I  U  P  A  C






News & Notices
.. Prize for Young Chemists
.... 2000 winners
.... 2001 winners

Organizations & People

Standing Committees

Divisions

Projects

Reports

Publications

Symposia

AMP

Links of Interest

Search the Site

Home Page

 

 

Winner of the IUPAC Prize
for Young Chemists - 2001

 

 

Teri Wang Odom wins one of the five IUPAC Prize for Young Chemists, for her Ph.D. thesis work entitled "Electronic Properties of single-walled carbon nanotubes"

Teri Odom was also the winner of the Young Scientist Award co-sponsored by the RACI and the Australian Journal of Chemistry
> see Chem. Int. 24(3), 2002

Current address

(at the time of application)

Harvard University
Department of Chemistry and Chemical Biology
12 Oxford Street
Cambridge, MA 02138 USA

Tel: +1 617 4959436
Fax: +1 617 4952500
E-mail: [email protected]

Academic degrees

  • Ph.D. Havard University, November 2000, Chemical Physics.
  • A.M. Havard University, June 1999, Chemistry.
  • B.S. Stanford University, June 1996, Chemistry.

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.


Page last modified 8 May 2002.
Copyright ©2001-2002 International Union of Pure and Applied Chemistry.
Questions or comments about IUPAC, please contact, the Secretariat.
Questions regarding the website, please contact Web Help.