Shaopeng Feng Qualifying Exam

Abstract: Moiré superlattices of twisted 2D van der Waals (vdW) layers are emerging as promising platforms for investigating novel electronic states causing Mott insulating states or superconductivity, due to electronic correlations by manipulating the twisted angles. However, characterizing and visualizing the local structure of moiré superlattices are still limited or require complex equipment such as transmission electron microscopy and scanning tunneling microscope, which need special sample preparation and low throughput, which far impedes progress in the field. Besides, using electrodes for transport measurements might not show the intrinsic properties of moiré superlattice due to the possible inhomogeneity. Recently, interfacial ferroelectric domains, regions exhibiting reversible spontaneous polarization, have been found in moiré superlattices such as twisted hexagonal boron nitride (h-BN) and transition-metal dichalcogenides (TMDC) such as WSe2, which enable the new ways of visualizing using PFM. Besides, microwave impedance microscopy (MIM) is a scanning probe technique, which is able to visualize local conductivity or permittivity of the material in a reasonable resolution. Twisted bilayer WSe2 are fabricated and characterize the polarized domains using scanning probe microscopy including piezoresponse force microscopy (PFM) and microwave impedance microscopy (MIM). Low-noise microwave impedance microscopy (MIM) in tapping mode under ambient conditions at room temperature were used to visualize the moiré superlattices. The polarized domains characterized in this study offer an alternative method for visualizing moiré superlattices and investigating local intrinsic properties, which could enhance the development of electronic devices.