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Title: Electronic and optoelectronic devices based on 2-dimensional heterostructures
Author: Huang, Hefu
ISNI:       0000 0004 8507 3574
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2020
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Two-dimensional materials, including graphene, h-BN, and monolayer transition metal dichalcogenides such as MoS2 and WS2, have attracted enormous attention due to their unique properties and potential application in electronics. A variety of heterostructures devices can be made based on 2D materials. In this thesis, we investigate the electronic and optoelectronic properties of heterostructures of 2D crystals grown by chemical vapor deposition. In our study on graphene-MoS2 heterojunction, we show that a rectification ratio over 104 can be achieved in Au-MoS2-graphene device. The rectifying behavior can be tuned by external gating and photo-irradiation. The decrease of rectification ratio under irradiation can be explained by the quick drop of Au-MoS2 Schottky barrier height, which is derived by fitting the output characteristics of Au-MoS2-Au and Gr-MoS2-Gr device as a metal-semiconductor-metal transistor. The thickness of 2D materials is found to have a great impact on the properties of WS2-graphene heterostructures. Heterojunction based on graphene and bilayer WS2 have much higher conductivity and photoresponsivity compared to monolayer WS2-graphene device. A greater than unity photogain is measured for both devices. The layer-dependent conductivity and photoresponsivity are due to different electron affinities and band gaps of monolayer and bilayer WS2. The fabrication methods of vertical heterostructures are also explored, including electron-beam lithography, wet transfer of pre-patterned materials, and graphene printing technique. Using graphene printing, a transfer resolution of 10 mm can be achieved, which is promising to be used in both lateral and vertical device making.
Supervisor: Warner, Jamie H. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available