Robin Nicholas

Abstract:

Miniband charge transport was investigated by Monte Carlo simulations of electronic motion in short period superlattices of type-II energy band alignment (InAs/GaSb composition). The strong decrease of the miniband energy width when the electronic in-plane energy increases, characteristic of type-II superlattices, leads to a conductivity that is very sensitive to a magnetic field applied parallel to the axis of the superlattice. For structures with a miniband energy width greater than the optic phonon energy, the differential conductance can be changed from positive to negative by the magnetic field, due to the suppression of optic phonon emission.

Abstract:

The first FIRM-PL measurements in InSb/GaSb quantum dots have been performed. At low power densities the FIR absorption causes a transfer of carriers between dots and effectively cools the system, preferentially populating large, low energy dots and increasing the photoluminescence (PL) intensity, At higher powers the carrier temperature increases and the PL intensity falls. The spectral dependence of the FIRM-PL signal measures the energy spectrum of the quantum dots, showing a peak at 14.5 meV corresponding to transitions between the first two energy levels of the quantum dot distribution. (C) 2003 Elsevier B.V. All rights reserved.

Abstract:

We present magnetotransport results for strongly coupled InAs/GaSb superlattices. The low effective mass of carriers in the InAs/GaSb superlattice allows the study of the transport processes at fields above the magnetophonon condition, where the cyclotron spacing homega(c) is greater than the LO phonon energy homega(LO). Magnetotransport results display Stark Cyclotron resonance and the Magnetophonon effect. Resonant tunnelling features change dramatically above the magnetophonon condition and suggest the superlattice undergoes formation of ordered electric field domain states linked to the observation of LO phonon mediated tunnelling. (C) 2003 Elsevier B.V. All rights reserved.

Abstract:

We report a comparative study on gas and liquid phase oxidative treatments of SWNTs, using a HiPCO produced raw material with a broad distribution of nanotube diameters. The effects of these two processes were correlated using absorption and Raman spectroscopy (radial breathing and G modes) and TEM. We conclude that liquid phase oxidation is a continuous diameter-selective process eliminating narrower SWNTs through oxidizing sidewalls of SWNTs. Gas phase oxidation is a more discrete process requiring high activation energy for diameter-selective purification. Unlike liquid phase oxidation the gas phase process preferentially oxidizes SWNTs without introducing sidewall defects. © 2003 Elsevier B.V. All rights reserved.