Phonon dynamics of exotic materials Bi₂Se₃, Bi₂Te₃, Sb₂Te₃ and WSe₂

Akintola, Kolawole (2014) Phonon dynamics of exotic materials Bi₂Se₃, Bi₂Te₃, Sb₂Te₃ and WSe₂. Masters thesis, Memorial University of Newfoundland.

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Abstract

Raman and Brillouin spectroscopy have been employed to investigate the phonon dynamics of Bi₂Se₃, Bi₂Te₃, Sb₂Te₃ (all topological insulators) and WSe₂ (a 2D semiconductor). In the frequency range studied, two peaks were observed in the Raman spectra of Bi₂Se₃, Bi₂Te₃, Sb₂Te₃. Polarization studies were done to study the nature of these peaks and assign them to particular vibrational modes. Due to the fact that all three materials belong to the same space group and have the same Wycko↵ positions, it stands to reason that their Raman spectra will be identical in every respect except for the position where the peaks appear. This was found only to be partially true from what we observed. The presence of Te atoms in Bi₂Te₃, and Sb₂Te₃ was observed to play a prominent role in determining mode intensity. We verified that the higher intensity of the peak observed due to the A₁g(2) 1g mode compared to that due to the Eg(2) g mode in Bi₂Te₃, and Sb₂Te₃, is most probably due some resonance effect induced by the laser excitation wavelength used. Raman scattering studies of WSe₂ have generated much controversy with regards mode assignment and the shifts at with spectral peaks appear while there are no reports from Brillouin scattering studies of this material. Our Raman experiment shows that the Raman active E₂g(1) 2g and A1g modes in WSe₂ initially thought to be degenerate are not degenerate. This was verified by collecting spectra at different polarization configurations. One of the polarization configurations showed two closely separated peaks, enabling us to identify the constituent modes from the originally near-degenerate mode in the unpolarized spectrum. The velocity of the Rayleigh surface wave was extracted from the Brillouin spectra collected at different angles of incidence ranging from 35 to 80 degrees in a Brillouin light scattering experiment. The value of the velocity compares well with those previously reported for related materials. This work reports new results on the vibrational properties of Bi₂Se₃, Bi₂Te₃, Sb₂Te₃ and WSe₂. Our results also resolves the inconsistencies reported in previous studies on the vibrational properties of these materials.

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