INVERSION SYMMETRY BREAKING IN MoTe2 PROBED BY SECOND HARMONIC GENERATION

摘要

While two-dimensional electron gases and heterostructures have been studied for decades, the discovery of two dimensional crystals has led to a new era of low-dimensional materials research. These materials are being explored for flexible electronics, photovoltaics, and chemical sensors [1, 2]. One of these materials is molybdenum ditelluride (MoTe2), which is of particular interest due to the phase transitions between metallic (1T or p-MoTe2) and semiconducting (2H or a-MoTe2) polymorphs with strong spin-orbit coupling, a near-infrared bandgap for 2H phase, and strong magnetic properties for 1T' phase [3-5]. In order to utilize the unique properties of these materials, a variety of characterization techniques are required. Recently second harmonic generation (SHG) has been used to study two dimensional semiconductors [6-9]. Due to the sensitivity of nonlinear processes to crystal symmetry, SHG provides an optical method for determining the crystal orientation and symmetry of flakes [6-9]. We used polarized SHG images to study the symmetry properties of 1T' MoTe2 flakes. While bulk and monolayer 1T' MoTe2 is inversion symmetric, this symmetry is broken for bilayer flakes and strong SHG is observed.