While two-dimensional electron gases and heterostructures have been studied for decades, the discovery of two dimensional (2D) crystals has led to a new era of low-dimensional materials research. One example is molybdenum ditelluride (MoTe2), which is of particular interest due to the phase transitions between metallic (1T' or β-MoTe2) and semiconducting (2H or α-MoTe2) polymorphs. In order to utilize the unique properties of these materials, a variety of characterization techniques are required. One such technique is second harmonic generation (SHG), which provides optical information about the crystal symmetry and orientation. 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. The 1T' MoTe2 single-crystalline platelets were grown using an iodine-assisted chemical vapor transport method and subsequently exfoliated onto a SiO2/Si substrate. Figure 1(a) a topographic image of a MoTe2 flake with the single layer (1L) and bilayer (2L) regions indicated. A strong SHG signal is observed for the bilayer region and no SHG signal from the single layer region. Figure 1(c) shows the SHG polarization dependence with the excitation polarization and collection analyzer parallel (black) and crossed (red). These measurements demonstrate that SHG is excellent for rapidly determining the crystal phase and orientation of a flake as well as probing the inversion symmetry of monolayer and multilayer 2D materials.

Inversion symmetry breaking in MoTe2 probed by second harmonic generation

252nd National Meeting (2016)

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