# Alumni Dissertations and Theses

• ### Systematic Tuning of Silicon Schottky Barrier Height by Atomic Interlayers with Low Electronegativities

Author:
Wei Long
Year of Dissertation:
2012
Program:
Physics
Raymond Tung
Abstract:

The Schottky barrier height (SBH) is of great importance to the functionality of semiconductor devices, as it governs the carrier transport across the metal-semiconductor (MS) interface. The presence of the Fermi level (FL) pinning phenomena makes tuning the SBH a difficult goal to achieve. The technique of "partisan interlayer" (PI) was proposed recently to modify the SBH, where stable adsorbate-terminated semiconductor (ATS) surfaces were used to form SBs with subsequently applied metal. When elements with large electronegativities were used to form the ATS, the PI technique was effective in reducing the n-type SBH and increasing the p-type SBH, driven by the expected transfer of charge from the semiconductor to the adsorbates. In this thesis work, elements with electronegativities smaller than that of the semiconductor are used as surface termination. SBHs for Ag, Au and In on Si surfaces are found to increase for the n-type and decrease for the p-type interfaces, by as much as 0.25eV, when Ga, Mg and K are used to terminate the Si surfaces. The present results are thus in agreement with the expected charge transfers from elements with smaller electronegativities to silicon and illustrate the general validity of the PI technique. The chemical stability of these surfaces likely weakens the MS interaction and leads to the (partial) preservation of the surface dipole at the MS interface. However, large degrees of SBH inhomogeneity are observed for diodes on these surfaces, likely due to insufficient stability of these surfaces to completely withstand metal interaction. These results are discussed within the basic models of SBH formation and the implications of these results for SBH control of MS systems are also addressed.

• ### Proton Pumping in Cytochrome c Oxidase

Author:
Jianxun Lu
Year of Dissertation:
2015
Program:
Physics
Marilyn Gunner
Abstract:

Cytochrome c oxidase (CcO) is a large trans-membrane protein, which is the final enzyme in the respiratory electron transport chain in mitochondria or aerobic bacte- ria. It implements proton pumping through the mitochondrial membrane against the electrochemical gradient, by utilizing the chemical energy released by reducing O2 to water. The active site of the chemical reaction is called the Binuclear Center (BNC) that is made up of heme a3, CuB, a Tyrosine residue and their ligands. The protein is reduced four times by electron from cytochromes c to reduce O2 and to generate four different BNC redox states step by step. In each reduction step a proton is delivered to the BNC and another proton is pumped across the protein to increase the trans-membrane proton gradient. In CcO, the pumped proton is firstly located in the proton loading site (PLS), and then is released out of the protein. In these processes, a high conserved Glutamate residue, plays an essential role on the proton translocation either to the BNC or the PLS. In this thesis, Multi-Conformational Continuum Electrostatics (MCCE) and Molecular Dynamics (MD) are combined to study the proton affinity (pKa) of the high conserved Glutamate residue and the identity of the PLS. This Glutamate residue is located in a hydrophobic cavity in the protein, and the simulations show that the hydration of the cavity is controlled by the protonation state of the propionic acid of heme a3, a group on the proton outlet pathway. The changes in hydration and elec- trostatic interactions lower the proton affinity by at least 5 kcal/mol. The identity of the residues in the PLS is another open question in CcO research, and various groups above the BNC have been considered as candidates. We designed a new model for the simulation via separating the catalytic cycle into smaller substates and monitoring the charge of all residues in the protein. The results demonstrates the PLS is a cluster rather than a single residue, and the proton affinity of the heme a3 propionic acids primarily determines the number of protons loaded into the PLS.

• ### Exploring Non-Equilibrium Dynamics in Time Dependent Density Functional Theory

Author:
Kai Luo
Year of Dissertation:
2015
Program:
Physics
Neepa Maitra
Abstract:

Time-dependent density functional theory(TDDFT) is a method of choice for calulations of excitation spectra and response properties in materials science and quantum chemistry. The many-body problem is mapped into a set of one-body Schr\"odinger equations, called the Kohn-Sham(KS) equations. In principle, the one-body potential can be chosen such that the density of the interacting system is exactly reproduced by the KS system. However, one component of the one-body potential has to be approximated and is typically adiabatic". Though in linear response regime adiabatic approximations give quite good spectra, it is important to explore their performances in non-equilibrium dynamics. \noindent In this thesis, I will present the results of the explorations on non-equilibrium dynamics in TDDFT. For the first study, a decomposition of exact exchange-correlation potential into kinetic and interaction components is derived. We compare the components with that of adiabatic" counterparts in non-perturbative dynamics and find that the interaction component is less poorly approximated adiabatically than the kinetic component. A salient feature is that step structures generically appear, of relevance in the second study. We prove that the step structures only appear in the non-linear response regime. We find an exact condition which is typically violated by the approximations in use today. Spuriously time-dependent spectra in TDDFT can be explained and we find that the more the condition is violated the worse the dynamics is. In last, we envision that orbital functionals are able to incorporate the memory effects and compensate the deficiencies of the adiabatic" approximations.

• ### Magnetic deflagration in the molecular magnet Mn12-ac

Author:
Sean McHugh
Year of Dissertation:
2009
Program:
Physics
Myriam Sarachik
Abstract:

In 1995, Paulsen and Park observed abrupt spontaneous reversals of the magnetization in crystals of the molecular magnet Mn12-ac, which they dubbed magnetic avalanches". They suggested that the magnetic avalanches were a thermal runaway process where the reversing spins release heat stimulating further relaxation. Various exotic phenomena were proposed as an alternative explanations. In 2005, Suzuki et al. established that this spontaneous magnetic relaxation occurs as a front" separating regions of opposing magnetization that propagates at a constant speed through the crystal. They suggested that this propagating front is analogous to a flame in chemical deflagration and introduced the thermal relaxation process, magnetic deflagration. The analysis presented there was limited by lack of data. A more thorough comparison with the theory would require the ability to trigger avalanches in a more controlled way rather than wait for their spontaneous occurrence. The work presented in this thesis is a continuation of the program initiated by Suzuki. Significant progress experimental progress has been made allowing us to trigger avalanches over a wide range of conditions. The magnetization dynamics and the ignition temperatures are studied in detail using an array of micro-sized Hall sensors and Germanium thermometers. In addition, we report the existence of a new species of avalanches consisting only of the fast-relaxing isomers of Mn12-ac, the so-called minor species". We explore avalanches of both species, as well as the interaction between them. Finally, a detailed analysis is performed to compare the experiment with the theory of magnetic deflagration. We find the theory of magnetic deflagration to be consistent with the data and extract values for the key physical quantities: the thermal diffusivity and avalanche front temperatures. Agreement between our predicted values and an independent measurement of these quantities would provide compelling verification of the theory.

• ### Solid State NMR Studies of Materials for Energy Technology

Author:
Chandana Nambukara Kodiweera Arachchilage
Year of Dissertation:
2010
Program:
Physics
Steve Greenbaum
Abstract:

Abstract SOLID STATE NMR STUDIES OF MATERIALS FOR ENERGY TECHNOLOGY by Chandana K. Nambukara Kodiweera Arachchilage Adviser: Professor Steve Greenbaum Presented in this thesis are NMR investigations of the dynamical and structural properties of materials for energy conversion and storage devices. 1H and 2H NMR was used to study water and methanol transportation in sulfonated poly(arylene ether ketone) based membranes for direct methanol fuel cells (DMFC). These results are presented in chapter 3. The amount of liquid in the membrane and ion exchange capacity (IEC) are two main factors that govern the dynamics in these membranes. Water and methanol diffusion coefficients also are comparable. Chapters 4 and 5 are concerned with 31P and 1H NMR in phosphoric acid doped PBI membranes (para-PBI and 2OH-PBI) as well as PBI membranes containing ionic liquids (H3PO4/PMIH2PO4/PBI). These membranes are designed for higher-temperature fuel cell operation. In general, stronger short and long range interactions were observed in the 2OH-PBI matrix, yielding reduced proton transport compared to that of para-PBI. In the case of H3PO4/PMIH2PO4/PBI, both conductivity and diffusion are higher for the sample with molar ratio 2/4/1. Finally, chapter 6 is devoted to the 31P NMR MAS study of phosphorus-containing structural groups on the surfaces of micro/mesoporous activated carbons. Two spectral features were observed and the narrow feature identifies surface phosphates while the broad component identifies heterogeneous subsurface phosphorus environments including phosphate and more complex structure multiple P-C, P-N and P=N bonds.

• ### Nuclear Magnetic Resonance Studies on Lithium and Sodium Electrode Materials for Rechargeable Batteries

Author:
Tetiana Nosach
Year of Dissertation:
2014
Program:
Physics
Steve Greenbaum
Abstract:

In this thesis, Nuclear Magnetic Resonance (NMR) spectroscopic techniques are used to study lithium and sodium electrode materials for advanced rechargeable batteries. Three projects are described in this thesis. The first two projects involve 6Li, 7Li and 31P NMR studies of two cathode materials for advanced rechargeable batteries. The third project is a study of sodium titanate cathode materials for Na-ion batteries, where 1H, 7Li, and 23Na static and magic angle spinning NMR were used in order to obtain detailed information on the chemical environments.

• ### Quantum Rotational Effects in Nanomagnetic Systems

Author:
Michael O'Keeffe
Year of Dissertation:
2013
Program:
Physics
Eugene Chudnovsky
Abstract:

Quantum tunneling of the magnetic moment in a nanomagnet must conserve the total angular momentum. For a nanomagnet embedded in a rigid body, reversal of the magnetic moment will cause the body to rotate as a whole. When embedded in an elastic environment, tunneling of the magnetic moment will cause local elastic twists of the crystal structure. In this thesis, I will present a theoretical study of the interplay between magnetization and rotations in a variety of nanomagnetic systems which have some degree of rotational freedom. We investigate the effect of rotational freedom on the tunnel splitting of a nanomagnet which is free to rotate about its easy axis. Calculating the exact instanton of the coupled equations of motion shows that mechanical freedom of the particle renormalizes the easy axis anisotropy, increasing the tunnel splitting. To understand magnetization dynamics in free particles, we study a quantum mechanical model of a tunneling spin embedded in a rigid rotor. The exact energy levels for a symmetric rotor exhibit first and second order quantum phase transitions between states with different values the magnetic moment. A quantum phase diagram is obtained in which the magnetic moment depends strongly on the moments of inertia. An intrinsic contribution to decoherence of current oscillations of a flux qubit must come from the angular momentum it transfers to the surrounding body. Within exactly solvable models of a qubit embedded in a rigid body and an elastic medium, we show that slow decoherence is permitted if the solid is macroscopically large. The spin-boson model is one of the simplest representations of a two-level system interacting with a quantum harmonic oscillator, yet has eluded a closed-form solution. I investigate some possible approaches to understanding its spectrum. The Landau-Zener dynamics of a tunneling spin coupled to a torsional resonator show that for certain parameter ranges the system exhibits multiple Landau-Zener transitions. These transitions coincide in time with changes in the oscillator dynamics. A large number of spins on a single oscillator coupled only through the in-phase oscillations behaves as a single large spin, greatly enhancing the spin-phonon coupling.

• ### Strategies for Discriminating and Comparing Unknown Unitary Transformations

Author:
Guy Okoko
Year of Dissertation:
2014
Program:
Physics
Janos Bergou
Abstract:

How to discriminate or compare two unitary transformations that are completely unknown? We first examine the unambiguous discrimination of two unknown unitary transformations; we show that the results are the same as those found for the programmable discrimination of two unknown quantum states. Next we consider the minimum-error comparison of two unknown unitary transformations; the results are obtained in the general case where the prior probabilities are different. Last we study the unambiguous discrimination of two unknown unitary transformations in the case where multiple copies of data are available.

• ### Colloidal Quantum Dot Based Photonic Circuits and Devices

Author:
Nicky Okoye
Year of Dissertation:
2014
Program:
Physics
Vinod Menon
Abstract:

Colloidal quantum dots have desirable optical properties which can be exploited to realize a variety of photonic devices and functionalities. However, colloidal dots have not had a pervasive utility in photonic devices because of the absence of patterning methods. The electronic chip industry is highly successful due to the well-established lithographic procedures. In this thesis we borrow ideas from the semiconductor industry to develop lithographic techniques that can be used to pattern colloidal quantum dots while ensuring that the optical properties of the quantum dots are not affected by the process. In this thesis we have developed colloidal quantum dot based waveguide structures for amplification and switching applications for all-optical signal processing. We have also developed colloidal quantum dot based light emitting diodes. We successfully introduced CdSe/ZnS quantum dots into a UV curable photo-resist, which was then patterned to realize active devices. In addition, "passive" devices (devices without quantum dots) were integrated to "active" devices via waveguide couplers. Use of photo-resist devices offers two distinct advantages. First, they have low scattering loss and secondly, they allow good fiber to waveguide coupling efficiency due to the low refractive index which allows for large waveguide cross-sections while supporting single mode operation. Practical planar photonic devices and circuits incorporating both active and passive structures can now be realized, now that we have patterning capabilities of quantum dots while maintaining the original optical attributes of the system. In addition to the photo-resist host, we also explored the incorporation of colloidal quantum dots into a dielectric silicon dioxide and silicon nitride one-dimensional microcavity structures using low temperature plasma enhanced chemical vapor deposition. This material system can be used to realize microcavity light emitting diodes that can be realized on any substrate. As a proof of concept demonstration we show a 1550 nm emitting all-dielectric vertical cavity structure embedded with PbS quantum dots. Enhancement in spontaneous emission from the dots embedded in the microcavity is also demonstrated.

• ### Wave Scattering in Random Layered Media

Author:
Jongchul Park
Year of Dissertation:
2010
Program:
Physics
Azriel Genack
Abstract:

The scaling and statistics of the transport of waves in random media depend strongly on the dimensionality of the medium. The statistic of transmission in one dimension (1D) and quasi-1D (Q1D) have been calculated and tested. However, the statistic for other dimensions has not been established. Exploring transport in a layered system of stacks of glass cover slips with transverse nonuniformity has allowed us to study a dimensional crossover in transport from 1D towards 3D. The crossover occurs when the lateral spread of the wave become larger than the transverse coherence length in the transmitted speckle pattern as the number of layers increases. In thin samples, in which light does not spread beyond a single coherence area of the field on the output surface, the statistics of normalized intensity follow 1D statistics associated with a segment of a log-normal distribution with a sharp drop below the log-normal distribution for low values of intensity. Once the lateral spread is larger than the transverse coherence length, the probability density of intensity becomes a mixture of a mesoscopic distribution and an intensity distribution of a Gaussian field. This distribution was originally found for Q1D. Beyond 1D, the intensity statistics have a same form as Q1D statistics which is a function of a single localization parameter, the “statistical conductance” g’. This transition from 1D to Q1D statistics reflects a topological change in the transmitted field. In 1D, the transmitted intensity never vanishes, while beyond 1D, a speckle pattern built upon a network of phase singularities forms.