EXAMPLE CONDENSED MATTER PHYSICS EXPERIMENT AND THEORY PROJECT DESCRIPTIONS
The experimental condensed matter physics (CMPE) effort is focused on thin films and surfaces, with a particularly strong effort in the area of thin-film photovoltaics (PV). This group and their labs are highly regarded as one of the top in the U.S. for thin-film PV. While a primary component of the PV research program is the study of thin-film solar cell devices which includes a wide range of different technologies and approaches including nonlinear optics and fast photon spectroscopy, spectroscopic ellipsometry, multi-exciton transitions in nanoparticles, semiconductor and conductor materials, organic PV materials, carbon nanotubes, nanoparticle hybrid devices, and earth-abundant materials. Besides enhanced materials and technologies for PV devices, another area is direct PV-hydrogen generation by water electrolysis and this leads to discussions of transportation fuels, infrastructure, and hydrogen fuel cells. Students get involved in a wide range of characterization techniques, including Raman scattering, photoluminescence, x-ray diffraction, scanning electron microscopy (SEM), atomic force microscopy, as well as current-voltage and spectral- quantum-efficiency.
The above figure shows work on nano-crystals of CoFeS4 by an REU student utilizing Raman Spectra on the left and SEM on the right.
Highlighted Project: Title: Composite Nanocrystalline Germanium + Amorphous Si Films, Mentors: Prof. N. Podraza (NP), graduate student Biwas SubediDescription: NP’s group is working on experimentally fabricating thin films with isolated fractions of germanium nano-crystallites (nc-Ge) embedded within and passivated by a hydrogenated amorphous silicon (a-Si:H) network in order to simultaneously manipulate electronic and optical response for photovoltaic applications. The target application involves inorganic-organic lead halide perovskite (CH3NH3PbI3) based photovoltaics (PV), which have very high initial conversion efficiencies (> 20%) with a band gap near 1.5-1.6 eV. The film will be tailored to control crystallite fraction and connectivity during deposition based on real-time spectroscopic ellipsometry (RTSE) monitoring. Composites will serve as absorbers in single-junction PV. RTSE monitoring of nc-Ge + a-Si:H composites will be used to study growth evolution, control the microstructure, and design fabrication processes. Structural, optical, and electrical properties will be characterized as functions of nc-Ge content and interconnectivity to identify the range over which fundamental properties can be manipulated. Device physics for PV with narrow band gap composites will be studied with the overall intention of enhancing long-wavelength absorption without decreasing Voc significantly. Tandem structure designs incorporating CH3NH3PbI3 based top junctions and nc-Ge + a-Si:H based bottom junctions with the potential to achieve 30% efficiency will be evaluated.
Professional Development: As this project is in the exploratory stage, the REU student researcher will be performing initial depositions, characterization of the composite film structure, and identifying any process-property relationships. The student will learn how to do these depositions via sputtering and plasma-enhanced chemical vapor deposition. After spectroscopic ellipsometer operation training, the student will utilize the ellipsometer as a sophisticated tool for optical metrology to help characterize their films. From their study, the ellipsometry should yield information on dielectric functions of layered optically isotropic or anisotropic materials, film thicknesses, interface roughness, and compositions (void and alloy fractions), and depth profiles of inhomogeneous thin films. These efforts will provide basic training in condensed matter experimental work applied to materials fabrication and characterization.
Other projects: “Influence of Activation on the Photoluminescence of Thin Film Cadmium Telluride” (Graduate student Niraj Shrestha, Profs. A. Phillips, M. Heben, R. Ellingson); We are studying defects in CdTe through optical photoluminescence (PL) studies. Through comparative measurements of the PL emission from CdTe processed with CdCl2, MgCl2, and ZnCl2, we can discover whether and how the activation process introduces optically active defects within the CdTe thin film. The REU student will conduct various PL measurements and analyze the resulting data sets.
“Non-Contact THz Metrology for Electrical Property Extraction”, (Prof. N. Podraza, student K. Subedi) The REU student researcher will be studying the long-wavelength optical and electrical response first of a commercial substrate, then coated by each subsequent layer until the PV device is complete—exposing the student both to device fabrication as well as fundamental materials characterization.
“Earth Abundant Thin-Film Solar Cells for Sustainable Solar Energy Production”, (Prof. Y. Yan group, graduate student Sandeep Bista)
We will study the fabrication of solar cells in the perovskite CH3NH3PbI3 system of earth-abundant materials. Our group has recently succeeded in creating a world-record efficiency device by reducing Pb requirements by 50% in this system. Further investigation of a complete replacement of Pb is on-going. The REU student will generate his/her own solar cells with our extant set-up, under Mr. Bista’s supervision, and characterize cell characteristics such as I-V curves, quantum efficiency, fill factor, Voc etc. The student will learn about the intricacies of tactful manipulation of experimental parameters for obtaining highly functional devices and the careful characterization, record-keeping, and organization of data that follows. A good grasp of the physics of solar cells and relevant materials will also emerge.
Theoretical and computational research in condensed matter is focused on the study of thin-film growth and atomic-scale growth processes on surfaces and bulk, the fundamental study of non- equilibrium processes and defects in materials, studies of whisker formation in conductors and aging in photovoltaics, the origin, and repair of non-uniformities in large-scale electronic devices such as photovoltaics solar cells and displays, resistive switching random access memory, and the theoretical search for new magnetic and hard coating materials applications. A variety of research methods are used including density functional theory, kinetic Monte Carlo simulations, regular and accelerated molecular dynamics simulations, as well as analytical and numerical methods. The research on thin-film growth and atomic-scale growth processes also involves the development and use of a variety of parallel computational methods and algorithms.
Highlighted Project: Title: Predicting High Performance Coatings of Transition Metal Nitrides by First Principles Computations, Mentors: Prof. S. V. Khare, graduate student Bishal Dumre
Description: Transition-metal nitrides are well known for their remarkable physical properties including high hardness and mechanical strength, chemical inertness, and high-temperature stability. As a result, they are widely studied and have become technologically important for applications such as hard wear-resistant coatings, diffusion barriers, and optical coatings. The annual world market of $20-$70 billion for cutting tools alone, has spurred worldwide research and development effort for the last three-and-a-half decades, as even modest improvements in tool lifetime or cutting speed have tremendous cost savings potential. Khare group has been investigating binary (M(1)xM(2)yN) nitrides for over two decades and has recently demonstrated an approach with the ATAT software to theoretically predict temperature- composition phase diagrams of ternary nitrides to discover new alloys for improved coating performance. The novelty of this method is that it is purely an ab initio computational method, not relying on experimental input. It saves significant experimental time, on the order of decades, by narrowing the search space of materials from a few thousand compounds to about tens of them. The REU student will be assigned a ternary system such as Ti-Cu-N to compute part of its phase diagram and some mechanical properties.
Professional Development: An REU student would be running of Python scripts developed in Khare’s group by former and current graduate students to perform ab initio computations with VASP software. Pre- and post- computation file generation, understanding of various crystal structure types and generation of lattice based solid-solution structures computationally with our existing SQS codes would be part of his/her learning. We have access to the Ohio Supercomputer cluster as well as other clusters outside Ohio due to Khare’s collaborations. The student would learn the basics on how to access these machines, learn the preliminary use of Linux and MPI for conducting computations on multiple (~256) parallel threads.
Other projects: “Parallel temperature-accelerated dynamics simulations of non-equilibrium processes”, Mentors: Prof. J. G. Amar, graduate student Indiras Khatri; We are developing and applying improved methods for parallel temperature-accelerated dynamics in order to simulate the evolution of driven condensed matter systems on extended time- and length-scales. An REU student will participate in the application of these methods to one of several possible projects including simulations of the evolution of the structure and morphology of thin-films during sub-mono- and/or multi-layer thin-film growth. Simulations will be carried out using resources at the Ohio Supercomputer Center.
Broader Impacts: There are several impacts for REU students in our program, a few are listed: (i) Learning to use a variety of experimental devices and equipment and understanding what physics has been obtained by performing the experiment. Estimating the uncertainty in the measured data, its origins, and the statistics of data analysis is a key part of all our research groups. (ii) Learning to use a variety of computational software, testing the software, and realizing that just because a program runs does not mean its output is meaningful. Learning to analyze and critically evaluate the output of computer programs by using test runs and basic knowledge of physics as applied to the computation. (iii) Organizing and presenting the data in a technical manner consistent with the norms of the field of science. (iv) Working with multi-cultural teams of postdocs, graduate students, technicians, and fellow undergraduates.