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  • Public Library of Science (PLoS)
  • American Institute of Physics (AIP)
  • 2015-2019  (6)
  • 1
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    Optimization of an exchange-correlation density functional for water (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-06-10
    Description: We describe a method, that we call data projection onto parameter space (DPPS), to optimize an energy functional of the electron density, so that it reproduces a dataset of experimental magnitudes. Our scheme, based on Bayes theorem, constrains the optimized functional not to depart unphysically from existing ab initio functionals. The resulting functional maximizes the probability of being the “correct” parameterization of a given functional form, in the sense of Bayes theory. The application of DPPS to water sheds new light on why density functional theory has performed rather poorly for liquid water, on what improvements are needed, and on the intrinsic limitations of the generalized gradient approximation to electron exchange and correlation. Finally, we present tests of our water-optimized functional, that we call vdW-DF-w, showing that it performs very well for a variety of condensed water systems.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    Characterizing solid-state ignition of runaway chemical reactions in Ni-Al nanoscale multilayers under uniform heating (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-10-08
    Description: Nanoscale layers of nickel and aluminum can mix rapidly to produce runaway reactions. While self-propagating high temperature synthesis reactions have been observed for decades, the solid-state ignition of these reactions has been challenging to study. Particularly elusive is characterization of the low-temperature chemical mixing that occurs just prior to the ignition of the runaway reaction. Characterization can be challenging due to inhomogeneous microstructures, uncontrollable heat losses, and the nonuniform distribution of heat throughout the material prior to ignition. To reduce the impact of these variables, we heat multilayered Ni/Al foils in a highly uniform manner and report ignition temperatures as low as 245 °C for heating rates ranging from 2000 °C/s to 50 000 °C/s. Igniting in this way reveals that there are four stages before the reaction is complete: heating to an ignition temperature, low temperature solid-state mixing, a separate high temperature solid-state mixing, and liquid-state mixing. Multiple bilayer spacings, heating rates, and heating times are compared to show that the ignition temperature is a function of the bilayer spacing. A symmetric numerical diffusion model is used to show that there is very little chemical mixing in the first 10 ms of heating but significant mixing after 50 ms. These predictions suggest that ignition temperatures should increase for the slowest heating rates but this trend could not be identified clearly. The modeling was also used to examine the kinetic parameters governing the early stages of solid-state diffusion and suggest that grain boundary diffusion is dominant.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 3
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    The creation of large-volume, gradient-free warm dense matter with an x-ray free-electron laser (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-03-25
    Description: The efficiency and uniformity of heating induced by hard x-ray free-electron laser pulse is investigated for 0.5  μ m silver foils using the X-ray Pump Probe instrument at the Linac Coherent Light Source facility. Intense 8.9 keV x-ray pulses of 60 fs duration deposit energy predominantly via inner-shell ionization to create a non-equilibrium Ag solid density plasma. The x-ray pulses are focused to 14 × 17  μ m 2 by means of beryllium lenses and by varying the total beam energy, the energy deposition is varied over a range of irradiances from 4.4 to 6.5 × 10 15  W/cm 2 . Two time-and-space resolved interferometers simultaneously probed the expansion of the front and rear sample surfaces and find evidence of a nearly symmetric expansion pointing to the uniformity of energy deposition over the full target thickness. The experimental results are compared with two different hydrodynamic simulations of the sample expansion. The agreement between experimental and theoretical results yields an estimate of the temperature evolution as a function of x-ray irradiance that varies from 8 to 10 eV for the x-ray irradiances studied.
    Print ISSN: 1070-664X
    Electronic ISSN: 1089-7674
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 4
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    p+-doping analysis of laser fired contacts for silicon solar cells by Kelvin probe force microscopy (2016)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2016-03-15
    Description: Local rear contacts for silicon passivated emitter and rear contact solar cells can be established by point-wise treating an Al layer with laser radiation and thereby establishing an electrical contact between Al and Si bulk through the dielectric passivation layer. In this laser fired contacts (LFC) process, Al can establish a few μ m thick p + -doped Si region below the metal/Si interface and forms in this way a local back surface field which reduces carrier recombination at the contacts. In this work, the applicability of Kelvin probe force microscopy (KPFM) to the investigation of LFCs considering the p + -doping distribution is demonstrated. The method is based on atomic force microscopy and enables the evaluation of the lateral 2D Fermi-level characteristics at sub-micrometer resolution. The distribution of the electrical potential and therefore the local hole concentration in and around the laser fired region can be measured. KPFM is performed on mechanically polished cross-sections of p + -doped Si regions formed by the LFC process. The sample preparation is of great importance because the KPFM signal is very surface sensitive. Furthermore, the measurement is responsive to sample illumination and the height of the applied voltage between tip and sample. With other measurement techniques like micro-Raman spectroscopy, electrochemical capacitance-voltage, and energy dispersive X-ray analysis, a high local hole concentration in the range of 10 19  cm −3 is demonstrated in the laser fired region. This provides, in combination with the high spatial resolution of the doping distribution measured by KPFM, a promising approach for microscopic understanding and further optimization of the LFC process.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 5
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    Platelets Guide Leukocytes to Their Sites of Extravasation (2016)
    Public Library of Science (PLoS)
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    Publication Date: 2016-05-07
    Description: Effective immune responses require the directed migration of leukocytes from the vasculature to the site of injury or infection. How immune cells “find” their site of extravasation remains largely obscure. Here, we identified a previously unrecognized role of platelets as pathfinders guiding leukocytes to their exit points in the microvasculature: upon onset of inflammation, circulating platelets were found to immediately adhere at distinct sites in venular microvessels enabling these cellular blood components to capture neutrophils and, in turn, inflammatory monocytes via CD40-CD40L-dependent interactions. In this cellular crosstalk, ligation of PSGL-1 by P-selectin leads to ERK1/2 MAPK-dependent conformational changes of leukocyte integrins, which promote the successive extravasation of neutrophils and monocytes to the perivascular tissue. Conversely, blockade of this cellular partnership resulted in misguided, inefficient leukocyte responses. Our experimental data uncover a platelet-directed, spatiotemporally organized, multicellular crosstalk that is essential for effective trafficking of leukocytes to the site of inflammation.
    Print ISSN: 1544-9173
    Electronic ISSN: 1545-7885
    Topics: Biology
    Published by Public Library of Science (PLoS)
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  • 6
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    PEDOT:PSS emitters on multicrystalline silicon thin-film absorbers for hybrid solar cells (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-02-26
    Description: We fabricated an efficient hybrid solar cell by spin coating poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) on planar multicrystalline Si (mc-Si) thin films. The only 5  μ m thin Si absorber layers were prepared by diode laser crystallization of amorphous Si deposited by electron beam evaporation on glass. On these absorber layers, we studied the effect of SiO x and Al 2 O 3 terminated Si surfaces. The short circuit density and power conversion efficiency (PCE) of the mc-Si/Al 2 O 3 /PEDOT:PSS solar cell increase from 20.6 to 25.4 mA/cm 2 and from 7.3% to 10.3%, respectively, as compared to the mc-Si/SiO x /PEDOT:PSS cell. Al 2 O 3 lowers the interface recombination and improves the adhesion of the polymer film on the hydrophobic mc-Si thin film. Open circuit voltages up to 604 mV were reached. This study demonstrates the highest PCE so far of a hybrid solar cell with a planar thin film Si absorber.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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