Electronic and optoelectronic devices using grown 2-dimensional materials

Thesis

Two-dimensional (2D) materials are a category of layered materials with atomic-level thickness. This thesis focuses on electronic and optoelectronic devices with both vertical and lateral configurations by using grown 2D materials, including graphene, tungsten disulfide (WS2), hexagonal boron nitride (h-BN), and platinum diselenide (PtSe2). Most of the 2D materials used in the project were synthesized by chemical vapor deposition (CVD).

Chapter 4 demonstrates an asymmetrical vertical structure of graphene/WS2/h-BN/graphene (GrB/WS2/h-BN/GrT) using all CVD-grown 2D materials. The device arrays were fabricated by vertically stacking graphene electrodes, h-BN continuous film, and WS2 single-crystal domains via DI water/ IPA-based wet transfer techniques. The photovoltaic effect of the asymmetrical vertical structure is around 7 times improved than that of the symmetrical structure without the h-BN layer. By changing the sequence of the h-BN layer in the vertical stack, the electron flow direction can be delicately controlled.

Chapter 5 focuses on high-performance electroluminescence (EL) devices with a vertical heterostructure of Gr/h-BN/WS2/h-BN/Gr by using all CVD-grown 2D materials. Long-lived persistent EL is demonstrated for more than 2 hours without significant degradation of the WS2. In the cycling test, the EL signal peak position and the intensity stay almost the same after several ON/OFF cycles under high bias, demonstrating good stability and durability when pulsed. Further investigation shows that the limiting factor for EL devices is not the degradation of the WS2 but the electroburning of the topmost graphene electrode exposed to the air.

Chapter 6 explores a photodetector with a lateral structure of PtSe2-WS2-PtSe2, where semimetal multi-layered PtSe2 thin film acts as electrodes and monolayer WS2 acts as the photoactive material. PtSe2 thin film was synthesized by thermally assisted selenization. Direct laser patterning was applied for the device fabrication. As-fabricated devices exhibit a satisfactory ON/OFF ratio and fair photoresponse. A comparison study shows that the device with shorter channel width has better photoresponsivity. The back-to-back Schottky diodes model well estimates the barrier height of PtSe2-WS2 heterojunction. The device exhibits the lowering of barrier height with the increasing laser power, arising from the photogating effect.

Authors: Hou, L

• DOI: 10.5287/ora-v0dagy95d
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: 2d materials; optoelectronics; nanofabrication
• Subjects: Materials