Robin Nicholas

Abstract:

Single-walled carbon nanotubes (SWCNTs) have been dispersed with three types of amphiphilic materials in aqueous solutions: (i) an anionic aliphatic surfactant, sodium dodecyl sulfate (SDS), (ii) a cyclic lipopeptide biosurfactant, surfactin, and (iii) a water-soluble polymer, polyvinylpyrrolidone (PVP). Solution photoluminescence (PL) studies suggest that SDS wrapping is very sensitive to the temperature whereas the other two give relatively robust wrapping around the SWCNTs. Low temperature PL spectra from both surfactin and PVP dispersed SWCNTs show that peak shifts are strongly dependent on their chiralities, which can be explained by the bandgap modulations with the environmental strain. The uniaxial and torsional strains deduced from their bandgap shifts were similar, indicating the strains are mainly due to the different rates of thermal expansion in SWCNTs and ice.

Abstract:

We report an efficient technique to separate ferromagnetic catalyst particles from an aqueous surfactant solution of single-walled carbon nanotubes (SWNTs) by the use of a 1.3 T permanent magnet. High resolution transmission electron microscopy (HRTEM) demonstrates that SWNTs are coated with a surfactant layer that stabilises the aqueous dispersions of SWNTs. The residual quantities of Fe catalyst (∼3%) can be effectively removed from a colloid solution of SWNTs in a magnetic field while absorbance spectra of the initial and purified solutions show that the nanotube diameter distribution remains unchanged. © 2005 Elsevier Ltd. All rights reserved.

Abstract:

Strain-induced band gap shifts that depend strongly on the chiral angle have been observed by optical spectroscopy in polyvinylpyrrolidone(PVP)- or Surfactin micelle-wrapped single-walled carbon nanotubes (SWCNTs). Uniaxial and torsional strains are generated by changing the environment surrounding the SWCNTs, using the surrounding D2O ice temperature or the hydration state of a wrapping polymer. These methods can be used as diagnostic tools to determine the quantum number, q, and examine chiral vector indices for specific nanotube species.

Abstract:

We investigate vertical magnetotransport measurements in strongly coupled InAs/GaSb superlattices within the miniband transport regime. Sample magnetoresistance curves display oscillations due to the successive un-nesting of Landau level minibands. This qualitative picture is strongly supported by Monte Carlo simulations, which include acoustic and optic phonon scattering as well as Umklapp processes, providing a semi-classical description of the miniband transport.