Author: Mak, Chun Hin
Title: Synthesis of two-dimensional layered materials for electrocatalysis and photodetection applications
Advisors: Lau, S. P. (AP)
Degree: Ph.D.
Year: 2020
Subject: Thin films
Layer structure (Solids)
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Physics
Pages: xv, 133 pages : color illustrations
Language: English
Abstract: Two dimensional layered materials (2DLMs) have been intensively researched for the electrical and optoelectronic applications in the past decade. The quantum confinement allows one to change the energy band level of the 2DLMs by changing their thickness. The optoelectronic and mechanical properties of 2DLMs exhibit a large difference between the bulk and their ultrathin counterparts. Also, the large surface to volume ratio of the nanomaterials allow one to easily modify their physical and chemical properties by surface modification. Depending on the specific application, the band structure or materials morphology can be engineered. In this work, we focused on the potential application of the 2DLMs. Firstly, graphene and palladium diselenide (PdSe₂) were synthesized and optimized for the phototransistors. For graphene-based transistor, the band structure modification was done by coating the zero-dimensional material bismuth silver sulfide (AgBiS₂) nanocrystals (NCs) on the graphene channel. Secondly, vertically aligned platinum diselenide (PtSe₂) films were synthesized for hydrogen evolution reaction (HER). AgBiS₂ NCs are emerging optoelectronic materials due to their high absorption coefficient and solution-processability. We report a facile method to prepare AgBiS₂ nanocrystals in ambient conditions. The nanocrystals are of high crystallinity and without byproducts, which make them suitable for solution processable optoelectronic devices. To overcome the low responsivity of pristine graphene-based transistor, AgBiS₂ NCs were incorporated into the transistors. As a light absorber on the graphene channel, the responsivity of the phototransistors was highly enhanced by the photogating effect. The transistor was suitable for near infrared detection application. Photodetectors with a high responsivity of 10⁵ A W⁻¹ for 895 nm wavelength at a low operation voltage of 0.1 V were demonstrated.
The wafer-scale polycrystalline PdSe₂ thin films with controllable thickness by the selenization of Pd films were prepared. The PdSe₂ film with the thickness of 9.2 nm was employed to be the transistor channel. A PdSe₂ photodetector can detect a broad wavelength ranging from 420 nm to 1200 nm. The responsivity and detectivity can reach 1.96 × 10³ A W⁻¹ and 1.72 × 10¹⁰ W Hz-1/2 at VDS = 3 V, respectively. As the phototransistor is driven by the photoconductive effect, a fast response time was achieved in hundreds of milliseconds. It shows a good balance between the responsivity and response time. This work demonstrated that the polycrystalline PdSe₂ films are promising for broadband photodetection. Besides the photodetection based on the 2DLMs, 2DLMs are shown to have pronounced catalytic effect in HER activity. We synthesized the large area, vertically aligned PtSe₂ films by selenization of the platinum films. The thicknesses of the PtSe₂ films can be varied from 3.8 nm to 76 nm. The density of the vertically aligned PtSe₂ is increased with the Pt film thickness. The HER activity of the PtSe₂ can be enhanced significantly as the active edge site density increases. The maximum cathodic current density of 227 mA cm⁻² can be obtained through increasing the edge density.
Rights: All rights reserved
Access: open access

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