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I.Electronic and Thermal and
Thermoelectric Transport in Carbon Nanotubes
and Nanowires
The recent discovery
of various 1-dimensional (1-d) nanomaterials
such as carbon nanotubes and
semiconductor/metal nanowires has ignited a
great deal of theoretical and experimental work. The electronic
properties of 1-dimensional (1D) nanoscaled
materials including carbon nanotubes and
semiconductor/metal nanowires have been
intensively studied at the single nanotube/nanowire
level, and have exhibited a variety of unique physical phenomena due to
the enhanced quantum confinement of electrons in reduced dimensions.
However, unlike the electronic properties, the experiments investigating
the thermal properties of these materials have mostly focused on 'bulk'
measurements. In thermal transport measurements such as thermal
conductivity and thermoelectricity, it is difficult to extract absolute
values for these quantities from the 'bulk' experiments due to the
presence of numerous junctions between individual molecular wires, which
often present extrinsic effects to the measurements.
Most importantly, it
is only at mesoscopic scales where one is able
to study the quantum limit of energy (thermal) transport and
thermoelectric effects. In this regard, mesoscopic
thermal transport measurements are necessary to elucidate the intrinsic
properties of these materials in the quantum limits. Such mesoscopic experiments in semiconducting
devices have been recently demonstrated in Kim's research group. Using
novel hybridized synthesis techniques in combination with semiconductor
device fabrication techniques, the Kim group is currently investigating
the quantum limit of thermal transport and mesoscopic
thermoelectric phenomena in 1D nanoscale materials.Specifically, the Kim group intends to
address following open questions:
- The energy
transport/dissipation in the quantum transport limit: How does
energy transport/dissipate when the quantum phase coherence length
of the energy carrier is comparable to the sample dimension?
- Thermoelectric phenomena
in nanoscale materials: What determines
the thermoelectricity in a confined nanoscale
system?
- Nanoscale engineering for
thermoelectric applications: Is efficient thermoelectric
cooling/generation attainable with 1D nanoscale
materials?
For this purpose, we
have established a collaboration work with sample providers outside Columbia University in order to obtain nanoscale materials for investigation of the
aforementioned mesoscopic
thermal/thermoelectric transport measurements.
In addition,
synthesizing and manipulating carbon nanotubes
have been one of the greatest challenges to investigate physical
properties and to use this material for device applications. The Kim
group has made a few innovative progresses in this field by demonstrating
(i) ultralong single
walled and multiwalled carbon nanotube growth (ii) manipulation of multiwalled nanotubes for
fabrication of hierarchical structures based on nanotubes;
and (iii) investigation of electrical transport in different nanotube based nanostructures including resistance
scaling in length of 1-dimensional channels.
II. Transport Properties of Novel 2-dimensional Nanocrystals
Applying an external
electric field across a gate insulator attracts or repels charge carriers
in a material and creates a thin charge accumulation or depletion layer
at the surface/interface of the sample. The ability to control the charge
carrier density through the electric field effect has provided new
opportunities to investigate materials, whose properties strongly depend
on carrier concentration. Those materials include organic conductors,
high temperature superconductors, metal chalcogenide
and semimetallic layered materials, such as
graphite.
The Kim group has
developed experimental techniques to create and meosocopic
graphite samples and graphene for transport measurement. We are now
applying this experimental technique to create novel 2D nanocrystals from other layered materials mentioned
above. In combination with new material preparation and manipulation
techniques using micromechanical and microfabrication
tools, we will investigate field effect transport phenomena in these 2D
nanostructures. Specifically, we will perform galvanomagnetic
measurements, such as magnetoresistance and
Hall measurements, at low temperatures and high magnetic field to
investigate electron transport in the strong quantum limit.
There are tremendous
intellectual merits for the proposed study. 2D electron systems studied
today fall into two classes. They consist either of strongly anisotropic
bulk materials in which conduction is preferentially planar. Otherwise
they are artificially structured, mostly by molecular beam epitaxial growth of semiconductors. Investigations of
the former, requires the measurement of many layers of quasi 2D systems
in parallel and includes all interactions between them. Investigations of
the later allow for the measurement of individual layers but represent an
"artificial" material.
We propose to push
the measurement of multilayer materials to the single atomic layer limit.
The exploration of electron transport in such "natural 2D
systems" may lead to discoveries of new phenomena, particularly when
the quantum phase coherence length of the charge carriers is comparable
to the sample dimensions. Our studies should also provide insight into
the importance of layer to layer interactions since we should be able to control
the number of layers under study. In general, this study will elucidate
the enhanced quantum mechanical effects in low-dimensional mesoscopic scale systems.
III. Electron Transport in Molecular Structures and Sensor
Applications
Single-walled carbon nanotubes are ideally suited as electrical wires to a
molecule. They are outstanding nanoscale
one-dimensional conductors (i.e., about the same size as the molecules
being probed), thus forming a switch in which all the elements are
naturally at small dimensions (this also allows for more effective
gating, as gate field screening is reduced relative to larger metal
electrodes); 2) the single crystal structure of the nanotube
makes it immune to the effects of grain migration that occurs in granular
gold films; 3) carbon nanotubes can be metallic
or semiconducting, offering different energy
band alignment scenarios with molecules; 4) carbon-based chemistry offers
greater flexibility than metals to form bonds to molecules.
Recently, we have
developed to fabricated nanometer-size gaps in SWCNTs.
Breaking local carbon-carbon bonds in a controlled environment allows us
to engineer covalent chemistry at the end of exposed reactive section of nanotubes. Initial experiments employing a conducting
molecule with synthesized amine end groups have yielded electrical
reconnection across the gap. We are presently working toward developing a
detailed understanding of the nature of the electrical response of these
SWCNT-molecule junctions. Our recent success in preparing and bridging
gaps in SWCNTs with an individual molecules
will allow for the study of a wide variety of molecules with specific end
group chemistry, thereby bringing us one step further along the road of
rationally designed molecular junctions and potential sensor applications
based on this newly developed techniques.
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