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Fabrication and Electrical Characterization of Graphene and Graphene-MOx Hybrid Nanostructures for Gas Sensing Applications

Esfandiar, Ali | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44860 (48)
  4. University: Sharif University of Technology
  5. Department: Nanoscience and Nanotechnology
  6. Advisor(s): Iraji Zad, Azam; Akhavan, Omid; Gholami, Mohammad Reza
  7. Abstract:
  8. In this project, two types of graphene obtained from chemical and chemical vapor deposition (CVD) methods were studied for gas/vapor sensing applications . In chemical method, oxidation and exfoliation of graphite was performed in liquid phase. AFM, XPS, Raman spectroscopy and Uv-visible spectroscopy were indicated that single layers synthesis of graphene oxide (GO) sheets was formed successfully. Reduction of the GO to reduced graphene oxide (RGO) was chemically done by hydrazine as a conventional reducing agent and melatonin as a green alternative. Photocatalytic reduction and hybridization of GO with TiO2 was also studied. TEM, SEM and AFM analyses confirmed nucleation and growth of TiO2 nanporticles on the GO sheets. The TiO2/GO hybrid structures were chemically doped by Pd and Pt catalysts, and hydrogen gas sensing measurements were studied on these samples. Variation of film’s electrical resistance of the samples after exposure gases was monitored for gas sensing. At the working temperature of 180 C, Pd doped TiO2/RGO in comparison with Pt doped samples demonstrated more sensitivity (> 90%) and fast response /recovery times (about 40 s). As another metal oxide, tungsten oxide was hybridized with GO by means of two methods: sol-gel and hydrothermal. In sol-gel method 0.5 % of GO relative to tungsten was optimized for hybrid gas sensors at 150 C with two orders of magnitude sensitivity. In hydrothermal route, oxidation state of GO was important key issue in crystal growth of Pd-WO3 hierarchical nanostructures on graphene sheets. Gas sensing measurements were performed for different concentrations of hydrogen ranging from 20 to 10,000 ppm at various temperatures. High sensitivity (>102), fast response time (less than 30 s) and complete recovery at 500 ppm of hydrogen were observed at low temperatures (100 °C and even room temperatures). In continue, CVD method was utilized for large scale growth of single layer graphene. Nanosphere lithography method was also used for fabrication of patterned graphene as nanomesh structure in centimeter scales. Graphene and graphene nanomesh field effect transistor arrays were fabricated by modified lithography process using a protective gold layer. We applied single stranded DNA (ssDNA) to a graphene nanomesh and investigated the sensing properties of a large array of such sensors. Sensing results indicated that the mesh structure of graphene provides more binding sites for analyte molecules, possibly because of oxidized edges and defects. Additionally, these sensors are able to discriminate between homologous vapors that differ by just one carbon atom with detection limits in few ppm ranges with fast response and recovery time at room temperature
  9. Keywords:
  10. Graphene Based Field Effect Nanotransistors ; Chemical Vapor Sensing ; Graphene Based Hybrid Gas Sensors ; Chemical Vapor Deposition (CVD)Growth of Graphene

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