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Nanoscience and
Nanotechnology: Science and Applications in Physical
Chemistry, Materials
Science, and Biophysics
Introduction
Working
on the nanometer scale, one billionth of a meter, requires the ability to
synthesize, manipulate, and organize matter in a controllable manner as well as
to predict and understand the properties of the resulting structure.
Fundamentally, the focus of the nanoscience research in this group is to study
discrete, molecular-scale intermolecular interactions. These are critical to
understanding problems such as (a) friction, adhesion, and lubrication,
important for physics applications; (b) binding energies on surfaces, essential
for the design of effective chemical and biological catalysts; as well as (c)
phenomena such as chemical and biological self-assembly.
More specifically, we are interested
in understanding the role of covalent chemistry in governing nanoscale
interactions. The model systems that we have primarily focused our efforts on
include two classes of interesting nanomaterials: carbon nanotubes and
semiconductor nanocrystals (quantum dots).
Carbon nanotubes consist of shells of
sp2-hybridized carbon atoms forming a hexagonal network, arranged
helically within a tubular motif. The advantage of carbon nanotubes is that
they are chemically and molecularly defined structures with reproducible
dimensions. In addition, single-walled carbon nanotubes in particular possess
interesting electronic properties, predominantly based on their diameter and
helicity. Moreover, they are stiffer and stronger than any potentially known
material with implications for the design of composite materials as well as
nanometer-scale devices.
Quantum
dots, such as CdS
and CdSe, alternately known as quantum dots, exhibit strongly size-dependent
optical and electrical properties. Because of their 2-50 nm size range,
nanocrystals are unique in that the number of surface atoms is a large fraction
of the total. Hence, their intrinsic properties are transformed by quantum size
effects due to the spatial confinement of excitations. The high luminescence
yield of these materials as well as the potential of adjusting emission and
absorption wavelengths by controllably selecting the nanocrystal size make
these materials attractive for the construction of a wide range of
optoelectronic devices with tailored properties.
Research Focus
Fundamentally, we are interested in
chemically derivatizing these nanostructures for the purpose of understanding
the structure-dependence of their electrical, optical, mechanical, and
electrochemical properties in order to exploit them for novel applications in
chemistry and biology. We present a few examples of our efforts to design,
create, and manipulate new and exciting classes of materials.
I. Rational Chemical
Functionalization of Nanotubes
Understanding the chemistry of
carbon nanotubes is critical to rational manipulation of their properties.
Chemical modification is essential to deposition of catalysts and other species
onto nanotube surfaces for nanocatalytic and sensor applications. Moreover,
such studies are crucial for oriented assembly of these nanostructures. The
ability to disperse and solubilize carbon nanotubes would also open up new
prospects in aligning and forming molecular devices. Nonetheless, this
objective necessitates controlled chemical functionalization of tubes, a
relatively unexplored area of research, compared with, for instance, fullerene
chemistry. From a fundamental scientific perspective, functionalization allows
for the exploration of the intrinsic molecular nature of these carbon nanotubes
and permits studies at the rich, structural interface between true molecules
and bulk materials.
(a).
Synthesis and characterization of nanotubes covalently complexed to molecular
coordination compounds. One of the complexes studied was Vaska's compound.
It has been found that Ir coordinates to these nanotubes by two distinctive
pathways. With raw nanotubes, the metal attaches as if the tubes behaved as
electron-deficient alkenes. With oxidized nanotubes, the reaction occurs by
coordination through the increased number of oxygen atoms, forming a
hexacoordinate structure around the Ir atom. Another compound analyzed was
Wilkinson's complex. It has been found that the Rh metal similarly
coordinates to these nanotubes through the increased number of oxygenated
species. The functionalization reaction, in general, appears to significantly
increase oxidized nanotube solubility in DMF (in the case of Vaska's)
and in DMSO (with Wilkinson's). A third set of experiments was performed with
lanthanide salts. It was discovered that the lanthanide ions likely
coordinate to shortened, oxidized nanotubes through the increased number of
oxygen atoms, forming predominantly ionic bonding arrangements and disrupting
hydrogen bonding in nanotube bundles. In addition, nanotubes were found to
quench lanthanide photoluminescence.
The derivatization process results in exfoliation
of larger bundles of nanotubes and may select for the presence of distributions
of smaller diameter tubes. Optical data on derivatized adducts suggest the
possibility of interesting charge-transfer behavior across the metal-nanotube
interface. An application has been made of this system as supports for
homogeneous catalysis.
Schematic for a nanotube-metal complex (Wilkinson's complex,
RhCl(PPh3)3) adduct. Figure shows a possible mode of
coordination, whereby oxygenated functionalities, such as two carboxylic acid
groups, at the opened ends of a (5, 5) single-walled carbon nanotube are able
to coordinate to the metal center. Oxygenated functionalities are expected to
be present at ends and defect sites.
(b). Selective Metallic
Tube Reactivity in the Solution-Phase Osmylation of Single-walled Carbon
Nanotubes. The interaction of
OsO4 in toluene with SWNTs in the presence of UV irradiation has
been found to demonstrate chemical specificity toward metallic nanotubes, with
a larger electron density near the Fermi level. The net results of
osmylation were
(a) covalent sidewall functionalization of these nanotubes through disruption
of the conjugated p-electron structure as well
as (b) reduction of the osmium tetroxide species to OsO2
nanoparticles, which were then templated onto the sidewall surface. A
systematic Raman study of our nanotube samples at three different excitation
wavelengths, probing different electronic populations of tubes, provided for
strong evidence of the higher reactivity of metallic tubes with respect to
osmylation, mainly because of the dramatic loss of resonances at 514.5 nm, as
compared with the minimal alterations observed with the peaks of primarily
semiconducting tubes at 1064 nm.
Schematic of the electronic density of states (DOS)
of metallic and semiconducting nanotubes overlaid on a scanning electron
micrograph (SEM) image of osmylated tubes.
Achieving a neat separation of metallic vs.
semiconducting nanotubes is critical to a variety of
applications. For instance, semiconducting nanotubes are useful for optical
sensing whereas isolated metallic tubes with quasi-ballistic transport could be
useful as leads in these nanoscale devices. Hence, covalent sidewall
functionalization, by exploiting subtle differences in reactivity between
different species of SWNTs, offers an important route to generating such
fundamentally interesting monodisperse samples of nanotubes.
(c). Generation of Carbon
Nanotube-Nanocrystal Heterostructures. Oxidized nanotubes have been covalently reacted
with functionalized CdSe quantum dots as well as with titanium dioxide
nanocrystals to form nanoscale heterostructures, characterized by transmission
electron microscopy (TEM) and infrared spectroscopy (FT-IR). Based on the types
of intermediary linking agents used, we have demonstrated a level of control
over the spatial distribution of nanocrystals on these tubes.
A Functionalized CdSe Quantum Dot -
Carbon Nanotube Heterostructure
(d). In Situ Growth of
Quantum Dots on Carbon Nanotube Surfaces The generation of nanoscale
interconnects and supramolecular, hierarchical assemblies enables the
development of a number of novel nanoscale applications. The route towards the
development of practical devices requires either the integration of these
nanoscale building blocks, such as nanotubes and nanocrystals, into existing
hardware or the 'bottom up' assembly of these structures into a complex,
functional arrangement. A rational approach towards engineering a robust system
is through chemical recognition. We have recently shown the in situ
mineralization of crystalline CdSe and CdTe quantum dots on (i) the
surfaces of oxidized multi-walled carbon nanotubes (MWNTs) and on
(ii) the surfaces of oxidized, ozonized single-walled carbon nanotubes
(SWNTs). We coordinate metallic precursors of quantum dots directly onto
nanotubes and then, proceed with in situ growth. The resulting network of
molecular-scale 'fused' nanotube-nanocrystal heterojunctions demonstrates a
controlled synthetic route to the synthesis of complex nanoscale
heterostructures and hierarchical assemblies.
Schematic
illustrating various steps in the growth of a nanotube-nanocrystal
heterostructure. Pristine nanotubes are oxidized to generate functional groups
at the nanotube ends and at a few defect sites. CdTe nanocrystals are then
grown in situ by coordination of Cd and injection of a Te solution.
(e). Solubilization of
Oxidized Single-walled Carbon Nanotubes in Organic and Aqueous Solvents through
Organic Derivatization. The solubilization of oxidized
carbon nanotubes has been achieved through derivatization using a
functionalized organic crown ether. The resultant, synthesized adduct
yielded concentrations of dissolved nanotubes on the order of ~1 g/L in water
as well as in methanol, according to optical measurements. The nanotube-crown
ether adduct can be readily redissolved in 10 different organic solvents
at substantially high concentrations. Characterization of these solubilized
adducts was performed with proton as well as lithium NMR spectroscopy. The
solutions were further analyzed using UV-visible, photoluminescence, and FT-IR
spectroscopies and were structurally characterized using atomic force
microscopy (AFM) and TEM.
Optimized geometry for crown ether-functionalized
carbon nanotubes. Adduct formation likely arises from a zwitterionic
interaction between the carboxylic acid groups on the carbon nanotube and the
amino functionality on the derivatized crown ether
(f). Rational Sidewall Functionalization and Purification of
Single-walled Carbon Nanotubes by Solution-Phase
Ozonolysis. We
have developed a 'one-pot' oxidative methodology with three major
objectives: first, the purification of as-prepared carbon
nanotubes to obtain a high-quality product by removing amorphous carbon and
metal impurities; secondly, the chemical functionalization of
nanotube sidewalls; and thirdly, a systematic procedure to
rationally skew the distribution of oxygenated functional groups to
favor (i.e. generate higher proportions of) one particular moiety, through a
reproducible chemical protocol, on the surfaces of the resultant purified
nanotubes.
Optimized Geometry for Sidewall-Ozonized Single-Walled
Carbon Nanotubes
These goals are accomplished
by favoring the generation of carboxylic acids, aldehydes/ketones, and alcohols
on the surfaces of carbon nanotubes through chemical treatment with hydrogen
peroxide (H2O2), dimethyl sulfide (DMS), and sodium
borohydride (NaBH4), respectively, that take advantage of the high
reactivity of primary ozonides, that are presumed to form upon the ozonolysis
of nanotube dispersions in solution. In effect, the reaction sequence ozonizes
(and hence, oxygenates) the sidewalls of these nanotubes, thereby broadening
the chemical processability and reactivity of these nanomaterials. The
derivatized materials have been characterized by means of scanning electron
microscopy (SEM) and TEM, and spectroscopically, using Raman, UV-Vis-Near IR,
and X-ray photoelectron spectroscopies.
Recently, we
have established that chemical reactivity of nanotubes in this sidewall
addition reaction, i.e., solution-phase ozonolysis, is dependent on
diameter. Smaller diameter nanotubes have greater strain energy per carbon
atom due to increased curvature strain and greater rehybridization. The radial
breathing modes in the low wavenumber region of nanotube Raman spectra indicate
that, after functionalization, features corresponding to small diameter tubes
are relatively diminished in intensity with a relatively minor alteration in
the profile of larger diameter tubes.
II. Synthesis
and Characterization of Novel Non-Carbon Nanostructures.
Understanding the behavior of ferroelectric
materials at the nanoscale is of importance to the development of molecular
electronics, in particular for random access memory and logic circuitry.
Indeed, transition metal oxides with a cubic perovskite structure are
noteworthy for their advantageous dielectric, piezoelectric, electrostrictive,
pyroelectric, and electro-optic properties with corresponding applications in
the electronics industry for transducers, actuators, and high-k-dielectrics.
These oxides, including BaTiO3 and SrTiO3, exhibit large
nonlinear optical coefficients and large dielectric constants. Because these
effects are dependent on structure and finite size, considerable effort has
been expended in the controllable synthesis of crystalline materials and thin
films of these ferroelectric oxides.
One-dimensional nanotube/nanowire
systems offer fundamental scientific opportunities for investigating the
influence of size and dimensionality of materials with respect to their
collective optical, magnetic, and electronic properties.
(a).
Hydrothermal Synthesis of Perovskite Nanotubes. We have been intent on developing a mild, low
temperature, and generalizable synthetic strategy to generate crystalline
1-D barium and strontium titanate perovskite nanotubes. To this end, we have
developed a wet-chemical, hydrothermal synthesis, using an aqueous medium under
alkaline conditions. Our strategy has been to utilize a titanium oxide
(TiO2) nanotube as a bona fide precursor template material in
order to generate the corresponding perovskite transition metal oxide nanotubes
in a rational manner.
Titania nanotubes as precursors in the hydrothermal
synthesis of BaTiO3 and SrTiO3 nanotubes under ambient
temperature and strong alkaline conditions.
(b). Large-scale Synthesis of
Single-Crystalline Perovskite Nanostructures.
Single-crystalline perovskite nanostructures of
reproducible shape have been prepared using a simple, readily
scaleable solid-state reaction in the presence of NaCl and a nonionic
surfactant. Pristine BaTiO3
nanowires have diameters ranging from 50 to 80 nm with an aspect
ratio larger than 25. Single-crystalline SrTiO3 nanocubes with a mean edge length of 80 nm have been produced using
a similar procedure. Extensive characterization of these nanostructures has
been performed using scanning electron microscopy (SEM), transmission electron
microscopy (TEM), high-resolution transmission electron microscopy (HRTEM),
energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD).
SEM
Image of Barium Titanate Nanorods
III. Efforts in Probe
Microscopy
One of the most promising techniques
for the study of intermolecular forces is the atomic force microscope (AFM). On
this instrument, the probe attached to the cantilever raster scans across the
surface of the sample, while sensing topography and measuring forces. The AFM
has the capability of measuring van der Waals, capillary, electrostatic, and
even magnetic forces as low as the piconewton range. As well, it has a capacity
for spatial resolution on a nanometer scale. Moreover, this technique can be
used to measure forces in any medium, be it in air, in vacuo, or in fluid. We
use AFM primarily as a characterization technique for investigations of
our nanomaterials, as well as for topographical imaging of biomolecular
structures, such as proteins. Nonetheless, we also have innovative projects,
involving tip manipulation and creation of chemically-specific probes with the
capability of spatially high-resolution imaging.
Controllable AFM
tip-initiated, in-situ reactions. We have used AFM tips, coated with a benign,
relatively safe reducing agent, to selectively reduce a spatially defined
region of a monolayer of imines. Confirmation of reaction completion came
through the use of surface mid-IR results as well as with the use of the
chloranil test. This process mimics an important, more general solution
reaction on a much smaller, localized scale without the use of electric current
or potentially hazardous reaction conditions.
Functionalized AFM Tip Reducing
Imines on a Surface
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