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  • Public Library of Science (PLoS)
  • American Institute of Physics (AIP)
  • 2015-2019  (6)
  • 1985-1989  (23)
  • 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)
    Details
    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)
    Details
    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)
    Details
    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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  • 7
    Electronic Resource
    Electronic Resource
    Correlation between forward voltage at low current and catastrophic metal migration failure mechanisms in (AlGa)As laser diodes (1989)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 66 (1989), S. 2819-2827 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thirty-seven of 97 AlxGa1−xAs laser diodes from three tests conducted in vacuum (10−6 Torr) at a −20 °C heat sink temperature (Tj∼0 °C) failed catastrophically. Causes of failure varied from testing error to defects in the metallization. In all cases, p-side anomalies allowed indium solder (and probably gold in addition to indium in one case) to migrate into the GaAs bulk, causing the observed failures. The results of this research show that the integrity of the p-side metallization is crucial for reliable operation. Stringent controls have been implemented in the design and processing of the p-side metallization and catastrophic failures due to p-side anomalies have not been observed in subsequent life tests. A correlation was made between shifts in the forward voltage at 1 mA (Vf at 1 mA) and the specific failure mechanisms found in this research. In all cases Vf at 1 mA was found to decrease. The amount of decrease depended on the failure mechanism. A quantitative model is presented that explains the shift in Vf as a function of the type of defect in the active region.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.344210
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  • 8
    Electronic Resource
    Electronic Resource
    Zinc-implantation-disordered (InGa)As/GaAs strained-layer superlattice diodes (1986)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 60 (1986), S. 1131-1134 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have examined the properties of (InGa)As/GaAs strained-layer superlattices (SLSs) that have been disordered by implantation of 5×1015/cm2, 250 keV 64Zn+ followed by controlled atmosphere annealing at 680 °C for 30 min. Ion channeling techniques indicate that the Zn-disordered regions of the SLS contain extensive crystalline damage after annealing. Simulations of the disordering process using an analytic ion range code predict that the electrical junction resulting from the implantation process is located outside the disordered region of the SLS in both the vertical and the lateral directions. Junction electroluminescence intensity for given drive current densities from the Zn-disordered SLS devices is comparable to that from reference Be-implantation-doped (SLS retained) devices and greatly exceeds that from heavily dislocated grown-junction mesa diodes in the homogeneous alloy of the average SLS composition; this result is consistent with the results of the simulations. This study demonstrates that implantation disordering can be as useful for strained-layer systems as for less severely mismatched heterojunction systems.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337356
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  • 9
    Electronic Resource
    Electronic Resource
    Back bias effects on two-dimensional electron gas (1988)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 63 (1988), S. 1121-1125 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A new technique is described to study heterostructure field-effect transistors. This technique is based on the application of an electric field perpendicular to the channel via a p-doped substrate. The measurements demonstrate that the centroid of the two-dimensional electron gas resides approximately 10 nm below the heterointerface and that application of a negative substrate potential increases the confinement of these charges towards the interface. A negative substrate voltage decreases the K' factor of the transistor and also reduces the well capacity. Capacitance voltage measurements confirm that a parasitic channel in the donor layer can be formed and that it is shielded from the substrate by the two-dimensional electron gas.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.341123
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  • 10
    Electronic Resource
    Electronic Resource
    Energy levels of finite-depth quantum wells in an electric field (1987)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 61 (1987), S. 2273-2276 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Numerical calculations of energy levels and wavefunctions for a particle in a finite quantum well subject to an electric field are described. The calculations are restricted to the regime where the tunneling rate out of the well is small. In this regime the results are in good agreement with results of an approximate calculation wherein the finite well is replaced by an infinitely deep well whose width has been adjusted (separately for each level) to obtain the correct zero-field eigenvalue, as recently proposed for the ground state by Miller et al. [D. A. B. Miller, D. S. Chemla, T. C. Damen, A. C. Gossard, W. Wiegmann, T. H. Wood, and C. A. Burrus, Phys. Rev. B 32, 1043 (1985)]. Over a significant range of well depths and fields (which are the only variables, provided that appropriately normalized units are used), it is found that the difference between the approximate and exact eigenvalues can be accurately estimated from a simple empirical formula. These results should be useful in studies of electro-optic effects in semiconductor quantum-well structures.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.337989
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