2H-NbSe₂ 和 1T-VSe₂ 單晶的光學研究

Abstract

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Two-dimensional layered transition metal dichalcogenides (TMDs) have potential for future optoelectronic applications due to their structural diversity and tunable electronic and optical properties. These properties are influenced by crystal symmetry, dimensionality, and interlayer interactions. This thesis investigates the vibrational and optical properties of 2H-NbSe2 and 1T-VSe2 single crystals using Raman spectroscopy and spectroscopic ellipsometry, supported by theoretical calculations.For 2H-NbSe2, we find that the first-order Raman-active A_1g and E_2g modes at 229 and 237 cm-1 are difficult to distinguish. Helicity-dependent Raman spectroscopy effectively separates these modes by leveraging phonon symmetries with circularly polarized light. Furthermore, two additional phonon modes, 〖ω'〗_b at 470 cm-1 and 〖ω'〗_a at 195 cm-1, emerge below 100 K and 10 K, respectively. Both modes are the M-point phonons and are related to the charge-density-wave (CDW) phase transition. The room-temperature optical absorption spectrum exhibits a Drude response at low energies and five prominent interband transition peaks at higher energies, primarily attributed to charge transfer between the Nb 4d and Se 4p orbitals. As the temperature decreases, both the plasma and collision frequencies decrease, exhibiting two notable anomalies: one at 32 K, corresponding to the CDW transition, and another at 7.2 K, linked to the emergence of superconductivity. Combined Raman and optical spectroscopy provide an insight into the vibrational and electronic properties of the material.For 1T-VSe2, we identify the first-order Raman-active A_1g mode at 199 cm-1. Additionally, five broad peaks are observed, which are attributed to second-order two-phonon processes. Notably, the prominent peak at 250 cm-1, previously assigned as the E_g mode, is reinterpreted as a combinational phonon mode (ω_2) resulting from a double-resonance Raman scattering process. This reassignment is supported by the first-principles phonon dispersion calculations and the two-phonon density of states. As the temperature decreases below 120 K, the ω_2 mode at 248 cm-1 splits, associated with the formation of the CDW phase. This lattice distortion caused by the CDW transition results in the appearance of new phonon modes at lower temperatures. The room-temperature optical absorption spectrum of 1T-VSe2 shows three interband transitions involving V 3d to Se 4p excitations. One occurs at the Γ point, while two higher-energy transitions at the M point likely involve bands with similar symmetry. These results reflect the complex band structure and strong V 3d–Se 4p hybridization.