Optoelectronic devices based on van der Waals heterostructures

Abstract

In this thesis we investigate the use of van der Waals heterostructures in optoelec- tronic devices. An improvement in the optical and electronic performance of specific devices can be made by combining two or more atomically thin materials in layered structures. We demonstrate a heterostructure photodetector formed by combining graphene with tungsten disulphide. These photodetectors were found to be highly sensitive to light due to a gain mechanism that produced over a million electrons per photon. This arises from the favourable electrical properties of graphene and the strong light-matter interaction in WS2 . An analysis of the photodetector per- formance shows that these devices are capable of detecting light under moonlight illuminations levels at video-frame-rate speeds with applications in night vision ima- ging envisaged. We also report a novel method for the direct laser writing of a high-k dielectric embedded inside a van der Waals heterostructure. Such structures were shown to be capable of both light-detection and light-emission within the same de- vice architecture, paving the way for future multifunctional optoelectronic devices. Finally we address a more fundamental problem in the properties of aligned grap- hene/hBN heterostructures. Strain distributions are shown to modify the electronic properties of graphene due to a change in the interlayer interaction. We demon- strates a method to engineer these strain patterns by contact geometry design and thermal annealing strategies.