Author: Ng, Ngai Hang
Title: Thin film transition metal chalcogenide based gas sensors
Advisors: Ong, C. W. (AP)
Zhang, Xuming (AP)
Degree: M.Phil.
Year: 2018
Subject: Hong Kong Polytechnic University -- Dissertations
Transition metal compounds
Gas detectors
Department: Department of Applied Physics
Pages: xvii, 122 pages : color illustrations
Language: English
Abstract: Dynamic gas sensing mechanism of nanocluster-assembled ZnSe film under UVillumination Dynamic gas sensing mechanism was studied using the sensing result of oxygen on nanocluster-assembled ZnSe film. The film was fabricated using Supersonic Cluster Beam Deposition (SCBD) method followed by post-selenization. It was characterized by Raman spectroscopy, UV-Vis spectroscopy and XRD. AFM images of nanocluster-assembled ZnSe film show a porous morphology which plays critical role in gas sensing. The time-dependent conductivity σ ph andgas sensor response S to oxygen at various concentrations were measured at different operation temperature. The sensor responses measured under UV illumination are varies from 306.2 to 0.3 when oxygen concentration increases from 0.01 % to 20 % at room temperature and from 177.4 to 1.7 when oxygen concentration increases from 0.02% to 10% at 80 °C. Results are well fitted to the proposed model derived by the sum of two exponential time-dependent terms. This model incorporated the mechanisms of photo generation and recombination of electron-hole pairs, adsorption and desorption of oxygen, and ionization of adsorbed oxygen. The rate constants and coefficients used in the model revealed the gas sensing details about the change of the relative contribution of individual mechanisms, which vary with oxygen concentration and temperature. More precise of sensing response could be predicted according to the proposed model.
Gas sensing properties of nanocluster-assembled, multilayer and monolayer MoS₂ films Gas sensing properties of different structures of MoS₂ were studied. Large-area monolayer MoS₂ film was successfully produced using a one-step reactive DC magnetron sputtering. The crystalline and uniform monolayer film was obtained from the processes using a reactant gas containing 9 % H₂S in Ar, a substrate temperature of 500 °C with 3 °C/min ramping down to room temperature, a sputtering pressure of 15mTorr and deposition time of 25 min. Achievement of the desired MoS₂ phase was confirmed by Raman spectroscopy and XRD analysis. Indirect-to-direct bandgap transition was detected by PL spectrum at 1.91 eV. The film thickness was found to be reduced to monolayer of about 0.61 nm according to the XS-TEM image. Multilayer MoS₂ film was produced using sputtering of the same setting but a longer deposition time of 60 min, while nanocluster-assembled MoS₂ film was produced using SCBD method followed by post-sulfurization. The dark sensor response of monolayer MoS₂ against 10 - 500 ppm H2S is 1.25 - 25.68%, while those of multilayer and nanocluster-assembled MoS₂ films are small and close to zero. Besides, photo-assisted monolayer MoS₂ film exhibits an enhanced gas sensor response of 4.04 - 52.07 % against H₂S varying in the same concentration range. It is also sensitive to 10 - 500 ppm NH3 with the sensor response of 53.8 - 247.4 %. In contrary, the sensor response to H2 is weaker, and is found to change slightly of 0.4 - 6.3 % for 40 - 500 ppm. These results indicated that only monolayer MoS₂ film with a direct bandgap can give remarkable enhanced gas sensor response to H₂S, which are found to be symbiotic.
Rights: All rights reserved
Access: open access

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