Novel Optoelectronic Devices Based on 2D Van der Waals Heterostructures

Abstract

This thesis presents an analysis of the fundamental properties of certain two dimensional materials, including graphene, hexagonal boron nitride, transition metal dichalcogenide, and recently emerging chromium triiodide. Following that, two dimensional materials isolation and device fabrication tehniques are explored. Various van der Waals heterostructure assembly methods are described as well as their advantages and disadvantages. With these assembly methods and nanofabrication techniques, we demonstrate two novel devices based on the van der Waals heterostructures.

Firstly, we primarily investigate an electrically-driven and site-controlled single photon emitting device. The device incorporates gold nano-pillars into a van der Waals heterostructure which consists of metallic graphene, insulating hexagonal boron nitride, and semiconducting monolayer tungsten diselenide. The presence of gold nano-pillars creates local strain within monolayer tungsten diselenide, inducing defect-related localized excitons that can be electrically pumped, which result in the single photon emission. Furthermore, through control of the hexagonal boron nitride thickness, the energy of these site-controlled localized excitons can be electrically controlled, with the quantum confined Stark shift of up to 40 meV. This work represents an important step for the integration of solid single photon emitters into current quantum photonic technologies.

The second focus of the thesis is the all optical switching of magnetization in two-dimensional magnets. We achieve this by fabricating a van der Waals heterostructure that combines monolayer tungsten diselenide with ferromagnetic fewlayer chromium triiodide. The magnetization of chromium triiodide in the van der Waals heterostructure can be toggled by pumping with multiple femtosecond laser 1pulses of either circular or linear polarization. The underpinning mechanism is ascribed to the spin-dependent charge transfer from monolayer tungsten diselenide to chromium triiodide. This work provides a new and ultrafast approach to manipulate the magnetic order in the two dimensional magnets.