Fabrication and Characterisation of van der Waals Heterostructures

Abstract

The properties displayed by the large family of layered materials is vast, and can range from semiconducting (MoS2) to superconducting (NbSe2) and ferromagnetic (CrBr3). Remarkably, the properties of these materials are often dramatically changed when they are exfoliated down to a single atomic layer, where quantum confinement can play a significant role. However, many of these materials are highly reactive towards oxygen and water present in the atmosphere. In bulk, a passivation layer usually forms which protects the interior of the material from further corrosion (the same mechanism which protects stainless steel). However, if we want to study pristine atomically-thin layers we need to develop protocols for handling these air-sensitive materials in a controlled atmosphere. In the first half of this thesis, we use a state-of-the-art system for the handling and fabrication of the air-sensitive layered superconductor NbSe2 and layered ferromagnet CrBr3 into heterostructures and investigate their properties at the two-dimensional limit. In the latter half of this thesis we study graphene/hexagonal boron nitride heterostructures using two newly developed scanning probe techniques. Here, not only did we require high-mobility samples but also large contamination free areas. Additionally, the surface of these structures is devoid of any postfabrication residues. We use a scanning single-electron transistor to map the two-dimensional electron flow, and by tuning the electron carrier density we are able to observe the shift from diffusive to ballistic transport with high sensitivity and minimal invasiveness. Also, we use a scanning superconducting quantum interference device as an ultra-sensitive thermometer to elucidate the heat dissipation mechanisms in high-mobility graphene/hexagonal boron nitride samples. We observe, for the first time, resonant inelastic scattering between electrons and localised atomic defects.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Andre Geim (Supervisor) & Irina Grigorieva (Supervisor)