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Raman spectra investigation of InAlGaN quaternary alloys grown by metalorganic chemical vapor deposition J. Appl. Phys. 112, 063111 (2012) Structural and micromechanical analyses by polarized Raman spectroscopy of wurtzitic GaN films grown on (0001) sapphire substrates J. Appl. Phys. 112, 053522 (2012) Improvement of near-infrared absorption linewidth in AlGaN/GaN superlattices by optimization of delta-doping location Appl. Phys. Lett. 101, 102104 Raman spectroscopy as a probe for the coupling of light into ensembles of sub-wavelength-sized nanowires Zinc oxide ͑ZnO͒ is a wide-band gap semiconductor that has attracted tremendous interest for optical, electronic, and mechanical applications. First-principles calculations by ͓C. G. Van de Walle, Phys. Rev. Lett. 85, 1012 ͑2000͔͒ have predicted that hydrogen impurities in ZnO are shallow donors. In order to determine the microscopic structure of hydrogen donors, we have used IR spectroscopy to measure local vibrational modes in ZnO annealed in hydrogen gas. An oxygenhydrogen stretch mode is observed at 3326.3 cm Ϫ1 at a temperature of 8 K, in good agreement with the theoretical predictions for hydrogen in an antibonding configuration. The results of this study suggest that hydrogen annealing may be a practical method for controlled n-type doping of ZnO.

The optical properties of PbSe nanocrystal quantum dots ͑NQDs͒ were studied as a function of applied hydrostatic pressure over the range from ambient to 5.4 GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement. Despite such strong confinement, the authors find that the energy gaps of 3, 5, and 7 nm diameter PbSe NQDs change monotonically with pressure with a dependence that is almost entirely determined by the bulk deformation potential. The sizable dependence of the NQD energy gap with pressure invites applications in the areas of high speed pressure sensing and tunable IR lasers.

GSE_ADA/RSV is a free software package for custom analysis of single crystal micro-X-ray diffraction (SCµXRD) data, developed with particular emphasis on data from samples enclosed in diamond anvil cells and subject to high pressure conditions. The package has been in extensive use at the high-pressure beamlines of Advanced The software is optimized for processing of widerotation images and includes a variety of peak intensity corrections and peak filtering features, which are custom-designed to make processing of high-pressure SCµXRD easier and more reliable.

We present isothermal volume compression behavior of two polycrystalline (Mg,Fe)O samples with FeO = 39 and 78 mol% up to *90 GPa at 300 K using synchrotron X-ray diffraction and neon as a pressuretransmitting medium. For the iron-rich (Mg 0.22 Fe 0.78 )O sample, a structural transition from the B1 structure to a rhombohedral structure was observed at 41.6 GPa, with no further indication of changes in structural or compression behavior changes up to 93 GPa. In contrast, a change in the compression behavior of (Mg 0.61 Fe 0.39 )O was observed during compression at P C 71 GPa and is indicative of a spin crossover occurring in the Fe 2? component of (Mg 0.61 Fe 0.39 )O. The low-spin state exhibited a volume collapse of *3.5%, which is a larger value than what was observed for a similar composition in a laser-heated NaCl medium. Upon decompression, the volume of the high-spin state was recovered at approximately 65 GPa. We therefore bracket the spin crossover at 65 B P (GPa) B 77 at 300 K (Mg 0.61 Fe 0.39 )O. We observed no deviation from the B1 structure in (Mg 0.61 Fe 0.39 )O throughout the pressure range investigated.

Mg x Zn 1−x O ͑x = 0.15͒ and ZnO nanocrystals of about 40 nm in diameter were studied using x-ray diffraction and diamond-anvil cells. The equation of state ͑EOS͒ for MgZnO is reported for the first time. Between pressures of 9.45 and 10.7 GPa, MgZnO transforms into the rocksalt ͑NaCl͒ structure, which persisted to 1.1 GPa upon decompression. The EOS parameters for ZnO are close to their bulk values and in good agreement with values obtained previously. The bulk modulus for MgZnO was slightly lower than that of ZnO. The pressure-induced decrease in c / a ratio was greater for MgZnO, consistent with the tendency for MgZnO to move toward a cubic structure. From previous photoluminescence measurements ͓J. Huso et al., Appl. Phys. Lett. 89, 171909 ͑2006͔͒, the band-gap volume deformation potentials for ZnO and Mg 0.15 Zn 0.85 O were determined to be Ϫ3.6 and Ϫ4.0 eV, respectively.

Changes in the conformations of conjugated molecules affect the optical and electronic properties significantly. Hydrostatic pressure has been used to probe the conformations of biphenyl and deuterated biphenyl at liquid-helium temperatures. Infrared (IR) spectra of these materials have been taken up to a pressure of 2 GPa. A disappearance of certain IR absorption peaks has been found to occur between 0.07 and 0.45 GPa, due to the phase transition from a twisted to a planar conformation. Numerical simulations together with ...

The conformation of p-terphenyl and deuterated p-terphenyl has been investigated, using high-pressure infrared spectroscopy at liquid-helium temperatures. First-principles calculations, together with the experimental results, were performed to determine the structure of p-terphenyl in the twisted conformation. At low temperatures and pressures, p-terphenyl belongs to the point group of symmetry. In this configuration, the central ring is twisted with respect to the plane of the outer rings. The symmetry of the molecule is nearly ...

The different conformations of molecular compounds play important roles in biochemistry and organic solid-state technology. Hydrostatic pressure has been applied to para-quaterphenyl to probe its molecular structure at liquid-helium temperatures. The molecules transform from a twisted to a planar conformation at a critical pressure of 0.9 GPa. This conformational change results in the abrupt disappearance of five infrared-absorption peaks. A group-theoretical analysis shows that the peaks, which correspond to out-of- ...

Using infrared spectroscopy and a diamond-anvil cell, we have observed carbon and carbon-hydrogen local vibrational modes (LVM's) in InP at hydrostatic pressures as high as 5.5 GPa at liquid-helium temperatures. For pressures beyond 4.5 GPa, the carbon-hydrogen mode was not observed, perhaps as a result of a transformation of the complex into a different configuration. The LVM arising from carbon substitutional impurities varies linearly with pressure, whereas the shift of the carbon-hydrogen mode has a positive curvature. ...

Crystal whiskers of bis (4-nitrophenyl) disulfide have been grown on a 100-nm thick Pb film from a supersaturated solution of p-nitrothiophenol. The whiskers preferentially grow on Pb but not on Si, In, or Ag. The crystals were characterized by electron microscopy and infrared spectroscopy. The optimized structure of the molecule was determined by ab initio calculations.

We report the structural transitions of pyridine as a function of pressure up to 26 GPa using in situ Raman spectroscopy and infrared absorption spectroscopy. By monitoring changes in the Raman shifts in the lattice region as well as the band profiles in both Raman and IR spectra, a liquid-to-solid transition at 1 GPa followed by solid-to-solid transitions at 2, 8, 11, and 16 GPa were observed upon compression. These transitions were found to be reversible upon decompression from 22 GPa. A further chemical transformation was observed when compressed beyond 22 GPa as evidenced by the substantial and irreversible changes in the Raman and infrared spectra, which could be attributed to the destruction of the ring structure. The observed transformations in pyridine were also compared to those for benzene. The similar transition sequence with well-aligned transition pressures suggests that these isoelectronic aromatics may have similar structures and stabilities under high pressure.

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We present results of concomitant measurements of synchrotron x-ray diffraction (XRD), Brillouin, and Raman spectroscopy on the single crystal samples of cubic silicon carbide (3C-SiC) under quasi-hydrostatic pressures up to 65 GPa, as well as x-ray diffraction and Raman spectroscopy up to 75 GPa. We determined the equation of state of 3C-SiC and pressure dependencies of the zone-center phonon, elastic tensor, and mode Gruneisen parameters. Cubic SiC lattice was found to be stable up to 75 GPa, but there is a tendency for destabilization above 40 GPa, based on softening of a transverse sound velocity. By applying the concomitant density and elasticity measurements, we determined the pressure on the SiC sample without referring to any other pressure scale thus establishing a new primary pressure scale with a 2%-4% precision up to 65 GPa. We proposed corrections to the existing ruby and neon pressure scales, and also calibrated cubic SiC as a pressure marker for the x-ray diffraction and Raman experiments. V

Methane is one of the most abundant hydrocarbon molecules in the universe and is expected to be a significant part of the icy giant planets (Uranus and Neptune) and their satellites. Ethane is one of the most predictable products of chemical reactivity of methane at extreme pressures and temperatures. In spite of numerous experimental and theoretical studies, the structure and relative stability of these materials even at room temperature remains controversial. We have performed a combined experimental and theoretical study of both methane and ethane up at high pressures up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopy and the ab-initio evolutionary algorithm, respectively. In the case of methane we have successfully solved the structure of phase B by determining the space group and the positional parameters of carbon atoms, and by completing these results for the hydrogen positions using the theoretical calculations. The general structural behavior under pressu...

The elastic properties of two single crystals of majoritic garnet (Mg3.24Al1.53Si3.23O12 and Mg3.01Fe0.17Al1.68Si3.15O12), have been measured using simultaneously single-crystal X-ray diffraction and Brillouin spectroscopy in an externally heated diamond anvil cell with Ne as pressure transmitting medium at conditions up to ~30 GPa and ~600 K. This combination of techniques makes it possible to use the bulk modulus and unit-cell volume at each condition to calculate the absolute pressure, independently of secondary pressure calibrants. Substitution of the majorite component into pyrope garnet lowers both the bulk (Ks) and shear modulus (G). The substitution of Fe was found to cause a small but resolvable increase in Ks that was accompanied by a decrease in ∂Ks / ∂P, the first pressure derivative of the bulk modulus. Fe substitution had no influence on either the shear modulus or its pressure derivative. The obtained elasticity data were used to derive a thermo-elastic model to describe Vs and Vp of complex garnet solid solutions. Using further elasticity data from the literature and thermodynamic models for mantle phase relations, velocities for mafic, harzburgitic and lherzolitic bulk compositions at the base of Earth's transition zone were calculated. The results show that Vs predicted by seismic reference models are faster than those calculated for all three types of lithologies along a typical mantle adiabat within the bottom 150 km of the transition zone. The anomalously fast seismic shear velocities might be explained if laterally extensive sections of subducted harzburgite-rich slabs pile up at the base of the transition zone and lower average mantle temperatures within this depth range.

We present the pressure-volume-temperature (P −V −T) equation of state of polycrystalline (Mg 0.06 Fe 0.94)O (Mw94) determined from laser-heated x-ray diffraction experiments up to 122 GPa and 2100 K, conditions approaching those of the deep mantle. We conducted two sets of experiments, one with an in situ Fe metal oxygen fugacity buffer and one without such a buffer. The internal pressure markers used in these experiments were B2-NaCl and hcp-Fe in the buffered experiment and B2-NaCl in the unbuffered experiment. In the sampled P −T range of the high temperature part of this study, only the B1 structure of Mw94 was observed, indicating that the addition of Mg to FeO stabilizes the B1 phase with respect to the B8 phase at these conditions. Both datasets were fit to a Birch-Murnaghan and Mie-Grüneisen-Debye thermal equation of state using a new open-source fitting routine, also presented here. Analysis of these data sets using the same internal pressure marker shows that the P-V-T data of Mw94 obtained in the unbuffered experiment are well explained by the equation of state parameters determined from the buffered data set. We have also compared the thermal equation of state of Mw94 with that of wüstite and conclude that Mw94 has measurably distinct thermoelastic properties compared with those of wüstite. We use the results obtained in the buffered experiment to

The microscopic structure of Mg-H complexes in GaN has been a subject of intense theoretical and experimental investigation. In order to probe the Mg-H structure, we have studied the effect of hydrostatic pressure on the local vibrational mode (LVM) frequency. At ambient pressure, the LVM frequency is 3125 cm-1, which corresponds to a N-H stretching mode. In this study, Fourier-transform spectroscopy was performed on free-standing GaN:Mg,H samples in a diamond-anvil cell, with nitrogen as a pressure-transmitting fluid. The samples had been removed from their sapphire substrate by the laser-liftoff technique. The LVM frequency was measured, at liquid helium temperatures, for pressures ranging from 0 to 5 GPa. The pressure dependence of the frequency is nonlinear: first it decreases with pressure, then it increases. Comparison with first-principles calculations allows us to derive information about the microscopic structure of the Mg-H complex. The calculated stable configuration inde...

The structure of acceptor-hydrogen complexes is a subject of fundamental and technological interest. To probe the interactions between hydrogen, the acceptor, and the surrounding host atoms, hydrostatic pressure may be applied over a wide range. Using infrared spectroscopy at liquid-helium temperatures, we have observed carbon and carbonhydrogen local vibrational mode (LVMs) in InP at hydrostatic pressures as high as 5.5 GPa. For pressures beyond 4.5 GPa, the carbon-hydrogen mode was not observed, perhaps as a result of a transformation of the complex into a different configuration. The LVM arising from carbon substitutional impurities varies linearly with pressure, whereas the shift of the carbon-hydrogen mode has a positive curvature. Both of these observations are in qualitative agreement with the pressure dependence of LVMs in GaAs. While the substitutional carbon impurities show very similar pressure shifts in the two materials, the linear pressure coefficient of the carbon-hydrogen stretch mode in InP is nearly three times that in GaAs.