Understanding layer-by-layer anisotropy in Transition Metal Dichalcogenides (TMDs) with Scanning Probe Microscopy

Abstract

The field of two-dimensional (2D) materials became very popular amongst scientists and engineers alike, after the discovery of graphene, showing promising physical, electrical, and optical properties. Since the advent of graphene, the research in the field of 2D materials caught up pace to understand the origin of these fascinating properties at the nanoscale. Different types of properties have been and are currently being studied by several research groups. Our choice of 2D materials to study the electrical properties, atomic structure, surface morphology, and layer-dependent vibration modes is ReSe2 and MoS2 2D materials using STM, AFM, and Raman Spectroscopy. Different sample preparation techniques proved the use of PDMS stamp to be the best substrate to exfoliate a few layered sample of ReSe2 and MoS2. The characterization of samples with the help of AFM confirmed the presence of one to five layers in the sample. Raman spectroscopy measurements were also carried out to identify the number of layers in the ReSe2 and MoS2 few layered samples by analyzing the shift in Raman peaks. STM studies of ReSe2 reveal the distorted chain-like atomic structure and its lattice parameters along with voltage dependent features. Scanning tunneling spectroscopy (STS) measurements were carried out to determine the band gap of a few layered samples. STM measurements of ReSe2 bulk flakes didn't show promising results due to high resistance in the range of a few MΩ, however the band gap information showed that a few layers of ReSe2 act as semiconductor as opposed to the bulk sample which is essentially insulating in nature. Bulk MoS2 was also explored using STM & STS. Our investigation points towards interesting anisotropic physical phenomena in ReSe2 and need for more detailed experimentation.