Atomic and electronic properties of 2D moire interfaces

Abstract

With a wide variety of 2-Dimensional (2D) metals, semiconductors and insulators available there has been significant interest in creating novel optoelectronic devices that can surpass the performance of traditional state-of-the-art electronics. More recently, a new degree of freedom in the meta-material design has been achieved by vertically stacking 2D crystals with a slight rotational misalignment. The work presented in this thesis details the fabrication of twisted bilayers of transition metal dichalcogenides (TMDs), with rotational alignment close to 0 or 180 degrees depending on the desired polytype (3R and 2H, respectively), to study the effects of lattice reconstruction on their electronic properties. The two main projects in this thesis focus firstly on lattice reconstruction at the atomic scale and secondly on the electrical properties induced by lattice reconstruction. To demonstrate lattice reconstruction at the atomic scale, scanning transmission electron microscopy (STEM) was employed. Our observations were combined with density functional theory (DFT) and multiscale calculations to determine the angle at which the transition from rigid to reconstructed lattices occurs and describe the atomic structure of the reconstructed lattices. The electronic properties of reconstructed TMDs were studied using scanning probe microscopy (SPM) techniques such as conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM), the latter of which revealed out-of-plane ferroelectricity where the oppositely polarised states were spatially confined to the triangular domains observed in 3R-polytypes but not in the 2H-polytype. Then, scanning electron microscopy (SEM) was used to study domain boundary dynamics as a function of an out-of-plane displacement to elucidate the ferroelectric switching phenomena. Finally, electrical transport measurements were performed to demonstrate such behaviour in prototype ferroelectric devices with a tunnelling junction geometry. Electronic SPM imaging techniques were employed prior to electrically contacting them.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andre Geim (Supervisor) & Roman Gorbachev (Supervisor)