Browsing by Subject "Si(100)"
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Item Alignment effects of hydrogen reflection by Si(100)(2015-08) Stevens, Robert Gardiner; Sitz, Greg Orman; Fink, Manfred; Downer, Michael; Keto, John; Henkelman, GraemePrevious scattering experiments have shown a dependence of reflectivity on the alignment of the angular momentum vector of molecular hydrogen incident on a surface of Pd(111), Isakson 2001. In these past experiments orthohydrogen was preferentially aligned relative to the surface by preparing the J=3 state. This J=3 state has multiple values for its magnetic quantum number and therefore there is a distribution in J=3 alignment. Within I will discuss the design of a new laser that can efficiently pump parahydrogen from the J=0 state, which has no distribution in magnetic quantum numbers and therefore the resulting J=2 state can be aligned with much better precision. Evidence suggests (Isakson, 2001) that the perpendicular alignment of the angular momentum vector with respect to the surface (helicopter-type motion) was less reflective than the parallel alignment (cartwheeler-type motion) for orthohydrogen when interacting with Pd(111). Within a study of the preservation of these alignments, both helicopter and cartwheeler, for aligned J=3 initial states will be attempted as they reflect off of the unreactive Si(100) surface. This study will strongly influence future studies, ones off of reactive surfaces such as Pd(111), and dictate if loss of alignment can explain the perceived decreased reflectivity.Item Effects of surface temperature in gas-surface interaction : quantum-state resolved studies of H₂ scattering from Si(100)(2010-12) Zhang, Shengyuan; Sitz, Greg Orman; Downer, Michael; Erskine, James L.; Mullins, C. B.; Keto, John W.The scattering of H2 from Si(100) has been studied using pulsed molecular beam techniques and quantum state-specific detection methods. These studies can be used to test theoretical calculations and give insight into new theories of molecule-surface interactions, a fundamental study in a diverse field of science and technology. In this work, time-of-flight (TOF) spectra of the elastic scattering of H2(v=1. J=1) and H2(v=0, J=1) from clean Si were recorded over a wide range of surface temperatures. Two data processing strategies were developed to extract rich kinematic information from the scattering experiments, e.g., mean translational energy exchange, absolutely survival probability, and angular and speed distribution of the scattered molecules. No such set of quantitative results has been reported before for this system. Compared with close packed metal surfaces, these scattering experiments from a covalently bonded semiconductor surface showed a completely distinct dynamics, e.g. the finding of energy gain instead of loss from the substrate, much broader angular distribution and some counterintuitive surface temperature effects. From the studies of molecules/surface scattering experiments, the thermal excitation on Si(100) surface which depends on surface temperature can substantially alter the adsorption barrier and its distribution, and therefore changes the kinematics of scattered molecules. As a result, even the most basic understanding of the dynamics has to include phonon excitation and deexcitation of the silicon substrate.Item Tailoring nanoscale metallic heterostructures with novel quantum properties(2013-05) Sanders, Charlotte E.; Shih, Chih-Kang; Raizen, Mark G.Silver (Ag) is an ideal low-loss platform for plasmonic applications, but from a materials standpoint it presents challenges. Development of plasmonic devices based on Ag thin film has been hindered both by the dificulty of fabricating such film and by its fragility out of vacuum. Silver is non-wetting on semiconducting and insulating substrates, but on certain semiconductors and insulators can adopt a metastable atomically at epitaxial film morphology if it is deposited using the "two-step" growth method. This method consists of deposition at low temperature and annealing to room temperature. However, epitaxial Ag is metastable, and dewets out of vacuum. The mechanisms of dewetting in this system remain little understood. The fragility of Ag film presents a particular problem for the engineering of plasmonic devices, which are predicted to have important industrial applications if robust low-loss platforms can be developed. This dissertation presents two sets of experiments. In the first set, scanning probe techniques and low energy electron microscopy have been used to characterize Ag(111) growth and dewetting on two orientations of silicon (Si), Si(111) and Si(100). These studies reveal that multiple mechanisms contribute to Ag film dewetting. Film stability is observed to increase with thickness, and thickness to play a decisive role in determining dewetting processes. A method has been developed to cap Ag film with germanium (Ge) to stabilize it against dewetting. The second set of experiments consists of optical studies that focus on the plasmonic properties of epitaxial Ag film. Because of the problems posed until now by epitaxial Ag growth and stabilization, research and development in the area of plasmonics has been limited to devices based on rough, thermally evaporated Ag film, which is robust and simple to produce. However, plasmonic damping in such film is higher than in epitaxial film. The optical studies presented here establish that Ag film can now be stabilized sufficiently to allow optical probing and device applications out of vacuum. Furthermore, they demonstrate the superiority of epitaxial Ag film relative to thermally evaporated film as a low-loss platform for plasmonic devices spanning the visible and infrared regimes.