Investigation of van der Waals interactions of carbon monoxide and water at the surfaces of pristine gallium nitride nanowires and gold decorated gallium nitride nanowires.

摘要

Gas phase interactions at the surfaces of bare and gold nanoparticle (Au NP) decorated gallium nitride (GaN) nanowires (NWs) have been extensively studied using photoelectron spectroscopy under ultra high vacuum conditions. GaN NWs were grown by the vapor-liquid-solid (VLS) mechanism, where the average NWs diameter was 105 ± 75 nm. Au NPs of average size 4-5 ± 0.5 nm were decorated over GaN NWs by plasma enhanced chemical vapor deposition (PECVD). First, the interaction of Au NPs as a function of CO, and H 2O exposures was investigated by examining the shift in an electron binding energy of the Au 4f, C1s and O1s electron core level states at 298 K, 77 K and 20 K using X-ray photoelectron spectroscopy (XPS). XPS analysis revealed for CO and H2O that for temperatures at 298 K and 77 K the interaction with the surface of the Au-GaN NWs was undetectable. However at 20K, the occurrence of binding energy shifts of the Au 4f, C1s, and O1s electron core level states indicated weak physisorption of CO and H2 O onto the Au-GaN NWs surface as opposed to chemisorption. The temperature dependence of the adsorption indicated that the interaction was physisorption and the interaction between the gases and Au-GaN NWs surface was Van Der Waal. Thermal desorption studies using XPS indicated that the adsorbates began to desorb at 298 K.;Ultraviolet photoelectron spectroscopy (UPS) has been used to explore the observed physisorption of CO and H2O with the surface of bare and Au NP-GaN NWs at 298 K, 77 K and 20 K at three different dosing pressures: 9 × 10-8, 9 × 10-7 and 9 × 10-6 Torr. CO and H2O did not bond to the surface of the bare GaN NWs at 298 K, 77 K, or 20 K, even at the highest dosing pressure. Temperature and pressure dependent UPS analysis revealed that CO and H 2O weakly physisorbed to the Au - GaN NWs. For the exposures up to 200 Langmuir(L), the static activation energy of adsorption of CO and H2 O was found to range from 9.0 ± 1.0 to 45.6 ± 4.3 kJ/mole and 2.0 ± 0.1 to 5.0 ± 0.1 kJ/mole, respectively. The adsorption at 298 K of 100 L of CO at all the three dosing pressures, followed by 100 L of H2O, showed that CO adsorption promotes H2O adsorption, while 100 L of H2O followed by 100 L of CO showed that H2 O inhibits CO adsorption. The findings of this study that the adsorption of H2O inhibits CO adsorption onto the Au NP-GaN NWs explain previous studies of the gas sensing properties of mats of Au NP-GaN NWs. The results of this dissertation work, in particular the co-adsorption results, demonstrate that the sensor response to CO and H2O, as well as the order of exposure, is due to competition for adsorption sites and CO-H2O interactions mediated by the order of occupation of the adsorption sites present on Au NPs.;Overall, the results of this research have led to a better understanding of the sensor response of Au-GaN NWs. Also, it was demonstrated that Au-GaN NWs are an excellent system to study the gas phase interaction with the Au NPs since bare GaN NWs are relatively inert.