Alumni Dissertations

 

Alumni Dissertations

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  • SYNTHESIS AND CHARACTERIZATION OF POLYCRYSTALLINE SEMICONDUCTOR CsSnI3 THIN-FILMS

    Author:
    Zhuo Chen
    Year of Dissertation:
    2013
    Program:
    Physics
    Advisor:
    Kai Shum
    Abstract:

    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.

  • Novel materials and techniques for renewable energy and biosensing applications

    Author:
    Yongki Choi
    Year of Dissertation:
    2010
    Program:
    Physics
    Advisor:
    Ramzi Khuri
    Abstract:

    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

  • Novel materials and techniques for renewable energy and biosensing applications

    Author:
    Yongki Choi
    Year of Dissertation:
    2010
    Program:
    Physics
    Advisor:
    Ramzi Khuri
    Abstract:

    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 Choun
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Sultan Catto
    Abstract:

    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 Cormier
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Richard Steinberg
    Abstract:

    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.

  • The Impact of Context on Learning and Epistemology in Physics

    Author:
    Sebastien Cormier
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Richard Steinberg
    Abstract:

    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 Cubero
    Year of Dissertation:
    2014
    Program:
    Physics
    Advisor:
    Peter Orland
    Abstract:

    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 Dong
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Carlos Meriles
    Abstract:

    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.

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

    Author:
    Wei Dong
    Year of Dissertation:
    2009
    Program:
    Physics
    Advisor:
    Carlos Meriles
    Abstract:

    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 Dzedzits
    Year of Dissertation:
    2012
    Program:
    Physics
    Advisor:
    Tobias Schafer
    Abstract:

    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.