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At least 595 records · Page 33

Theory of activated glassy relaxation, mobility gradients, surface diffusion, and vitrification in free standing thin films

We have constructed a quantitative, force level, statistical mechanical theory for how confinement in free standing thin films introduces a spatial mobility gradient of the alpha relaxation time as a function of temperature, film thickness, and location in the film. The crucial idea is that relaxation speeds up due to the reduction of both near-surface barriers associated with the loss of neighbors in the local cage and the spatial cutoff and dynamical softening near the vapor interface of the spatially longer range collective elasticity cost for large amplitude hopping. These two effects are fundamentally coupled. Quantitative predictions are made for how an apparent glass temperature depends on the film thickness and experimental probe technique, the emergence of a two-step decay and mobile layers in time domain measurements, signatures of confinement in frequency-domain dielectric loss experiments, the dependence of film-averaged relaxation times and dynamic fragility on temperature and film thickness, surface diffusion, and the relationship between kinetic experiments and pseudo-thermodynamic measurements such as ellipsometry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Effect of asymmetric concentration profile on thermal conductivity in Ge/SiGe superlattices

The effect of the chemical composition in Si/Ge-based superlattices on their thermal conductivity has been investigated using molecular dynamics simulations. Simulation cells of Ge/SiGe superlattices have been generated with different concentration profiles such that the Si concentration follows a step-like, a tooth-saw, a Gaussian, and a gamma-type function in direction of the heat flux. The step-like and tooth-saw profiles mimic ideally sharp interfaces, whereas Gaussian and gamma-type profiles are smooth functions imitating atomic diffusion at the interface as obtained experimentally. Symmetry effects have been investigated comparing the symmetric profiles of the step-like and the Gaussian function to the asymmetric profiles of the tooth-saw and the gamma-type function. At longer sample length and similar degree of interdiffusion, the thermal conductivity is found to be lower in asymmetric profiles. Furthermore, it is found that with smooth concentration profiles where atomic diffusion at the interface takes place the thermal conductivity is higher compared to systems with atomically sharp concentration profiles.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Ab-initio simulations of higher Miller index Si:SiO{sub 2} interfaces for fin field effect transistor and nanowire transistors

Models of three representative higher Miller index interfaces, Si(310):SiO{sub 2}, Si(410):SiO{sub 2}, and Si(331):SiO{sub 2}, have been built by an ab-initio molecular dynamics method. We show that each interface can be made as a fully bonded network without any defects and has a reasonable electronic structure for use in fin field effect transistors or gate-all-around nanowire devices. The differences in numbers of oxygen bridges are attributed to the intermediate sub-oxide components and the atomic step structure. The interface bonding schemes to passivate different densities of dangling bonds on different facets are also analyzed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

A Revisited Mechanism of the Graphite-to-Diamond Transition at High Temperature

The graphite-diamond transition, under high-pressure and high-temperature conditions, has been a central subject in physical science. However, its atomistic mechanism remains under debate. Employing large-scale molecular dynamics (MD) simulations, we report a mechanism whereby the diamond nuclei in the graphite matrix propagate in two preferred directions, among which the graphite [120] is about 2.5 times faster than [001]. Consequently, cubic diamond (CD) is the kinetically favorable product, while only a few hexagonal diamonds (HDs) can exist as the twins of CDs. The coherent interface of t-(100)gr//(11-1)cd + [010]gr//[1-10]cd observed in MD simulation was confirmed by our high-resolution transmission electron microscopy experiment. This proposed mechanism not only clarifies the role of HD in graphite-diamond transition but also yields atomistic insight into strengthening synthetic diamond via microstructure engineering.

36 MATERIALS SCIENCE↗

Investigation of design principles for metal-binding and conductive protein assemblies

Throughout the lifetime of this initiative, including renewals, we focused on understanding the fundamental principles of protein-protein interface design that enable predictable and modular spatial and kinetic control of multi-component protein self-assembly in 1D, 2D, and 3D, including the interface with inorganic materials, small molecules, and metal ions. We designed individual protein components that bind specific metal ions, including REEs and transport ions across lipid membranes. We created helical 1D filaments of repeating units with programmed periodicity, pitch, and multi-component environmentally responsive self-assembling protein fibers. We showed that these filaments reversibly assemble and disassemble under specific pH conditions and created end-specific caps that independently tune the balance of attachment and detachment rates at each terminus of the filament. Using similar filaments, we succeeded in binding arrays of heme and chlorophyll molecules and assembling patterned helical coatings around carbon nanotubes in efforts to create de novo conductive nanowires. By arraying REE binding sites in a large circular tandem array with a repeat protein-based cyclic oligomer, we created a molecular scaffold for superradiance and paramagnetic quantum sensing. We created a range of one-component and two-component self-assembling 2D arrays and showed that when designed to engage cell receptors, these arrays can control cell behavior from outside the cell signal to inside the cell. We designed helical repeat proteins with variable lengths displaying charged residues in a pattern matched to the cation lattice of mica. achieved a range of ordered states with an epitaxial match to the underlying crystal lattice. We further applied the learned principles of protein-induced biomineralization to design proteins with an interface lattice matching CaCO 3 and guide the formation of specific crystal forms of CaCO 3 from solution, a significant advance toward the global need to manage carbon. In all cases of mineral lattice matching and biomineralization, we followed assembly using molecularly resolved in situ AFM imaging and extracted information about assembly pathways and energetics, applying deep learning to quantify the dynamics of protein self-organization. We developed techniques for using dynamic metal-dependent interfaces on protein nanopores for discriminatively sensing dilute REEs in solution and demonstrated the use of strong metal-binding interfaces to drive nanocage disassembly for conditional nanocompartmentalization applications. This grant supported 11 people, including Asim Bera, Evans Brackenbrough, Andrew Borst, Nikita Hanikel, Timothy Huddy, Emily Joyce, Alex Young-Seug Kang, Ryan Kibler, Joshua Morris Lubner, Harley Pyles, and Shuai Zhang. The research effort culminated in the production of published papers and theses. Electronic Thesis/Dissertation are distributed by ProQuest/UMI Dissertation Publishing and made available on an open access basis through UW Libraries ResearchWorks Service.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

X-ray Photon Correlation Spectroscopy Studies of Surfaces and Thin Films

The technique of X-ray Photon Correlation Spectroscopy (XPCS) is reviewed as a method for studying the relatively slow dynamics of materials on time scales ranging from microseconds to thousands of seconds and length scales ranging from microns down to nanometers. We focus on the application of this technique to study dynamical fluctuations of surfaces, interfaces and thin films. We first discuss instrumental issues such as the effects of partial coherence (or alternatively finite instrumental resolution) and optimization of signal-to-noise ratios in the experiments. We then review what has been learned from recent XPCS studies of capillary wave fluctuations on liquid surfaces and polymer films, of nanoparticles used as probes to study the interior dynamics of polymer films, of liquid crystals and multilamellar surfactant films, and of metal surfaces, and magnetic domain wall fluctuations in antiferromagnets. We then discuss studies of non-equilibrium dynamics described by 2-time correlation functions. Finally, we briefly speculate on possible future XPCS experiments at new synchrotron sources currently under development including studies of dynamics on time scales down to femtoseconds.

Sinha, Sunil K.↗

Quantitative in-situ scanning electron microscope pull-out experiments and molecular dynamics simulations of carbon nanotubes embedded in palladium

In this paper, we present our results of experimental and numerical pull-out tests on carbon nanotubes (CNTs) embedded in palladium. We prepared simple specimens by employing standard silicon wafers, physical vapor deposition of palladium and deposition of CNTs with a simple drop coating technique. An AFM cantilever with known stiffness connected to a nanomanipulation system was utilized inside a scanning electron microscope (SEM) as a force sensor to determine forces acting on a CNT during the pull-out process. SEM-images of the cantilever attached to a CNT have been evaluated for subsequent displacement steps with greyscale correlation to determine the cantilever deflection. We compare the experimentally obtained pull-out forces with values of numerical investigations by means of molecular dynamics and give interpretations for deviations according to material impurities or defects and their influence on the pull-out data. We find a very good agreement of force data from simulation and experiment, which is 17 nN and in the range of 10–61 nN, respectively. Our findings contribute to the ongoing research of the mechanical characterization of CNT-metal interfaces. This is of significant interest for the design of future mechanical sensors utilizing the intrinsic piezoresistive effect of CNTs or other future devices incorporating CNT-metal interfaces.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Stability of SiC and SiN interfaces in titanium carbide and nitride based heterostructures

First-principles molecular dynamics simulations of the stability of the NaCl-type (B1) SiC and SiN interfaces in TiX/one multilayer SiY (X,Y=C,N) heterostructures were carried out as functions of temperature. It was previously shown that the SiX interfaces in the heterostructures based on transition metal compounds could be dynamically unstable owing to the elongated interfacial Si-X bonds compared to the Si-X bonds in the B1-SiX phases in equilibrium. However, this criterion is not satisfied for some SiX(111) interfaces. The aim of this work is to study the temperature-induced changes in the structure of the heterostructures under consideration and to clarify a possible origin of instability of the interfaces in them. It is shown that, at finite temperatures, the interfacial layer in the TiN(001)/SiC heterostructure transforms into the zinc blende-type (B3) SiC-like layer, whereas the interfaces in other (001) heterostructures transform into amorphous-like interfacial layers. The TiC(111)/SiC and TiN(111)/SiN heterostructures are stable up to 1400 K. The TiN(111)/B3-like SiC and TiC(111)/B3-like SiN systems form during static relaxation of the initial heterostructures. The phase transformations of the interfaces are explained in terms of dynamical and elastic stability criteria. In contrast to the (111) interfacial layers, all the B1-(001) interfaces are found to be dynamically unstable. The formation of the B3-(111) interfaces occurs due to the elastic instability of the corresponding B1-(111) interface

36 MATERIALS SCIENCE↗

Dislocation and Structural Studies at Metal–Metallic Glass Interface at Low Temperature

In this paper, molecular dynamics (MD) simulation deformation studies on the Al (metal)–Cu{sub 50}Zr{sub 50} (metallic glass) model interface is carried out based on cohesive zone model. The interface is subjected to mode-I loading at a strain rate of 10{sup 9} s{sup −1} and temperature of 100 K. The dislocations reactions and evolution of dislocation densities during the deformation have been investigated. Atomic interactions between Al, Cu and Zr atoms are modeled using EAM (embedded atom method) potential, and a timestep of 0.002 ps is used for performing the MD simulations. A circular crack and rectangular notch are introduced at the interface to investigate the effect on the deformation behavior and fracture. Further, scale size effect is also investigated. The structural changes and evolution of dislocation density are also examined. It is found that the dominant deformation mechanism is by Shockley partial dislocation nucleation. Amorphization is observed in the Al regions close to the interface and occurs at a lower strain in the presence of a crack. The total dislocation density is found to be maximum after the first yield in both the perfect and defect interface models and is highest in the case of perfect interface with a density of 6.31 × 10{sup 17} m{sup −2}. In the perfect and circular crack defect interface models, it is observed that the fraction of Shockley partial dislocation density decreases, whereas that of strain rod dislocations increases with increase in strain.

36 MATERIALS SCIENCE↗

Ab Initio Study of the Interface of the Solid-State Electrolyte Li 9 N 2 Cl 3 with a Li-Metal Electrode

Ab initio molecular dynamics study of an electrochemical interface between a solid-state-electrolyte Li 9 N 2 Cl 3 and a Li-metal is performed to analyze interphase formation due to electrochemical instability. An external electric field is applied to emulate the interface under charging conditions. At open circuit, we observed electrochemical stability at both sides of the interface, without electrolyte decomposition, or new phase formation, in good agreement with reported experiments. However, a structural re-accommodation takes place at both sides of the interface. Some oxidation of the surface Li-atoms takes place when Cl and N anions capture Li from the metal to complete octahedral structures, a Bader charge and electron density analyses explain the difference between Cl and N behavior. At the interface with the Li 9 N 2 Cl 3 (002) facet, the larger negative charge on low-coordinated N atoms is able to retain more Li atoms than the Cl atoms do. At charging conditions applying an external electric field (0.5 V/Å), no major changes are observed in atomic diffusion or structural changes at the interphase; however, the main difference is the change of the Li-metal crystal structure. Due to migration of Li from the metal anode to the electrolyte, vacancies are created on the anode, causing a structural re-arrangement at the anode surface.

36 MATERIALS SCIENCE↗

Phonon scattering due to van der Waals forces in the lattice thermal conductivity of Bi{sub 2}Te{sub 3} thin films

In this work, we calculate the thermal conductivity of layered bismuth telluride (Bi{sub 2}Te{sub 3}) thin films by solving the Boltzmann transport equation in the relaxation-time approximation using full phonon dispersion and compare our results with recently published experimental data and molecular dynamics simulation. The group velocity of each phonon mode is readily extracted from the full phonon dispersion obtained from first-principle density-functional theory calculation and is used along with the phonon frequency to compute the various scattering terms. Our model incorporates the typical interactions impeding thermal transport (e.g., umklapp, isotope, and boundary scatterings) and introduces a new interaction capturing the reduction of phonon transmission through van der Waals interfaces of adjacent Bi{sub 2}Te{sub 3} quintuple layers forming the virtual superlattice thin film. We find that this novel approach extends the empirical Klemens-Callaway relaxation model in such anisotropic materials and recovers the experimental anisotropy while using a minimal set of parameters.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Dynamic reflectance changes in high-purity zirconium when it is shocked from the α to ω phase

Flash lamp-illuminated integrating spheres have been used to measure the dynamic reflectance at the interface between a shocked metal target and a window maintaining the shock stress. In conjunction with radiance measurements, this method was recently used to study the pressure versus temperature phase boundary of the beta phase to body-centered-tetragonal transition of tin at around 13 GPa and, in separate experiments, to show a relationship between the reflectance and the phase fraction. We have used this technique to measure the ratio, R, of the dynamic reflectance relative to the ambient value for a high-purity zirconium sample attached to a sapphire window to study the alpha to omega phase transition. Nine experiments cover the range of stress, σ, from 4 to 16 GPa at six wavelengths from 500 to 1550 nm. The two phases in zirconium differ in volume by <1%, so a phase change between them can be difficult to identify from anomalies in the particle velocity measured on the shock Hugoniot. The phase boundary is believed to cross the Hugoniot around 3 GPa, but dynamically it is observed at much higher stresses >6 GPa. Our results show that the relative reflectance decreases upon initial shock in the alpha state but increases as the material transforms to its omega state. The slope dR/dσ changes the sign around the phase transition. For shocks below 6 GPa, dR/dσ < 0, and for σ above 10 GPa, dR/dσ > 0. In the stress region between 6 and 10 GPa, dR/dσ varies somewhat, perhaps due to the band structure or because the transition is incomplete there. I. INTRODUCTION

74 ATOMIC AND MOLECULAR PHYSICS↗

Doping effect on photoabsorption and charge-separation dynamics in light-harvesting organic molecule

Using ab-initio theoretical methods, we demonstrate possible enhancement of photo-conversion efficiency of an organic solar cell via intentional doping in molecular graphene-fullerene heterojunction [the hexabenzocoronene (HBC)-triethylene glycol (TEG)–C{sub 60} molecule]. Photoabsorption analysis indicates oxygen substitution into HBC leads to an extension of the spectra up to an infrared regime. A quantum-mechanical molecular dynamics simulation incorporating nonadiabatic electronic transitions reveals that a dissociated charge state (D{sup +} and A{sup -}) in the O-doped system is more stable than the pristine case due to the presence of an effective barrier by the TEG HOMO/LUMO level. We also find that oxygen doping in HBC enhances the intermolecular carrier mobility after charge separation. On the other hand, the pristine molecule undergoes rapid recombination between donor and acceptor charges at the interface. These analyses suggest that the graphene oxidation opens a new window in the application of organic super-molecules to solar cells.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Dynamic behavior of nanoscale liquids in graphene liquid cells revealed by in situ transmission electron microscopy

Recent advances in graphene liquid cells for in situ transmission electron microscopy (TEM) have opened many opportunities for the study of materials transformations and chemical reactions in liquids with high spatial resolution. However, the behavior of thin liquids encapsulated in a graphene liquid cell has not been fully understood. Here, we report real time TEM imaging of the nanoscale dynamic behavior of liquids in graphene nanocapillaries. Our observations reveal that the interfaces between liquid and gas bubble can fluctuate, leading to the generation of liquid nanodroplets near the interfaces. Liquid nanodroplets often show irregular shape with dynamic changes of their configuration under the electron beam. We consider that the dynamic motion of liquid-gas interfaces might be introduced by the electrostatic energy from transiently charged interfaces. We find that improving the wettability of graphene liquid cells by ultraviolet-ozone treatment can significantly modify the dynamic motion of the encapsulated liquids. Our study provides valuable information of the interactions between liquid and graphene under the electron beam, and it also offers key insights on the nanoscale fluid dynamics in confined spaces.

36 MATERIALS SCIENCE↗

Molecular simulation of the structural and thermodynamic properties of n-alkane/brine interfacial systems with nonionic surfactants

All-atom molecular dynamics (MD) simulations are used to study the structural and thermodynamic properties of water + n-heptane interfacial systems in the presence of salinity (NaCl, NaI, KCl, and KI) and two different nonionic surfactants. Excellent qualitative and quantitative agreement with interfacial tension experimental data is obtained, but this requires applying a charge scaling factor to the ionic species, suggesting the likely role of polarizability effects. Tensoactive behavior is observed for the NaI and KI systems, coinciding with layering of the iodide species near the interface. Further, there are significant interaction differences between the surfactants and the different ionic species, but the structural and thermodynamic behavior of the surfactant molecules is relatively unaffected by the specific salt species. Our analysis suggest that the surfactant-surfactant interactions play a critical role in determining the interfacial behavior, with a much smaller impact associated with the composition of the brine phase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

On the influence of polarization effects in predicting the interfacial structure and capacitance of graphene-like electrodes in ionic liquids

The electric double layer (C{sub D}) and electrode quantum (C{sub Q}) capacitances of graphene-based supercapacitors are investigated using a combined molecular dynamics and density functional theory approach. In particular, we compare an approach that includes electronic polarization to one that is polarization-free by evaluating both C{sub D} and C{sub Q} using [EMIM][BF{sub 4}] ionic liquid as a model electrolyte. Our results indicate that the inclusion of polarization effects can yield higher C{sub D} values—in this study by up to 40% around ±2 V—which we attribute primarily to the presence of charge smearing at the electrode-electrolyte interface. On the other hand, we find that the polarization-induced distortion of the electronic structure of graphene does not noticeably alter the predicted C{sub Q}. Our analysis suggests that an accurate description of the spatial charge distribution at the graphene interface due to polarization is necessary to improve our predictive capabilities, though more notably for C{sub D}. However, the conventional polarization-free approximation can serve as an efficient tool to study trends associated with both the C{sub Q} and C{sub D} at the interface of various graphene-like materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗