Vibrational and electronic excitations of two atom diameter mercury telluride nanowires studied by resonance Raman spectroscopy

Abstract

This thesis presents the results from a temperature dependant resonance Raman spectroscopy study of HgTe extreme nanowires embedded within SWCNTs. Extreme nanowires are nanowires at the absolute limits of the nanoscale, in this case, just 1-2 atoms in diameter.This work demonstrates that due to the effect of quantum confinement on the electronic wavefunction and the reordering of atoms creating a new allotrope of HgTe never previously measured, new physics of 1D materials is observed. In this body of work we perform resonance Raman spectroscopy experiments with excitation photon energies of 1.65eV to 1.90eV and sample temperatures between 4-300K on an ensemble of HgTe filled single-walled carbon nanotubes. The Raman spectra are analysed and show that 1D HgTe within a SWCNT exhibits new Raman peaks not associated with modification of the bulk material. We couple the Raman results with HRTEM and utilise symmetry arguments to propose two of the fundamental vibrational modes at 47cm^-1 and 52cm^-1 are associated with vibrations with B_g and A_g symmetry respectively. Most strikingly, our results indicate a decrease in the rate of Raman scattering as a function of increasing temperature, not clearly in-line with the expected behaviour of Raman scattering due to an increase in the thermal phonon population. Through detailed analysis of temperature dependent resonance Raman data we can understand this effect in terms of broadening of the linewidth of the optical transition. We set out the evidence that this result can be understood by a model in which the resonance’s broadening is dominated by the coherence lifetime broadening. This allows us to determine the coherence lifetime of the underlying optical transition: 9fs at 295K and 18fs at 50K. The results are compared with similar results on carbon nanotubes which suggests that the optical transitions responsible for the Raman resonances are excitonic and is likely general to small diameter nanowires. A review of existing and comparable Raman measurements on such nanowires is presented and the implications of the main results in this thesis are discussed in terms of a general interest to the wider physics community.

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