Physics
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Item Analyzing Dynamical Processes with Local Molecular Field Theory(2023) Zhao, Renjie; Weeks, John D; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Local molecular field (LMF) theory provides a framework for describing the collective response of a system to long-range interactions in nonuniform liquids. Based on this theory, different roles played by the short and long-range components of the intermolecular interactions can be disentangled in determining relevant structural and thermodynamic properties in equilibrium. Furthermore, in dynamical processes, nonlocal long-range interactions are often associated with long relaxation times, and can contribute significantly to the stability of the system in different phases. In this thesis, LMF theory is utilized to quantify and analyze the dynamical effects arising from long-range Coulomb interactions in aqueous solutions, while elucidating how they are connected to strong local forces and fluctuations. The first half of the work concerns ionic and dipolar solvation dynamics, which plays an essential role in many solution phase chemical reactions. The physical models of Gaussian-smoothed charge and dipole distributions are conceptualized from LMF theory to investigate the molecular origins of linear and nonlinear effects in solvation dynamics. The long-range component of the solute-solvent electrostatic interaction is shown to underlie the linear response behavior of the system, while the short-range interactions introduce additional nonlinear effects. The LMF-based solvation models further demonstrate their functionality in probing the intrinsic dielectric dispersion of solvent water. The second half of the work is focused on the nucleation processes in the aqueous environment. Simulating crystal nucleation from solutions requires efficient treatments for intermolecular interactions to drive the transitions on time scales affordable to molecular dynamics simulations. For this purpose, a LMF-based molecular model is employed to capture the renormalized long-range interactions, and well-tempered metadynamics is adopted to enhance the fluctuations arising from short-range interactions. By comparing to a short-range reference model, the necessity of long-range interactions in explaining metastability is revealed. Temporal fluctuations and direct evidence for the two-step nucleation mechanism are observed through the analysis using a deep learning-based approach. The results about these two types of dynamical processes contribute to a deeper understanding of the roles of short and long-ranges interactions in the aqueous systems.Item RAPID HEATING AND CHEMICAL SPECIATION CHARACTERIZATION FOR COMBUSTION PERFORMANCE ANALYSIS OF METALLIZED, NANOSCALE THERMITES AND PVDF BOUND SOLID PROPELLANT COMPOSITIONS(2021) Rehwoldt, Miles Christian; Rodriguez, Efrain; Zachariah, Michael R; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Energetic materials research focuses on performance analysis of cost-effective solid materials which safely, precisely, and efficiently transitions stored chemical potential energy to kinetic energy at a rate throttled through chemical or architectural means. Heterogenous compositions of metal fuels and solid materials with a high storage capacity of condensed oxidizing elements, such as oxygen and/or fluorine, is a class of energetic material of interest given its relatively high reaction enthalpies and adiabatic flame temperatures. In the wake of the earliest instances of metal fuels being used as a high energy additive during World War II, characterizing the reaction mechanisms of micron and nanoparticle aluminum fuels with various oxidizer sources has been a primary subject of research within the solid energetics community. The advent of nanotechnologies within the past two decades brought with it the promise of a prospective revolution within the energetics community to expand the utility and characterization of metallized energetic materials in solid propellants and pyrotechnics. Significant prior research has mapped reactivity advantages, as well as the many short comings of aluminum-based nanoscale energetic formulations. Examples of short comings include difficulties of materials processing, relative increase in native oxide shell thickness, and particle aggregate sintering before primary reaction. The less than flaw-less promises of nanoscale aluminum fuels have thus become the impetus for the development of novel architectural solutions and material formulations to eliminate drawbacks of nanomaterial energetics while maintaining and improving the benefits. This dissertation focuses on further understanding reaction mechanisms and overall combustion behavior of nanoscale solid energetic composite materials and their potential future applications. My research branches out from the heavy research involved in binary, aluminum centric systems by developing generalized intuition of reaction and combustion behaviors through modeling efforts and coupling time-of-flight mass spectrometry to rapid heating techniques and novel modes of product sampling. The studies emphasize reaction mechanisms and microwave sensitivities of under-utilized compositions using metal fuels such as titanium, generalize the understanding of the interaction of fluoropolymer binders with metal fuels and oxidizer particles, and characterize how multi-scale architectural structure-function relations of materials effect ignition properties and energy release rates.Item INNOVATIVE SCANNING PROBE METHODS FOR ENERGY STORAGE SCIENCE: ELUCIDATING THE PHYSICS OF BATTERY MATERIALS AT THE NANO-TO-MICROSCALE(2017) Larson, Jonathan; Reutt-Robey, Janice E; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In recent decades, approaches to generate electrical energy through renewable means has greatly benefited from technological advancements. However, the need for robust schemes to store that energy in safe and cost-effective manners persists. Thus, there is a shared global call to advance electrical energy storage science and technology. Breakthroughs in the field stand to impact humans, ecosystems, environments, economies, and even international security. Currently, many innovative routes rooted in basic science are being taken to develop novel concepts, chemistries, electrolytes, and geometries for electrical energy storage. Many of these approaches make use of nano-to-mesoscale structures and technologies which increases the demand for new methods of characterization and scientific discovery at those scales. Still, progress to address this demand is stymied by practical scientific and technological challenges associated with the buried interfaces in battery systems. In this dissertation, I present how my PhD work has precisely targeted this need within the energy storage community, and made lasting impact. I detail why, and how, I have pioneered scanning-probe based technologies and techniques that make use of “battery probes” consisting of electrochemically active materials. A suite of techniques is developed and leveraged for basic electrical energy storage science: scanning nanopipette and probe microscopy, pascalammetry with microbattery probes, inverted scanning tunneling spectroscopy, and nanoscale solid-state electrochemistry with nanobattery probes. The use of these techniques motivated finite-element numerical simulations of electrostatic potentials, and electric fields, at play during field-driven lithiation of multi-walled carbon nanotubes. Also motivated were analytical models for surface diffusion and diffusion through a stressed electrolyte simultaneously experiencing latent-species activation.Item MANIPULATING AND SIMPLIFYING THE INTERMOLECULAR INTERACTIONS IN LIQUID MIXTURES(2017) Gao, Ang; Weeks, John D; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Long ranged intermolecular interactions have significant influence on the structure of the liquid and present serious challenges for computer simulations. In particular, the long ranged tail of Coulomb interaction usually needs to be calculated using Ewald summation or related techniques in computer simulation, which can be too time consuming to be carried out for large systems. Local Molecular Field(LMF) theory has been developed to simplify long-ranged Coulomb and Van der Waals interactions for nonuniform liquids by approximating these long ranged interactions as effective static single-particle fields. Despite the success LMF theory made in describing the structure of nonuniform liquids, it is not appropriate to use LMF theory to describe the structure of uniform liquid mixtures, since the dynamically moving unbalanced forces produced in mixture can not be captured by the framework of LMF theory. In this thesis, we propose a new framework which approximates the unbalanced forces produced in a mixture as effective intermolecular interactions. This new framework can simplify the long ranged intermolecular interactions and produce a mimic system with short ranged solvent-solvent interactions, which is much easier to simulate or analyze. Based on this framework and other techniques introduced in this thesis, we have constructed a "Short Solvent Model", which has noticeable advantages compared to the explicit solvent model and implicit solvent model. This framework has also been used to simplify the interactions of phase-separating mixtures. The impact of using this framework on the diffusion dynamics of the solutes has also been discussed. Possible application of this framework and the Short Solvent Model to biopolymers folding problems is briefly discussed.Item COLLISION DYNAMICS OF HIGHLY ORIENTED SUPER ROTOR MOLECULES FROM AN OPTICAL CENTRIFUGE(2017) Murray, Matthew J.; Mullin, Amy S; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Sophisticated optical methods provide some of the most promising tools for complete control of a molecule’s energy and orientation, which enables a more complete understanding of chemical reactivity and structure. This dissertation investigates the collision dynamics of molecular super rotors with oriented angular momentum prepared in an optical centrifuge. Molecules with anisotropic polarizabilities are trapped in the electric field of linearly polarized light and then angularly accelerated from 0 to 35 THz over the duration of the optical pulse. This process drives molecules to extreme rotational states and the ensemble of molecules has a unidirectional sense of rotation determined by the propagation of the optical field. High resolution transient IR absorption spectroscopy of the super rotor molecules reveals the dynamics of collisional energy transfer. These studies show that high energy CO2 and CO rotors release large amounts of translational energy through impulsive collisions. Time-evolution of the translational energy distribution of the CO2 J=0-100 state shows that depletion from low J states involves molecules with sub-thermal velocities. Polarization-dependent Doppler profiles of CO rotors show anisotropic kinetic energy release and reveal a majority population of molecular rotors in the initial plane of rotation. Experimental modifications improved signal to noise levels by a factor of 10, enabling new transient studies in the low-pressure, single-collision regime. Polarization-dependent studies show that CO2 rotors in the J=54-100 states retain their initial angular momentum orientation, and that this effect increases as a function of rotational angular momentum. These studies show that rotating molecules behave like classical gyroscopes. Polarization-dependent measurements of CO2 rotors in the presence of He and Ar buffer gases show that CO2 super rotors are more strongly relaxed by He collisions, demonstrating the importance of rotational adiabaticity in the relaxation process. Quantum scattering calculations of the He-CO2 and Ar-CO2 collision systems were performed to interpret the qualitative features of the experimental results. This work provides a detailed mechanistic understanding of the unique collisional dynamics of super rotor molecules.Item Visualizing Quantum Reactive Scattering Dynamics(2015) Warehime, Michael; Alexander, Millard H; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Born-Oppenheimer approximation, which allows a decoupling of electronic and nuclear motion, underlies the investigation of molecular dynamics. In some cases this decoupling is not possible, so that nuclear motion can induce changes in electronic state. It is then necessary to account for collision-induced transitions between multiple potential energy surfaces. This is an inherently quantum phenomena. In this dissertation we present a new way to visualize these non-adiabatic transitions in chemical reactions of open-shell atoms. Toward this end, we have developed new algorithms and developed a MATLAB-based software suite for simulating non-adiabatic reactions. We have also determined new molecular potential energy surfaces and their couplings required to simulate the reactive dynamics.Item Jamming effects in glasses and biopolymers(2014) Kang, Hongsuk; Thirumalai, Devarajan; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this dissertation, jamming effects in highly packed systems are studied in two specific materials: glasses and biopolymers in cellular environments. Suspensions consisting of highly charged colloids, which are well-known glass-forming systems, are investigated using molecular dynamics simulations in order to test Random First Order Transition (RFOT) theory. I found that there is a critical volume fraction at which ergodic-to-nonergodic transitions for three dynamic observables take place in accordance with RFOT. Based on numerical observations, it is also proposed that the dynamic heterogeneity can be attributed to the violation of law of large numbers. In addition, the bond orientational order of colloidal suspensions and soft-spheres is discussed in the context of liquid-glass transitions. The response of biopolymers to a crowded environment is another interesting issue because 20-40% volume of a cell is occupied by various cellular components such as ribosomes and proteins in vivo. In this work, using low-friction langevin dynamics simulations with explicit crowding particles, I examined the conformational change of biopolymers in the presence of crowders of various sizes and shapes. The simulation results reveal that cylindrical crowders induce much greater compaction of the polymers than spherical ones at low volume fractions and the stronger crowding effects disappear at higher volume fractions due to local nematic ordering of cylindrical particles. The reduction in the size of polymer is even more dramatic in a mixture of spherical and cylindrical shapes because of cooperative crowding effects explained by the phase separation of spheres and rodlike particles. Finally, the crowding effects of cellular components on bacterial chromosomes are estimated using a mixture of spherical crowders with the composition found in bacterial cytoplasms.Item Organic Molecular Thin Films on Device-Relevant Substrates(2013) Groce, Michelle Anne; Einstein, Theodore L; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Organic thin films are central to many cutting-edge electronic devices. Improving our understanding of the characteristics of thin films is important not only to the development of condensed matter physics but also to our ability to engineer specialized devices that we demand be ever smaller, less expensive, and more efficient. This thesis applies the experimental techniques of scanning tunneling microscopy and spectroscopy to the task of characterizing submonolayer thin films of two types: the organic semiconductor C60 on silicon oxide, and self-assembling porous networks of trimesic acid on graphite. Capture zone analysis of the initial nucleation regime for C60 on ultrathin silicon oxide is reported. The critical nucleus size, reflecting the largest unstable cluster of particles on a surface, is found to have a parabolic dependence on temperature rather than a monotonically increasing one. Between stages of stable monomers (i=0$) at < 300 K and > 480 K, a peak corresponding to i=1 is found at 386±3 K. This unique temperature dependence is attributed to defect-like variation in the silicon oxide surface. The first successful room-temperature UHV STM of trimesic acid on graphite is also presented here. These exploratory studies indicate the potential for a variety of porous hexagonal networks of trimesic acid to exist on a graphitic surface at room temperature. Significant electronic effects on graphite from trimesic acid lattices are shown via scanning tunneling spectroscopy, including an electronic state at -0.14 V that appears in networks whose pores are filled with excess TMA guest molecules. Ultimately, if the growth of TMA films could be extended to graphene, then the periodicity of electronegative oxygen atoms in molecules physisorbed on the graphene surface is predicted to provide a slight energy shift between the degenerate sublattices, opening a band gap. Promising directions for future research in these areas are also discussed.Item Exploring Equilibrium Systems with Nonequilibrium Simulations(2012) Ballard, Andrew James; Jarzynski, Christopher; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Equilibrium sampling is at the core of computational thermodynamics, aiding our understanding of various phenomena in the natural sciences including phase coexistence, molecular solvation, and protein folding. Despite the widespread development of novel sampling strategies over the years, efficient simulation of large complex systems remains a challenge. While the majority of current methods such as simulated tempering, replica exchange, and Monte Carlo methods rely solely on the use of equilibrium techniques, recent results in statistical physics have uncovered the possibility to sample equilibrium states through nonequilibrium simulations. In our first study we present a new replica exchange sampling strategy, "Replica Exchange with Nonequilibrium Switches," which uses nonequilibrium simulations to enhance equilibrium sampling. In our method, trial swap configurations between replicas are generated through nonequilibrium switching simulations which act to drive the replicas towards each other in phase space. By means of these switching simulations we can increase an effective overlap between replicas, enhancing the probability that these moves are accepted and ultimately leading to more effective sampling of the underlying energy landscape. Simulations on model systems reveal that our method can be beneficial in the case of low replica overlap, able to match the efficiency of traditional replica exchange while using fewer processors. We also demonstrate how our method can be applied for the calculation of solvation free energies. In a second, separate study, we investigate the dynamics leading to the dissociation of Na-Cl in water. Here we employ tools of rare event sampling to deduce the role of the surrounding water molecules in promoting the dissociation of the ion pair. We first study the thermodynamic forces leading to dissociation, finding it to be driven energetically and opposed entropically. In further analysis of the system dynamics, we deduce a) the spatial extent over which solvent fluctuations influence dissociation, b) the role of sterics and electrostatics, and c) the importance of inertia in enhancing the reaction probability.Item Nanofabrication on engineered silicon (100) surfaces using scanning probe microscopy(2011) Li, Kai; Silver, Richard M; Einstein, Theodore L; Chemical Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Fabricating and measuring sub-5 nanometer features brings to light several pressing issues in future semiconductor industry manufacturing and dimensional metrology. This dissertation presents a feasible process to create nanostructures using scanning probes with applications in dimensional metrology and nanomanufacturing processes. Using the lattice spacing of a crystal as the fundamental "ruler" or scale, sub 5 nm critical dimension reference standards can be created with atomic scale dimensional control. This technique relies on atomically sharp tips to provide robust imaging and patterning (nanolithography) capabilities. We have developed a comprehensive process to routinely produce high quality scanning tunneling microscope (STM) tips. The quality of STM tips are a critical factor in achieving reproducible patterning. A modified electrochemical etching method has been used to create sharp tips with preferred apex geometry. By using a field ion microscope (FIM), tip surfaces have been cleaned by field evaporation. Finally, a thermal ultra-high-vacuum (UHV) process is implemented to stabilize the atoms on the tip apex for improved performance. The process is also found to be capable of restructuring the apex to regain atomic resolution when tips fail during imaging or patterning. Silicon (100) samples with pre-patterned micrometer-size fiducial marks are used as templates in this technique. The fiducial marks are used as 2D references to relocate the tip scanned area and the lithographic patterns. Large atomically-flat reconstructed (100) surfaces are obtained after a wet chemical cleaning process and a high temperature annealing process. After the high temperature annealing process, we observed reproducible step-terrace patterns formed on surfaces due to the fiducial marks. A kinetic Monte-Carlo simulation was used to study quantitatively the evolution of surface morphology under the influence of fiducial marks. Some of the key aspects, such as the electromigration effect and step permeability have been extensively studied. Hydrogen-passivated silicon (100) reconstructed surfaces are used to create nanopatterns by selective depassivation lithography. Optimized depassivation procedures enable us to fabricate patterns from the microscale to the atomic scale consistently using an UHV STM. To preserve and later enhance the nanopatterns, SiO2 hard etch mask marks are formed by oxidizing the patterns using ambient humidity or gaseous oxygen. A reactive ion etching (RIE) process is used to further enhance the aspect ratio of oxidized nanopatterns so that they can be served as 3D nanostructures on silicon surfaces.