Department of Physics
http://hdl.handle.net/2104/4776
2018-04-26T22:43:16ZProbing interactions among molecules, substrate, and tip using tip-enhanced Raman spectroscopy.
http://hdl.handle.net/2104/10160
Probing interactions among molecules, substrate, and tip using tip-enhanced Raman spectroscopy.
Tip-Enhanced Raman Spectroscopy (TERS) has shown that spatial resolution <1 nm is possible, but is technically challenging and difficult even in Ultra-High Vacuum at low temperatures with plasmonic substrates. Here we demonstrate that in ambient conditions it is possible to obtain Raman signal from isolated molecules on Au and submonolayer molecular islands on bulk MoS2. Analysis of the relative Raman peak ratio and the Raman peak position shift from the spatial TERS mapping show differences in the adsorbates-adsorbates interaction and the adsorbates-substrates interaction on Au and MoS2. TERS tips can influence the shape and diffusion of surfaces and adsorbates. We show the directional growth of Ag island stripes at room temperature via tip-assisted coarsening of hexagonal islands on Ag(111) using scanning tunneling microscope (STM). Tip-assisted island growth is not directly related to the tunneling current, bias voltage, tip composition or atmospheric O2 content but strongly depends on tip morphology.
2017-07-31T00:00:00ZEncrypting the universe.
http://hdl.handle.net/2104/10130
Encrypting the universe.
Layer one N = 4 SUSY Weakly Coupled Free Fermionic Heterotic String (WCFFHS) Models that include string-scale massive gauge sectors are statistically investigated for even orders through 12 for uniqueness and gauge content. A focus is given to models containing the standard model or gauge group content with the embedded standard model. Results are compared with those of prior searches of N = 4 models without explicit string-scale massive gauge sectors. This search revealed models with standard model gauge content at the string scale, which were not observed in prior searches. Additionally, research regarding an image encryption algorithm is presented. The RGB values of each pixel in an image are encrypted using three intertwined Mandelbrot summations. Runtimes for decryption depend primarily on the size of the image. Modifications to increase complexity and runtime are explored. Advantages include adaptability, reasonable runtime on a typical personal computer and that it is novel, enhancing its security.
2017-08-02T00:00:00ZLEED and STM studies of rutile (1x1) and (1x2) TiO2(110).
http://hdl.handle.net/2104/9662
LEED and STM studies of rutile (1x1) and (1x2) TiO2(110).
The structures and properties of rutile (1x1) and (1x2) TiO2(110) surfaces are studied using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). For (1x1) TiO2, LEED analysis shows common relaxation features reported in literatures: The bond length for bridging oxygen is contracted to 1.852 Å and 5-fold coordinated titanium is sagged downward by 0.182 Å. In addition, the sequence of alternatively contracted-elongated Ti-O bonds has been observed and discussed. The partial reduced (1x1) surface exposed under formaldehyde (CH2O) environment is also investigated using variable temperature (75-300 K) STM. At low temperature, a diolate formed via coupling between two CH2O molecules is discovered. At room temperature, such diolate is further found forming a stable coverage of adsorption. For (1x2) reconstructed TiO2(110), LEED simulation of Ti2O added row model is performed with low temperature (T = 190K) experimental spectra of fractional diffraction spots. The resultant structure yields a Rp value of 0.51 which suggests an indefinite structural correlation to the experimental data. The modeling details are discussed to explain such LEED result and suggestions are given to improve it. A novel method using a chemical probe to investigate (1x2) TiO2(110) is also utilized. TMAA as a chemical probe is dosed on the surface and imaged using room temperature STM. The adsorptions of TMAA are observed on sites of (1x2) trough and strand, and also on cross-linked sites with varying preferences. Three proposed (1x2) added row models, Ti2O, Ti2O3 and Ti3O6, are compared to the observed adsorption behaviors. Ti2O added row model is found consistent with such adsorption behaviors and models of TMAA adsorptions are proposed.
2016-04-20T00:00:00ZHořava-Lifshitz theory of gravity and its applications to cosmology.
http://hdl.handle.net/2104/8762
Hořava-Lifshitz theory of gravity and its applications to cosmology.
Huang, Yongqing, 1984-
In this dissertation, I studied Hořava-Lifshitz gravity and its applications to inflationary cosmology. After introducing the original proposal with the projectability and detailed balance conditions, I discussed its attracting features as well as the problems it faces. An extended model without the detailed balance condition was then studied and found to be stable in de Sitter spacetime, but still possess an extra scalar mode in the gravity sector. I then studied a model with an extra U(1) symmetry dubbed as the projectable general covariant HL gravity and showed that it has the same degree of freedom as general relativity, and is free of the stability problem. It was found that the FLRW universe is necessarily flat, given that the coupling with matter takes a specific form. I also studied the scalar perturbations around the FLRW metric and presented all the possible gauge choices. Applications of the general covariant model in inflationary cosmology were studied in the second part. After deriving the slow-roll conditions in this model, I showed that in the super-horizon regions the scalar perturbations become adiabatic, and the comoving curvature perturbation is constant, because of this slow-roll condition. By using the uniform approximation technique, power spectra and indices of primordial scalar and tensor perturbations under the slow-roll approximations were expressed explicitly in terms of the slow-roll parameters and the various coupling constants. I found that they are in general different from, but reducible to, the values in the class of simplest inflation models. Next I studied the non-Gaussianities of these perturbations. For scalar perturbations, by properly choosing the coupling constants, a large nonlinearity parameter fNL is possible. I also found that the bispectrum favors the equilateral shape as a result of the higher order spatial derivatives, and that folded shape is enhanced when the vacuum is from the Bunch-Davis vacuum. Both the squeezed and the equilateral shapes appear in the bispectrum for primordial gravitational waves. In addition, the polarization tensors of the tensor fields have strong effects on the shapes of the bispectrum.
2013-09-16T00:00:00Z