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Title: Optical properties of 2D transition metal dichalcogenides enhanced by plasmonic nanostructures
Author: Wang, Zhuo
ISNI:       0000 0004 8504 5151
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2017
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As a typical two dimensional (2D) materials system, transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe2) and molybdenum disulfide (MoS2) have been demonstrated to be potentially useful building blocks for optoelectronic integrated circuits. However, being atomically thin limits light-matter interaction, resulting in low light-to-light or light-to-electron conversion efficiency. Hence, enhancing their light emission or photoelectron extraction efficiency are critical for integrating these materials in devices. The first part of this thesis focuses on enhancing the linear light absorption and emission of WSe2 by placement on sub-20-nm-wide gold trenches. We report a giant photoluminescence (PL) with an enhancement factor of 20,000 from monolayer WSe2 over the trenches, due to enhanced absorption and Purcell factor by lateral gap plasmons confined in the trenches. This work paves a way towards plasmon-enhanced nonlinear optical properties, photodetectors and sensors of TMDCs. The second part reports enhanced second harmonic generation (SHG) at 400 nm from monolayer WSe2 over the trenches, owing to the intensification of local fields at the excitation wavelength induced by gap plasmons in the trenches. Furthermore, the trenches aligned in a single direction allows for dynamic tuning of the SHG amplitude from WSe2 by selective excitation of the gap plasmon. This work demonstrates a practical approach for optimizing the nonlinear optical properties of TMDCs, which can be utilized to configure a tunable nonlinear light emitter. Chapter 5 investigates the hot-carrier relaxation and extraction mechanisms in MoS2, revealing the photo-electron conversion processes. We report a slower hot-carrier cooling for C-excitons in MoS2 owing to favorable band alignment and transient excited-state Coulomb environment and demonstrate a high hot-carrier extraction efficiency (80%) in MoS2 monolayer/graphene heterostructures. This work provides experimental and theoretical physics insights for the photo-electron conversion process in TMDCs.
Supervisor: Wee, Andrew T. S. ; Maier, Stefan A. Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral