Recent Research activities in the Dresselhaus group that have attracted
wide attention are in the areas of carbon nanotubes, bismuth nanowires and low
dimensional thermoelectricty. Regarding carbon nanotubes, which were previously
predicted to be either semiconducting or metallic depending on their geometries,
we have been studying the dependence of their Raman spectra as a function of
laser excitation energy and were in this way able to identify a characteristic
difference between the spectra for metallic and semiconducting nanotubes.
By attributing the difference in their spectra to a resonant
process between the incident (and scattered) laser photon energy with interband
transitions between singularities in the one dimensional density of electronic
states in the valence and conduction bands, we have been able to use this
principle to excite either metallic or semiconducting tubes selectively
(within a sample containing both semiconducting and metallic tubes), using both
the Stokes and anti-Stokes excitation mechanisms. We have also carried out
surface enhanced Raman scattering (SERS) studies using carbon nanotubes
adsorbed with nanostructured silver (20 nm) particles. This technique allowed
enhancement factors of many orders of magnitude, thereby making it possible
to observe Raman spectra for presumably a single rope of nanotubes.
We have devised a way to prepare arrays of aligned bismuth nanowires
down to 7 nm diameter (embedded in an anodic alumina template), 50-100
microns in length, with a wire density of ~ 1011/cm2, with
their wire axes along a common crystalline orientation, and preserving
the crystal structure of bulk bismuth. We previously predicted a semimetal-
semiconductor transition as a function of nanowire diameter due to quantum
confinement effects, and we have now succeeded in observing this effect.
We are now developing techniques to study the transport and optical properties
of the nanowire arrays, and have recently succeeded in the measurement of the
resistance of a single quantum wire 70 nm in diameter using a 4-probe method.
Progress has been made with the demonstration of enhanced thermoelectricity
performance in low-dimensional systems such as Si/Ge multiquantum well
superlattices and bismuth nanowires.