Kinetics of Molecular Photodesorption at Graphene and Conductance Response to Photodesorption
| Autor: | Yang-Yang Hsu, 許泱泱 |
|---|---|
| Rok vydání: | 2014 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 102 Graphene is applicable to gas sensing, because of its extremely high surface area to volume ratio, its surface as long as there is any atom or molecule adsorption contact adsorbate outer electrons will overlap with the Pz orbital of graphene, resulting in a charge transfer, for one atomic layer thickness (0.34 nm) of graphene doping effect will be very significant. The photodesorption is regarded that the change in carrier density is proportional to the density of adsorbed molecules; namely the conductivity response due to adsorption/desorption can be described by the kinetics of the density of adsorbed molecules. When graphene is illuminated, the conductance of graphene decreases, attributable to the molecular photodesorption. We study the conductance response of CVD growth of graphene on photodesorption in air. The CVD growth of graphene transferred onto the SiO2/Si substrate. Silver paste is used to make electrodes on graphene. The graphene is placed in chamber to keep the relative humidity at 40 % at 1 atm. Using 325 nm UV helium - cadmium laser, due to molecular photodesorption (oxygen species), resulting in the hole de-doping, so that the current decreases exponentially (exp(-t/d), d is the desorption time constant); continuous illumination reach static current, stopping illumination, molecules adsorbed on graphene, resulting hole doping, so that the current increases exponential (1-exp(-t/a), a is the adsorption time constant). Under illumination in the atmosphere, the rate equation of molecular adsorption, which is proposed, contains molecular photodesorption and re-adsorption. Illuminated desorption can be obtained from the analytical solution: the desorption time constant (d), the current modulation (I), as well as the static current (Is), the three parameters are the function of the adsorption time constant (a), cross section () and the photon flux density (F). The graphene is irradiated from UV laser with different power density. At low light power density, I proportional to the power density, but at high light power density, I is not proportional to the power density. The reason for molecular photodesorption, then the contribution of molecular adsorption is not negligible. In order to understand the mechanism of photodesorption, using 325 nm UV light helium - cadmium laser, 405 nm, 473 nm and 532 nm solid-state laser irradiation, respectively, different wavelengths of light affect the cross section. Therefore, the discussion of the cross section and the photon energy of wavelengths (Eph) of the relationship, the result is . |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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