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Molecular dynamics simulations of uranyl adsorption and structure on the basal surface of muscovite
Anthropogenic activities have led to an increased concentration of uranium on the Earth’s surface and potentially in the subsurface with the development of nuclear waste repositories. Uranium is soluble in groundwater, and its mobility is strongly affected by the presence of clay minerals in soils and in subsurface sediments. We use molecular dynamics simulations to probe the adsorption of aqueous uranyl (UO 2 2+ ) ions onto the basal surface of muscovite, a suitable proxy for typically ultrafine-grained clay phases. Model systems include the competitive adsorption between potassium counterions and aqueous ions (0.1 M and 1.0 M UO 2 Cl 2 , 0.1 M NaCl). We find that for systems with potassium and uranyl ions present, potassium ions dominate the adsorption phenomenon. Potassium ions adsorb entirely as inner-sphere complexes associated with the ditrigonal cavity of the basal surface. Uranyl ions adsorb in two configurations when it is the only ion species present, and in a single configuration in the presence of potassium. Finally, the majority of adsorbed uranyl ions are tilted less than 45° relative to the muscovite surface, and are associated with the Si 4 Al 2 rings near aluminum substitution sites.
Utilizing dynamic laser speckle to probe nanoscale morphology evolution in nanoporous gold thin films
Not Available
Improved atmosphere-ocean coupled modeling in the tropics for climate prediction
We investigated the initial development of the double ITCZ in the Community Climate System Model (CCSM Version 3) in the central Pacific. Starting from a resting initial condition of the ocean in January, the model developed a warm bias of sea-surface temperature (SST) in the central Pacific from 5oS to 10oS in the first three months. We found this initial bias to be caused by excessive surface shortwave radiation that is also present in the standalone atmospheric model. The initial bias is further amplified by biases in both surface latent heat flux and horizontal heat transport in the upper ocean. These biases are caused by the responses of surface winds to SST bias and the thermocline structure to surface wind curls. We also showed that the warming biases in surface solar radiation and latent heat fluxes are seasonally offset by cooling biases from reduced solar radiation after the austral summer due to cloud responses and in the austral fall due to enhanced evaporation when the maximum SST is closest to the equator. The warming biases from the dynamic heat transport by ocean currents however stay throughout all seasons once they are developed, which are eventually balanced by enhanced energy exchange and penetration of solar radiation below the mixed layer. Our results also showed that the equatorial cold tongue develops after the warm biases in the south central Pacific, and the overestimation of surface shortwave radiation recurs in the austral summer in each year.
High aspect ratio structures for meta-surface optics
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Brief communication: Decadal changes in topography, surface water and subsurface structure across an Arctic coastal tundra site
Abstract. In ice-rich polygonal tundra, spatiotemporal heterogeneity in ground-ice melt reshapes topography, impacting infrastructure, water and carbon cycles. This study evaluates changes in topography and subsurface structure at a coastal Arctic site by comparing data from two surveys conducted a decade apart. Each survey includes electrical resistivity tomography, active layer thickness, photogrammetry, and topographic data. Results reveal subsidence and decrease in permafrost table elevation with varying intensity and spatial distribution across polygons, alongside diverse thermal-hydrological responses, such as thermokarst pool formation in high-centered-polygons and more even subsidence in flat-centered-polygons. The study also underscores the value and limitations of sporadic surveys.
Surface and Bulk Electronic Structures of Lafeaso Studied by Angle-Resolved Photoemission Spectroscopy
Abstract Not Provided
Effects of Thermodynamic Ensemble and Mineral Surface on Interfacial Water Structure.
Abstract not provided.
SiGe on sSOI: Nanoscale Engineering of Structures and Devices on Surfaces.
Abstract not provided.
Quantifying Electrocatalytic Structure-Property Relationships with Surface Science, Electrochemistry, and Density Functional Theory
Presented by NETL at Electrochemical Society Meeting, 10/19
Radiation tolerance of gradient grain-structured copper processed by surface mechanical attrition treatment.
Abstract not provided.
Inferring Structural Parameters of Low-Surface-Brightness-Galaxies with Uncertainty Quantification using Bayesian Neural Networks
Measuring the structural parameters (size, total brightness, light concentration, etc.) of galaxies is a significant first step towards a quantitative description of different galaxy populations. In this work, we demonstrate that a Bayesian Neural Network (BNN) can be used for the inference, with uncertainty quantification, of such morphological parameters from simulated low-surface-brightness galaxy images. Compared to traditional profile-fitting methods, we show that the uncertainties obtained using BNNs are comparable in magnitude, well-calibrated, and the point estimates of the parameters are closer to the true values. Our method is also significantly faster, which is very important with the advent of the era of large galaxy surveys and big data in astrophysics.
Experimental and theoretical investigations on the adherent behaviors of high viscosity liquid: The effect of surface topography
Wetting test and classical molecular dynamics (MD) simulations were employed to investigate the adherent behaviors of hydroxyl-terminated polybutadiene (HTPB), a typical high viscosity liquid widely used as a binder in rocket motor propellants, onto the copper (Cu) surfaces with different topographies. Experimental data show that the microstructures protruding above the substrate surfaces can be ‘sensed’ by the HTPB droplet, which has been indicated by the smaller contact angles and higher interface tension for this specific liquid on the rough surfaces (featured with sinusoidal structures or square pillars) than that on the smooth surface. Simulation results reveal the atomic details of spreading process of HTPB chains, with diverse diffusion propensities presented on different surfaces. The HTPB droplet spreads widely on the smooth surface, exhibiting a roughly isotropic trend. On the contrary, the state of this high viscosity liquid on the sinusoidal-structured surface is featured by wide contact diffusion along the run of groove, but relatively restricted extension in the direction perpendicular to the groove, as a result of the barriers exerted by the nearest neighbouring peaks. The existence of nanoscale pillars increases the exterior surface area of Cu substrate, therefore the HTPB chains can penetrate deep to envelop the peripheries of the pillars, forming abundant direct contacts with Cu surface and producing a highest HTPB–Cu interaction energy, which well accords with the experimental results about the adherent strength of HTPB.
Simple formula for the interspaces of periodic grating structures self-organized on metal surfaces by femtosecond laser ablation
Self-organized grating structures formed on Mo and Ti metal surfaces irradiated with femtosecond laser pulses at wavelengths of 800 and 400 nm are investigated by electron microscopy. We observe the formation of the self-organized grating structures on the metals irradiated with 400-nm laser pulses at low laser fluence in narrow fluence ranges. The interspaces of the grating structure depend on the wavelength and fluence of the laser. We find that the dependence of the grating interspaces on laser fluence can be explained by a simple formula for induction of a surface-plasma wave through the parametric decay of laser light.
Quantitative surface studies of moiré intercalated Pb structures under Gr/SiC
Quantitative growth studies using surface diffraction and scanning tunneling microscopy are carried out to determine the intercalation conditions for Pb under Gr/SiC, with initial phase a mixture of buffer-layer and single-layer graphene. Intercalation is a kinetically complex, multistage process with intercalated Pb moving between different subsurface locations with annealing, eventually to the location of higher stability. The transfer is determined by monitoring the difference of a given spot intensity from its clean value, as a function of growth parameters. In addition, a broad, low-intensity (10×10) gr moiré phase is observed exclusively originating from intercalated Pb. Well-defined growth conditions are identified for the moiré spot to be observed reversibly. The (10×10) gr phase follows changes of the Pb(10) spot during growth and annealing experiments, instead of changes of the other Gr/SiC spots indicating preferred nucleation of Pb islands on the (10×10) gr domains. These structural results are discussed in connection to different models proposed for the (10×10) gr supercell and the subsurface Pb coverage. In conclusion, mass transport of subsurface Pb is unusually high, possibly because of the very high compression of the intercalated Pb, when compared to metallic Pb(111).
Interface Architecture Determined Electrocatalytic Activity of Pt on Vertically Oriented TiO2 Nanotubes
The surface atomic structure and chemical state of Pt is consequential in a variety of surface-intensive devices. Herein we present the direct interrelationship between the growth scheme of Pt films, the resulting atomic and electronic structure of Pt species, and the consequent activity for methanol electro-oxidation in Pt/TiO{sub 2} nanotube hybrid electrodes. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed to relate the observed electrocatalytic activity to the oxidation state and the atomic structure of the deposited Pt species. The atomic structure as well as the oxidation state of the deposited Pt was found to depend on the pretreatment of the TiO{sub 2} nanotube surfaces with electrodeposited Cu. Pt growth through Cu replacement increases Pt dispersion, and a separation of surface Pt atoms beyond a threshold distance from the TiO{sub 2} substrate renders them metallic, rather than cationic. The increased dispersion and the metallic character of Pt results in strongly enhanced electrocatalytic activity toward methanol oxidation. This study points to a general phenomenon whereby the growth scheme and the substrate-to-surface-Pt distance dictates the chemical state of the surface Pt atoms, and thereby, the performance of Pt-based surface-intensive devices.
Stabilization of the Metastable Lead Iodide Perovskite Phase via Surface Functionalization
Metastable structural polymorphs can have superior properties and applications to their thermodynamically stable phases, but the rational synthesis of metastable phases is a challenge. Here in this paper, a new strategy for stabilizing metastable phases using surface functionalization is demonstrated using the example of formamidinium lead iodide (FAPbI 3 ) perovskite, which is metastable at room temperature (RT) but holds great promises in solar and light-emitting applications. We show that, through surface ligand functionalization during direct solution growth at RT, pure FAPbI 3 in the cubic perovskite phase can be stabilized in nanostructures and thin films at RT without cation or anion alloying. Surface characterizations reveal that long-chain alkyl or aromatic ammonium (LA) cations bind to the surface of perovskite structure. Calculations show that such functionalization reduces the surface energy and plays a dominant role in stabilizing the metastable perovskite phase. Excellent photophysics and optically pumped lasing from the stabilized single-crystal FAPbI 3 nanoplates with low thresholds were demonstrated. High-performance solar cells can be fabricated with such directly synthesized stabilized phase-pure FAPbI3 with a lower bandgap. Our results offer new insights on the surface chemistry of perovskite materials and provide a new strategy for stabilizing metastable perovskites and metastable polymorphs of solid materials in general.
Nano Pattern on n-Si (100) Surface by Ion Irradiation
Nano structuring of silicon surface by low energy ion irradiation is reported. Semi insulating n-Si (100) has been irradiated by 50 keV Ar{sup +} ion beam at an angle of 50 deg. with respect to surface normal. The irradiated sample's surfaces were analyzed by Atomic Force Microscopy. Irradiation caused formation of nano-sized elliptical dots aligned in rows perpendicular to ion beam direction at fluence of 1x10{sup 17} ions/cm{sup 2}. At higher fluences of 3x10{sup 17} ions/cm{sup 2} and 7x10{sup 17} ions/cm{sup 2} self organized ripples were developed on the surface.