prepare PMMA nanotubes doped
with tiny amounts of perylene as the fluorescent material.  Perylene was added to a 5% PMMA solution in
acetone. The PMMA solution was allowed to pass through an AAO membrane. This
process forms perylene doped PMMA nanotubes. While the perylene/PMMA nanotubes were
still inside the AAO template “chemically isolated and inert”, an aqueous
solution of disodium fluorescene (DSF) was passed through. When the AAO was
excited with UV light, absorbed by the perylene and not the DSF, the perylene
fluorescence was quenched while the DSF fluorescence evolved. This was
indicative of an efficient Förster type energy transfer from the perylene (donor)
inside the PMMA nanotube to the DSF (acceptor) in solution [42].Solution-based formation of polymer nanotubes is a straightforward method for
fabricating them. Yet another rout was investigated by which an AAO template is
wetted with the molten polymer. This method is limited to polymers that have a
low melting point or could flow through nanopores. For an amorphous polymer, it
was heated well above its glass transition temperature or melting point in case
of a partially crystalline polymer. Polymer nanotubes made from polytetrafluoroethylene
(PTFE), polystyrene, or polymethyl methacrylate (PMMA) were successfully
fabricated following the above-mentioned method. The nanopore size and period
of wetting determine whether researchers obtain a nanotube or a nanowire. For a
200 nm AAO template brief wetting resulted in polymer nanotubes with wall
thickness between 10 and 20 nm, while extended wetting yielded nanowires. For
smaller size AAO nanopores wetting the template with molten polymer for brief periods
of time produced nanowires [43–45].
Therefore, nanotube or nanowire formation is a function of AAO template pore
size. The smaller the nanopore size, the better the chances were for nanowire
formation.Electropolymerization
is a different method of forming polymer nanowires or nanotubes. This method is
limited to certain electron rich aromatic monomers, or monomers that can be
electropolymerized. The back of the AAO template is sputter coated with a thin
Au film and fixed to the cathode end of an electrochemical cell. The AAO
template is immersed in a conducting solution of the monomer. During
electrolysis the monomer is attracted to the inside of the AAO template toward
the gold layer. Polymerization occurs inside the nanopores gradually forming a
polymer nanowire. The length of the nanowire depends on the electrolysis
period. The polymer nanowire is then released from the template by first
scraping off the metal coating from the back of the AAO template then
dissolving away the template with an acid or base. Polypyrrole nanotubes with a
60 nm external diameter were synthesized by alternating current
electropolymerization of a pyrrole solution [46, 47]. It was also revealed that
formation of nanotubes or nanowires through electropolymerization depends on
the potential as well as monomer concentration. 3,4-ethylenedioxythiophene could
be electropolymerized to Poly(3,4-ethylenedioxythiophene) (PEDOT) nanowires or
nanotubes. PEDOT nanotubes were grown at a potential lower than 1.4 V versus. SCE
regardless of the initial concentration of the monomer [48].Another
kind of highly crystalline polymer nanowires or nanorods was developed by Al-Kaysi
et al. [40]. Photopolymerziation of a
photoactive monomer was achieved inside the AAO templates. The first step was
to load the AAO template with a concentrated solution of the monomer, in this
case 9AC-ME. The solvent was slowly evaporated at elevated temperatures, to
yield highly crystalline monomer nanorods. The A-A type monomer units were
aligned in an alternating face to face sandwich packing at a distance less than
4 Å. Irradiating the loaded AAO template with UV light photopolymerized the
monomer to give regioregular poly(9AC-ME) (Scheme 2). The polymer nanorods
retained the crystalline order of the monomer to yield very rigid, highly
insoluble, highly crystalline polymer. Different diameter poly(9AC-ME) nanowires were
synthesized (Figure 3). Those nanorods can be unzipped by heating them above
280C° [40]Scheme 2
[4π+4π]
photodimerization of 9AC-ME.(a) 200 nm diameter
poly(9AC-ME) nanowires [40]. (b) 35 nm
diameter  poly(9AC-ME) nanowires [40].
(a)(b)


## 5. Template Assisted Synthesis of Small Molecule Nanowires, Nanotubes, and Nanorods

It
has been previously described in the current manuscript that 1D polymer
nanotubes or nanowires can be formed via AAO template method. Polymers are long
molecules with flexible structure that can maintain the 1D nanostructure
morphology after removal from the template. What about small molecules? Can we
use AAO template to form 1D nanostructures? The term small molecules will not be
limited to molecules with a certain molecular weight threshold. Monomers,
PAHs, amino acids, and so forth will be considered. Small molecules, like in the case
of polymer chains, are held together by weak van der Waals forces. Unlike in
polymers those weak forces are far fewer per molecular unit. Small molecule
crystals are softer and more brittle. Nonetheless scientists were able to utilize
AAO templates to make small molecule nanotubes, nanowires, or nanorods. The
chemical literature is still ripe with few examples of small molecule 1D
nanostructures. Several methods of 1D small molecule nanostructures will be
surveyed.


### 5.1. Solution-Based Template Wetting Method

When
an AAO template is dipped in a solution of the small molecule, the capillary
action of the alumina nanopores will soak up the solution rapidly. When the
solvent evaporates, the solute is deposited on the nanopore walls because the
high surface energy of the pores is stronger than the intermolecular forces.
Thus a nanotube is formed. Not only organic small molecules but also
organosilanes such as hexaphenylsilole [49] and organometallic complexes
such as (Porphryinato)(phthalocyaninato)europium complex [50] were investigated using this
methodology. Several small molecules were utilized to fabricate nanotubes or
nanowires via solution-based template wetting.Before
venturing into the realm of small organic molecule 1D nanostructure, let us consider
the case of generating Fullerene nanotubes.C60 is not a small
molecule and might not fit in this section of the review, but the method used
to form C60 nanotubes paved the way for the incorporation of smaller
molecules later on. In this process the cleaned AAO template (200 nm) was
repeatedly dipped-dry in a toluene solution of C60. After several
cycles the loaded AAO template was thermally annealed at 500C° for several
hours. The C60 nanotubes were liberated from the AAO template by
etching it out with aqueous NaOH. SEM and TEM microscopy confirmed the hollow
tubular nature of the C60 nanotubes. Energy-dispersive X-microanalysis
confirmed the exclusive carbon content of the nanotube. Electron diffraction
pattern revealed that the nanotubes were polycrystalline [51].This
method was first used in 2002 to prepare perylene nanotubs. A clean AAO
template (200 nm) was dipped-dry several times in a saturated perylene
solution. The perylene laden AAO template was thermally annealed at 240C°
to form perylene nanotubes. The template was removed by dissolving it in an
aqueous NaOH solution. The perylene nanotubes were polycrystalline with a
diameter equal the pore diameter and the length was close to the thickness of
the template. The tube wall thickness was around 25 nm. Optical spectroscopy of
the nanotubes revealed that the perylene was not chemically modified in the
process and the perylene units avoided face to face stacking thus preventing
the formation of an excimer type fluorescence emission [52]. In an independent attempt to
repeat the same experiment using similar preparation techniques and conditions,
we found that during the thermal annealing process the perylene usually
sublimes out of the nanopores. This would in fact explain the low yield of
perylene nanotubes.Pyrene
nanotubes (100 nm diameter) were synthesized by the dip-dry method. The formed
nanotubes had an evenly deposited thick pyrene coating. These nanotubes serve
as a coating for inorganic material to be deposited inside them. An equimolar
solution ofAgNO3 and KI in water was allowed to diffuse, with the
aid of external pressure, through the templated pyrene nanotubes. The reaction
between the Ag+ and I− ions precipitated out AgI inside
the nanotubes. The AgI nanorods were now coated with the pyrene nanotubes.
After the reaction was completed, the template was removed with NaOH (Figure 4).
The composite nature of the nanowire was confirmed by TEM. The electron
diffraction pattern showed that the AgI nanowire inside the pyrene nanotube was
polycrystalline, so was the pyrene nanotube [53].Figure 4
Composite
pyrene/AgI nanowire.Nanotubes
made from chiral molecules were also investigated. Concentrated solutions of
chiral quinidine in chloroform were filtered through 100 or 140 nm AAO
templates with the aid of an external pressure. The thickness of the nanotube
was proportional to the concentration of the solution. Very concentrated solutions
yielded quinidine nanowires, while dilute solutions of quinidine failed to
nucleate and form nanotubes or nanowires. The loaded AAO templates were then annealed
in an oven at 120C°. This method is different than the dip-dry
method previously described. The nanotubes had wall thickness around 30 nm of
crystallized quinidine molecules. The circular dichroism (CD) spectrum of these
nanotubes showed ordered interaction between the molecules. This was not
observed in the CD spectra of quinidine cast films [54].


### 5.2. Melting-Recrystallization Template Wetting Method

In some cases small
organic molecules have very low solubility in organic solvents, especially high
molecular weight PAHs and perylene-diimide derivatives. The alternative to
solution-based template wetting is to melt the organic molecule and wet the AAO
template with the molten material. This method can be extended to soluble
organic small molecules as well.Di-benzoylmethane (DBM)
is a small organic molecule with a relatively low melting point (80C°).
A molten sample of DBM was prepared and 200 nm pore diameter AAO template was
dipped inside the molten compound. The organic liquid immediately wetted the
nanopore walls. It was observed that the longer the immersion time, the thicker
the deposited organic layer on the nanopore walls. Tube wall thickness from 20
to 70 nm was observed with increasing immersion time from 10 to 120 minutes,
respectively. Complete filling of the nanopores was prevented due to the strong
adhesive forces between the AAO walls and the organic molecules. These forces
are stronger than the cohesive forces between the molecules themselves. An X-ray
diffraction (XRD) of the DBM nanotubes inside the template revealed that the
DBM molecules prefer to arrange themselves along the (121) plane. It was also
observed that the fluorescence emission from the nanotubes was 4.6 times
greater than that from the bulk [55]Pentacene is an
important PAH used in organic semiconductor technology. It has a very low
solubility in organic solvents.  When
powder pentacene was melted over a 200 nm AAO template inside a tube furnace
under a gentle stream of Argon gas, pentacene nanotubes were formed. Longer
thermal annealing times of the molten pentacene result in the formation of
nanowires, but not without partial thermal degradation of pentacene. The
pentacene nanotubes were liberated from the AAO template and their tubular
structure confirmed by TEM and SEM. The tubes had an average length of 15μm
and an external diameter close to 200 nm. The tube wall thickness was 40 nm.
Powder XRD of the templated pentacene nanotubes revealed that the pentacene
units have a short range herringbone packing with the pentacene long axis
parallel to the pore length. The low degree of crystallinity of the nanotube
did not effect its resistivity which was around 5×105Ω⋅m
[56].


### 5.3. Sublimation Method

Another
way of introducing small molecules inside AAO templates is by sublimation. The sublimed
small molecules are percolated through the nanochannels where they condense. In
the literature, this method was used to make pyrene and 1,4-bis[2-(5-phenyloxazolyl)] benzene (POPOP) nanowires (20,
70, and 130 nm in diameter and several micrometers long). The nanowires are most
likely polycrystalline in nature, although no evidence of that was presented.
The fluorescence spectra of these nanowires showed dependence on the diameter
of the nanowire. Thus the 20 nm diameter nanowires showed monomer like
fluorescence, most likely due to the inability of the molecules at the surface to
form pairs. The ratio of unpaired molecules at the surface was greater than
those paired in the middle of the nanowire. Large diameter nanowires showed
fluorescence similar to that of bulk crystals. Unfortunately, the authors of
the paper did not elaborate on the experimental conditions for preparing these
nanowires [57]. From our experience,
subliming small molecules such as anthracene, naphthalene, POPOP, or pyrene
close to AAO template results in the immediate clogging of the AAO template pores
and growth of crystals on the surface. The authors of [57] must have used heated AAO templates or sophisticated vacuum techniques. This
method is limited to small organic molecules that are stable enough to be
sublimed, while most small organic molecules melt or decompose instead at
elevated temperatures.


### 5.4. Electrophoretic Deposition Method

Solution
and melting recrystalization methods yield nanotubes or nanowires depending on
the processing conditions. Electrophoretic
deposition is another method of making small molecule nanowires or nanotubes. When
one face of an AAO template is coated with a thin film of Au and affixed to an
anode, the bottom side of the pores becomes positively charged and will attract
small organic molecules with high electron density.A
0.1 molar solution of pyrene is prepared in an electrolytic solution of boron
trifluoride diethyl etherate (BFEE). When the coated AAO template was used as a
working electrode, the pyrene units were eletro-oligomerized, thus forming a
continuous nanowire. 200 nm thick and 60 micron long nanowires of oligopyrene
(average number of repeat units = 6) were synthesized. The length of the
nanowire could be controlled by the time of electodeposition. The formed
nanowires were highly crystalline and exhibited multicolored fluorescence
emission depending on the degree of conjugation between the pyrene units in
each oligomer [58].Electrophoretic
deposition was also used to make charge-transfer salt nanotubes of
Bis(ethylenedithio) tetrathiafulvalene (BEDT-TTF). The charge-transfer salt was
composed of an alternating layer of BEDT-TTF cation and an anion. A similar
method to the one used to prepapre oligopyrene was utilized. The deposited 1D
nanostructure was a nanotube instead of a nanowire [59].Perylene-diimide
derivatives (PDDs) have important use as dyes and in photoconduction devices. PDDs
contain electron rich nitrogen which makes them attracted to the anode in an
electrophoretic deposition. Custom made AAO templates (80 nm pore diameter)
with the aluminum back still attached to the template served as the anode.
Electrophoretic deposition was carried out in a chloroform solution containing10−5 M PDD and trifluoroacetic acid as the protonating agent. The
length of the PDD nanowires was proportional to the electrophoretic deposition
time. Photoconductive measurement on aligned PDD nanowires showed an order of
magnitude more photosensitivity than that of PDD cast films. Indeed this method
is a general way for preparing nanowires of n-type PDD molecules. This method
is also limited by the law solubility of PDD in organic solvents. Thus longer
electrophoretic deposition times will be required [60].


### 5.5. Solvent Annealing Method

At
this point different methods for making 1D small molecule organic
nanostructures have been discussed. Nanotubular structures are
thermodynamically favored over a fully packed nanowire due to the strong
surface area energy of the AAO nanopores. All these 1D nanostructures have one
thing in common; they are polycrystalline. Regardless of the method of
preparation, the 1D nanostructure formed is held together by polycrystalline or
randomly oriented molecular units. The polycrystalline nature of these
nanotubes or nanowires was adequately confirmed by the concentric circular pattern
of the electron diffraction (ED) from isolated nanotube or nanowire. The
polycrystalline nature of these organic nanostructures makes them inferior to
their inorganic counterparts, since the latter are highly crystalline and in
many cases the whole 1D structure is a single crystal. The hope was to find a
general method for making highly crystalline organic 1D nanostructure using AAO
templates.One
can start with a large single crystal of an organic small molecule then shave
it down “top-down method” to the desirable nanosize. This is an expensive
procedure and not for bulk production. Another way of generating highly
crystalline 1D organic nanostructures is via self-assembly [6, 8]. This technique is limited to certain type of organic
molecules and yields 1D nanostructures with varied dimensions.


#### 5.5.1. Experimental Details

A
commercially available AAO template (Whatman-13, 200 nm, 25
mm diameter) has enough pores to accommodate a maximum of 6–9 mg of organic material, assuming that the density of bulk organic small
molecules is around 1g/mL. Suspend the AAO membrane using specially designed
Teflon holder that holds the AAO template from the edge, inside a
standard 16 oz jar. Add around 0.2 mL a
concentrated solution of the organic small molecule (10–20 mg in every 0.1 mL
of good solvent) to the surface of the template. The organic solvent used is
usually volatile such as dichloromethane, tetrahydrofuran (THF), or Benzene. THF
was more often used because it is a good solvent for most organic small
molecules. After the solvent has evaporated from the surface of the template, a
couple of milliliters of the organic solvent were added to the bottom of the
jar. A
small vial containing phosphorous pentoxide was placed inside the jar to
prevent water contamination and keep the system dry. The jar was tightly
covered and a long needle was inserted through the lid to the bottom of the
jar. Another needle was inserted in the cover coupled with a gas bubbler. Argon
gas was introduced into the jar at a rate of 1 bubble every 8 to 10 second. The slow
room-temperature solvent vapor annealing causes the deposited organic layer on
the surface of the AAO template to dissolve in the solvent vapors, while
capillary action inside the template prevented the solution from dripping. The
organic compound slowly crystallizes in the nanochannels after all the solvent
has evaporated (ca. 48 hour). The annealed AAO template surface was polished using
2000 grit sandpaper to remove excess organic layer. The template is etched out
using phosphoric or aqueous sodium hydroxide, liberating the nanorods (Figure5).Figure 5
Steps for fabricating solvent annealed molecular
crystal nanorods.Several small
organic molecules were used to prepare highly crystalline organic nanowires “referred
to as nanorods.”  2,7-di-t-butylpyrene [61] was used to prepare the
first sample of crystalline nanorods. A modified solvent annealing procedure
was used. Instead of room temperature solvent annealing, the jar/AAO template
was placed in an oven for 2 days. The solvent used in this case was THF (B.P.
65C°) and the oven temperature was set at 3 degrees above the
boiling point of THF. This procedure yielded highly crystalline nanorods. The
evidence was established by the unique dot pattern of an ED of a single
nanorod. Dark field experiments revealed that the nanorod is a single crystal
over a minimum of 15 μm length
(Figure 6(c)). Other compounds were also used to prepare nanorods, for example,
m-terphenyl and 9-cyanoanthracene [61].
It is important to remark that m-terphenyl nanotubes are also obtained by
dipping an AAO template in a molten sample. The main drawback associated with
the solvent annealing method is the difficulty to prepare nanorods from an
organic material that does not have a high solubility in a volatile organic
solvent. Thus compounds of electronic importance such as Anthracene and
9,10-diphenylanthracene have a low solubility in THF or dichloromethane; hence,
we could not form nanorods. Table 1 contains of a list of some small molecules and
solvent annealing conditions used to successfully form nanorods.Table 1
Some of the small molecules used to make nanorods.
MoleculeNanorod size/solvent/annealing temperature200 nm diameter/60μmSolvent: THFTemperature: 70C°[61]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: Room temperature. (22C°)[63]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°) or 5C°
gave similar results[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: room temperature (22C°)[63]200 nm diameter/60μmSolvent: THFTemperature: 70C°[63]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: THFTemperature: (68C°)[63]200 nm diameter/60μm35 nm
diameter/50μmSolvent: THFTemperature: 70C°[63]200 nm diameter/60μmSolvent: THFTemperature: 68C°[63]200 nm diameter/60μm35 nm
diameter/50μm13 nm diameter/50μmSolvent: THF or dichloromethaneTemperature: room temperature (22C°)[64]200 nm diameter/60μmSolvent: THF, or dichloromethaneTemperature: room temperature (22C°)[65]200 nm diameter/60μmSolvent:
dichloromethaneTemperature: 68C°[63]200 nm diameter/60μm35 nm
diameter/50μm13 nm
diameter/50μmSolvent: THFTemperature: room temperature (22C°)[39, 62]200 nm diameter/60μm35 nm
diameter/50μmSolvent: THFTemperature: 70C°[40](a) SEM image of a 200 nm 2,7-di-t-butylpyrene
nanorods [61]. (b) SEM image of a 200 nm
9-anthracene carboxylic acid nanorod bundle [62].
(c) Electron Diffraction pattern of 2,7-di-t-butylpyrene nanorod [61]. (d) Fluorescence microscope image of fluorescent
200 nm thick 60 micron long nanorods [63].
(a)(b)(c)(d)


## 5.1. Solution-Based Template Wetting Method

When
an AAO template is dipped in a solution of the small molecule, the capillary
action of the alumina nanopores will soak up the solution rapidly. When the
solvent evaporates, the solute is deposited on the nanopore walls because the
high surface energy of the pores is stronger than the intermolecular forces.
Thus a nanotube is formed. Not only organic small molecules but also
organosilanes such as hexaphenylsilole [49] and organometallic complexes
such as (Porphryinato)(phthalocyaninato)europium complex [50] were investigated using this
methodology. Several small molecules were utilized to fabricate nanotubes or
nanowires via solution-based template wetting.Before
venturing into the realm of small organic molecule 1D nanostructure, let us consider
the case of generating Fullerene nanotubes.C60 is not a small
molecule and might not fit in this section of the review, but the method used
to form C60 nanotubes paved the way for the incorporation of smaller
molecules later on. In this process the cleaned AAO template (200 nm) was
repeatedly dipped-dry in a toluene solution of C60. After several
cycles the loaded AAO template was thermally annealed at 500C° for several
hours. The C60 nanotubes were liberated from the AAO template by
etching it out with aqueous NaOH. SEM and TEM microscopy confirmed the hollow
tubular nature of the C60 nanotubes. Energy-dispersive X-microanalysis
confirmed the exclusive carbon content of the nanotube. Electron diffraction
pattern revealed that the nanotubes were polycrystalline [51].This
method was first used in 2002 to prepare perylene nanotubs. A clean AAO
template (200 nm) was dipped-dry several times in a saturated perylene
solution. The perylene laden AAO template was thermally annealed at 240C°
to form perylene nanotubes. The template was removed by dissolving it in an
aqueous NaOH solution. The perylene nanotubes were polycrystalline with a
diameter equal the pore diameter and the length was close to the thickness of
the template. The tube wall thickness was around 25 nm. Optical spectroscopy of
the nanotubes revealed that the perylene was not chemically modified in the
process and the perylene units avoided face to face stacking thus preventing
the formation of an excimer type fluorescence emission [52]. In an independent attempt to
repeat the same experiment using similar preparation techniques and conditions,
we found that during the thermal annealing process the perylene usually
sublimes out of the nanopores. This would in fact explain the low yield of
perylene nanotubes.Pyrene
nanotubes (100 nm diameter) were synthesized by the dip-dry method. The formed
nanotubes had an evenly deposited thick pyrene coating. These nanotubes serve
as a coating for inorganic material to be deposited inside them. An equimolar
solution ofAgNO3 and KI in water was allowed to diffuse, with the
aid of external pressure, through the templated pyrene nanotubes. The reaction
between the Ag+ and I− ions precipitated out AgI inside
the nanotubes. The AgI nanorods were now coated with the pyrene nanotubes.
After the reaction was completed, the template was removed with NaOH (Figure 4).
The composite nature of the nanowire was confirmed by TEM. The electron
diffraction pattern showed that the AgI nanowire inside the pyrene nanotube was
polycrystalline, so was the pyrene nanotube [53].Figure 4
Composite
pyrene/AgI nanowire.Nanotubes
made from chiral molecules were also investigated. Concentrated solutions of
chiral quinidine in chloroform were filtered through 100 or 140 nm AAO
templates with the aid of an external pressure. The thickness of the nanotube
was proportional to the concentration of the solution. Very concentrated solutions
yielded quinidine nanowires, while dilute solutions of quinidine failed to
nucleate and form nanotubes or nanowires. The loaded AAO templates were then annealed
in an oven at 120C°. This method is different than the dip-dry
method previously described. The nanotubes had wall thickness around 30 nm of
crystallized quinidine molecules. The circular dichroism (CD) spectrum of these
nanotubes showed ordered interaction between the molecules. This was not
observed in the CD spectra of quinidine cast films [54].


## 5.2. Melting-Recrystallization Template Wetting Method

In some cases small
organic molecules have very low solubility in organic solvents, especially high
molecular weight PAHs and perylene-diimide derivatives. The alternative to
solution-based template wetting is to melt the organic molecule and wet the AAO
template with the molten material. This method can be extended to soluble
organic small molecules as well.Di-benzoylmethane (DBM)
is a small organic molecule with a relatively low melting point (80C°).
A molten sample of DBM was prepared and 200 nm pore diameter AAO template was
dipped inside the molten compound. The organic liquid immediately wetted the
nanopore walls. It was observed that the longer the immersion time, the thicker
the deposited organic layer on the nanopore walls. Tube wall thickness from 20
to 70 nm was observed with increasing immersion time from 10 to 120 minutes,
respectively. Complete filling of the nanopores was prevented due to the strong
adhesive forces between the AAO walls and the organic molecules. These forces
are stronger than the cohesive forces between the molecules themselves. An X-ray
diffraction (XRD) of the DBM nanotubes inside the template revealed that the
DBM molecules prefer to arrange themselves along the (121) plane. It was also
observed that the fluorescence emission from the nanotubes was 4.6 times
greater than that from the bulk [55]Pentacene is an
important PAH used in organic semiconductor technology. It has a very low
solubility in organic solvents.  When
powder pentacene was melted over a 200 nm AAO template inside a tube furnace
under a gentle stream of Argon gas, pentacene nanotubes were formed. Longer
thermal annealing times of the molten pentacene result in the formation of
nanowires, but not without partial thermal degradation of pentacene. The
pentacene nanotubes were liberated from the AAO template and their tubular
structure confirmed by TEM and SEM. The tubes had an average length of 15μm
and an external diameter close to 200 nm. The tube wall thickness was 40 nm.
Powder XRD of the templated pentacene nanotubes revealed that the pentacene
units have a short range herringbone packing with the pentacene long axis
parallel to the pore length. The low degree of crystallinity of the nanotube
did not effect its resistivity which was around 5×105Ω⋅m
[56].


## 5.3. Sublimation Method

Another
way of introducing small molecules inside AAO templates is by sublimation. The sublimed
small molecules are percolated through the nanochannels where they condense. In
the literature, this method was used to make pyrene and 1,4-bis[2-(5-phenyloxazolyl)] benzene (POPOP) nanowires (20,
70, and 130 nm in diameter and several micrometers long). The nanowires are most
likely polycrystalline in nature, although no evidence of that was presented.
The fluorescence spectra of these nanowires showed dependence on the diameter
of the nanowire. Thus the 20 nm diameter nanowires showed monomer like
fluorescence, most likely due to the inability of the molecules at the surface to
form pairs. The ratio of unpaired molecules at the surface was greater than
those paired in the middle of the nanowire. Large diameter nanowires showed
fluorescence similar to that of bulk crystals. Unfortunately, the authors of
the paper did not elaborate on the experimental conditions for preparing these
nanowires [57]. From our experience,
subliming small molecules such as anthracene, naphthalene, POPOP, or pyrene
close to AAO template results in the immediate clogging of the AAO template pores
and growth of crystals on the surface. The authors of [57] must have used heated AAO templates or sophisticated vacuum techniques. This
method is limited to small organic molecules that are stable enough to be
sublimed, while most small organic molecules melt or decompose instead at
elevated temperatures.


## 5.4. Electrophoretic Deposition Method

Solution
and melting recrystalization methods yield nanotubes or nanowires depending on
the processing conditions. Electrophoretic
deposition is another method of making small molecule nanowires or nanotubes. When
one face of an AAO template is coated with a thin film of Au and affixed to an
anode, the bottom side of the pores becomes positively charged and will attract
small organic molecules with high electron density.A
0.1 molar solution of pyrene is prepared in an electrolytic solution of boron
trifluoride diethyl etherate (BFEE). When the coated AAO template was used as a
working electrode, the pyrene units were eletro-oligomerized, thus forming a
continuous nanowire. 200 nm thick and 60 micron long nanowires of oligopyrene
(average number of repeat units = 6) were synthesized. The length of the
nanowire could be controlled by the time of electodeposition. The formed
nanowires were highly crystalline and exhibited multicolored fluorescence
emission depending on the degree of conjugation between the pyrene units in
each oligomer [58].Electrophoretic
deposition was also used to make charge-transfer salt nanotubes of
Bis(ethylenedithio) tetrathiafulvalene (BEDT-TTF). The charge-transfer salt was
composed of an alternating layer of BEDT-TTF cation and an anion. A similar
method to the one used to prepapre oligopyrene was utilized. The deposited 1D
nanostructure was a nanotube instead of a nanowire [59].Perylene-diimide
derivatives (PDDs) have important use as dyes and in photoconduction devices. PDDs
contain electron rich nitrogen which makes them attracted to the anode in an
electrophoretic deposition. Custom made AAO templates (80 nm pore diameter)
with the aluminum back still attached to the template served as the anode.
Electrophoretic deposition was carried out in a chloroform solution containing10−5 M PDD and trifluoroacetic acid as the protonating agent. The
length of the PDD nanowires was proportional to the electrophoretic deposition
time. Photoconductive measurement on aligned PDD nanowires showed an order of
magnitude more photosensitivity than that of PDD cast films. Indeed this method
is a general way for preparing nanowires of n-type PDD molecules. This method
is also limited by the law solubility of PDD in organic solvents. Thus longer
electrophoretic deposition times will be required [60].


## 5.5. Solvent Annealing Method

At
this point different methods for making 1D small molecule organic
nanostructures have been discussed. Nanotubular structures are
thermodynamically favored over a fully packed nanowire due to the strong
surface area energy of the AAO nanopores. All these 1D nanostructures have one
thing in common; they are polycrystalline. Regardless of the method of
preparation, the 1D nanostructure formed is held together by polycrystalline or
randomly oriented molecular units. The polycrystalline nature of these
nanotubes or nanowires was adequately confirmed by the concentric circular pattern
of the electron diffraction (ED) from isolated nanotube or nanowire. The
polycrystalline nature of these organic nanostructures makes them inferior to
their inorganic counterparts, since the latter are highly crystalline and in
many cases the whole 1D structure is a single crystal. The hope was to find a
general method for making highly crystalline organic 1D nanostructure using AAO
templates.One
can start with a large single crystal of an organic small molecule then shave
it down “top-down method” to the desirable nanosize. This is an expensive
procedure and not for bulk production. Another way of generating highly
crystalline 1D organic nanostructures is via self-assembly [6, 8]. This technique is limited to certain type of organic
molecules and yields 1D nanostructures with varied dimensions.


### 5.5.1. Experimental Details

A
commercially available AAO template (Whatman-13, 200 nm, 25
mm diameter) has enough pores to accommodate a maximum of 6–9 mg of organic material, assuming that the density of bulk organic small
molecules is around 1g/mL. Suspend the AAO membrane using specially designed
Teflon holder that holds the AAO template from the edge, inside a
standard 16 oz jar. Add around 0.2 mL a
concentrated solution of the organic small molecule (10–20 mg in every 0.1 mL
of good solvent) to the surface of the template. The organic solvent used is
usually volatile such as dichloromethane, tetrahydrofuran (THF), or Benzene. THF
was more often used because it is a good solvent for most organic small
molecules. After the solvent has evaporated from the surface of the template, a
couple of milliliters of the organic solvent were added to the bottom of the
jar. A
small vial containing phosphorous pentoxide was placed inside the jar to
prevent water contamination and keep the system dry. The jar was tightly
covered and a long needle was inserted through the lid to the bottom of the
jar. Another needle was inserted in the cover coupled with a gas bubbler. Argon
gas was introduced into the jar at a rate of 1 bubble every 8 to 10 second. The slow
room-temperature solvent vapor annealing causes the deposited organic layer on
the surface of the AAO template to dissolve in the solvent vapors, while
capillary action inside the template prevented the solution from dripping. The
organic compound slowly crystallizes in the nanochannels after all the solvent
has evaporated (ca. 48 hour). The annealed AAO template surface was polished using
2000 grit sandpaper to remove excess organic layer. The template is etched out
using phosphoric or aqueous sodium hydroxide, liberating the nanorods (Figure5).Figure 5
Steps for fabricating solvent annealed molecular
crystal nanorods.Several small
organic molecules were used to prepare highly crystalline organic nanowires “referred
to as nanorods.”  2,7-di-t-butylpyrene [61] was used to prepare the
first sample of crystalline nanorods. A modified solvent annealing procedure
was used. Instead of room temperature solvent annealing, the jar/AAO template
was placed in an oven for 2 days. The solvent used in this case was THF (B.P.
65C°) and the oven temperature was set at 3 degrees above the
boiling point of THF. This procedure yielded highly crystalline nanorods. The
evidence was established by the unique dot pattern of an ED of a single
nanorod. Dark field experiments revealed that the nanorod is a single crystal
over a minimum of 15 μm length
(Figure 6(c)). Other compounds were also used to prepare nanorods, for example,
m-terphenyl and 9-cyanoanthracene [61].
It is important to remark that m-terphenyl nanotubes are also obtained by
dipping an AAO template in a molten sample. The main drawback associated with
the solvent annealing method is the difficulty to prepare nanorods from an
organic material that does not have a high solubility in a volatile organic
solvent. Thus compounds of electronic importance such as Anthracene and
9,10-diphenylanthracene have a low solubility in THF or dichloromethane; hence,
we could not form nanorods. Table 1 contains of a list of some small molecules and
solvent annealing conditions used to successfully form nanorods.Table 1
Some of the small molecules used to make nanorods.
MoleculeNanorod size/solvent/annealing temperature200 nm diameter/60μmSolvent: THFTemperature: 70C°[61]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: Room temperature. (22C°)[63]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°) or 5C°
gave similar results[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: room temperature (22C°)[63]200 nm diameter/60μmSolvent: THFTemperature: 70C°[63]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: THFTemperature: (68C°)[63]200 nm diameter/60μm35 nm
diameter/50μmSolvent: THFTemperature: 70C°[63]200 nm diameter/60μmSolvent: THFTemperature: 68C°[63]200 nm diameter/60μm35 nm
diameter/50μm13 nm diameter/50μmSolvent: THF or dichloromethaneTemperature: room temperature (22C°)[64]200 nm diameter/60μmSolvent: THF, or dichloromethaneTemperature: room temperature (22C°)[65]200 nm diameter/60μmSolvent:
dichloromethaneTemperature: 68C°[63]200 nm diameter/60μm35 nm
diameter/50μm13 nm
diameter/50μmSolvent: THFTemperature: room temperature (22C°)[39, 62]200 nm diameter/60μm35 nm
diameter/50μmSolvent: THFTemperature: 70C°[40](a) SEM image of a 200 nm 2,7-di-t-butylpyrene
nanorods [61]. (b) SEM image of a 200 nm
9-anthracene carboxylic acid nanorod bundle [62].
(c) Electron Diffraction pattern of 2,7-di-t-butylpyrene nanorod [61]. (d) Fluorescence microscope image of fluorescent
200 nm thick 60 micron long nanorods [63].
(a)(b)(c)(d)


## 5.5.1. Experimental Details

A
commercially available AAO template (Whatman-13, 200 nm, 25
mm diameter) has enough pores to accommodate a maximum of 6–9 mg of organic material, assuming that the density of bulk organic small
molecules is around 1g/mL. Suspend the AAO membrane using specially designed
Teflon holder that holds the AAO template from the edge, inside a
standard 16 oz jar. Add around 0.2 mL a
concentrated solution of the organic small molecule (10–20 mg in every 0.1 mL
of good solvent) to the surface of the template. The organic solvent used is
usually volatile such as dichloromethane, tetrahydrofuran (THF), or Benzene. THF
was more often used because it is a good solvent for most organic small
molecules. After the solvent has evaporated from the surface of the template, a
couple of milliliters of the organic solvent were added to the bottom of the
jar. A
small vial containing phosphorous pentoxide was placed inside the jar to
prevent water contamination and keep the system dry. The jar was tightly
covered and a long needle was inserted through the lid to the bottom of the
jar. Another needle was inserted in the cover coupled with a gas bubbler. Argon
gas was introduced into the jar at a rate of 1 bubble every 8 to 10 second. The slow
room-temperature solvent vapor annealing causes the deposited organic layer on
the surface of the AAO template to dissolve in the solvent vapors, while
capillary action inside the template prevented the solution from dripping. The
organic compound slowly crystallizes in the nanochannels after all the solvent
has evaporated (ca. 48 hour). The annealed AAO template surface was polished using
2000 grit sandpaper to remove excess organic layer. The template is etched out
using phosphoric or aqueous sodium hydroxide, liberating the nanorods (Figure5).Figure 5
Steps for fabricating solvent annealed molecular
crystal nanorods.Several small
organic molecules were used to prepare highly crystalline organic nanowires “referred
to as nanorods.”  2,7-di-t-butylpyrene [61] was used to prepare the
first sample of crystalline nanorods. A modified solvent annealing procedure
was used. Instead of room temperature solvent annealing, the jar/AAO template
was placed in an oven for 2 days. The solvent used in this case was THF (B.P.
65C°) and the oven temperature was set at 3 degrees above the
boiling point of THF. This procedure yielded highly crystalline nanorods. The
evidence was established by the unique dot pattern of an ED of a single
nanorod. Dark field experiments revealed that the nanorod is a single crystal
over a minimum of 15 μm length
(Figure 6(c)). Other compounds were also used to prepare nanorods, for example,
m-terphenyl and 9-cyanoanthracene [61].
It is important to remark that m-terphenyl nanotubes are also obtained by
dipping an AAO template in a molten sample. The main drawback associated with
the solvent annealing method is the difficulty to prepare nanorods from an
organic material that does not have a high solubility in a volatile organic
solvent. Thus compounds of electronic importance such as Anthracene and
9,10-diphenylanthracene have a low solubility in THF or dichloromethane; hence,
we could not form nanorods. Table 1 contains of a list of some small molecules and
solvent annealing conditions used to successfully form nanorods.Table 1
Some of the small molecules used to make nanorods.
MoleculeNanorod size/solvent/annealing temperature200 nm diameter/60μmSolvent: THFTemperature: 70C°[61]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: Room temperature. (22C°)[63]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°) or 5C°
gave similar results[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: dichloromethaneTemperature: room temperature (22C°)[63]200 nm diameter/60μmSolvent: THFTemperature: 70C°[63]200 nm diameter/60μmSolvent: THFTemperature: room temperature (22C°)[61]200 nm diameter/60μmSolvent: THFTemperature: (68C°)[63]200 nm diameter/60μm35 nm
diameter/50μmSolvent: THFTemperature: 70C°[63]200 nm diameter/60μmSolvent: THFTemperature: 68C°[63]200 nm diameter/60μm35 nm
diameter/50μm13 nm diameter/50μmSolvent: THF or dichloromethaneTemperature: room temperature (22C°)[64]200 nm diameter/60μmSolvent: THF, or dichloromethaneTemperature: room temperature (22C°)[65]200 nm diameter/60μmSolvent:
dichloromethaneTemperature: 68C°[63]200 nm diameter/60μm35 nm
diameter/50μm13 nm
diameter/50μmSolvent: THFTemperature: room temperature (22C°)[39, 62]200 nm diameter/60μm35 nm
diameter/50μmSolvent: THFTemperature: 70C°[40](a) SEM image of a 200 nm 2,7-di-t-butylpyrene
nanorods [61]. (b) SEM image of a 200 nm
9-anthracene carboxylic acid nanorod bundle [62].
(c) Electron Diffraction pattern of 2,7-di-t-butylpyrene nanorod [61]. (d) Fluorescence microscope image of fluorescent
200 nm thick 60 micron long nanorods [63].
(a)(b)(c)(d)


## 6. Some Unique Properties of Highly Crystalline Small Molecule Nanorods Made Using Solvent Annealing Method

An interesting case that clearly differentiated 1D organic nanostructures from the bulk was
in the use of photoreactive ancthracene ester derivatives. Highly crystalline
nanorods of 9-anthracene-t-butylester (9-TBAE) [64], 9-methylanthracene and
9-Anthracene carboxylic acid (9-AC) [62], which were made using
the solvent annealing method, exhibit unique photomechanical properties. When
large micron-sized crystals of 9-TBAE or 9-AC were irradiated with UV light
(365 nm), the crystal photodimerizes via a [4+4] photocycloaddition to form the photodimer. The photochemical reaction produced
internal stress that resulted in crystal disintegration. When 200 nm nanorods of
9-TBAE were irradiated, a clean crystal-crystal transformation was observed
with an anisotropic expansion along the long axis. In other cases when 200 nm
(9-AC) nanorods were asymmetrically or partially irradiated, the result is
wiggling and micron scale translation of the whole nanorod. The [4+4] photodimerization of 9-AC is thermally
reversible; thus, the nanorod regains its original structure and the mechanical
motion could be recycled several times [66]. Coating the 9-AC with a
thin layer of silica did not hinder this photomechanical response, which
suggests that the amount of energy produced is large enough to overcome the
constriction due to a layer of silica, propel the nanorod through solution, and
push the micron-sized nanostructures across [39]. Nanorods made from
9-methylanthracene ester (9-MAE) tend to contract along the long axis when
irradiated with UV light. This contraction was unexpected since 9-TBAE expands
instead. This expansion, contraction, or  wiggling behavior could be directly
explained by the way the crystals pack and rearrange inside the nanorod. C. J.
Bardeen group of the department of chemistry at the University of California-Riverside
are currently investigating the correlation between crystal structure and
photomechanical response of some anthracene derivative nanorods.Another
interesting behavior of some highly crystalline organic nanorods is their
ability to form crystalline cocrystal nanorods without loosing their
original shape or crystallinity. 9-methyl anthracene nanorods with 200 nm diameter
were fabricated via solvent annealing method. The nanorods were kept inside the
template which was suspended in an aqueous suspension of
1,2,4,5-tetracyanobenzene. After several days a solid state diffusion of the
TCNB molecules inside the 9-MA nanorods was observed. After removal of the
template the nanorods have changed into an intimate 1 : 1 charge-transfer salt of
9-MA:TCNB. The charge-transfer nanorods were single crystals as confirmed by
TEM and ED experiments.  A similar
diffusion experiment on microns-sized single crystal of 9-MA resulted in
disintegration of that crystal and formation of the polycrystalline CT complex [65].


## 7. Conclusion

AAO template method is a general way of
obtaining organic nanotubes, nanowires, or nanorods of variable length and
diameter (7–400 nm). The use of these templates to
generate polymer nanotubes or nanowires has taken precedents over small
molecule 1D nanostructures. There are ways for making small molecule nanowires
and nanotubes that can withstand the rigger of template fabrication. It is also
shown that organic 1D nanostructures could behave in a unique way that
differentiates them from the isolated molecule or the bulk. Polymer nanowires
made from semiconducting polymers have enhanced resistivity when compared to
the bulk or cast film. The unique shape of polymer nanotubes and nanowires
exposes a large surface area of the material in a confined space. Thus many
polymer nanotubes and nanowires have seen applications in gas sensing analysis
and as smart filters. Highly crystalline organic nanorods made from photoreactive
small molecules can expand or contract when irradiated with UV light without
disintegrating like the larger micron-sized crystals.



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*Source: 436375-2009-04-29.xml*