Browsing by Author "Chris Roland, Committee Member"

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Atomic Structure Optical Properties and Electron Transport in Self-assembled Monolayers on Surfaces
(2007-07-18) Wang, Shuchun; Marco Buongiorno Nardelli, Committee Member; Wenchang Lu, Committee Member; Jerry Bernholc, Committee Chair; Chris Roland, Committee Member; Zhilin Li, Committee Member
Adsorbate-induced modification of semiconductor or metal surfaces creates a nano-scale quantum structure which offers a rich vein of exotic physical phenomena for investigation. Human desire to harness these properties for technological or scientific purposes has led to extensive experimental and theoretical investigations. This dissertation focuses on the ab initio simulations of atomic, electronic, optical, and transport properties of nano-scale systems. The calculated results for indium nanowires on the Si(111) surface identify their atomic structure and reveal a phase transition at low temperature. Transport simulations on the self-assembled monolayer of ferrocenyl-alkanethiolate on Au(111) surface show negative difference resistance, which is in very good agreement with experimental observations. This opens a new opportunity for applications in nanoscale molecular devices.
Computational Modeling: Nanoindentation and An Ad Hoc Molecular Dynamics-Finite Difference Thermostat
(2004-07-20) Schall, James David; Phil Russell, Committee Member; Ron Scattergood, Committee Member; Donald W. Brenner, Committee Chair; Chris Roland, Committee Member
Due to anharmonicities in atomic interactions it is expected that the indentation modulus should very with pre-existing stress in the substrate. However, Tsui, Pharr and Oliver have shown in experiment that the indentation modulus for indentations where plasticity is present is essentially independent of the pre-existing stress if the true contact area is used to make the calculation. They show that the dependence is due to errors in the empirical estimates used to determine contact area. Their experiment is repeated using molecular dynamics simulation and the results of various empirical estimates for the contact area have been compared to the true contact determined from the simulation. The results show that the empirical estimates lead to large errors in contact area. As a result, the hardness and modulus are in error. When true contact areas are used the results agree with experiment. By using shallow elastic indentations, it is shown that indentation can be used to predict the true dependence of pre-stress on the indentation modulus predicted given the knowledge of the anharmonicity in the atomic potential which may be predicted using molecular statics calculations. In addition, a new method for temperature control in molecular dynamics simulations is presented. In metals, electronic interactions account for the majority of the heat flow. Approaches such as the embedded atom method do not account for electrons explicitly and thermal transport cannot be accurately modeled. To overcome this, an ad hoc feedback between the molecular dynamics simulation and the continuum heat flow equation has been developed. The method relies on experimental values for the thermal conductivity and heat capacity as inputs for a finite difference solution to the continuum equation. The thermostat was tested for a simple quasi one-dimensional case. Results are in excellent agreement with the analytical solution for heat flow. The method was extended to three dimensions and applied to the problem of substrate heating due to tip sliding. Although the temperature changes in the substrate due to the sliding tip are small, results show significant differences in the force felt by the tip when the thermostat is turned on or off.
Investigations of grain size dependent sediment transport phenomena on multiple scales
(2004-04-20) Thaxton, Christopher S; Helena Mitasova, Committee Co-Chair; Lubos Mitas, Committee Co-Chair; Chris Roland, Committee Member; Rich McLaughlin, Committee Member
Sediment transport in coastal and fluvial environments resulting from short time-scale processes of disturbance such as urbanization, mining, agriculture and military operations have significant impact on channel network and shoreline morphology, downstream water quality and ecosystems, and the integrity of land use applications. The scale and spatial distribution of these effects are largely attributable to the size distribution of the sediment grains that become eligible for transport due to disturbance. An improved understanding of advective and diffusive grain size dependent sediment transport phenomena will lead to the development of more accurate predictive models and preventative measures. To this end, three studies were performed that investigate grain-size dependent sediment transport on three different scales. Discrete particle computer simulations of sheet flow bedload transport on the scale of 0.1-100 millimeters were performed on a heterogeneous population of grains of various grain sizes. The relative transport rates and diffusivities of grains under both oscillatory and uniform, steady flow conditions were quantified. These findings suggest that, due to preferential vertical sorting of the largest grains to the top of the bed, a representative grain size that is functionally dependent on the applied flow parameters should be employed when parameterizing bed roughness. On the scale of 1-10m, experiments were performed to quantify the hydrodynamics and sediment capture efficiency of various baffles installed in a sediment retention pond, a commonly used sedimentation control measure in watershed applications. Analysis indicates that optimum sediment capture effectiveness may be achieved based on baffle permeability, pond geometry, and/or flow rate. Finally, on the scale of 10-1,000m, simulations were performed using a path sampling bivariate watershed erosion / deposition model in which grain size dependent terrain modification and pattern formation were integrated. Results correspond well to field observations and suggest that, with further refinements, the presented model may prove a valuable tool for further scientific advancement and engineering applications. Although a unique set of governing equations applies to each scale, an improved physics-based understanding of small and medium scale behavior may yield more accurate parameterization of key variables used in large scale predictive models.