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Growth of Two-Dimensional Molybdenum Disulfide Structures for Gas Sensing Application

Barzegar, Maryam | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 52879 (48)
  4. University: Sharif University of Technology
  5. Department: Institute for Nanoscience and Nanotechnology
  6. Advisor(s): Irajizad, Azam; Asgari, Reza
  7. Abstract:
  8. Molybdenum disulfide (MoS2) nanoflakes, a two-dimensional crystal with tunable bandgap depends on number of layers, is a promising candidate for future nanoelectronic devices. In the present research, flower shaped MoS2 nanoflakes have been synthesized via hydrothermal method. As part of the research, the growth of triangular MoS2 monolayer up to 10 micrometers has optimized by carring out several chemical vapor deposition experiments with varying deposition parameters. The prepared samples were characterized using optical, scanning electron, and atomic force microscopes, x-ray diffraction analysis, and Raman spectroscopy. Based on the experimental results, the growth mechanism has been suggested. Chemiresistor gas sensor based on flower shaped MoS2 nanoflakes which were prepared by a facile hydrothermal method were fabricated. Gas sensing characteristics of these sensors were evaluated upon exposure to 200 to 400 ppm of xylene and methanol vapors. As working temperature increased, the sensitivity was improved from 25 to 55 for 200 °C and the response time decreased from about 800 s to 120 s. The results showed higher sensitivity and shorter response times for methanol at 200 °C. The gas sensing mechanisms for both vapors were discussed. Furthermore, the vertical MoS2 nanoflake films grown by CVD method, were used as n-channel for back-gated FET gas sensors, for which the contacts were made by sputtering of 50 nm titanium and then 100 nm gold. Dynamic gas sensing behavior of the produced transistors were investigated at room temperature and 50 °C in the presence of different concentrations (1 ppm, 10 ppm) of ethanol and methanol vapors. As the temperature increased to 50 ºC, the sensor response was enhanced from about 2.7 to almost 9.7 for 1 ppm ethanol, while for methanol the response increased from about 2.2 to 4.6. The gas sensing mechanism for both vapors at room temperature and 50 ºC were suggested. For better understanding the gas sensing mechanism, the adsorption of methanol and xylene gas molecules were investigated by means of density functional computations. The adsorption parameters such as adsorption energy and energy band gap, density of states, and Mulliken charge transfer were studied. The adsorption energy and charge transfer for xylene adsorption shows a significant improvement after nickel decoration on monolayer MoS2. Therefore, surface modification of monolayer MoS2 can improve the adsorption of xylene vapor comparing to methanol molecules
  9. Keywords:
  10. Chemical Vapor Deposition (CVD) ; Field Effect Transistors ; Gas Sensor ; Molybdenum Desulfide ; Two Dimentional Materials

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