Ph.D. Thesis
Title Electronic Properties of single-walled
carbon nanotubes
Adviser Professor Charles M. Lieber
Thesis Committee X. Sunney Xie, Chemistry; and Michael Tinkham,
Physics, Havard University.
Essay
Carbon nanotubes are cylindrical, extended-fullerene
structures that are currently the focus of intense interest worldwide.
This attention to carbon nanotubes is not surprising in light of their
promise to exhibit unique physical properties that could impact broad
areas of science and technology, ranging from super strong composites
to nanoelectronics. Experimental studies have shown that carbon nanotubes
are the stiffest known material and buckle elastically (versus fracture)
under large bending or compressive strains. These mechanical characteristics
demonstrate clearly that nanotubes may have significant impact on
advanced composites. The remarkable electronic properties of carbon
nanotubes offer the greatest intellectual challenges and potential
for novel applications. Theoretical calculations first predicted that
single-walled carbon nanotubes (SWNTs) could exhibit either metallic
or semiconducting behavior depending only on diameter and helicity.
This ability to display fundamentally distinct electronic properties
within an all-carbon, sp2 -hybridized lattice, without
changing the local bonding, sets nanotubes apart from all other nanowire
materials.
My dissertation focused on experimental investigations
of these unique electronic properties using scanning tunneling microscopy
(STM). STM and tunneling spectroscopy are ideal techniques to probe
predictions about SWNT electronic properties, as well as to investigate
how these properties are affected by local perturbations, due to their
ability to resolve simultaneously atomic structure and measure electronic
properties of materials.
Topographic STM images of these samples revealed a rich
variety of SWNT structures, with diameters ranging from 0.8 to 1.4
nm and chiral angles ranging from 0� to 30�. The ability to characterize
the electronic properties of the atomically resolved tubes by tunneling
spectroscopy enabled us to determine whether the electronic properties
depend on structure. Indeed, we observed two classes of electronic
behavior, metallic and semiconducting, which can be correlated in
detail with specific diameter and angle measurements. In addition,
we found that independent of chiral angle, the energy gaps of semiconducting
nanotubes (Eave = 0.7 eV) were inversely proportional to
their diameter, in agreement with theoretical predictions. The characterization
of semiconducting and metallic SWNTs with subtle changes in structure
represented a significant step forward in understanding these 1D nanostructures.
Moreover, by extending the energy range of our tunneling spectroscopic
measurements (� 2 eV), we observed sharp singularities that are due
to the 1D nanotube band structure. The details of these singularities
(e.g. peak energy position and relative peak spacing) were found to
depend explicitly on whether the nanotube was semiconducting or metallic,
and we were able to compare our experimental findings with tight-binding
calculations on specific structures that were determined from STM
images.
Besides investigating the intrinsic electronic properties
of perfect SWNTs, we also characterized structural defects, namely
mechanical bends and capped ends. How structural deformations impact
the electronic nature of individual nanotubes is crucial to understanding
increased chemical reactivity at the bend areas and field-emission
properties from the ends, respectively. We found low energy peaks
in the spectroscopy (0.2, 0.5 eV) due to the presence of the bend.
In addition, we characterized spectroscopically capped ends and found
new features in the density of states near the tube end. Tight-binding
calculations of a proposed structural model suggested that these features
arose from the specific arrangement of carbon atoms that close the
end.
The tip in an STM experiment usually functions as a
non-invasive probe of the local electronic density of the surface.
We exploited the STM tip, however, to manipulate controllably the
nanotube length in order to interrogate their electronic properties
as a function of finite length. We discovered that reducing the nanotube
length in metallic nanotubes resulted in equally spaced peaks in the
tunneling spectra, which correspond to discrete electron energy levels,
and whose spacing scaled as 1/length. In contrast, we found that reducing
the length of semiconducting nanotubes produced no effect on their
electronic properties down to 5 nm. In other cases, the current-voltage
(I-V) curves of 5-6 nm long segments exhibited a zero current
region around V = 0 and irregularly spaced steps at higher
current levels. These characteristics were attributed to the interplay
of electron energy levels and single-electron tunneling effects, and
we fit the data to a modified orthodox coulomb blockade theory.
The final section of my thesis focused on the effects
of external perturbations on the carbon nanotube system. We chose
to decorate the nanotubes with magnetic impurities, motivated by the
lack of understanding between such impurities and low-dimensional
electron systems. The interaction between a magnetic moment of an
atom with the conduction electrons of its non-magnetic host is traditionally
known as the Kondo effect. We created an analogous magnetic nanostructure
by thermally evaporating Co clusters on SWNT samples. Tunneling spectroscopy
measured above the Co on metallic nanotubes revealed a narrow peak
(with width d = 8 meV) near zero bias,
which suggests strongly the presence of a Kondo resonance. When Co
clusters were situated on semiconducting nanotubes, no peaks were
observed in the tunneling spectra. This observation suggests that
the peak feature near zero bias is not due to the bare Co d-orbital
resonance and emphasizes the necessity of conduction electrons in
the host needed to interact with the magnetic cluster in order to
observe the Kondo resonance. Furthermore, spatially resolved measurements
indicated that the Kondo effect due to the magnetic impurities disappeared
after 2 nm. We also created the ultimate magnetic nanostructure�Co
on finite-sized nanotubes�and found that spectroscopic measurements
over the Co exhibited enhanced conductance at zero bias as well as
higher order peaks due to finite size.
In summary, my thesis characterized in detail the electronic
properties of SWNTs. These studies have confirmed theoretical predictions
about the unique electronic behavior of nanotubes, tested fundamental
physics ideas in one-dimension, uncovered new phenomena in low-dimensional
systems, and contributed to an understanding of how nanotubes may
be exploited in future technological applications, such as molecule-based
electronics.
