# Alumni Dissertations

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### SYNTHESIS AND CHARACTERIZATION OF POLYCRYSTALLINE SEMICONDUCTOR CsSnI3 THIN-FILMS

Author:Zhuo ChenYear of Dissertation:2013Program:PhysicsAdvisor:Kai ShumAbstract:This thesis deals with a virtually unexplored semiconductor material CsSnI3 from material synthesis, structural, optical, and electrical characterization to the fabrication and validation of CsSnI3 thin-film solar cells. We started with synthesizing CsSnI3 thin films based on CsI and SnCl2 (or SnI2) by using an apparatus which consists of e-beam and thermal evaporators. The quality of polycrystalline CsSnI3 thin-films were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Experimental data on XRD and electron diffraction patterns taking from the synthesized thin-films match very well to the theoretically calculated ones based the first principles calculations, confirming that the synthesized CsSnI3 thin-films have an orthorhombic crystal structure. With the well-defined crystal structure, we theoretically studied the electronic band structure of CsSnI3. Extensive optical characterizations of CsSnI3 thin-films were then carried out revealing many extraordinary properties such as 1) direct band gap energy of 1.32 eV at 300 K with its abnormal temperature dependence, 2) extremely high photoluminescence quantum yield, 3) large exciton binding energy, and 4) strong two-phonon assisted excitonic absorption near band edge. These properties are interpreted in terms of the unique electronic and structural properties of CsSnI3. The value of 1.3 eV for the energy band gap of CsSnI3 suggests a unique application of CsSnI3 thin-films on solar cells. This is because this value is right in the small range of the optimal band gaps for the Shockley-Queisser maximum efficiency limit of a single-junction solar cell. A prototype Schottky solar cell was designed, fabricated, and validated. The measured power conversion efficiency (PCE) is 0.9 % which is presently limited by the series and shunt resistance. The improvement strategy on PCE is given at the end of my thesis. In order to make the CsSnI3 thin-film solar cells cost effective, various low cost materials synthesis methods for CsSnI3 are also described in this thesis. CsSnI3 thin-films can be now inexpensively deposited on to glass or other low-cost substrates. I believe that the CsSnI3 based materials are ideally suited for many applications such as lasers, light-emitting diodes, integrated photonic devices such as infrared electro-optic modulator, solar cells, and even more specialized applications such as spectral solar concentrators.

### Dynamics of Nanoparticles in Fluids and at Interfaces

Author:Weikang ChenYear of Dissertation:2014Program:PhysicsAdvisor:Joel KoplikAbstract:In this thesis, we use molecular dynamics simulation to study three basic behaviors or properties of nanoparticles: deposition during droplets evaporation, slip boundary condition and Brownian motion. These three problems address the need for an in-depth understanding of the dynamics of nanoparticles in fluids and at interfaces. In the first problem, evaporation of the droplets dispersed with particles, we investigated the distribution of evaporative flux, inner flow field, density and temperature. And we use these numerical experiments to check on our hydrodynamic theory of the "coffee ring" phenomenon. The simulations reveal the connection between the particle interactions and the deposit structure, and indicate some limitations in continuum modeling. In the second problem, we explore the slip boundary conditions for curved surfaces, which is one of the desired information in modeling the hydrodynamics of micro-fluidic objects. The conclusion we draw is strong: the slip length, defined in a consistent tensorial manner, depends only on the physical properties of the solid and fluid involved and does not vary with the flow configuration. The final part is devoted to the Brownian motion of Janus particle, where we use a simple model to explain the increase of diffusivity of self-propelling Janus particles. We also show that the hydrodynamic image could be used to account for the self-aligning phenomenon at liquid-solid interfaces. The coupling between the translation and rotation is investigated by Brownian simulation, where we modify the standard Langevin equation with coupling terms which derive from the hydrodynamic interaction with the liquid-solid interfaces. The resultant individual trajectories and their diffusivities are consistent with both the laboratory observations and theoretical calculations.

### SECOND QUANTUM STATE TRANSITION IN GaAs/AlGaAs RESONANT BRAGG STRUCTURE PROBED BY MODULATION REFLECTANCE SPECTROSCOPY

Author:Yuechao ChenYear of Dissertation:2014Program:PhysicsAdvisor:Mim NakarmiAbstract:Modulation spectroscopy, ever since its introduction by B.O. Seraphin in 1964, has been considered and widely used as a sensitive experiment technique for studying and characterizing the properties of varieties of semiconductor materials. Compared to general optical reflectance spectrum which measures the absolute reflection, the modulation spectroscopy evaluates the interpretation of the changes in the optical response from the sample caused by a periodic physical perturbation applied to the sample, such as temperature, electric fields, hydrostatic pressure, uniaxial stress, \emph{etc}. Those modulation spectroscopies with an external electric field perturbation are known as electroreflectance spectroscopy, which provides sharp and derivative-like spectral features in the energy region of excitonic transitions in the semiconductors while suppressing uninteresting background effects that are not affected by the electro-modulation. One interesting phenomenon is that when the excitonic transition energy in a periodic dielectric structure, for example multiple quantum well (MQW) structure, meets the Bragg resonance condition, the reflectance spectrum shows an enlarged responding effect with enhanced reflectivity and broadened transition features. This kind of a structure is known as a resonant Bragg structure (RBS) and the coincidence of the exciton and Bragg resonances is called the double resonance condition. In this thesis, we employed both electroreflectance and optical reflectance spectroscopies to probe excitonic transitions in a GaAs/AlGaAs RBS MQW structure. The sample structure was specially designed and fabricated to tune the double resonance condition around the second state of the heavy-hole exciton x(e2-hh2) transitions by variation of the incident angle and temperature. The sample used in this experiment consists of 60 periods of quantum well structures with GaAs well layer (13 nm) and AlGaAs barrier layer (94 nm), grown by solid source molecular beam expitaxy on a semi-insulating GaAs substrate. We observed a significant enhancement of excitonic features at the x(e2-hh2) exciton transitions around incident angle of 23$^\circ$ in both techniques, revealing the double resonance condition at low temperature. Additionally, heavy-hole and light-hole ground state exciton transitions x(e1-hh1) and x(e1-lh1) were also evaluated. In the temperature dependence of optical reflectance and electroreflectance from the double resonance condition, we observed redshift of the excitonic features. The electric field dependence measurement of electroreflectance exhibited a broadening effect for the x(e2-hh2) exciton transition.

### Novel materials and techniques for renewable energy and biosensing applications

Author:Yongki ChoiYear of Dissertation:2010Program:PhysicsAdvisor:Ramzi KhuriAbstract:Ultrasmall (1 nm and 2.8 nm) colloidal silicon nanoparticles behave as electrocatalysts for the electrooxidation of the renewable energy sources such as ethanol, methanol and glucose. Particle-immobilized electrodes show an onset of electrocatalysis occurring at potentials between -0.4 V and 0.05 V vs. Ag/AgCl at neutral pH. Both the onset potential and the strength of electrocatalysis are dependent on particle size. Tafel measurements show that electrooxidation of the fuels is a first order reaction with the transfer of one electron. The electrocatalytic activity of the particles to the fuels undergoes at least a 50-fold increase under alkaline condition compared to under acidic condition. A significant increase in the electrocatalytic current is obtained when the electrocatalysis is performed in darkness. Prototype single-compartment and double-compartment hybrid fuel cells have been constructed and tested, using the particles as the anode electrocatalyst, in order to demonstrate the potential of the particles in fuel cell applications. Voltage-controlled amplification of the output current of an enzymatic transistor has been demonstrated. By applying external voltage between the gating and the working electrode on which the enzyme glucose oxidase was immobilized, the biocatalytic output current was increased significantly, allowing the detection limit of glucose to be lowered from the milli-molar to the zepto-molar level. The current amplification was reversibly controlled by the applied voltage. Applying this technique to the ethanol-alcohol dehydrogenase system showed similar results. The enzyme's bio-specificity was preserved in the presence of the field. The detector, with its output current controlled by the voltage applied at a third electrode, behaves as a field-effect transistor, whose current-generating mechanism is the conversion of analytes to products using an enzyme as catalyst. In addition, voltage-controlled reaction kinetics of biological catalysis is achieved using the microperoxidase-11 and hydrogen peroxide system. The interfacial electron transfer of the system was manipulated by applying the voltage to the electrode. The manipulated electron transfer causes kinetic parameters of the catalysis to acquire nonlinear dependences on the voltage. The nonlinearity indicates the feasibility of effectively controlling the efficiency of a bio-catalytic reaction or a conversion process using the voltage

### Generalization of the three-term recurrence formula and its applications

Author:Yoon Seok ChounYear of Dissertation:2012Program:PhysicsAdvisor:Sultan CattoAbstract:In an earlier paper we showed development of a bilocal baryon-meson field from two quark-antiquark fields. In the local approximation the hadron field was shown to exhibit supersymmetry which was then extended to hadronic mother trajectories and to inclusion of multiquark states. The Hamiltonian in the case of vanishing quark masses was shown to have a very good agreement with experiments. The theory for vanishing mass was solved using confluent hypergeometric functions. In order to solve the spin-free Hamiltonian with light quark masses we are led to develop a totally new kind of special function theory in mathematics that generalize all existing theories of confluent hypergeometric types. We call it the `Grand Confluent Hypergeometric Function.' Our new solution produces previously unknown extra "hidden" quantum numbers relevant for description of supersymmetry and for generating new mass formulas. Furthermore, we show for the first time how to solve mathematical equations having three term recursion relations and go on producing the exact solutions of some of the well-known special function theories that include Mathieu, Heun, Lame and the Grand Confluent Hypergeometric Function. We hope these new functions and their solutions will produce remarkable new range of applications not only in supersymmetric field theories as is shown here, but in the areas of all different classes of mathematical physics, applied mathematics and in engineering applications.

### The Impact of Context on Learning and Epistemology in Physics

Author:Sebastien CormierYear of Dissertation:2009Program:PhysicsAdvisor:Richard SteinbergAbstract:This dissertation investigates the impact that various contexts have on student learning and epistemology. This is accomplished by analyzing diverse student populations learning various subjects in physics in distinctive classroom environments at City College New York (CCNY). Studies in Physics Education Research (PER) have found that many students lack proper conceptual understanding after instruction in physics and that their epistemology, or approaches to learning and doing science, is different from those of experts. The PER community has used these results to develop models of learning and tools for teaching introductory and modern physics with goals that include improving the conceptual understanding, problem solving abilities, and epistemologies. These curricula are applied in a wide variety of contexts here at CCNY. The student contexts in this dissertation range from high school to graduate school, and the topics range from introductory to modern physics. We apply many tools commonly used in PER, such as multiple-choice surveys, essay questions, and guided interviews to study these classrooms. We find that PER- based curriculum implemented in these different contexts is able to improve both conceptual understanding and epistemology.

### Yang-Mills Theories as Deformations of Massive Integrable Models

Author:Axel Cortes CuberoYear of Dissertation:2014Program:PhysicsAdvisor:Peter OrlandAbstract:Yang Mills theory in 2+1 dimensions can be expressed as an array of coupled (1+1)-dimensional principal chiral sigma models. The SU(N) principal chiral sigma model in 1+1 dimensions is integrable, asymptotically free and has massive excitations. We calculate all the form factors and two- point correlation functions of the Noether current and energy-momentum tensor, in 't Hooft's large-N limit (some form factors can be found even at finite N). We use these new form factors to calculate physical quantities in (2+1)-dimensional Yang-Mills theory, generalizing previous SU(2) by P. Orland to SU(N). The anisotropic gauge theory is related to standard isotropic one by a Wilsonian renormalization group with ellipsoidal cutoffs in momentum. We calculate quantum corrections to the effective action of QED and QCD, as the theory flows from isotropic to anisotropic. The exact principal chiral sigma model S-matrix is used to examine the spectrum of (1+1)-dimensional massive Yang Mills theory.

### LONG-RANGE DIPOLAR FIELDS AS A TOOL FOR NUCLEAR MAGNETIC RESONANCE MICROSCOPY

Author:Wei DongYear of Dissertation:2009Program:PhysicsAdvisor:Carlos MerilesAbstract:ABSTRACT Long-Range Dipolar Fields as a Tool for Nuclear Magnetic Resonance Microscopy By Wei Dong Mentor: Prof. Carlos Meriles Nuclear Magnetic Resonance (NMR) is widely used today for structural and dynamical studies of the properties of diverse materials. However, due to the relatively low sensitivity of the standard induction detection method, NMR is strongly constrained when probing samples whose effective dimensions are less than a few microns. To overcome these limitations, our novel strategy based on the manipulation of the long-range dipolar interactions between the sample and a hyperpolarized semiconductor tip located close to its surface. These interactions are used to modulate the tip nuclear magnetization in a way proportional to the local sample magnetization. The advantage of this strategy lies in that the highly sensitive detection methods - e.g., optical detection - can be used to monitor the semiconductor tip, thus providing the opportunity to indirectly probe the sample neighboring the tip with a favorable signal-to-noise ratio. Because the detected portion of the sample is comparable to the size of the tip, resolution exceeding the currently attainable could be possible. As an initial demonstration of our methodology we designed an experiment in which a 3 mm diameter distilled water droplet - playing the role of a sensor - was used to detect the NMR signal of the sample surrounding the droplet, in this case, silicon oil (Sigma-Aldrich) contained in a 5 mm diameter glass tube. Notice the sample (oil) and the detection center (water) are distinct and discernible objects only connected through long range intermolecular dipolar couplings. A special pulse sequence was designed and applied in the experiment to encode the sample magnetization for detection. By utilizing the Runge-Kutta algorithm, I modeled a 2000 spins ensemble based on the given geometry and numerically calculated the Bloch differential equations of this coupled spins system. Experimental results have a very good agreement with the numerical calculations. This preliminary experiment proves that not only the sample NMR signal can be indirectly detected, also many other sample information - e.g., relaxation time, sample spectrum, etc. - are attainable. Minimizing the short range dipolar couplings is a very crucial part to achieve the final goal of this strategy when the solid state semiconductor was used as the sensor. At the second stage, a modified MREV16 decoupling pulse sequence was designed and applied to greatly reduce the short range dipolar couplings inside the solid state sensor. A 3 mm thick disk GaAs crystal has been chosen as the sensor due to its excellent optical hyperpolarization properties. By acquiring 71Ga NMR signal, I successfully indirectly detected a tiny nuclear dipolar field induced by proton spins from an adjacent organic sample (as small as 7 nT). Optical enhancing the bulk averaged nuclear spin polarization in semiconductors is another critical technique that will be integrated into our strategy. Comparing to the thermal nuclear magnetization, we achieved 2-3 orders of magnitude optical enhancement for 71Ga in GaAs crystal and 3 orders for 125Te in CdTe crystal. Finally optical Faraday rotation will be used as an ultrasensitive detection to incorporate into the strategy. Optical reading of the electronic Larmor frequency shift in the semiconductor by using Time-Resolved Faraday Rotation (TRFR) to probe the sample magnetization change is the basic idea of our optical detection scheme. Through collaboration with Attocube AG, a leading company specialized on low temperature optical microscopy, our cutting-edge cryogenic optical NMR probe has been finished. Certainly, integrating optical hyperpolarization and optical detection with modern NMR technique is very challenging and requires tremendous work. However given the steady progress in the area of nanotechnology, our strategy's future still appears quite promising.

### Averaged dynamics of the advection-diffusion equation and applications to ocean flows.

Author:Yauheni DzedzitsYear of Dissertation:2012Program:PhysicsAdvisor:Tobias SchaferAbstract:This dissertation presents some aspects of an advection-diffusion equation and its applications to physical oceanography. We propose a perturbative scheme of averaging the advection-diffusion equation in the limit of vanishing diffusivity. Under the restriction that the time-dependence of the advective field is completely separable we construct an exact solution of the purely advective part via action-angle coordinates and treat diffusion as a perturbation using Lie transform techniques. The developed method is applied to a regularized vortical flow field which is periodically modulated in time. Numerical simulations of the vortical flow advection in presence of small diffusion are discussed. We present numerical evidence that the spectrum of of the averaged time-independent advection-diffusion operator converges to the spectrum of the operator with fully enabled time dynamics. A formal generalization of the method for three-dimensional time-periodic flows is discussed. We also discuss the importance of advection and diffusion in problems of transport and mixing in complicated dynamical systems, such as hydrodynamical systems, in particular describing ocean currents. We propose a method to visualize and analyze the structure of complex flows using data from HYbrid Coordinate Ocean Model (HYCOM) as an example. We present results of simulations obtained with highly parallel Co-array Fortran code that can be run on modern computing systems that support partitioned global address space (PGAS) programming model.

### OPTICAL SPECTROSCOPY OF XENON-RELATED DEFECTS IN DIAMOND

Author:Yury DziashkoYear of Dissertation:2014Program:PhysicsAdvisor:Anshel GorokhovskyAbstract:The work presents the results of optical studies of Xe-related defect in diamond. This defect is one of a few having narrow zero-phonon line in the near-infrared part of the photo-luminescence spectra. It appears in diamond after Xe+ ion implantation followed by thermal annealing. Given unique physical properties of diamond (hardness, optical transparency in wide spectral range, chemical inertness, high thermal conductivity, low thermal expansion coefficient) and stability of Xe-related center it can be viewed as an potential candidate for the source of single-photons, or as optically manipulated qubit, not unlike nitrogen-vacancy center. However, compared to the latter Xe-related center is not as well understood and it is necessary to gain thorough understanding of its spatial structure, symmetry, electronic states and mechanisms of interaction with the host diamond lattice. This thesis addresses several questions, in regard to Xe-related center: 1) How efficiently this center is formed in the implantation/annealing process; 2) How does it interact with the host lattice to produce both homogeneous and inhomogeneous shape of the 811.7 nm zero-phonon line; and 3) What is the type and nature of transitions responsible for the 811.7 nm zero-phonon line. In answering the first question two methods for measuring the conversion efficiency of any kind of optically active defect have been developed and applied to Xe-related center in diamond. The study of line-broadening mechanisms showed how this center interacts with the strain produced by other neighboring Xe-centers, distributed in a plane-like geometry. Also, interaction with vibrational motions of the diamond lattice localized in the vicinity of the Xe-centers is analyzed and the parameters of this pseudo-local mode are found. Finally, using the effects of optical saturation in photo-luminescence under excitation with Gaussian laser beams it has been shown that 811.7 nm zero-phonon line is due to absorption and emission by circular magnetic dipole. Also, the site symmetry of Xe-related center has been confirmed as trigonal with the threefold axis oriented along <111> direction in the diamond crystal.