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Amorphous titanium dioxide (a-TiO2) is widely used as a coating material in applications such as electrochemistry and self-cleaning surfaces where its interface with water has a central role. However, little is known about the structures of the a-TiO2 surface and aqueous interface, particularly at the microscopic level. In this work, we construct a model of the a-TiO2 surface via a cut-melt-and-quench procedure based on molecular dynamics simulations with deep neural network potentials (DPs) trained on density functional theory data. After interfacing the a-TiO2 surface with water, we investigate the structure and dynamics of the resulting system using a combination of DP-based molecular dynamics (DPMD) and ab initio molecular dynamics (AIMD) simulations. Both AIMD and DPMD simulations reveal that the distribution of water on the a-TiO2 surface lacks distinct layers normally found at the aqueous interface of crystalline TiO2, leading to an ∼10 times faster diffusion of water at the interface. Bridging hydroxyls (Ti2–ObH) resulting from water dissociation decay several times more slowly than terminal hydroxyls (Ti–OwH) due to fast Ti–OwH2 → Ti–OwH proton exchange events. These results provide a basis for a detailed understanding of the properties of a-TiO2 in electrochemical environments. Moreover, the procedure of generating the a-TiO2-interface employed here is generally applicable to studying the aqueous interfaces of amorphous metal oxides. [ABSTRACT FROM AUTHOR]

The degradation of microplastics in relation to marine pollution has been receiving increasing attention. Because the spherulites that comprise microplastics have a highly ordered lamellar structure, their decomposition is thought to involve a lamellar structure collapse process. However, even in the simplest case of an order–disorder transition between lamellae and melt upon heating, the microscopic details of the transition have yet to be elucidated. In particular, it is unclear whether nucleation occurs at defects in the crystalline portion or at the interface between the crystalline and amorphous portions. To observe the transition in molecular simulations, an approach that distinguishes between the crystalline and amorphous structures that make up the lamella is needed. Local order parameters (LOPs) are an attempt to define the degree of order on a particle-by-particle basis and have demonstrated the ability to precisely render complex order structure transitions during phase transitions. In this study, 274 LOPs were considered to classify the crystalline and amorphous structures of polymers. Supervised machine learning was used to automatically and systematically search for the parameters. The identified optimal LOP does not require macroscopic information such as the overall orientation direction of the lamella layers but can precisely distinguish the crystalline and amorphous portions of the lamella layers using only a small amount of neighboring particle information. [ABSTRACT FROM AUTHOR]

The interfacial stiffness for nematic–smectic B (nm–smB) interface in a liquid crystalline (LC) material is calculated using Capillary Wave Theory (CWT) and molecular dynamics simulations. The Gay–Berne (GB) pair potential with parameters κ, κ′, μ, and ν equal to 3, 5, 2, and 1 is used to model the LC material. Using a smart three-step recipe, we have obtained an nm–smB phase coexistence in our simulations where the nm and smB directors are nearly parallel to each other and perpendicular to the interface normal. The density profiles are used to compute the nm–smB coexisting density range, the interfacial width, and its position. The smectic phase is differentiated from the nematic phase by using the local bond order parameter (q6q6), which has helped us to demonstrate that the interface is indeed rough. Finally, the interfacial stiffness of the nm–smB interface is computed by following the CWT analysis and is found to be γ ̃ n m − s m B = 0.398 61 k B T / σ e e 2 = 0.044 29 / σ s s 2 , where σee and σss are the length and diameter of the GB LC particles. [ABSTRACT FROM AUTHOR]

We calculate the degree to which the final structure of the local groundstate in a liquid is a function of the strength of a perturbing potential applied during energy minimization. This structural susceptibility is shown to correlate well with the observed tendency of the liquid adjacent to a crystal interface to exhibit a crystalline groundstate, a feature that has been strongly linked to the observation of ultra-fast crystal growth in pure metals and ionic melts. It is shown that the structural susceptibility increases dramatically as the interaction potential between atoms is softened. [ABSTRACT FROM AUTHOR]

We investigate how varying molecular topology of polymers influences crystallization in thin polymer films. In particular, we simulate linear and star polymers of fixed mass having a progressively increasing number of arms (f ≤ 16) in a system where the linear polymer exhibits crystallization in a thin film geometry, but no apparent crystallization in the corresponding bulk material. The degree of crystallization of the polymer film at long times decreases progressively with increasing f, and no crystallization is observed beyond f = 8. Crystallization for smaller values of f develops as a sigmoidally shaped wavefront initiating from the supporting crystalline interface. We suggest that large shape fluctuations and the competition of length scales of star polymers with high f lead to inhibited crystallization. [ABSTRACT FROM AUTHOR]

The contact of heterogeneous materials with different atomic structures and concentrations becomes omnipresent in modern materials. The differences across the interface introduce various gradients that could alter thermodynamic and kinetic behaviors of the materials. Here, we report a spontaneous melting of the interface formed by a crystalline metal and an amorphous solid at the temperature below their respective equilibrium melting temperatures. The transformation can even become continuous with disappearance of all known features of the first-order melting transition. We show that the change in the nature of melting in the interface region is caused by random disordering induced by interdiffusing atomic elements or impurities in the crystalline phase and the presence of a strong concentration gradient constrained by the sample geometry. [ABSTRACT FROM AUTHOR]

Extensive molecular dynamics simulations of xenon in two classes of zeolite crystal systems, one consisting of purely intra-crystalline space and the other with both intra- and inter-crystalline space are reported. The latter mimics a typical poly-crystalline sample of zeolite. Comparison of results from these two systems provides insights into the structure and dynamics in the presence of inter-crystalline space. The temperature, as well as the distance between the crystallites, has been varied. The density distribution and diffusivities calculated inside the poly-crystalline system show that the interfacial region between the crystal and the inter-crystalline region acts as a bottleneck for diffusion through the system. At lower temperatures, the particles are trapped at the interface due to the pronounced energy minima present in that region. With the increase in temperature, the particles are able to overcome this barrier frequently, and the transport across the inter-crystalline region is increased. A ballistic or superdiffusive motion is seen in the inter-crystalline region along all the axes except along the axis which has the inter-crystalline space. The transition time for ballistic to diffusive motion increases with the increase in the length of the inter-crystalline space. Velocity auto- and cross correlation functions exhibit strong oscillations and exchange of kinetic energy along directions perpendicular to the direction of the inter-crystalline space. These results explain why uptake and PFGNMRmeasurements exhibit lower values for diffusivity for the same system when compared to Quasi- Elastic Neutron Scattering. Thus, using molecular dynamics simulations, we were able to correlate the difference of diffusivity values measured using various experimental methods where these intercrystalline regions are common. [ABSTRACT FROM AUTHOR]

The potential of zero charge (PZC) is a fundamental property that describes the electrode/electrolyte interface. The determination of the PZC at electrode/ionic liquid interfaces has been challenging due to the lack of models that fully describe these complex interfaces as well as the non-standardized approaches used to characterize them. In this work, we present a method that combines electrode immersion transient and impedance measurements for the determination of the PZC. This combined approach allows the distinction of the potential of zero free charge (pzfc), related to fast double layer charging on a millisecond timescale, from a potential of zero charge on a timescale of tens of seconds related to slower ion transport processes at the interface. Our method highlights the complementarity of these electrochemical techniques and the importance of selecting the correct timescale to execute experiments and interpret the results. [ABSTRACT FROM AUTHOR]

We present a density functional theory study of the oxidation of 1D periodic rods supported along the [001] direction on the rutile TiO2(110) surface. The study shows evidence for an oxidation of the interface between the supported Au and the TiO2 crystal. The added O atoms adsorb at the 5f-Ti atoms in the through under the Au rod and are stabilized by charge transfer from the nearest Au atoms. Despite an extensive search, we find no low energy barrier pathways for CO oxidation involving CO adsorbed on Au and O at the perimeter of the Au/TiO2 interface. This is in part attributed the weak adsorption of CO on cationic Au at the perimeter. [ABSTRACT FROM AUTHOR]

We present the results of molecular dynamics simulations of the crystal nucleation rate in a supercooled Lennard-Jones liquid. The nucleation rate as a function of the pressure has been calculated by the method of determining the expectation time for liquid crystallization at temperatures higher than that of the triple point (T*=0.865), close to the temperature of the terminal critical point of the metastable extension of the melting curve (T*=0.55) and below this temperature (T*=0.4). In computer experiments the nucleation rate varied from 1032 to 1035 s-1 m-3. The dimensions of critical nuclei and the pressure inside them, the surface free energy at a critical crystal nucleus-liquid interface, the height of the nucleation barrier, and the Zeldovich factor have been determined from the results of molecular dynamics simulations and their comparison with classical homogeneous nucleation theory. It is shown that the surface free energy at a curved crystal-liquid interface, as distinct from a flat interface, has also been determined at temperatures lower than the temperature of the terminal critical point of the melting curve and is a monotonically increasing function of the temperature. [ABSTRACT FROM AUTHOR]

In the effort to develop atomistic models capable of accurately describing nanoscale systems with complex interfaces, it has become clear that simple treatments with rigid charge distributions and dispersion coefficients selected to generate bulk properties are insufficient to predict important physical properties. The quantum Drude oscillator model, a system of one-electron pseudoatoms whose “pseudoelectrons” are harmonically bound to their respective “pseudonuclei,” is capable of treating many-body polarization and dispersion interactions in molecular systems on an equal footing due to the ability of the pseudoatoms to mimic the long-range interactions that characterize real materials. Using imaginary time path integration, the Drude oscillator model can, in principle, be solved in computer operation counts that scale linearly with the number of atoms in the system. In practice, however, standard expressions for the energy and pressure, including the commonly used virial estimator, have extremely large variances that require untenably long simulation times to generate converged averages. In this paper, low-variance estimators for the internal energy are derived, in which the large zero-point energy of the oscillators does not contribute to the variance. The new estimators are applicable to any system of harmonic oscillators coupled to one another (or to the environment) via an arbitrary set of anharmonic interactions. The variance of the new estimators is found to be much smaller than standard estimators in three example problems, a one-dimensional anharmonic oscillator and quantum Drude models of the xenon dimer and solid (fcc) xenon, respectively, yielding 2–3 orders of magnitude improvement in computational efficiency. [ABSTRACT FROM AUTHOR]

The location of the liquidus in the low-pressure crystalline phase of SnI4 was determined utilizing in situ x-ray diffraction measurements under pressures up to approximately 3.5 GPa. The liquidus is not well fitted to a monotonically increasing curve such as Simon's equation, but breaks near 1.5 GPa and then becomes almost flat. The results are compared to those from molecular dynamics simulations. Ways to improve the model potential adopted in the simulations are discussed. [ABSTRACT FROM AUTHOR]

Thin films of solid ammonia (NH3 and ND3) have been characterized using low temperature (25–110 K) Fourier-transform infrared (FTIR) spectroscopy, and the three solid phase (amorphous, metastable, and crystalline) spectra are reported. This work has been motivated by confusion in the literature about the metastable and crystalline phases as a result of an early erroneous report by Staats and Morgan [(J. Chem. Phys. 31, 553 (1959)]. Although the crystalline phase has subsequently been reported correctly, the metastable phase has not been described in the literature in detail. The unique characteristics of the metastable phase, reported here for the first time, include multiple peaks in the ν2 and ν3 regions and peak intensities that are dependent on the deposition temperature. This behavior may be the result of (a) preferential molecular orientations in the solid, or (b) exciton splitting due to different crystal shapes in the solid. The amorphous and metastable phases of deuterated ammonia are also reported for the first time. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A study of the energy accommodation of neon colliding with a crystalline self-assembled 1-decanethiol monolayer adsorbed on Au(111) is presented. The intensity and velocity dependencies of the scattered neon as a function of incident angle and energy were experimentally measured. Scattering calculations show good agreement with these results, which allows us to examine the detailed dynamics of the energy and momentum exchange at the surface. Simulation results show that interaction times are, at most, a few picoseconds. Even for these short times, energy exchange with the surface, both normal and in-plane, is very rapid. An important factor in determining the efficiency of energy exchange is the location at which the neon collides with the highly corrugated and structurally dynamic unit cell. Moreover, our combined experimental and theoretical results confirm that these are truly surface collisions in that neon penetration into the organic boundary layer does not occur, even for the highest incident energies explored, 560 meV. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Polycrystalline AgNd(WO[sub 4])[sub 2] has been prepared by a in solid-state reaction. X-ray diffraction measurements indicate that this compound crystallizes with a scheelite-type tetragonal structure. Absorption and emission spectra have been measured, using the technique of the crystal field parameters, to obtain the energy levels scheme for Nd[sup 3+] ions in the crystalline matrix. These data indicate the existence of a structural local disorder associated with a random distribution of the ions of Ag[sup +] and Nd[sup 3+]. The temperature variation of the paramagnetic susceptibility in the range from 2 to 300 K has been studied. The Van Vleck formula has been used to calculate the temperature variation of the paramagnetic susceptibility and it was compared with the experimental results. Particularly at low temperatures, satisfactory agreement has been obtained. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The development of a methanol multilayer on Pd{110} at 124 K has been examined with reflection–absorption infrared spectroscopy (RAIRS). The spectra indicate that a crystalline multilayer grows in which hydrogen-bonded chains of molecules are parallel to the substrate. A “sandwich” phase is identified and consists of several physisorbed layers in between the chemisorbed layer and the crystalline phase. Methanol desorption and decomposition on heating have been followed with simultaneous temperature-programmed RAIRS and TPD measurements. Desorption of chemisorbed methanol is observed at 214 K, whereas the decomposition reaction yielding adsorbed CO and H proceeds at ∼295 K on heating at 2 K s-1. Spectral bands observed on adsorption at 180 K to low exposure, which precede the appearance of the CO signal, are tentatively attributed to chemisorbed methanol. No conclusive evidence for a methoxy intermediate in the decomposition reaction is found. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Some hexa-substituted benzenes are known to exhibit molecular rotation in their crystalline phase, but this phenomenon is not very well understood. In this context, we have critically examined two materials, namely pentachloronitrobenzene and 2,3,4,5,6-pentabromotoluene over a wide range of temperatures, using dielectric spectroscopy and differential scanning calorimetry (DSC) to ascertain the nature of the above said relaxation. A relaxation process is found in the crystalline phase, which is slightly non-Arrhenius in its temperature dependence in the former and deviation from Debye behavior has been found in both the samples. This behavior is similar to that of the primary (or α-) relaxation found in glass forming materials. This phenomenon is found to correspond to a steplike (small) change in the specific heat in the DSC measurements indicating that it is a glass transition phenomenon. The sub-T[sub g] relaxation data do not indicate any secondary (or β-) process (of measurable magnitude). However, the same materials when studied in small quantities in the liquid matrix of o-terphenyl (nonpolar) indicated a considerable magnitude for the β-process of molecules even below the liquid T[sub g] of these mixtures. The results are discussed in the context of the glass transition phenomena of the above materials. [ABSTRACT FROM AUTHOR]

We analyze two anisotropic atom–atom models used to describe the crystalline α, β, and γ phases of S[sub 8] crystals, the most stable compound of elemental sulfur in solid phases, at ambient pressure and T≤400 K. The calculations are performed via a series of classical molecular dynamics (MD) simulations, with flexible molecular models and using a constant pressure–constant temperature algorithm for the numerical simulations. All intramolecular modes that mix with lattice modes, and are therefore relevant to the onset of structural phase transitions, are taken into account. Comparisons with experimental data and previous results obtained with an isotropic atom–atom molecular model are also performed. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Investigates density profiles and first layer adsorption isotherms of a network forming fluid near a crystalline surface using the Percus-Yevick approximation. Connectivity properties of the interface; Estimation of the interfacial percolation threshold; Thickness of adsorbed layer; Connectedness wall-particle correlation functions.

The crystalline structures of hydroxy-functionalized hexanethiol self-assembled monolayers on Au(111) were characterized using gas-phase transport of molecular precursors and an ultrahigh vacuum scanning tunneling microscope. In the pristine state the monolayer exhibits a commensurate lattice with an oblique primitive unit-cell of dimensions a=3, b=[Square_Root]13, α=a tan(2[square_root_of_3]); a structure that is fundamentally different than the hexagonal lattice observed for methyl-terminated monolayers. Hydration of the monolayer results in conversion to a polymorphic phase. These results demonstrate the importance of end-group chemical bonding in the molecular packing of this widely studied class of materials. [ABSTRACT FROM AUTHOR]

The crystalline structure of hydroxy-functionalized hexanethiol self-assembled monolayers on Au(111) were characterized using gas-phase transport of molecular precursors and an ultrahigh vacuum scanning tunneling microscope. In the pristine state the monolayer exhibits a commensurate lattice with an oblique primitive unit-cell of dimensions a=3, b=[Square_Root]13, α=a tan(2[Square_Root]3); a structure that is fundamentally different than the hexagonal lattice observed for methyl-terminated monolayers. Hydration of the monolayer results in conversion to a polymorphic phase. These results demonstrate the importance of end-group chemical bonding in the molecular packing of this widely studied class of materials. [ABSTRACT FROM AUTHOR]

Molecular-dynamics simulations of several water-crystalline urea interfaces have been performed. The structure and dynamics of water close to the urea crystal surface are discussed in terms of density profiles, positional and orientational distribution functions, and diffusion coefficients. The water structure close to the interface is strongly determined by the structure of the crystal surface: the (001) and (111) interfaces reveal strong adsorption of water while the (110) and (111) interfaces do so to a lesser extent. Assuming that the growth rate of a specific crystal face decreases with increasing solvent adsorption, the appearance of only (111) on the urea growth form is predicted. We argue that on the other hand the dominance of (110) over (001) cannot be explained using a simple layer growth model. [ABSTRACT FROM AUTHOR]

A continuum thermodynamic treatment of the local equilibrium concentration at a two-phase interface during a first-order diffusional phase transition in crystalline solids is presented. It is assumed that the second-phase domain has a spherical morphology, is coherent with the matrix, and possesses different elastic constants than the matrix. It is shown that the equilibrium concentration at the solid–solid interface is strongly coupled to the stress states of the matrix, precipitate, and interface as well as the interfacial curvature. This coupling gives rise to many new phenomena, such as the dependence of the equilibrium interfacial concentrations on the magnitude of the supersaturation in the parent phase, the misfit between the precipitate and matrix and the difference in the partial molar volumes of the diffusing species. Capillarity is also strongly influenced by the crystallinity through the surface stress and deformation, supersaturation of the parent phase, and the degree of elastic inhomogeneity. It is concluded that approximating the equilibrium interfacial conditions during phase transformations in solids using thermodynamics which is valid only for fluids may be erroneous in many cases. [ABSTRACT FROM AUTHOR]

We have studied the ZrO[sub 2](111)/Ni(111) interface using the ultrasoft pseudopotential formalism within density functional theory. We find that ZrO[sub 2](111) adheres relatively strongly at the monolayer level but thicker ceramic films interact weakly with the Ni-substrate. We argue that the cohesion changes character from dominantly image charge interactions for thick ceramic films to more covalent for monolayer ZrO[sub 2](111) films. We provide an analysis of energetic, structural and electronic aspects of the ZrO[sub 2]/Ni interface as a function of the thickness of the oxide layer. We also address the role of the exchange-correlation density functional parameterization for modeling the oxide and metal/oxide interface and discuss the sensitivity of the supercell approximation for metal/oxide interface properties. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present an energy-based algorithm, POINTER, which can determine the permissible alignments of a polypeptide (or other macromolecule) with respect to the lattice vectors of an interfacial surface (this alignment is defined by the angle θ). The algorithm represents both the interface and the macromolecule in three dimensions. For each value of θ, incremental moves of the macromolecule occur in the x, y, z direction along the θ orientation, as well as rotation (ω, γ, ζ) of either the macromolecular chain or the interfacial slab. We utilized a simple forcefield that consists of a dipole-dipole, dipole-charge, or charge-charge electrostatic interaction term and a Lennard-Jones attraction-repulsion term to describe the nonbonding interactions between macromolecular atoms and interfacial atoms. We benchmarked our method by modeling ice- and mineral-interaction polypeptides on various Miller planes of hexagonal ice and inorganic solids, respectively. In addition, we searched phase space for a simpler, nonpolypeptide system: The ice-nucleating C31 alcohol monolayer (comprised of 61 C31 molecules) in contact with the {001} plane of hexagonal ice. Our results indicate that the POINTER simulation method can reproduce the macromolecule orientation observed for each benchmark system. In addition, our simulations point to a number of factors--polypeptide binding site structure, the positioning of hydrophobic residues near the interface, and interface topology--which can influence the adsorption orientation of polypeptides on hexagonal ice and inorganic solids. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]