Study of the structural transition of two-dimensional metal chalcogenides

Abstract

Vanadium diselenide (VSe2), a member of the transition metal dichalcogenides family, is emerging as a promising two-dimensional candidate for electronic devices with exotic properties including charge density wave and potential ferromagnetism. The bulk crystal of VSe2 exists in a crystallographic form of the 1T phase with a metallic characteristic. In this thesis, I report the first observation of a structural phase transition of multilayer VSe2 from 1T to 2H, which occurs at about 650 K, accompanied a metal-semiconductor transition. The phase transition is verified by Raman spectra, as different polymorphs have different Raman signals related to different characteristic vibration modes. The electrical characteristics are also studied on VSe2 nanoflakes, as the phase transition occurs along with the metal-semiconductor transition, which is consistent with the electronic band structure calculation. The experimental observation also indicates that the 2H phase is more thermodynamically stable than the 1T phase in the multilayer form, which is consistent with the theoretical prediction. Besides VSe2, a layered III-VI semiconductor, gallium selenide (GaSe) in 2D form, also received attention from electronic and optoelectronic applications. In this thesis, second harmonic generation and photoluminescence measurements are carried on the bulk and few-layer GaSe. The result indicates the coexistence of β-GaSe and ε-GaSe both in bulk and few-layer forms, with different proportion. The change of peak position and intensity of photoluminescence is also consistent with the band structure calculation result.