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In this study, the femtosecond laser-induced crystallization of CuNiSnP amorphous ribbons was investigated by utilizing an amplified Ti:sapphire laser system. X-ray diffraction and scanning electronic microscope were applied to examine the phase and morphology changes of the amorphous ribbons. Micromachining without crystallization, surface patterning, and selective crystallization were successfully achieved by changing laser parameters. Obvious crystallization occurred under the condition that the laser fluence was smaller than the ablation threshold, indicating that the structural evolution of the material depends strongly on the laser parameters. Back cooling method was used to inhibit heat accumulation; a reversible transformation between the disordered amorphous and crystalline phases can be achieved by using this method. [ABSTRACT FROM AUTHOR]

NbN/W2N nano-multilayer films with a modulation periodicity, Λ, ranging from 5.1 to 157.4 nm have been deposited on a Si(100) substrate by reactive magnetron sputtering in Ar/N2 mixtures. The Λ dependent structural and mechanical properties for the resulting NbN/W2N multilayers have been evaluated by means of low-angle x-ray reflectivity, x-ray diffraction, high-resolution transmission electron microscope, and nanoindentation measurements. The finding is that for films with Λ ≤ 10.6 nm, fcc NbN layers are coherent with cubic W2N layers, resulting in NbN layers and W2N layers that are in the compressive and tensile states, respectively. In contrast, as Λ is larger than 10.6 nm, a phase transition from W2N to W occurs in the W2N layer, which is a result of the coherent interface strain relaxation. For this case, all layers are in the compressive state, and the coherent interface disappears. The intrinsic compressive stress evolution with Λ can be interpreted in terms of interface stress. The formation of coherent interface at small Λ (≤10.6 nm) is helpful for releasing point defects in layers, leading to a low compressive stress (≤1.1 GPa). The hardness for the obtained multilayer film increases with decreasing Λ, and approaches a maximum value of 43.7 GPa when Λ is 7.4 nm. The maximum strengthen at lower Λ is mainly attributed to coherent interface stresses and the modulus difference between the NbN and W2N layers. The increase in hardness with a decrease in Λ is interpreted by the Lehoczky model. [ABSTRACT FROM AUTHOR]

The high-pressure behavior of molybdenum trioxides (MoO3) has been investigated by angle-dispersive synchrotron x-ray powder diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 43 and 30 GPa, respectively. In the pressure range of up to 43 GPa, structural phase transitions from the orthorhombic α-MoO3 phase (Pbnm) to the monoclinic MoO3-II phase (P21/m), and then to the monoclinic MoO3-III phase (P21/c), occurred at pressures of about 12 and 25 GPa at room temperature, respectively. Our observation of the transition from the orthorhombic α-MoO3 to the monoclinic MoO3-II phase is in disagreement with earlier studies in which the phase transition could not be obtained when only pressure is applied. The changes in the Mo–O distances and O–Mo–O and Mo–O–Mo angles may explain the changes in Raman spectrum. The pressure dependence of the volume of two monoclinic high-pressure phases is described by a third-order Birch–Murnaghan equation of state, which yields a bulk modulus value of B0=143.41(3) GPa with B0′=12, and B0=261.9(3) GPa with B0′=3.5. [ABSTRACT FROM AUTHOR]

High-pressure synchrotron radiation x-ray diffraction and Raman spectroscopy measurements of indium oxide (In2O3) were carried out at room temperature up to 27.8 and 26.2 GPa, respectively. A pressure-induced phase transition from cubic-phase (Ia3) was observed at a pressure above 12.8–15.3 GPa, which disagrees with earlier theoretical prediction (3.8 GPa). According to the x-ray diffraction experimental data, the high-pressure phase is isostructural with hexagonal corundum-type structure (R3c symmetry). However, broad peaks observed in Raman spectra suggest that the high-pressure structure is disordered. The volume change from cubic phase to corundum phase is about 4% and the axial ratio c/a in the corundum phase decreases with increasing pressure. [ABSTRACT FROM AUTHOR]

We report on a Raman study of nanocrystalline GaN with the wurtzite structure synthesized by arc plasma method. Resonant Raman scattering is observed using 514.5 nm (2.41 eV) laser excitation, which is near the band gap of the "yellow band" (2.2-2.3 eV). Under such near-resonant excitation, new Raman bands unexpected in an ideal wurtzite GaN crystal were found. The transverse optical modes of A[sub 1] (531 cm[sup -1]) and E[sub 1] (560 cm[sup -1]), and the nonpolar modes of E[sub 2] [567 cm[sup-1] (high) and 143 cm[sup -1] (low)] normally observed in bulk crystals, were recorded and were observed to be resonantly enhanced. Two new bands (680 and 344 cm[sup -1]) were assigned to the inactive optical phonon modes B[sub 1] (high) and B[sub 1] (low), respectively. A broadband centered at 710 cm[sup -1] was attributed to surface modes of the nanocrystals, providing good agreement with the calculated result based on Fröhlich theory. As a result of this study, Raman scattering of GaN nanocrystals has been characterized. [ABSTRACT FROM AUTHOR]

Focuses on a study which characterized the titanium tungsten-gallium arsenide interface after rapid thermal annealing. Thermal stability of tungsten-based films on gallium arsenide; Methodology of the study; Results and discussion.

Photoluminescence (PL) spectra of single-crystal ZnO bulk under hydrostatic pressure are studied using the diamond-anvil-cell technique at room temperature. The PL spectrum of ZnO single crystal taken at atmospheric pressure was dominated by a strong near-band-edge exciton emission. The emission line was found to shift towards higher energy with increasing applied pressure. By examining the pressure-dependent PL spectral features, the pressure coefficient of the direct Γ band gap of the wurtzite ZnO was determined. The hydrostatic deformation potential of the band gap has also been deduced from the experimental results. [ABSTRACT FROM AUTHOR]

Photoluminescence and Raman spectra of ZnO single-crystal nanosheets have been studied as a function of applied hydrostatic pressure using the diamond-anvil-cell technique at room temperature. The ZnO nanosheets synthesized via a vapor transport process have uniform plane surfaces with lateral dimensions up to several microns and thickness of ∼100 nm. In terms of Raman results, the ZnO nanosheets underwent a transition from wurtzite to rock-salt structure with an increase of pressure, and the phase-transition pressure was measured to be about 11.2 GPa. However, a strong near-band-edge UV emission of ZnO nanosheets was observed with the applied pressure up to 20.0 GPa. Simultaneously, the emission peak shifted to higher-energy side with increasing pressure. By examining the dependence of the near-band-edge emission peak on the applied pressure, the pressure coefficient of the direct Γ band gap in the wurtzite ZnO nanosheets was determined. [ABSTRACT FROM AUTHOR]

The discharge gas pressure is a key factor to influence the extracted current of ion source. In this paper, the dependence of extracted current on discharge gas pressure was investigated in detail at different arc discharge currents. The discharge gas pressure with a very broad range (0.1 Pa-2.7 Pa) was scanned for the first time. It is turned out that, with the increasing of discharge gas pressure, the extracted current increases and the arc voltage decreases at different arc currents; however, when the discharge gas pressure exceeds a certain value, the extracted current decreases. For the same discharge gas pressure, the higher the arc current, the higher the arc voltage and the extracted current are. The arc efficiency was also calculated, and its dependence on gas pressure was almost the same with the dependence of extracted current on gas pressure, but at the same discharge gas pressure, the lower the arc current, the higher the arc efficiency is and the lower the extracted current is. [ABSTRACT FROM AUTHOR]

Nanocomposite (BiFeO3)0.5:(Sm2O3)0.5 films were deposited on (001) oriented Nb-doped SrTiO3 substrates by pulsed laser deposition. The leakage current density versus electric field characteristics were investigated and compared with those of as-deposited and annealed pure BiFeO3 (BFO) thin films. The dominant leakage mechanisms of nanocomposite films were space-charge-limited current and Poole–Frenkle emission for positive and negative biases, respectively. The leakage current density of nanocomposite films was reduced three orders of magnitude in comparison with the as-deposited pure BFO films. The less oxygen vacancies in the BFO phase in the nanocomposite is believed to contribute to the leakage reduction. [ABSTRACT FROM AUTHOR]

Photoluminescence of ZnO single crystal nanotubes grown on sapphire substrate by metal organic chemical vapor deposition has been studied as a function of applied hydrostatic pressure using the diamond-anvil-cell technique. The photoluminescence spectra of the ZnO nanotubes at atmospheric pressure are dominated by strong near-band-edge ΓFX and ΓBX excitonic emission lines accompanied by a weak broad deep-level (DL) emission band. The pressure-induced shifts of all observed emission lines are followed up to 15 Gpa, when ZnO nanotubes undergo a phase transition from a direct-gap wurtzite structure to an indirect-gap rocksalt structure. The ΓFX emission is found to shift toward higher energy with applied pressure at a rate of 29.6 meV/GPa, which provides a method to measure the pressure coefficient of the direct Γ band gap in the wurtzite ZnO nanotubes. The ΓBX emission has a pressure coefficient of 21.6 meV/GPa, about 30% smaller than that of the ZnO band gap, which suggests that it might originate from the radiative recombination of the excitons bound to donorlike deep centers rather than shallow donors. The pressure coefficient of the broad DL emission band in ZnO tube is 16.8 meV/GPa, which indicates that the initial states involved in the emission process are deep localized states. [ABSTRACT FROM AUTHOR]

Simulation studies for single atomic lithium in solid hydrogen using the path integral Monte Carlo (PIMC) method with a constant pressure ensemble are reported. The pressure dependencies of the absorption spectra of Li in solid H[sub 2] for various trapping sites are extensively investigated at T=5 K and different pressures in the range from 0 to 4.8 GPa. The local structure around the Li atom cannot be rearranged with respect to the presence of pressure. Effects of pressure on the trapping site structures refer only to the decrease in the Li-H[sub 2] and H[sub 2]-H[sub 2] separations. The general pressure-induced effects on Li atomic absorption spectra associate with an increase in half-width and a centroid spectral shift (CSS), which experiences a blue shift below a turning pressure (P[sub t]) and a red shift above P[sub t]. For higher symmetrical trapping site structures, the peak splitting of the highly symmetrical triplet absorption pattern is mainly ascribed to the perturbation of the H[sub 2] molecules around the Li atom. For lower symmetrical trapping site structures, application of pressure also results in a change in absorption line shape from a low symmetrical triplet feature to a doublet plus singlet pattern, in which the doublet to singlet splitting increases remarkably with the increasing of pressure, while the perturbation of the H[sub 2] around the dopant largely determines the peak splitting for the "doublet." We demonstrate that the pressure provides a powerful means to probe the properties, which cannot be observed at zero pressure. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Ultrafine gallium nitride (GaN) powder has been synthesized by the dc arc plasma method through the reaction of metal gallium (Ga) with the mixture gas of nitrogen ( N2) and ammonia ( NH3). The analyses of the produced powder by x-ray diffraction, transmission electron microscopy, and selected-area electron diffraction showed that the GaN particles in wurtzite structure consisted of nanometer-sized polycrystals and monocrystals. The conversion of Ga to GaN was determined by the mixture ratio of NH3 and N2 in the mixture gas. The morphology of the GaN particles was mainly hexagonal with the size about 20–200 nm. When heated in air or nitrogen atmosphere, the thermostablity of the GaN powder was different. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ion beam optics for the neutral beam injection system on HL-2A Tokomak is studied by two- dimensional numerical simulation program firstly, where the emitting surface is taken at 100 Debye lengths from the plasma electrode. The mathematical formulation, computation techniques are described. Typical ion orbits, equipotential contours, and emittance diagram are shown. For a fixed geometry electrode, the effect of plasma density, plasma potential and plasma electron temperature on ion beam optics is examined, and the calculation reliability is confirmed by experimental results. In order to improve ion beam optics, the application of a small pre-acceleration voltage (∼100 V) between the plasma electrode and the arc discharge anode is reasonable, and a lower plasma electron temperature is desired. The results allow optimization of the ion beam optics in the neutral beam injection system on HL-2A Tokomak and provide guidelines for designing future neutral beam injection system on HL-2M Tokomak. [ABSTRACT FROM AUTHOR]

Raman spectra of single-wall carbon nanotubes (SWCNTs) were measured at different temperatures by varying the incident laser power. The elevated temperature of the SWCNTs and multiwall carbon nanotubes (MWCNTs) is confirmed to be due to the presence of impurities, defects, and disorder. The temperature coefficient of the frequency of the C-C stretching mode E[sub 2g] (GM) and that of the radial breathing mode in the SWCNT were determined to be ∼-0.038 and ∼-0.013 cm[sup -1]/K, respectively. It is found that the temperature coefficient of the GM in the SWCNT is larger than that of the MWCNT, highly oriented pyrolytic graphite, and the graphite. This is attributed to the structural characteristic of the SWCNT--a single tubular carbon sheet with smaller diameter. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The high-pressure behavior of molybdenum trioxides (MoO3) has been investigated by angle-dispersive synchrotron x-ray powder diffraction and Raman spectroscopy techniques in a diamond anvil cell up to 43 and 30 GPa, respectively. In the pressure range of up to 43 GPa, structural phase transitions from the orthorhombic α-MoO3 phase (Pbnm) to the monoclinic MoO3-II phase (P21/m), and then to the monoclinic MoO3-III phase (P21/c), occurred at pressures of about 12 and 25 GPa at room temperature, respectively. Our observation of the transition from the orthorhombic α-MoO3 to the monoclinic MoO3-II phase is in disagreement with earlier studies in which the phase transition could not be obtained when only pressure is applied. The changes in the Mo–O distances and O–Mo–O and Mo–O–Mo angles may explain the changes in Raman spectrum. The pressure dependence of the volume of two monoclinic high-pressure phases is described by a third-order Birch–Murnaghan equation of state, which yields a bulk modulus value of B0=143.41(3) GPa with B0′=12, and B0=261.9(3) GPa with B0′=3.5. [ABSTRACT FROM AUTHOR]

High-pressure behaviors of hydrogen-bonded molecular crystal, sulfamic acid (NH3(+)SO3(-), SA), have been investigated using Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques up to the pressure of ~20 GPa. Under ambient conditions, molecules of SA are arranged in puckered layers and held together by hydrogen bonding and electrostatic interactions. It is proved by the Raman results that SA undergoes the molecular conformation changes in the pressure range 8.1-10.2 GPa. Then between 10.2 and 12.7 GPa, a phase transition is observed in both Raman and XRD patterns. Both the ambient and high-pressure phases of SA crystallize in Pbca symmetry with similar unit-cell dimensions. The mechanism of the phase transition involves relative movements of adjacent hydrogen-bonded molecules, accompanied by the rearrangement of hydrogen bonds and the enhancement of electrostatic interactions.

High-pressure Raman scattering and synchrotron X-ray diffraction measurements of sodium squarate (Na(2)C(4)O(4), SS) are performed in a diamond anvil cell. SS possesses a rare, but typical structure, which can show the effect of face-to-face π-stacking without interference of other interactions. At ~11 GPa, it undergoes a phase transition, identified as a symmetry transformation from P2(1)/c to P2(1). From high-pressure Raman patterns and the calculated model of SS, it can be proved that the phase transition results from the distorted squarate rings. We infer it is the enhancement of π-stacking that dominates the distortion. For comparison, high-pressure Raman spectra of sodium squarate trihydrate (Na(2)C(4)O(4)●3H(2)O, SST) are also investigated. The structure of SST is determined by both face-to-face π-stacking and hydrogen bonding. SST can be regarded as a deformation of SS. A phase transition, with the similar mechanism as SS, is observed at ~10.3 GPa. Our results can be well supported by the previous high-pressure studies of ammonium squarate ((NH(4))(2)C(4)O(4), AS), and vice versa. High-pressure behaviors of the noncovalent interactions in SS, SST, and AS are compared to show the impacts of hydrogen bonding and the role of electrostatic interaction in releasing process.

We report a joint experimental and theoretical study on high pressure structures of nanoporous mayenite (12CaO·7Al(2)O(3)). A pressure-induced amorphization was identified at ∼13 GPa by in situ high pressure x-ray experiments. The amorphous product can be decompressed to ambient conditions. Ab initio calculations reveal that the anisotropic deformations of empty cages break the crystallographic long-range orders, leading to the formation of high-pressure amorphous state. Furthermore, the deformation routes of empty cages are found fully dependent on their individual locally structural geometry relevant to the distribution of extra-framework oxygen ions., (© 2011 American Institute of Physics)

The structural and dynamical properties of solid ammonia borane were investigated by means of extensive density functional theory calculation up to 60 GPa. Molecular dynamics simulations suggest that the Cmc2(1) phase found by recent room-temperature x-ray diffraction experiments can be obtained from the Pmn2(1) structure at high pressure and low temperature. Two new high-pressure phases were found on further compression at room temperature. We also found that all three high-pressure phases have proton-ordered structures, and the separation of the NH(3) and BH(3) rotation observed in the simulations can be explained by their distinct rotational energy barriers. The role of dihydrogen bonds in the high-pressure phases is discussed.

Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P2(1)2(1)2(1), Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C(2)H(6)) at 95 GPa, butane (C(4)H(10)) at 158 GPa, and further, carbon (diamond) and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.

The structural and dynamical properties of phase IV and V of hydrogen sulfide were investigated by means of extensive ab initio molecular dynamics simulations. Starting from an experimental proposal for the structure of phase IV, an Ibca symmetry with a stable hydrogen bonding network is found at 15 GPa and 100 K. Molecular dynamics simulations at increasing temperature and at the pressure of 15 GPa suggest that phase IV will transform to a proton disordered structure at 15 GPa and 350 K. The newfound structure has a hexagonal lattice of P63/mmc symmetry, which is believed to be the remaining crystalline structure of phase V. The high mobility of protons in phase V is believed to be the key point to the dissociation and decomposition of hydrogen sulfide.

By use of electrical field analysis method, the accuracy of electrical resistivity measurement with the van der Pauw method in a diamond anvil cell (DAC) was investigated for the situation that sample and gasket were electrically shorted. It is revealed that metal gasket could not be used in electrical measurement in DAC if the inside wall of the sample chamber was not insulated. When the shorted area was less than 20% of the inside wall of the sample chamber, the relative error was smaller than 10%. Once the shorted area exceeded 25%, the relative error increased rapidly.

The search for the stable single-bonded (polymeric) solid nitrogen attracted much attention in view of its potential application as a high energy density material. In this study, the stability of different candidate polymeric structures of nitrogen has been studied using ab initio calculations based on density-functional theory for the first time, from the angles of thermodynamic stabilities, mechanical stabilities, and dynamical stabilities in the pressure range from 0 to 360 GPa, respectively. According to our results, only Cmcm, A7, rcg, cg, BP, P2(1)2(1)2(1), and Pba2 are competitive structures and more favorable than sc, ch, LB, and cw strcutrues; their stable pressure range were also presented. Among the competitive structures, BP, Pba2, and P2(1)2(1)2(1) are the novel ones for their enthalpies are lower than the cg structure above 170 GPa. We further identify that the P2(1)2(1)2(1) phase can transform to cg structure at pressure below 60 GPa. Also a new phase transition sequence with increasing pressure has been presented, which is from the molecular phase epsilon-N(2) to cg at 47 GPa, to Pba2 at 170 GPa, and then to P2(1)2(1)2(1) at 307 GPa.

Two-electrode configuration was developed for in situ electrical impedance detecting on diamond anvil cell under high pressure. The metal gasket was used as one electrode and the risk coming from electrical short between sample and interside wall of the gasket was eliminated. The configuration was evaluated and proved to be effective by measuring the electric impedance of nanocrystalline ZnS under high pressure.

The high temperature and high pressure Brillouin scattering studies of liquid ammonia have been performed in a diamond anvil cell. Acoustic velocity, refractive index, adiabatic bulk modulus, and the equation of state of liquid ammonia were determined at temperatures up to 410 K and at pressures up to the solidification point. Velocity and refractive index increase smoothly with increasing pressure along isothermals but decrease slightly with the temperature increase. The bulk modulus increases linearly with pressure and its slope dB/dP decreases slightly with increasing temperature from 6.67 at 297 K to 5.94 at 410 K.

Brillouin scattering measurements were performed on liquid argon in a diamond anvil cell at various solidification points up to 503 K. With the measured results from the 60 degree platelet- and 180 degree back-scattering geometries, the sound velocity, refractive index, experimental equation of state, and adiabatic bulk modulus of liquid argon as a function of pressure were determined. The discrepancy between experimental and previous calculated equation of state indicates that the many-body contribution to the density of liquid argon increases with increasing pressure and decreases with increasing temperature. By analyzing the Brillouin spectra in the coexistence of liquid and solid phase regions, the volume change and latent heat of solid-liquid transformation along the equilibrium curve have been also obtained for the first time.

We present the calculations of the electronic structure and transport properties on the zinc-blende (ZB) and cinnabar phases of HgTe using the full-potential linearized augmented plane-wave method and the semiclassical Boltzmann theory. Our results show that n-doped cinnabar HgTe has a significant larger Seebeck coefficient and electrical conductivity along the z axis than those of the n-doped ZB phase. This is mainly attributed to the large structural anisotropy originated from its chainlike bonding characters along the z axis, resulting in the anisotropic energy distribution in the lowest conduction band of cinnabar structure. The resulting ZT values along the z axis of the n-doped cinnabar HgTe are predicted to reach very high values of 0.61 at room temperature and 1.74 at 600 K. Therefore, the current theory suggests that the cinnabar structure of HgTe could be a good thermoelectric material. Future experiments are thus demanded to explore its thermoelectric performance by making use of the high ZT.

We report on an original method that measures sample thickness in a diamond anvil cell under high pressures. The method is based on two hypotheses: completely plastic deformation on the gasket and completely elastic deformation of the diamonds. This method can further eliminate the effect of diamond deformation on the thickness measurement of a sample, which permits us to measure the thickness of alumina up to 41.4 GPa.

The structural transformations occurring to water from low-density (LDW) to high-density (HDW) regimes have been studied by Brillouin scattering for the first time at temperatures up to 453 K and at pressures up to the solidification point. At ambient temperature (293 K) a discontinuity in pressure response of the sound velocity is observed. Furthermore, there are evident breaks in the linear behavior of log10 C11 versus log10(rho/rho0) when pressure increases up to 0.29, 0.21, and 0.19 GPa at the temperature of 293, 316, and 353 K, respectively. It is supposed to indicate the structural transition from LDW to HDW, and the possible transition boundary between LDW and HDW is in good agreement with the molecular-dynamics simulation.