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Journal of Applied Physics

Issue 19 • Date May 2014

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Displaying Results 1 - 25 of 70
  • Defect correlated fluorescent quenching and electron phonon coupling in the spectral transition of Eu3+ in CaTiO3 for red emission in display application

    Page(s): 193101 - 193101-14
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    This paper reports on the defect correlated self-quenching and spectroscopic investigation of calcium titanate (CaTiO3) phosphors. A series of CaTiO3 phosphors doped with trivalent europium (Eu3+) and codoped with potassium (K+) ions were prepared by the solid state reaction method. The X-ray diffraction results revealed that the obtained powder phosphors consisted out of a single-phase orthorhombic structure and it also indicated that the incorporation of the dopants/co-dopants did not affect the crystal structure. The scanning electron microscopy images revealed the irregular morphology of the prepared phosphors consisting out of μm sized diameter particles. The Eu3+ doped phosphors illuminated with ultraviolet light showed the characteristic red luminescence corresponding to the 5D07FJ transitions of Eu3+. As a charge compensator, K+ ions were incorporated into the CaTiO3:Eu3+ phosphors, which enhanced the photoluminescence (PL) intensities depending on the doping concentration of K+. The concentration quenching of Eu3+ in this host is discussed in the light of ion-ion interaction, electron phonon coupling, and defect to ion energy transfer. The spectral characteristics and the Eu-O ligand behaviour were determined using the Judd-Ofelt theory from the PL spectra instead of the absorption spectra. The CIE (International Commission on Illumination) parameters were calculated using spectral energy distribution functions and McCamy's empirical formula. Photometric characterization indicated the suitability of K+ compensated the CaTiO3:Eu3+ phosphor for pure red emission in light-emitting diode applications. View full abstract»

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  • Enhancing the optical transmittance by using circular silver nanowire networks

    Page(s): 193102 - 193102-6
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    The optical transmittance of the circular and square silver nanowire networks with different surrounding dielectric environments is systematically investigated. Key parameters determining the plasmonic effect of the silver nanowires are identified and compared with the change of the nanowire density, the nanowire shape, the substrate materials, and the dielectric embedding materials. The total transmittance as a function of the sheet resistance of the nanowire networks for each case is evaluated. It is revealed that while the sheet resistance of the circular nanowire networks is maintained the same as that of the square networks, the optical transmittance in the former case is increased by up to 3% for the networks on glass, 15% on Si and 13% on Si covered with a 75 nm thick SiNx layer. These results give an improved guidance on the future design of the networks for various photovoltaic applications. View full abstract»

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  • Optical Salisbury screen with design-tunable resonant absorption bands

    Page(s): 193103 - 193103-5
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    A thin-film selective absorber at visible and near infra-red wavelengths is demonstrated. The structure consists of an optically thick layer of gold, a SiO2 dielectric spacer and a partially transparent gold film on top. The optical cavity so formed traps and absorbs light at a resonance wavelength determined by the film thicknesses. Observed fundamental-resonance absorption strengths are in the range 93%–97%. The absorption red-shifts and broadens as the thickness of the top gold layer is decreased with little change in absorption strength. Thus, strong absorption with design-tunable wavelength and width is achieved easily by unstructured blanket depositions. Observed angle-dependent spectra agree well with the recent three-layer analytical model of Shu et al. [Opt. Express 21, 25307 (2013)], if effective medium approximation is used to calculate the permittivity of the top gold film when it becomes discontinuous at the lowest thicknesses. View full abstract»

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  • Green synthesis of highly efficient CdSe quantum dots for quantum-dots-sensitized solar cells

    Page(s): 193104 - 193104-6
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    Green synthesis of CdSe quantum dots for application in the quantum-dots-sensitized solar cells (QDSCs) is investigated in this work. The CdSe QDs were prepared with glycerol as the solvent, with sharp emission peak, full width at half maximum around 30 nm, and absorption peak from 475 nm to 510 nm. The reaction is environmental friendly and energy saving. What's more, the green synthesized CdSe QDs are coherence to the maximum remittance region of the solar spectrum and suitable as sensitizers to assemble onto TiO2 electrodes for cell devices application. What's more, the dynamic procedure of the carriers' excitation, transportation, and recombination in the QDSCs are discussed. Because the recombination of the electrons from the conduction band of TiO2's to the electrolyte affects the efficiency of the solar cells greatly, 3-Mercaptopropionic acid capped water-dispersible QDs were used to cover the surface of TiO2. The resulting green synthesized CdSe QDSCs with Cu2S as the electrode show a photovoltaic performance with a conversion efficiency of 3.39%. View full abstract»

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  • Accuracy of sample material parameters reconstruction using terahertz pulsed spectroscopy

    Page(s): 193105 - 193105-9
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    New experimental and theoretical results for the material parameter reconstruction using terahertz (THz) pulsed spectroscopy (TPS) are presented. The material parameter reconstruction algorithm was realized and experimentally implemented to study the test sample. In order to both verify the algorithm and to estimate the reconstruction accuracy, test sample material parameters obtained with the TPS were compared with the results of the same sample studying by the use of the backward-wave oscillator (BWO) spectroscopy. Thus, high reconstruction accuracy was demonstrated for the spectral range, corresponding to the BWO sensitivity and located between 0.2 and 1.2 THz. The numerical simulations were applied for determining the material parameter reconstruction stability in the presence of white Gaussian noise in TPS waveforms as well as fluctuations in the femtosecond (FS) optical pulse duration. We report a strong dependence of the inverse problem solution stability on these factors. We found that the instability of the FS optical pulse duration used for THz pulses generation and detection limits the material parameter reconstruction with TPS. View full abstract»

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  • Estimation of the internal electric field inside (11-22) semipolar GaN/Al0.5Ga0.5N nanostructures and the radiative efficiency at low temperature

    Page(s): 193106 - 193106-7
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    We report on time-integrated and resolved photoluminescence data on self-assembled semipolar (11-22) GaN nanostructures embedded in Al0.5Ga0.5N. It is confirmed that the internal electric field is reduced for semipolar (11-22) orientation. It is shown in particular that the value of the electric field is 450–500 kV/cm for this orientation. The photoluminescence decay time of excitons is used as a probe of the reduction of the internal electric field in the case of semipolar GaN nanostructures. The measured decays are not only controlled by radiative lifetimes, which depend on the fields inside GaN nanostructures, but also on the nonradiative escape of carriers through barriers. The correspondent decay time is found equal to 330 ps. By the study of the decay time as a function of the emission energy, we can determine the evolution of the internal quantum efficiency as a function of the nanostructures height (energy) and to have access to the nonradiative lifetime at low temperature. View full abstract»

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  • Polarization ratio enhancement of a-plane GaN light emitting diodes by asymmetric two-dimensional photonic crystals

    Page(s): 193107 - 193107-5
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    Fabricating photonic crystals (PhCs) on GaN based non-polar light emitting diodes (LEDs) is an effective way to increase light extraction and meanwhile to preserve or improve polarization ratio. In this work, a-plane GaN LEDs with two-dimensional PhCs were demonstrated. With the E // m polarized modes (which mean the optical polarization with the electric field parallel to m-axis) as the target of diffraction, we matched E//m modes to the photonic bands and aligned E//c modes to fall within the photonic band gap. The results show stronger E//m but weaker E//c mode diffractions on both c- and m-axes. At the vertical direction, the polarization ratio is enhanced from 45.8% for the planar device to 52.3% for the LEDs with PhCs. View full abstract»

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  • Development of a cost effective surface-patterned transparent conductive coating as top-contact of light emitting diodes

    Page(s): 193108 - 193108-8
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    Sol-gel process has been used to form indium zinc oxide films using an optimized combination of zinc to indium concentration in the precursor solutions. Different structures, like one (1D) and two-dimensional (2D) gratings and diffractive optical elements (DOEs) in the form of Fresnel lens are fabricated on the film surface of proposed top metal contact of LED by imprint soft lithography technique. These structures can enhance the LED's light extraction efficiency (LEE) or can shape the output beam pattern, respectively. Several characterizations are done to analyze the material and structural properties of the films. The presence of 1D and 2D gratings as well as DOEs is confirmed from field emission scanning electron and atomic force microscopes analyses. Although, X-ray diffraction shows amorphous nature of the film, but transmission electron microscopy study shows that it is nano crystalline in nature having fine particles (∼8 nm) of hexagonal ZnO. Shrinkage behaviour of gratings as a function of curing temperature is explained by Fourier transform infra-red spectra and thermo gravimetric-differential thermal analysis. The visible transmission and sheet resistance of the sample are found comparable to tin doped indium oxide (ITO). Therefore, the film can compete as low cost substitute of ITO as top metal contact of LEDs. View full abstract»

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  • Dual band complementary metamaterial absorber in near infrared region

    Page(s): 193109 - 193109-6
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    In this paper, we present the dual band absorption characteristics of complementary metamaterial absorber in near infrared (1.3–2.5 μm) region. The dual band absorption is caused by two distinct resonance mechanisms—electrical resonance and cavity resonance. Electrical resonance occurs in the metal layer—top complementary metamaterial and the cavity resonance occurs in the spacer cavity formed between the top complementary metamaterial and bottom metal reflector layers. In order to elucidate the resonant mechanisms and study the effects of geometrical variations on both the resonant absorption behaviours, two sets of experiment were performed. It was seen that with increasing complementary metamaterial pattern dimension, the electrical resonance absorption peak showed a blue shift, while the cavity resonance showed a slight red shift. However, on the other hand, for the increase in spacer thickness, the cavity resonance peak showed a strong red shift, while the electrical resonance peak remained uninfluenced. The reason for these geometrical dependencies, for both resonances, is conceptually analysed. Furthermore, the design was optimized to attain single absorption band by engineering the cavity and electrical resonances to be at the same wavelength. The single absorption band was successfully realized, however, the peak wavelength showed a red shift from the electrical resonance as in dual band absorber case. The reason for the shift was further explored to be caused due to the strong coupling of electrical and cavity resonances. This approach of utilizing different resonant mechanisms for absorption at different wavelengths provides the means to achieve multiband absorbers, using a simple design and low cost fabrication process. View full abstract»

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  • Computational study of plasma sustainability in radio frequency micro-discharges

    Page(s): 193301 - 193301-11
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    We apply an implicit particle-in-cell Monte-Carlo (PIC-MC) method to study a radio-frequency argon microdischarge at steady state in the glow discharge limit, in which the microdischarge is sustained by secondary electron emission from the electrodes. The plasma density, electron energy distribution function (EEDF), and electron temperature are calculated in a wide range of operating conditions, including driving voltage, microdischarge gap, and pressure. Also, the effect of gap size scaling (in the range of 50-1000 μm) on the plasma sustaining voltage and peak electron density at atmospheric pressure is examined, which has not been explored before. In our simulations, three different EEDFs, i.e., a so-called three temperature hybrid mode, a two temperature α mode, and a two temperature γ mode distribution, are identified at different gaps and voltages. The maximum sustaining voltage to avoid a transition from the glow mode to an arc is predicted, as well as the minimum sustaining voltage for a steady glow discharge. Our calculations elucidate that secondary electrons play an essential role in sustaining the discharge, and as a result the relationship between breakdown voltage and gap spacing is far away from the Paschen law at atmospheric pressure. View full abstract»

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  • Measurements of an expanding surface flashover plasma

    Page(s): 193302 - 193302-8
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    A better understanding of vacuum surface flashover and the plasma produced by it is of importance for electron and ion sources, as well as advanced accelerators and other vacuum electronic devices. This article describes time-of-flight and biased-probe measurements made on the expanding plasma generated from a vacuum surface flashover discharge. The plasma expanded at velocities of 1.2–6.5 cm/μs, and had typical densities of 1010–1012 cm−3. The expansion velocity of the plasma leading edge often exhibited a sharp increase at distances of about 50 mm from the discharge site. Comparison with biased-probe data suggests that, under most conditions, the plasma leading edge was dominated by negative ions, with the apparent increase in velocity being due to fast H overtaking slower, heavier ions. In some cases, biased-probe data also showed abrupt discontinuities in the plasma energy distribution co-located with large changes in the intercepted plasma current, suggesting the presence of a shock in the leading edge of the expanding plasma. View full abstract»

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  • Assessing the role of secondary electron emission on the characteristics of 6-cavity magnetrons with transparent cathode through particle-in-cell simulations

    Page(s): 193303 - 193303-11
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    Effects of secondary electron emission (SEE) on the performance of a 6-cavity relativistic magnetron with transparent cathodes are probed through particle-in-cell simulations. Appropriate relations for the secondary electron yield have been developed and used. For comparisons, separate simulations have been performed with- and without electron cascading. Simulation results seem to indicate SEE to be detrimental to the power output due to deviations in the starting trajectories of secondary electrons, and the reduced fraction with synchronized rotational velocity. A higher reduction in output power is predicted with electron cascading, though mode competition was not seen at the 0.65 T field. A possible solution to mitigating SEE in magnetrons for high power microwave applications would be to alter the surface properties of emitting electrodes through irradiation, which can lead to graphitic film formation. View full abstract»

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  • Thermocyclic stability of candidate Seebeck coefficient standard reference materials at high temperature

    Page(s): 193501 - 193501-6
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    The Seebeck coefficient is the most widely measured property specific to thermoelectric materials. There is currently no consensus on measurement protocols, and researchers employ a variety of techniques to measure the Seebeck coefficient. The implementation of standardized measurement protocols and the use of reliable Seebeck Coefficient Standard Reference Materials (SRMs®) will allow the accurate interlaboratory comparison and validation of materials data, thereby accelerating the development and commercialization of more efficient thermoelectric materials and devices. To enable members of the thermoelectric materials community the means to calibrate Seebeck coefficient measurement equipment, NIST certified SRM® 3451 “Low Temperature Seebeck Coefficient Standard (10 K to 390 K)”. Due to different practical requirements in instrumentation, sample contact methodology, and thermal stability, a complementary SRM® is required for the high temperature regime (300 K to 900 K). The principal requirement of a SRM® for the Seebeck coefficient at high temperature is thermocyclic stability. We therefore characterized the thermocyclic behavior of the Seebeck coefficient for a series of candidate materials: constantan, p-type single crystal SiGe, and p-type polycrystalline SiGe, by measuring the temperature dependence of the Seebeck coefficient as a function of 10 sequential thermal cycles, between 300 K and 900 K. We employed multiple regression analysis to interpolate and analyze the thermocyclic variability in the measurement curves. View full abstract»

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  • Magnetic field role on the structure and optical response of photonic crystals based on ferrofluids containing Co0.25Zn0.75Fe2O4 nanoparticles

    Page(s): 193502 - 193502-7
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    Ferrofluids based on magnetic Co0.25Zn0.75Fe2O4 ferrite nanoparticles were prepared by co-precipitation method from aqueous salt solutions of Co (II), ZnSO4, and Fe (III) in an alkaline medium. Ferrofluids placed in an external magnetic field show properties that make them interesting as magneto-controllable soft photonic crystals. Morphological and structural characterizations of the samples were obtained from Scanning Electron Microscopy and Transmission Electron Microscopy studies. Magnetic properties were investigated with the aid of a vibrating sample magnetometer at room temperature. Herein, the Co0.25Zn0.75Fe2O4 samples showed superparamagnetic behavior, according to hysteresis loop results. Taking in mind that the Co-Zn ferrite hysteresis loop is very small, our magnetic nanoparticles can be considered soft magnetic material with interesting technological applications. In addition, by using the plane-wave expansion method, we studied the photonic band structure of 2D photonic crystals made of ferrofluids with the same nanoparticles. Previous experimental results show that a magnetic field applied perpendicular to the ferrofluid plane agglomerates the magnetic nanoparticles in parallel rods to form a hexagonal 2D photonic crystal. We calculated the photonic band structure of photonic crystals by means of the effective refractive index of the magnetic fluid, basing the study on the Maxwell-Garnett theory, finding that the photonic band structure does not present any band gaps under the action of applied magnetic field strengths used in our experimental conditions. View full abstract»

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  • Photoluminescence lineshape and dynamics of localized excitonic transitions in InAsP epitaxial layers

    Page(s): 193503 - 193503-8
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    The excitonic radiative transitions of InAsxP1−x (x = 0.13 and x = 0.40) alloy epitaxial layers were studied through magnetic field and temperature dependent photoluminescence and time-resolved photoluminescence spectroscopy. While the linewidth and lineshape of the exciton transition for x = 0.40 indicate the presence of alloy broadening due to random anion distribution and the existence of localized exciton states, those of x = 0.13 suggest that this type of compositional disorder is absent in x = 0.13. This localization is further supported by the behavior of the exciton transitions at low temperature and high magnetic fields. InAs0.4P0.6 exhibits anomalous “S-shaped” temperature dependence of the excition emission peak below 100 K as well as linewidth broadening at high magnetic fields due to the compression of the excitonic volume amid compositional fluctuations. Finally, photoluminescence decay patterns suggest that the excitons radiatively relax through two channels, a fast and a slow decay. While the lifetime of the fast decay is comparable for both compositions (∼30 ps), that of the slow decay increases from 206 ps to 427 ps as x increases from 0.13 to 0.40, attributable to carrier migration between the localization states of InAs0.4P0.6. View full abstract»

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  • Relative light yield and temporal response of a stilbene-doped bibenzyl organic scintillator for neutron detection

    Page(s): 193504 - 193504-5
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    The neutron time-of-flight (nTOF) diagnostics used to characterize implosions at the National Ignition Facility (NIF) has necessitated the development of novel scintillators that exhibit a rapid temporal response and high light yield. One such material, a bibenzyl-stilbene mixed single-crystal organic scintillator grown in a 99.5:0.5 ratio in solution, has become the standard scintillator used for nTOF diagnostics at NIF. The prompt fluorescence lifetime and relative light yield as a function of proton energy were determined to calibrate this material as a neutron detector. The temporal evolution of the intensity of the prompt fluorescent response was modeled using first-order reaction kinetics and the prompt fluorescence decay constant was determined to be 2.46 ± 0.01 (fit) ± 0.13 (systematic) ns. The relative response of the bibenzyl-stilbene mixed crystal generated by recoiling protons was measured, and results were analyzed using Birks' relation to quantify the non-radiative quenching of excitation energy in the scintillator. View full abstract»

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  • The impact of argon admixture on the c-axis oriented growth of direct current magnetron sputtered ScxAl1−xN thin films

    Page(s): 193505 - 193505-8
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    The piezoelectric properties of wurtzite aluminium nitride (w-AlN) are enhanced by alloying with scandium (Sc), thus offering superior properties for applications in micro electro-mechanical systems devices. ScxAl1−xN thin films have been prepared by DC reactive magnetron sputtering on Si (100) substrates from a single target. When targeting a concentration range from x = 0 up to x = 0.15, the preparation conditions have been optimized by varying the Ar/N2 ratio in the sputtering gas. To incorporate an increasing Sc concentration, a higher Ar/N2 ratio has to be applied during the deposition process. Hence, the argon concentration in the sputtering gas becomes a crucial parameter for microstructure-related parameters. To determine phase purity, degree of c-axis orientation, lattice parameter, and grain size, the ScxAl1−xN thin films were investigated by techniques, such as scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. View full abstract»

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  • Growth of Au nanoparticle films and the effect of nanoparticle shape on plasmon peak wavelength

    Page(s): 193506 - 193506-8
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    Metal nanoparticles (NPs) exhibit localized surface plasmon resonance (LSPR) and thus have potential for use in a wide range of applications. A facile technique for the preparation of NP films using an electron-cyclotron-resonance plasma sputtering method without a dewetting process is described. Field emission scanning electron microscopy (FE-SEM) observations revealed that the Au NPs grew independently as island-like particles during the first stage of sputtering and then coalesced with one another as sputtering time increased to ultimately form a continuous film. A plasmon absorption peak was observed via optical measurement of absorption efficiency. The LSPR peak shifted toward longer wavelengths (red shift) with an increase in sputtering time. The cause of this plasmon peak shift was theoretically investigated using the finite-difference time-domain calculation method. A realistic statistical distribution of the particle shapes based on FE-SEM observations was applied for the analysis, which has not been previously reported. It was determined that the change in the shape of the NPs from spheroidal to oval or slender due to coalescence with neighbouring NPs caused the LSPR peak shift. These results may enable the design of LSPR devices by controlling the characteristics of the nanoparticles, such as their size, shape, number density, and coverage. View full abstract»

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  • Metallic nanoparticles grown in the core of femtosecond laser micromachined waveguides

    Page(s): 193507 - 193507-5
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    3D-waveguides containing silver nanoparticles have been fabricated in tungsten lead–pyrophosphate glass by femtosecond laser micromachining. Nucleation and growth of nanoparticles occur in a single step process when high repetition rate laser (MHz) is employed, while an additional annealing is required for the irradiation using kHz laser system. The presence of nanoparticles locally changes the refractive index, and, therefore, the elliptical structures produced by direct laser writing were able to guide light. By increasing the pulse energy applied during the micromachining, the waveguide size increased from 2 to 30 μm, while their propagation loss decrease from 1.4 to 0.5 dB/mm at 632.8 nm. View full abstract»

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  • Molecular dynamics simulation of shock induced ejection on fused silica surface

    Page(s): 193508 - 193508-9
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    Shock response and surface ejection behaviors of fused silica are studied by using non-equilibrium molecular dynamics combining with the Tersoff potential. First, bulk structure and Hugoniot curves of fused silica are calculated and compared with experimental results. Then, the dynamical process of surface ejection behavior is simulated under different loading velocities ranging from 3.5 to 5.0 km∕s, corresponding to shock wave velocities from 7.1 to 8.8 km∕s. The local atomistic shear strain parameter is used to describe the local plastic deformation under conditions of shock compression or releasing. Our result shows that the shear strain is localized in the bottom area of groove under the shock compression. Surface ejection is observed when the loading velocity exceeds 4.0 km∕s. Meanwhile, the temperature of the micro-jet is ∼5574.7 K, which is close to experiment measurement. Several kinds of structural defects including non-bridging oxygen are found in the bulk area of the sample after ejection. View full abstract»

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  • Critical sizes for the stabilization of coherent precipitates

    Page(s): 193509 - 193509-5
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    On growth of a precipitate beyond a critical size (r*), interfacial misfit dislocation loops are energetically stabilized. In the case of precipitation in (small) finite crystals, the energy of both the coherent precipitate and the dislocation loop are altered with respect to bulk crystals. Thus, as the crystal (domain) size approaches nanoscale, the critical size (r*) is expected to be altered with respect to bulk crystals. In the current investigation, finite element simulations are performed to study the variation of critical size (r*) with crystal/domain size and it is shown that below a critical domain size, the coherent precipitate is stabilized. Important findings include: (i) strain energy versus precipitate-size plot shows change in curvature for large precipitate sizes and (ii) coherent state is stable in two distinct regimes of precipitate sizes. Further, a phase diagram showing the stability regions of the coherent and semi-coherent states of the precipitate is drawn. View full abstract»

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  • Influence of grain size and surface condition on minority-carrier lifetime in undoped n-BaSi2 on Si(111)

    Page(s): 193510 - 193510-7
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    We have fabricated approximately 0.5-μm-thick undoped n-BaSi2 epitaxial films with various average grain areas ranging from 2.6 to 23.3 μm2 on Si(111) by molecular beam epitaxy, and investigated their minority-carrier lifetime properties by the microwave-detected photoconductivity decay method at room temperature. The measured excess-carrier decay curves were divided into three parts in terms of decay rate. We characterized the BaSi2 films using the decay time of the second decay mode, τSRH, caused by Shockley-Read-Hall recombination without the carrier trapping effect, as a measure of the minority-carrier properties in the BaSi2 films. The measured τSRH was grouped into two, independently of the average grain area of BaSi2. BaSi2 films with cloudy surfaces or capped intentionally with a 3 nm Ba or Si layer, showed large τSRH (ca. 8 μs), whereas those with mirror surfaces much smaller τSRH (ca. 0.4 μs). X-ray photoelectron spectroscopy measurements were performed to discuss the surface region of the BaSi2 films. View full abstract»

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  • Enhanced absorption of monolayer MoS2 with resonant back reflector

    Page(s): 193511 - 193511-5
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    The optical absorption of monolayer MoS2 on top of one-dimensional photonic crystal (1DPC) or metal films with spacer layers is theoretically investigated by extracting the permittivity of monolayer MoS2 from existing experimental results [K. F. Mak et al., Phys. Rev. Lett. 105, 136805 (2010)]. The absorption of graphene with 1DPC across a broad spectral range is substantially enhanced because of the photonic localization at the optical micro-cavity on top of the 1DPC or metal films. The absorption of monolayer MoS2 can be tuned by varying either the distance between the monolayer MoS2 and the back reflector or the thickness of the cover layers. View full abstract»

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  • Pressure-induced amorphization in orthorhombic Ta2O5: An intrinsic character of crystal

    Page(s): 193512 - 193512-4
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    The phase transition of orthorhombic Ta2O5 was investigated by in situ synchrotron X-ray diffraction and Raman spectroscopy. The orthorhombic phase transforms into an amorphous form completely at 24.7 GPa. A bulk modulus B0 = 139 (9) GPa for the orthorhombic Ta2O5 is derived from the P-V data. We suggest that the pressure-induced amorphization (PIA) in Ta2O5 can be attributed to the unstability of the a axis under high pressure leads to the connections of polyhedral breaking down and even triggers disorder of the whole crystal frame. These results demonstrate that the PIA is an intrinsic character of Ta2O5 which depends on its orthorhombic crystal structure rather than nanosize effects. This study provides a new kind of bulk material for investigating PIA in metal oxides. View full abstract»

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  • Cu2ZnSnSxO4−x and Cu2ZnSnSxSe4−x: First principles simulations of optimal alloy configurations and their energies

    Page(s): 193513 - 193513-12
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    With the aim of exploring oxidation and selenization of the photovoltaic material Cu2ZnSnS4, we used first principles methods to study the structure and stability of Cu2ZnSnSxO4−x and Cu2ZnSnSxSe4−x alloys for 0 ≤ x ≤ 4. Pure Cu2ZnSnO4 was found to have the lowest heat of formation, followed by Cu2ZnSnS4, and finally Cu2ZnSnSe4. This suggests that oxidization is very likely to occur, whereas selenization can only be accomplished under high temperature. For the alloys, the energetically favorable chalcogen configurations are very different for oxygen and selenium. While the energies of the selenium alloys are insensitive to the distribution of S and Se configurations, the lowest energy oxygen alloys have alternating S and O sites in the a–b planes. In considering the heats of formation of the Cu2ZnSnSxO4−x alloys, we find that they are unstable with respect to decomposition into binary oxides and sulfides except for small concentrations of O. Our results also show that it is energetically more favorable to sulfurize Cu2ZnSnSe4 than to selenize Cu2ZnSnS4. View full abstract»

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Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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P. James Viccaro
Argonne National Laboratory