Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications
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Chapter
9
Applications
of
Nanomaterials
9.1.
Introduction
Nanotechnology offers an extremely broad range of potential applications
from electronics, optical communications and biological systems to new
materials. Many possible applications have been explored and many
devices and systems have been studied. More potential applications and
new devices are being proposed in literature. It is obviously impossible to
summarize all the devices and applications that have been studied and it is
impossible to predict new applications and devices. This chapter will sim-
ply provide some examples to illustrate the possibilities of nanostructures
and nanomaterials in device fabrication and applications. It is interesting
to note that the applications
of
nanotechnology in different fields have dis-
tinctly different demands, and thus face very different challenges, which
require different approaches.
For
example, for applications in medicine, or
in nanomedicine, the major challenge is “miniaturization”: new instruments
to
analyze tissues literally down to the molecular level, sensors smaller than
a cell allowing to look at ongoing fhctions, and small machines that lit-
erally circulate within a human body pursuing pathogens and neutralizing
chemical toxins.
Applications of nanostructures and nanomaterials are based on
(i) the peculiar physical properties of nanosized materials, e.g. gold
39
1
392
Nanostructures and Nanomaterials
nanoparticles used as inorganic dye to introduce colors into glass and as
low temperature catalyst, (ii) the huge surface area, such as mesoporous
titania for photoelectrochemical cells, and nanoparticles for various sen-
sors, and (iii) the small size that offers extra possibilities for manipulation
and room for accommodating multiple hnctionalities. For many applica-
tions, new materials and new properties are introduced. For example, var-
ious organic molecules are incorporated into electronic devices, such as
sensors.2 This chapter intends to provide some examples that have been
explored to illustrate the vast range of applications of nanostructures and
nanomaterials.
9.2.
Molecular Electronics and Nanoelectronics
Tremendous efforts and progress have been made in the molecular elec-
tronics and nanoelectr~nics.~-'~ In molecular electronics, single molecules
are expected to be able to control electron transport, which offers the
promise of exploring the vast variety of molecular functions for electronic
devices, and molecules can now be crafted into a working circuit as shown
schematically in Fig.
9.1
.3
When the molecules are biologically active,
bioelectronic devices could be de~eloped.~.'~ In molecular electronics,
control over the electronic energy levels at the surface of conventional
semiconductors and metals is achieved by assembling on the solid
Fig.
9.1.
Schematic showing that molecules can now be crafted into working circuit,
though constructing real molecular chips remains a big challenge.
[R.
E
Service,
Science
293,
782 (2001).]
Applications
of
Nanomaterials
393
surfaces, poorly organized, partial monolayers of molecules instead of the
more commonly used ideal ones. Once those surfaces become interfaces,
these layers exert electrostatic rather than electrodynamic control over the
resulting devices, based on both electrical monopole and dipole effects of
the molecules. Thus electronic transport devices, incorporating organic
molecules, can be constructed without current flow through the mole-
cules. The simplest molecular electronics are sensors that translate unique
molecular properties into electrical signals. Sensors using a field effect
transistor (FET) configuration with its gate displaced into a liquid elec-
trolyte, and an active layer of molecules for molecular recognition were
reported in early
197O’s.l4
A
selective membrane is inserted on the insu-
lator surface of the FET, and this permits the diffusion of specific analyte
ions and construction of a surface dipole layer at the insulator surface.
Such a surface dipole changes the electric potential at the insulator surface
and, thus, permits the current going through the device. Such devices are
also known as ion-selective FET (ISFET) or chemical FET (CHEM-
FET).2,15,’6 Thin films attached to metal nanoparticles have been shown to
change their electrical conductivity rapidly and reproducibly in the pres-
ence of organic vapors, and this has been exploited for the development of
novel gas
sensor^.'^^'^
The monolayer on metal nanoparticles can
reversibly adsorb and desorb the organic vapor, resulting in swelling and
shrinking of the thickness of the monolayer, thus changing the distance
between the metal cores. Since the electron hopping conductivity through
the monolayers is sensitively dependent on the distance, the adsorption of
organic vapor increases the distance and leads to a sharp decrease in elec-
trical conductivity.
Many nanoscale electronic devices have been demonstrated: tunneling
junctions,
19-21
devices with negative differential electrically
configurable s~itches,2~?~~ carbon nanotube
transistor^,^^^^^
and single
molecular
transistor^.^^>^^
Devices have also been connected together
to form circuits capable of performing single functions such as basic
memory23,24929 and logic
function^.^^^^
Ultrahigh density nanowires lat-
tices and circuits with metal and semiconductor nanowires have also been
dem~nstrated.~~ Computer architecture based on nanoelectronics (also
known as nanocomputers) has also been st~died,~~~~~ though very limited.
Various processing techniques have been applied in the fabrication of
nanoelectronics such as focused ion beam (FIB),37-39 electron beam
lithogra~hy,~~9~O and imprint lith~graphy.~~ Major obstacles preventing the
development of such devices include addressing nanometer-sized objects
such as nanoparticles and molecules, molecular vibrations, robustness and
the poor electrical conductivity.
3
94
Nanostructures and Nanomaterials
Au nanoparticles have been widely used in nanoelectronics and molecu-
lar electronics using its surface chemistry and uniform size. For example,
Au
nanoparticles function as carrier vehicles to accommodate multiple func-
tionalities through attaching various functional organic molecules or bio-
components.'" Au nanoparticles can also function as mediators to connect
different functionalities together in the construction of nanoscale electronics
for the applications of sensors and detectors. Various electronic devices based
on Au nanoparticles and AuS5 clusters have been In particular,
single electron transistor action has been demonstrated for systems that con-
tain ideally only one nanoparticle in the gap between
two
electrodes sepa-
rated by only a few nanometers. This central metal particle represents a
Coulomb blockade and exhibits single electron charging effects due to its
extremely small capacitance. It can also act as a gate if it is independently
addressable by a third terminal. An electrochemically addressable nano-
switch, consisting of a single gold particle covered with a small number of
dithiol molecules containing a redox-active viologen moiety has been
demonstrated, and the electron transfer between the gold substrate and the
gold nanoparticle depend strongly on the redox-state of the vi~logen.~~
Single-walled carbon nanotubes have also been intensively studied for
nanoelectronic devices, due to the semiconducting behavior of different
allotrope^.^^
Examples
of
single-walled carbon nanotube nanoelectronic
devices include single-electron
FET,30950,51
sen~ors,~~~~~
and a molecular electronics toolbox.55 Carbon nanotubes have
been explored for many other applications, such as
actuator^,^^?^^
sen-
sor~,~*~~~ and thermometers made of multiple-walled carbon nanotubes
filled with gallium.60
9.3.
Nanobots
A
very promising and fast growing field for the applications of nanotech-
nology is in the practice of medicine, which, in general, is often referred
to as nanomedicine. One of the attractive applications in nanomedicine
has been the creation of nanoscale devices for improved therapy and
diagnostics. Such nanoscale devices are known as nanorobots or more
simply as nanobots.61 These nanobots have the potential to serve as vehi-
cles for delivery of therapeutic agents, detectors or guardians against early
disease and perhaps repair of metabolic or genetic defects. Similar to the
conventional or macroscopic robots, nanobots would be programmed to
perform specific functions and be remotely controlled, but possess a much
smaller size,
so
that they can travel and perform desired functions inside
Applications
of
Nanomaterials
395
the human body. Such devices were first described by Drexler in his book,
Engines of Creation in
1986.62
Haberzettl6I described what nanobots would do in practice in medicine,
which is briefly summarized below. Nanobots applied to medicine would
be able to seek out a target within the body such as a cancer cell or an
invading virus, and perform some function to fix the target. The fix deliv-
ered by the nanobots may be that of releasing a drug in a localized area,
thus minimizing the potential side effects of generalized drug therapy, or
it may bind to a target and prevent it from further activity thus, for exam-
ple, preventing a virus from infecting a cell. Further in the future, gene
replacement, tissue regeneration or nanosurgery are all possibilities as the
technology becomes more mature and sophisticated.
Although such capable and sophisticated nanobots are not yet realized,
many functions in much simplified nanobots are being investigated and
tested in the lab. It is also being argued that the nanobots would not take
the conventional approach of macroscopic robots. Examples include:
(1)
Architecture or structure to carry the payload, known as carrier: Three
groups of nanoscale materials have been studied extensively as a
structure or vehicle to carry various payloads. The first group is car-
bon nanotubes or buckyballs, the second group various dendrimers,
and the third various nanoparticles and nanocrystals.
(2)
Targeting mechanisms to guide the nanobots to the desired site of action:
The most likely mechanisms to be employed are based on antigen or
antibody interactions or binding of target molecules to membrane-bound
receptors. The navigation system for nanobots would be most likely to
use the same method that the human body uses, going with the flow and
“dropping anchor” when the nanobots reached its target.
(3)
Communication and information processing: Single molecular elec-
tronics may offer simple switch function of on and off, optical label-
ing would be a more readily achievable reality.
(4)
Retrieve of nanobots from human body: Retrieve of nanobots from
human body would be another challenge in the development of
nanobots. Most nanodevices could be eliminated from the body
through natural mechanisms of metabolism and excretion.
Nanodevices made of biodegradable or naturally occurring sub-
stances, such as calcium phosphate, would be another favorable
approach. “Homing” nanobots would be ideal, which can be collected
and removed after performing the desire function. The possible nega-
tive impacts of nanobots include the pollution and clog of systems in
human body, and the nanobots may become “out of control” when
some functions are lost or nanobots malfunction.