Robin Nicholas

Abstract:

Solubilizing and purifying carbon nanotubes remains one of the foremost technological hurdles in their investigation and application. We report a dramatic improvement in the preparation of single-walled carbon nanotube solutions based on the ability of specific aromatic polymers to efficiently disperse certain nanotube species with a high degree of selectivity. Evidence of this is provided by optical absorbance and photoluminescence excitation spectra, which show suspensions corresponding to up to approximately 60% relative concentration of a single species of isolated nanotubes with fluorescence quantum yields of up to 1.5%. Different polymers show the ability to discriminate between nanotube species in terms of either diameter or chiral angle. Modelling suggests that rigid-backbone polymers form ordered molecular structures surrounding the nanotubes with n-fold symmetry determined by the tube diameter.

Abstract:

The authors report photoluminescence (PL) measurement on a series of SiSiGe quantum-well structures that had different internal strain distributions. When each sample was placed in a high magnetic field, the field-dependent energy shift of the relevant PL peaks revealed either type-I or type-II exciton formation depending on the strain distribution. This observation is in agreement with theoretical modeling. The present investigation shows that type-I band alignment-desired for electroluminescent devices-can be achieved by strain engineering. © 2007 American Institute of Physics.

Abstract:

We report a detailed study of the magnetophotoluminescence of single-wall carbon nanotubes at various temperatures in fields up to 58 T. We give direct experimental evidence of the diameter dependence of the Aharanov-Bohm phase-induced band gap shifts. Large increases in intensity are produced by the magnetic field at low temperatures which are also significantly chiral index [(n,m)] dependent. These increases are attributed to the magnetic field induced mixing of the wave functions of the exciton states. A study of the emission from nanotubes aligned perpendicular to the applied magnetic field shows even larger field-induced photoluminescence intensity enhancements and unexpectedly large redshifts in band gap energies, not predicted theoretically. © 2007 The American Physical Society.

Abstract:

Photoluminescence as a function of temperature and magnetic field from single walled carbon nanotube solutions is described. This is modelled assuming that it is dominated by the small energy splitting between the dark and bright states of the singlet excitons which are found to be in the region of 1-5 meV for nanotubes of 0.8-1.2nm. The emission energies show a large red-shift due to the introduction of an Aharanov-Bohm phase by magnetic field along the tube axis and the luminescence intensity is strongly enhanced at low temperatures due to the mixing of the different valley states of the excitons. © World Scientific Publishing Company.