陰離子空缺對於層狀多晶與單晶1T-TiSe2-d的能帶與侷域結構之影響

Abstract

無中文摘要

A systematic study of 1T-TiSe2 polycrystalline and single crystalline with controlled Se deficiency level indicates that a significant Se loss could be responsible for the controversial charge density wave (CDW) phase and on whether the nominal 1T-TiSe2 should be categorized as a semiconductor or a semimetal at room temperature. In the polycrystalline form, the second order CDW phase transition near ~200 K is found to be most pronounced in samples with δ ~0.12, corresponding to about one Se atom missing per eight formula units in average, which is incommensurate to the hexagonal symmetry and naturally leads to the charge ordering of 2a × 2a × 2c superlattice via exciton-phonon coupling. The anomalous resistivity ρ(T) peak between 100 and 200 K indicates not only resistivity increase due to charge ordering, but also a concomitant p- to n- carrier type change. An interpretation using band model for an extrinsic p-type semiconductor with an impurity band (IB) in proximity to the valence band (VB) is proposed to explain the evolution of Se vacancy level and electronic structure change for 1T-TiSe2-δ, from the low doping bound (δ ~0.08) of semiconducting behavior to the heavily doped (δ ~0.17) dirty semiconductor showing metallic-like n-type conduction. Supporting experimental evidences for the Se vacancy existence are provided by the integrated chemical and physical property analyses, including electron probe microanalysis (EPMA), Hall coefficient, and magnetic susceptibility. In single crystal form, the Se vacancy and Ti-intercalation are dominant near the crystal surface as explored by the scanning tunneling microscopy (STM). The Se vacancy level is found reduced on the crystal surface after prolonged annealing at high temperature, but the intercalated Ti level grows, which implies the occurrence of local re-structuring near the Se vacancy sites. Room temperature Raman scattering spectrum shows a red shift of A1g phonon mode and a blue shift of Eg phonon mode after the long time high temperature post-annealing. The high temperature post-annealing procedure has different impact on polycrystalline and single crystal samples, while samples of small grain size (~10-30 μm) have dominant Se deficiency in equilibrium, samples of large grain size (≳ 1 mm) shows significant amount of Ti-intercalation. This is most likely due to the different level of local re-structuring near the Se vacancy sites.

Keywords: Transition metal dichalcogenides (TMDCs), Charge density wave (CDW), Excitonic insulator, Semiconductor.