Browsing by Subject "Density functional theory"

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A parallel eigensolver for real-space pseudopotential density functional theory calculations
(2022-06-23) Liou, Kai-Hsin; Chelikowsky, James R.; Demkov, Alexander A.; Hwang, Gyeong S.; Korgel, Brian A.
First-principles electronic structure calculations are a popular tool for understanding and predicting properties of materials. Among such methods, the combination of real-space density functional theory and pseudopotentials to solve the Kohn–Sham equation has several advantages. Real-space methods, such as finite differences and finite elements, avoid the global communication needed in fast Fourier transformation and offer better scalability for large calculations on hundreds or thousands of compute nodes. Besides, finite-difference methods with a uniform real-space grid are easy to implement, e.g., the convergence of a Kohn–Sham solution is controlled by a single parameter – the grid spacing. One promising algorithm for solving the Kohn–Sham eigenvalue problem in real space is the Chebyshev-filtered subspace iteration method (CheFSI). Within this algorithm, the charge density is constructed without regard to a solution for individual eigenvalues. However, for large systems CheFSI may suffer from super-linear scaling operations such as orthonormalization and the Rayleigh–Ritz procedure. In the dissertation I will present two improvements in CheFSI to enhance its scalability and accelerate calculation. The first one is a hybrid method that combines a spectrum slicing method and CheFSI. The spectrum slicing method divides a Kohn–Sham eigenvalue problem into subproblems, wherein each subproblem can be solved in parallel using CheFSI. We will show that, by the simulations of confined systems with thousands of atoms, this hybrid method can be faster and possesses better scalability than CheFSI. The second improvement is a grid partitioning method based on space-filling curves. Space-filling curves based grid partitioning improves the efficiency of the sparse matrix–vector multiplication, which is the key component of CheFSI. We will show that, by computations of confined systems with 50,000 atoms or 200,000 electrons, this method effectively reduces the communication overhead and improves the utilization of the vector processing capabilities provided by most modern parallel computers. Along with the improvements, I will also present three applications. One is the study of the evolution of density of states of silicon nanocrystals from small ones to their bulk limit. The simulations can hardly be performed without the improvement in sparse matrix–vector multiplication enhanced by space-filling curves based grid partitioning. The other two applications are the studies of proton transfer in liquid water and the adsorption of water on titanium dioxide surfaces.
Ab initio simulation methods for the electronic and structural properties of materials applied to molecules, clusters, nanocrystals, and liquids
(2014-05) Kim, Minjung, active 21st century; Chelikowsky, James R.
Computational approaches play an important role in today's materials science owing to the remarkable advances in modern supercomputing architecture and algorithms. Ab initio simulations solely based on a quantum description of matter are now very able to tackle materials problems in which the system contains up to a few thousands atoms. This dissertation aims to address the modern electronic structure calculation methods applied to a range of various materials such as liquid and amorphous phase materials, nanostructures, and small organic molecules. Our simulations were performed within the density functional theory framework, emphasizing the use of real-space ab initio pseudopotentials. On the first part of our study, we performed liquid and amorphous phase simulations by employing a molecular dynamics technique accelerated by a Chebyshev-subspace filtering algorithm. We applied this technique to find l- and a- SiO₂ structural properties that were in a good agreement with experiments. On the second part, we studied nanostructured semiconducting oxide materials, i.e., SnO₂ and TiO₂, focusing on the electronic structures and optical properties. Lastly, we developed an efficient simulation method for non-contact atomic force microscopy. This fast and simple method was found to be a very powerful tool for predicting AFM images for many surface and molecular systems.
Ab-initio electronic structure and quantum transport calculations on quasi-two-dimensional materials for beyond Si-CMOS devices
(2013-05) Chang, Jiwon, active 2013; Banerjee, Sanjay; Register, Leonard F.
Atomically two-dimensional (2-D) graphene, as well as the hexagonal boron nitride dielectric have been and are continuing to be widely investigated for the next generation nanoelectronic devices. More recently, other 2-D materials and electronic systems including the surface states of topological insulators (TIs) and monolayers of transition metal dichalcogenides (TMDs) have also attracted considerable interest. In this work I have focused on these latter two material systems on possible device applications. TIs are characterized by an insulating bulk band gap and metallic Dirac surface states which are spin-polarized. Here, the electronic structures of bulk and thin film TIs are studied using ab-initio density functional theory (DFT). Band inversion, an essential characteristic of TIs, is shown in the bulk band structures. Properties of TI surface bands in thin film such as the critical film thickness to induce a gap, the thickness dependent gap size, and the localization length of surface states are reported. Effects of crystalline dielectric materials on TI surface states are also addressed by ab-initio calculations. I discuss the sensitivity of Dirac point degeneracy and linear band dispersion of TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and TI. Additionally, this work presents research on exciton condensation in TI using a tight-binding model combined with self-consistent non-local Hartree-Fock mean-field theory. Possibility of exciton condensation in the TI Bi₂Se₃ thin film is assessed. Non-equilibrium Green's function (NEGF) simulations with the atomistic tight-binding (TB) Hamiltonian are carried out to explore the performance of metal-oxide-semiconductor field-effect-transistor (MOSFET) and tunnel field-effect-transistor (TFET) based on the Bi₂Se₃ TI thin film. How the high dielectric constant of Bi₂Se₃ affects the performance of MOSFET and TFET is presented. Bulk TMDs such as MoS₂, WS₂ and others are the van der Waals-bonded layered material, much like graphite, except monolayer (and Bulk) TMDs have a large band gap in-contrast to graphene (and graphite). Here, the performance of nanoscale monolayer MoS₂ n-channel MOSFETs are examined through NEGF simulations using an atomistic TB Hamiltonian. N- and p-channel MOSFETs of various monolayer TMDs are also compared by the same approach. I correlate the performance differences with the band structure differences. Finally, ab-initio calculations of adatom doping effects on the monolayer MoS₂ is shown. I discuss the most stable atomic configurations, the bonding type and the amount of charge transfer from adatom to the monolayer MoS₂.
Catalytic reactions at alloy surfaces
(2020-03-27) Li, Hao, Ph. D.; Henkelman, Graeme; Mullins, Charles B; Hwang, Gyeong; Humphrey, Simon
Alloys have been widely studied for heterogeneous catalysis. Many bi- and multi-metallic alloys have enhanced performance as compared to their monometallic counterparts. However, a full understanding of the alloying effects was not well-established. In my Ph.D. works, density functional theory (DFT) was employed to disentangle the atomic ensemble, ligand, and strain effects of surfaces alloyed by transition metals. It is found that alloying elements with strong and weak adsorption properties could produce a surface ensemble with optimally tuned adsorbate binding, which can help to understand the mechanisms of catalytic reactions and design high-performance alloy catalysts. We developed a Tunability theory that quantifies the tuning of adsorbate bindings at the specific atomic ensembles on surface, which provides predictive power of theory for experiments. Using combined theoretical and experimental methods, we designed and studied new alloy catalysts for many industrially significant reactions including electrocatalysis, vapor-phase catalysis, and liquid-phase catalysis. We developed comprehensive theories that predominantly based on the atomic ensemble effect to unify theories and experiments for alloy heterogeneous catalysts. Most importantly, we show how fundamental understandings from theories can be precisely applied to efficient energy and environmental reactions. In addition, to accelerate atomistic simulations and materials design, we have been developing a machine learning framework that can fit the potential energy surfaces from the quantum mechanical data, which can help to partly replace expensive DFT calculations and reduce scientific costs
Computational investigation of functional perovskites
(2018-06-12) Li, Xinyu, Ph. D.; Henkelman, Graeme; Zhou, Jianshi; Goodenough, John; Hwang, Gyeong
Functional perovskites have been investigated extensively for many years. Thousands of new perovskites are synthesized and studied every year. Many functional perovskites have been widely employed in industry. Density functional theory (DFT) calculations have been used to obtain a better understanding of functional perovskites, especially their electronic and structural properties. During my graduate study, I investigated perovskite’s properties on ionic transport, magnetic ordering, ferroelectricity, physical property and phase transition using DFT calculations. In the first case, I simulated the ionic transport process in several Ruddlesden- Popper (RP) phases. Climbing image nudged elastic band (CI-NEB) calculation was used to get accurate oxygen interstitial migration barrier. I established a linkage between interstitial migration barrier and perovskite’s octahedral rotation with symmetry mode approach. Two factors, including A-site atom radius and epitaxial strain, were used to reduce interstitial migration barrier in my simulation. My study on ionic transport in RP phases provides guidance on the design of fast ionic transport in perovskite oxides. In the second case, DFT calculation was employed to investigate a double perovskite’s magnetic and electronic properties. A new ferroelectric mechanism in perovskite, associated with the displacement of coplanar Mn²⁺, was discovered experimentally. My DFT calculation explained the origin of coplanar displacement from an orbital point of view. In addition, DFT simulations were used in the design of ferroelectricity enhancement perovskite. In the last case, I simulated structural behaviors under pressure of several double perovskites. The results show that these double perovskites can be divided into two groups based on their octahedral rotations under pressure. The origin of their distinct volume reduction mechanisms was studied through DFT simulations. The difference between the two mechanisms and their influence on bulk modulus were discussed based on my computational results.
Computing accurate solutions to the Kohn-Sham problem quickly in real space
(2014-08) Schofield, Grady Lynn; Chelikowsky, James R.
Matter on a length scale comparable to that of a chemical bond is governed by the theory of quantum mechanics, but quantum mechanics is a many body theory, hence for the sake of chemistry or solid state physics, finding solutions to the governing equation, Schrodinger's equation, is hopeless for all but the smallest of systems. As the number of electrons increases, the complexity of solving the equations grows rapidly without bound. One way to make progress is to treat the electrons in a system as independent particles and to attempt to capture the many-body effects in a functional of the electrons' density distribution. When this approximation is made, the resulting equation is called the Kohn-Sham equation, and instead of requiring solving for one function of many variables, it requires solving for many functions of the three spatial variables. This problem turns out to be easier than the many body problem, but it still scales cubically in the number of electrons. In this work we will explore ways of obtaining the solutions to the Kohn-Sham equation in the framework of real-space pseudopotential density functional theory. The Kohn-Sham equation itself is an eigenvalue problem, just as Schrodinger's equation. For each electron in the system, there is a corresponding eigenvector. So the task of solving the equation is to compute many eigenpairs of a large Hermitian matrix. In order to mitigate the problem of cubic scaling, we develop an algorithm to slice the spectrum into disjoint segments. This allows a smaller eigenproblem to be solved in each segment where a post-processing step combines the results from each segment and prevents double counting of the eigenpairs. The efficacy of this method depends on the use of high order polynomial filters that enhance only a segment of the spectrum. The order of the filter is the number of matrix-vector multiplication operations that must be done with the Hamiltonian. Therefore the performance of these operations is critical. We develop a scalable algorithm for computing these multiplications and introduce a new density functional theory code implementing the algorithm.
Data-driven prediction of nonequilibrium chemistry in plasma enhanced atomic layer etching of silicon nitride
(2023-05) Cheng, Erik S.; Hwang, Gyeong S.; Raja, Laxminarayan L; Henkelman, Graeme A; Ekerdt, John G
In the semiconductor industry, plasma enhanced atomic layer etching (PEALE) has attracted significant attention due to its potential for high quality etch in nanometer-scale and high-aspect ratio features, where atomic layer precision is critical. PEALE consists of two cyclic steps: 1) treatment of a surface with some precursor, and 2) ion bombardment-assisted removal of material. However, these processes can be challenging to study both experimentally and computationally; they involve timescales too short to be experimentally observable, but also involve complex dynamics that can be difficult to comprehensively model in simulations. Thus, fundamental mechanisms of PEALE are poorly understood. In this dissertation, a computational framework based on self-consistent-charge density functional tight binding (SCC-DFTB) and density functional theory (DFT) have been developed and applied to the case of PEALE of silicon nitride (SiN) with hydrofluorocarbons (HFCs). This framework includes an extension to the pbc-0-3 parameter set as well as tooling for automated execution and analysis of parallel TBMD simulations. We begin by investigating the possibility of thermal reactions between HFCs and SiN substrates, both under ambient conditions as well as under the influence of bombardment-induced local heating. Through this, we show that thermal reactions are unlikely to be the primary contributor to achieving PEALE of SiN with HFCs. Then, the structural and compositional evolution of SiN under ion bombardment without HFCs are assessed. Following this, we identify key mechanisms that govern bombardment-induced decomposition of physisorbed HFCs, with emphasis placed on analyzing the assisting role of the supporting SiN substrate. The surface reactions that can occur during bombardment, as well as after, have been described and used to predict the formation pathways of the quasi-equilibrium films that are generated during PEALE. Finally, the implications of these reactions for enhancing SiN etch through chemical and physical modification are discussed, especially the complexity of non-equilibrium etch products that play a key role. In this work, new atomic-level insight is gained into the mechanisms of SiN PEALE, with implications for process design in the semiconductor industry. Furthermore, this new computational approach is a general strategy that can be applied to many other plasma-driven processes, and for other non equilibrium chemistry as a whole.
Density functional theory calculations in materials science and catalysis
(2019-06-13) Chai, Wenrui; Henkelman, Graeme; humphrey, simon; Mullins, Charles B; hwang, gyeong
As scientists are pushing the limit of technology, experimental trial and error explorations are becoming increasingly unaffordable, especially when accurate manipulation of materials at the atomic level is essential. The need for theoretical oversight is ever increasing while traditional empirical theories are becoming obsolete. Density Functional Theory (DFT) is therefore developed as a portable theoretical tool that can shed light on many different fields based on first principle quantum mechanics. It is enjoying increasing popularity for its ability to provide information complementary to experimental characterization methods, as well as its astounding prediction power that can potentially vastly increases experimental output. In this dissertation, I showcase how DFT can be used to support experimental endeavors by providing insights and validations otherwise unobtainable and deepen our understanding of a variety of subjects. DFT calculations are also used to validate and explain experimental characterization results. For isolated Pt atom and Pt clusters with few atoms, the Pt dband center and hydrogen binding energy were calculated, used in conjunction with cyclicvoltammetry data to characterize the Pt atom/clusters and explain the observed activity towards hydrogen evolution reaction. For methodology development, DFT calculations are used to provide a plausible mechanism for the technique of hydrogen elimination monitoring in ultraviolet photodissociation of proteins for mass-spectroscopy to solve protein structures. It shows that depending on the degree of hydrogen bonding engagement, backbone cleavage can take place first and prevent succeeding hydrogen transfer. The results explained why the technique is a reliable method for finding protein structure information. Towards the development of materials, DFT calculations are used to find reaction mechanisms for hydrogenation using carbon-nitrogen-phosphorous pincer Fe catalysts and to find causes for geometry change for post-synthetically modified metal-organic frameworks
Experimental and theoretical investigation of electrochemically synthesized AuPt dendrimer-encapsulated nanoparticles (DENs)
(2020-06-22) Lapp, Aliya Siegel; Crooks, Richard M. (Richard McConnell); Henkelman, Graeme; Mullins, Charles B; Humphrey, Simon M; Roberts, Sean T
Herein, experiment and theory are combined to study the efficacy of synthesizing core@shell Au@Pt dendrimer-encapsulated nanoparticles (DENs) through electrochemical means. DENs are small (~1-2 nm), catalytically active nanoparticles (NPs) with precise sizes and compositions. These features assist pairing experiment with theory and catalytic interpretation. The small sizes of DENs can impart interesting physical and chemical properties that differ from bulk phase materials. Core@Pt shell NPs with monolayer (ML)-thick shells minimize the use of Pt, which is a key catalyst for various reactions but is expensive and scarce. Owing to the fact that the available techniques for Pt ML deposition were originally developed for bulk Au, their applicability to ~1.6 nm Au DEN cores is not trivial. In this dissertation, we explore several electrochemical strategies for synthesizing Au@Pt DENS: hydride-terminated (HT) Pt electrodeposition and underpotential deposition, followed by galvanic exchange with Pt (UPD/Pt GE) using two different UPD metals (Cu and Pb). Each of these techniques deposits a single Pt ML onto bulk Au surfaces. However, when they are applied to ~1.6 nm Au DENs, the AuPt NP structures that form are both dissimilar to one another and to the corresponding structures for bulk Au. The HT synthesis is found to lead to an alloy structure, whereas AuPt NP structure formed upon UPD/Pt GE depends on the choice of the UPD metal (Cu or Pb). More specifically, Cu UPD/Pt GE produces a core@shell structure, whereas an alloy structure is afforded by Pb UPD/Pt GE. These conclusions are supported by extensive experimental characterization and density functional theory (DFT) calculations. For the HT method, a core@shell structure can ultimately be obtained, but requires 3-5 total HT pulses. Due to the fact that catalysis tends to be highly structure sensitive, the results of these studies are important for rational design of catalysts. Indeed, we show that varying the number of HT pulses (from 1-10) can be used to tune electrocatalysis for formic acid oxidation (FAO).
First principles calculations of Raman spectra for nanostructures and improved high order forces
(2015-12) Bobbitt, Nathaniel Scott; Chelikowsky, James R.; Demkov, Alexander A; Ekerdt, John G; Hwang, Gyeong S; Korgel, Brian A
Advances in computing technology coupled with theoretical developments on the electronic structure problem have laid the foundation for the rapidly growing field of computational materials science. Modern supercomputers are able to perform ab initio calculations of realistic systems containing thousands of atoms. This is an important step forward in the maturation of the field because computational insight can be used to make predictions about or predict experimental data. This dissertation aims to address contemporary theory and practice of solving the electronic structure problem for a variety of nanoscale systems, most of which are of interest for energy application such as photovoltaics or Li-ion batteries. Our calculations are performed within density functional theory using real-space pseudopotentials. In the first part, we examine nanocrystals. We calculate size-dependent properties for ZnO nanocrystals with Al and Ga dopants. Next, we calculate Raman spectra for Si nanocrystals with Li impurities and Si-Ge core-shell structures, which gives us insight into the structure of these nanocrystals. In the second portion, we examine in depth the calculation of interatomic forces within density functional theory and propose a new integration scheme which we demonstrate calculates more accurate bond lengths and vibrational frequencies and improves the stability of molecular dynamics simulations.
First principles study of silicon-based nanomaterials for lithium ion battery anodes
(2014-05) Chou, Chia-Yun Ph. D.; Hwang, Gyeong S.; Mullins, Charles B; Manthiram, Arunmugam; Ekerdt, John G; Stevenson, Keith
Silicon (Si)-based materials have recently emerged as a promising candidate for anodes in lithium-ion batteries because they exhibit much higher energy-storage capacities than the conventional graphite anode. However, the practical use of Si is hampered by its poor cycleability; during lithiation, Si forms alloys with Li and undergoes significant structural and volume changes, which can cause severe cracking/pulverization and consequent capacity fading arising from the loss of electrical contacts. To overcome these drawbacks, many innovative approaches have been explored with encouraging results; however, many fundamental aspects of the lithiation behavior remain ambiguous. Hence, the focus of this work is to develop a better understanding of the lithiation process at the atomistic scale using quantum mechanical calculations. In addition, based on the improved understanding, we attempt to address the fundamental mechanisms behind the successful approaches to enhance the anode performance. To lay a foundation for the investigation of alloy-type anodes, in Chapter 3, we first examine how lithiation occurs in Si and the formation of crystalline and amorphous LixSi alloys (0 ≤ x ≤ 4); followed by assessing the lithiation-induced changes in the energetics, atomic structure, electronic and mechanical properties, and Li diffusivity. The same approach is then extended to analyze the lithiation behavior of germanium (Ge) and tin (Sn) for developing a generalized understanding on the Group IV alloy-type anodes. Along this comparative study, we notice a few distinguishing features pertain only to Si (or Ge), such as the facile Li diffusion in Ge and facet-dependent lithiation in Si, which are discussed in Chapter 4. Beyond the fundamental research, we also look into factors that may contribute to the improved anode performance, including (i) finetuning of the oxidation effects in Si-rich oxides, [alpha] -SiO [subscript 1/3] (Chapter 5), (ii) maximizing the surface effects through nano-engineered structures (Chapters 6 & 7), and finally (iii) the role of interface in Si-graphene (carbon) composites (Chapter 8).
First-principles investigation of carbon-based nanomaterials for supercapacitors
(2016-06-29) Pak, Alexander Jin; Hwang, Gyeong S.; Ekerdt, John G; Manthiram, Arumugam; Akinwande, Deji; Ren, Pengyu
Supercapacitors are electrochemical energy storage devices known for their large power densities and long lifetimes yet limited energy densities. A conventional understanding of supercapacitors relates the high power to fast ion accumulation at the polarized electrode interface, forming the so-called electric double layer (EDL), and the low energy to limited electrode surface area (SA). To improve the energy density, the capacitance may be enhanced by using high SA electrode materials such as carbon-based nanomaterials. While promising results have been experimentally reported, capacitances have also been noted to exhibit a highly non-linear relationship with SA. These interesting observations suggest that a gap exists in our fundamental understanding of charge storage mechanisms in the EDL of carbon nanomaterials. Given that EDLs are typically on the order of 1-3 nm thick, theoretical simulations can elucidate these unknown physical insights in order to identify new design principles for future electrode materials. In this dissertation, we explore two broad types of carbon-based nanomaterials, which are separated into two Parts, using a combined density functional theory and classical molecular dynamics computational approach. In Part I, we study the capacitance using various chemically and/or structurally modified graphene (or graphene-derived) materials which is motivated by previous accounts of the limited capacitance using pristine graphene. Our analysis demonstrates the viability of dramatically improving the capacitance using graphene-derived materials owing to enhancements in the quantum capacitance with marginal effects on the double layer capacitance. In Part II, we investigate the capacitance using nanoporous carbon materials which is motivated by experimental observations that relate capacitance to pore width rather than SA. Our findings confirm that promoting ion confinement through pore width control can enhance capacitance, but also identify pore shape dispersity as another important structural feature that facilitates fast ion dynamics during charging/discharging. The work in this dissertation presents an overview of new insights into charge storage mechanisms using low-dimensional carbon-based nanomaterials and future directions for materials development. Moreover, we anticipate that the established methodologies and analyses can be broadly applicable to the study of other applications utilizing electrified interfaces, including capacitive deionization and liquid-gated field effect transistors.
First-principles studies on degradation of aqueous amines for carbon dioxide capture
(2022-04-27) Yoon, Bohak; Hwang, Gyeong S.; Rochelle, Gary T.; Hildebrandt-Ruiz, Lea; Ren, Pengyu
Chemical absorption with aqueous amine-based solvents has been the most promising incumbent technology for post-combustion CO₂ capture from flue gas. However, its extensive operation is severely limited by the large cost attributed to the enormous energy requirement for solvent regeneration and degradation issues leading to makeup of amine solvent loss. First-principles atomistic modeling can provide key insights into elucidating chemical phenomena pertinent to degradation behavior in CO₂-loaded aqueous amine solution, which is often extremely challenging to be experimentally characterized. In this dissertation, our first-principles works on illuminating the molecular mechanisms governing solvent degradation of aqueous amine during CO₂ capture are presented. Using density functional theory based ab initio molecular dynamics with enhanced sampling techniques, we identify elementary reactions governing CO₂ capture and degradation. Molecular mechanisms of thermal and oxidative degradation of aqueous amine solvents are discussed in perspective of both thermodynamics and kinetics. We systematically investigate on the factors prevailing key reaction rates, such as amine functional groups, the steric hindrances, classes of amines (primary and secondary), concentration of amines, solvation nature, and temperature conditions. These factors may largely affect relative strengths of both inter- and intramolecular hydrogen bond interactions in CO₂-loaded aqueous amine solution. Our theoretical studies further illustrate the importance of an atomistic-level description of solvation structure and dynamics that may primarily govern CO₂ reaction with aqueous amine solvents and associated degradation mechanisms. This dissertation highlights the key role of first-principles computational modelling in accurately describing mechanistic understandings on CO₂ capture by aqueous amine solvents and associated degradation processes. The enhanced atomisticlevel descriptions will provide more complete explanations for experimental characterizations and valuable suggestions on how to optimize existing solvents and design more cost-efficient solvents for carbon capture processes.
First-principles study of electronic and topological properties of graphene and graphene-like materials
(2013-08) Jadaun, Priyamvada, 1983-; Banerjee, Sanjay; Register, Leonard F.
This dissertation includes work done on graphene and related materials, examining their electronic and topological properties using first-principles methods. Ab-initio computational methods, like density functional theory (DFT), have become increasingly popular in condensed matter and material science. Motivated by the search for novel materials that would help us devise fast, low-power, post-CMOS transistors, we explore the properties of some of these promising materials. We begin by studying graphene and its interaction with dielectric oxides. Graphene has recently inspired a flurry of research activity due to its interesting electronic and mechanical properties. For the device community, graphene's high charge carrier mobility and continuous gap tunability can have immense use in novel transistors. In Chapter 3 we examine the properties of graphene placed on two oxides, namely quartz and alumina. We find that oxygen-terminated quartz is a useful oxide for the purpose of graphene based FETs. Inspired by a recent surge of interest in topological insulators, we then explore the topological properties of two-dimensional materials. We conduct a theoretical study to examine the relationship between crystal space group symmetry and the electric polarization of a two-dimensional crystal. We show that the presence of symmetry restricts the polarization values to a small number of distinct groups. There groups in turn are topologically inequivalent, making polarization a topological index. We also conduct density functional theory calculations to obtain actual polarization values of materials belonging to C3 symmetry and show that our results are consistent with our theoretical analysis. Finally we prove that any transformation from one class of polarization to another is a topological phase transition. In Chapter 5 we use density functional theory to examine the electronic properties of graphene intercalation compounds. Bilayer pseudospin field effect transistor (BiSFET) has been proposed as an interesting low-power, efficient post-CMOS switch. In order to implement this device we need bilayer graphene with reduced interlayer interaction. One way of achieving that is by inserting foreign molecules between the layers, a process which is called intercalation. In this chapter we examine the electronic properties of bilayer graphene intercalated with iodine monochloride and iodine monobromide molecules. We find that intercalation of graphene indeed makes it promising for the implementation of BiSFET, by reducing interlayer interaction. As an interesting side problem, we also use hybrid, more extensive approaches in DFT, to examine the electronic and optical properties of dilute nitrides. Dilute nitrides are highly promising and interesting materials for the purposes of optoelectronic applications. Together, we hope this work helps in elucidating the electronic properties of promising material systems as well as act as a guide for experimentalists.
From polymer collapse to confined fluids : investigating the implications of nterfacial structuring
(2009-08) Goel, Gaurav; Truskett, Thomas Michael, 1973-
In the first part of this thesis, we present results from extensive molecular dynamics simulations of the collapse transitions of hydrophobic polymers in explicit water. The focus is to understand the roles that curvature and interactions associated with the polymer-water “interface” have on collapse thermodynamics. We show that model hydrophobic polymers can have parabolic, protein-like, temperature-dependent free energies of unfolding. Analysis of the water structure shows that the polymer-water interface can be characterized as soft and weakly dewetted. We also show that an appropriately defined surface tension for the polymer-water interface is independent of the attractive polymer-water interactions. This helped us to develop a perturbation model for predicting the effect of attractions on polymer collapse thermodynamics. In the second part, we explore connections between structure, thermodynamics, and dynamics of inhomogeneous fluids. First, we use molecular dynamics simulations and classical density functional theory (DFT) to study the hard-sphere fluid at approximately 103 equilibrium state points, spanning different confining geometries and particle-boundary interactions. We provide strong empirical evidence that both excess entropy and a new generalized measure of available volume for inhomogeneous fluids correlate excellently with self-diffusivity, approximately independent of the degree of confinement. Next, we study via simulations how tuning particle-wall interactions to flatten or enhance the particle layering of a model confined fluid impacts its self-diffusivity, viscosity, and entropy. Interestingly, interactions that eliminate particle layering can significantly reduce confined fluid mobility, whereas those that enhance layering can have the opposite effect. Excess entropy helps to understand and predict these trends. Finally, we explore the relationships between the effective interparticle interactions, static structure, and tracer diffusivity of a solute in a mixture. We show that knowledge of these relationships can allow one to “tune” the effective interparticle interactions of the solute in a way that increases its tracer diffusivity. One interesting consequence is that the mobility of a hard-sphere solute can be increased by adding a soft-repulsion to its interaction, effectively making it bigger.
Functional oxide heterostructures on semiconductors
(2013-08) Seo, Hosung; Demkov, Alexander A.
Complex oxides exhibiting a wide variety of novel functional properties such as ferromagnetism and ferroelectricity have been extensively studied during the past decades. Recent advances in the field of oxide heteroepitaxy have made it possible to create and control hybrid oxide heterostructures with abrupt epitaxial interfaces. The oxide heteroepitaxy with the capability of controlling interface composition, strain, length scales, etc. has opened the totally new and exciting scientific avenue and has offered potential device applications to be explored. Epitaxial integration of functional oxides on semiconductor such as Si (001) and Ge(001) is of great interest, as it potentially leads to further technological development of these interesting oxide systems. In this dissertation, using density functional theory we explore physics and chemistry of novel oxide heterostructures and issues related to the integration of functional oxides on semiconductors. Oxide materials that are studied in this dissertation include polar LaAlO₃, high-k dielectric SrTiO₃, photocatalytic anatase TiO₂ and CoO, and strongly correlated magnetic oxide LaCoO₃.
Metal-to-insulator transitions in transition metal oxides : a first principles study
(2015-08) O'Hara, Andrew; Demkov, Alexander A.; Chelikowsky, James R; MacDonald, Allan H; Tsoi, Maxim; Henkelman, Graeme
Transition metal oxides have received significant attention in recent decades due to their ability to display a wide range of novel functional properties. In particular, many oxides are able to undergo metal-to-insulator transitions as a function of external stimuli such as temperature, pressure, and electric field or through doping and defect formation. In the present dissertation, density functional theory is used to explore these phenomena in three systems: (1) the Peierls transition in NbO2, (2) defect formation necessary for HfO2’s resistive switching, and (3) La-doping of SrTiO3 and trap states that may limit conductivity. For NbO2, we use successive improvements to the exchange-correlation energy combined with experiment to improve understanding of the material’s band gap in the insulating phase and show it to be close to 1.2 eV for the direct gap with an indirect gap just below 1.0 eV. Furthermore, we are able to explain the orbital contributions to the dielectric function. Using a combination of transition state theory and phonon dispersion, we demonstrate that the phase transition is driven by a second-order structural transition of the Peierls type. For HfO2, we explore the nature of the metallic gettering layer used to create substoichiometric HfO2-x for resistive switching via an atomistic model of the hafnia-hafnium interface and use transition state theory to study the ability for oxygen to diffuse across the interface. Our investigation shows that the presence of hafnium lowers the formation energy of oxygen vacancies in hafnia, but more importantly the oxidation of hafnium through oxygen migration is energetically favored. In La-doped SrTiO3, the calculations are first used to corroborate optical and electrical measurements by giving values for the density of states effective mass as well as understanding the effect of La-doping on the conductivity and DC relaxation time. Motivated by the experimental observation that even after annealing in oxygen rich environments, heavily n-type doped SrTiO3 shows carrier concentrations inconsistent with dopant concentration, we explore the role that interstitial oxygen may play as a trapping state in SrTiO3. We find three meta-stable sites and that for n-type SrTiO3, interstitials with mid-gap states are favored.
Scalable electronic structure methods to solve the Kohn-Sham equation
(2018-01-23) Lena, Charles Manuel; Chelikowsky, James R.; Demkov, Alexander A; Ekerdt, John G; Hwang, Gyeong S; Gamba, Irene M
From the single hydrogen to proteins in the hundreds of thousands of kilodaltons, scientists can use the electronic structure of interacting atoms to predict their material properties. Knowing the material properties through solving the electronic structure problem, would allow for the controlled prediction and corresponding design of materials. The Kohn-Sham equations, based on density functional theory, transform a many-body problem impossible to solve for anything but the smallest molecules, into a practical problem which can be used to predict material properties. Although KSDFT scales as the cube of the number of electrons in the system, there are additional well documented approximations to further reduce the number of electrons, such as the pseudopotential method. The incoming exascale era will lead to unavoidable challenges in solving the Kohn-Sham equations. These challenges include communication and hardware considerations. Old paradigms, epitomized by repeated series of globally forced synchronization points, will give way to new breeds of algorithms to maximizing scaling performance while maintaining portability. This thesis focuses on the solution to Kohn-Sham DFT in real space at scale. Key to this effort is a parallel treatment of numerical elements involving the Rayleigh-Ritz method. At minimum, the Rayleigh-Ritz projection requires a number of distributed matrix vector operations equal to the number of electrons solved for in a system. Furthermore, the projection requires that number, squared and then halved, of dot products. The memory cost for such an algorithm also grows very large quickly, and explicit intelligent management is not an option. I demonstrate the computational requirements for the various steps in solving for the electronic structure problem for both large and small molecular systems. This thesis also discusses opportunities in real space Kohn-Sham DFT to further utilize floating point optimized hardware the with higher order stencils.
Simulation tools for predicting the atomic configuration of bimetallic catalytic surfaces
(2012-12) Stephens, John Adam; Hwang, Gyeong S.
Transition metal alloys are an important class of materials in heterogeneous catalysis due in no small part to the often greatly enhanced activity and selectivity they exhibit compared to their monometallic constituents. A host of experimental and theoretical studies have demonstrated that, in many cases, these synergistic effects can be attributed to atomic-scale features of the catalyst surface. Realizing the goal of designing -- rather than serendipitously discovering -- new alloy catalysts thus depends on our ability to predict their atomic configuration under technologically relevant conditions. This dissertation presents original research into the development and use of computational tools to accomplish this objective. These tools are all based on a similar strategy: For each of the alloy systems examined, cluster expansion (CE) Hamiltonians were constructed from the results of density functional theory (DFT) calculations, and then used in Metropolis Monte Carlo (MC) simulations to predict properties of interest. Following a detailed description of the DFT+CE+MC simulation scheme, results for the AuPd/Pd(111) and AuPt/Pt(111) surface alloys are presented. These two systems exhibit considerably different trends in their atomic arrangement, which are explicable in terms of their interatomic interactions. In AuPd, a preference for heteronuclear, Au-Pd interactions results in the preferential formation of Pd monomers and other small ensembles, while in AuPt, a preference for homonuclear interactions results in the opposite. AuPd/Pd(100) and AuPt/Pt(100) were similarly examined, revealing not only the effects of the same heteronuclear/homonuclear preferences in this facet, but also a propensity for the formation of second nearest-neighbor pairs of Pd monomers, in close agreement with experiment. Subsequent simulations of the AuPd/Pd(100) surface suggest the application of biaxial compressive strain as a means increasing the population of this catalytically important ensemble of atoms. A method to incorporate the effects of subsurface atomic configuration is also presented, using AuPd as an example. This method represents several improvements over others previously reported in the literature, especially in terms of its simplicity. Finally, we introduce the dimensionless scaled pair interaction, whereby the finite-temperature atomic configuration of any bimetallic surface alloy may be predicted from a small number of relatively inexpensive calculations.
Structural phase transitions in hafnia and zirconia at ambient pressure
(2010-08) Luo, Xuhui; Demkov, Alexander A.; Chelikowsky, James R.; Ekerdt, John G.; MacDonald, Allan H.; Kleinman, Leonard
In recent years, both hafnia and zirconia have been looked at closely in the quest for a high permittivity gate dielectric to replace silicon dioxide in advanced metal oxide semiconductor field effect transistors (MOSFET). Hafnium dioxide or HfO2 is chosen for its high dielectric constant (five times that of SiO2) and compatibility with stringent requirements of the Si process. As deposited, thin hafnia films are typically amorphous but turn polycrystalline after a post-deposition anneal. To control the phase composition in hafnia films understanding of structural phase transitions is a first step. In this dissertation using first principles methods we consider three phase transitions of hafnia and zirconia: monoclinic to tetragonal, tetragonal to cubic and amorphous to crystalline. Because the high surface to volume ratio in hafnia films and powders plays an important role in phase transitions, we also study the surface properties of hafnia. We discuss the mechanisms of various phase transitions and theoretically estimate the transition temperatures. We find two types of amorphous hafnia and show that they have different structural and electronic properties. The small energy barrier between the amorphous and crystalline structures is found to cause the low crystallization temperature. Moreover, we calculate work functions and surface energies for hafnia surfaces and show the surface suppression of the phase transitions.