Electromagnetic and thermal actuation in van der Waals heterostructures

Abstract

Assembling two-dimensional materials in vertical stacks—van der Waals heterostructures—leads to the creation of new properties and functionality in these systems. Electromagnetic and thermal effects in van der Waals materials, particularly the use of these effects for mechanical actuation, remains largely uncharted. Therefore, exploring electromagnetic and thermal actuation in van der Waals heterostructures is the main aim of this thesis. A range of existing nano- and microfabrication techniques was optimised to improve both quality and performance of the devices made of van der Waals heterostructures. Heterostructures made of graphene encapsulated by hexagonal boron nitride (hBN) were fabricated using a modified microfabrication procedure to produce much cleaner graphene-hBN interfaces. Bright light emission from graphene induced by incandescent heating was demonstrated and characterised using multiple techniques, including Raman spectroscopy and transport measurements. The results of this study reveal that boron nitride provides a protective environment for graphene hot filaments, thereby enabling a prolonged light emission and operation at temperatures exceeding 2000 K. Then, the quality of hBN/graphene heterostructures was explored further by fabrication and characterisation of tunnelling transistors based on graphene/hBN/graphene heterostructures. In particular, the defects that exist within the hBN flakes used as a dielectric layer for graphene-based tunnelling transistors were exploited for probing the density of states in graphene electrodes. Diverging from graphene-hBN systems, the behaviour of water in a common layered mineral like gypsum was explored by adopting a simple measurement procedure. This was motivated by recent results on unusual dielectric permittivity of confined water molecules. Capacitance measurements were performed for water confined in two different systems—in nano channels fabricated by the use of 2D materials and in gypsum (CaSO4-2H2O) system. Electrostatic actuation was studied in various fabricated designs of 2D materials heterostructures including microelectromechanical systems based on graphite/hBN/graphite capacitors. Moreover, atomic force microscopy technique was used to attempt to initiate transition between ABA and ABC stacking in graphite films. It was found that AFM nanoindentation indicates insufficient capability of achieving this transition.

Date of Award	31 Dec 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Vladimir Falko (Supervisor) & Artem Mishchenko (Supervisor)