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We report Raman studies of the Si–Si phonon band in Si1−xGex alloys, where the excitation is by visible and ultraviolet (351 nm) light. At a wavelength 351 nm, the optical penetration depth is extremely shallow (≈5 nm). By varying the excitation from 351 to 514 nm, the optical penetration depth spans from 5 to 300 nm. Two sets of samples were examined. Thin layers grown using molecular beam epitaxy were coherently strained to match the lattice constant of the silicon substrate. Thick layers grown using organo–metallic chemical vapor deposition were strain relaxed. For the thin, strained layers, visible excitation produces a spectrum, which is a superposition of the substrate and the epilayer phonon bands. Reducing the wavelength (and, consequently, penetration depth) allows us to isolate the epilayer spectrum. Phonon energies obtained using all excitation wavelengths agree. We conclude that Raman scattering from these alloys using 351 nm laser light gives us bulk alloy properties pertinent to the near-sur...

We present a study of the infrared absorption and Raman scattering intensity of the local carbon mode in Si1−yCy alloys grown by direct carbon implantation followed by different recrystallization procedures. For the case of laser-induced recrystallization, the integrated infrared absorbances are found to agree with an extrapolation of the calibration curve previously determined for very low substitutional carbon concentrations in Si. We argue that this finding provides strong evidence for the achievement of nearly perfect substitutionality in laser-recrystallized films, even though their carbon concentrations are three orders of magnitude beyond the solubility limit of carbon in Si. This conclusion is found to be consistent with measurements of the intensity of defect-induced Si Raman scattering relative to the Raman intensity of the local carbon mode. The Raman intensity of the local carbon mode at 605 cm−1 relative to the first-order Si Raman line at 521 cm−1 provides an ideal spectroscopic tool for the...

Carbon nano-onions (CNOs) with an average diameter of 43 nm have been studied under pressure. The interlayer d-spacings of the CNOs are expanded by about 3% compared to those of the larger CNOs with average diameter of 150 nm studied earlier. High pressure study on the G-band of the small CNOs indicates that a bonding change was initiated at 23.4 GPa, which is higher than that of the larger CNOs. The small CNOs were destroyed into amorphous fragments at above 48 GPa with a large applied deviatoric stress, showing a lower high pressure stability compared with the larger CNOs. These features are qualitatively similar to the size effects observed in the compression behavior of some nanocrystalline materials, showing that a reduced cluster size gives similar physical effects in the two classes of materials. The present results for CNOs can be rationalized by the interlayer expansion and the highly turbostratic layer structure of the studied material. The fact that CNOs with different diameters behave differently upon compression is important when selecting materials for applications.

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 B-0 = 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. (C) 2014 AIP Publishing LLC.

We studied the phase transition behavior of cubic BaZrO3 perovskite by in situ high pressure synchrotron X-ray diffraction experiments up to 46.4 GPa at room temperature. The phase transition from cubic phase to tetragonal phase was observed in BaZrO3 for the first time, which takes place at 17.2 GPa. A bulk modulus 189 (26) GPa for cubic BaZrO3 is derived from the pressure–volume data. Upon decompression, the high pressure phase transforms into the initial cubic phase. It is suggested that the unstable phonon mode caused by the rotation of oxygen octahedra plays a crucial role in the high pressure phase transition behavior of BaZrO3.

The structural phase transition of ZnO/SiO2 core/shell nanospheres was studied under high pressure using synchrotron X-ray diffraction. The results indicated that the wurtzite structure of the ZnO core is stable up to 11.5 GPa, and then transforms into rocksalt phase. The onset transition pressure is higher than those of the bulk and nano ZnO. It is worth noting that the phase transition from wurtzite to rocksalt is irreversible, which is obviously different from the uncapped bulk and nano ZnO. The pure rocksalt structure ZnO was first obtained at ambient conditions without catalyst or high temperature treatment. We suggested that the SiO2 shells play important roles in the phase transition of inner ZnO cores. The effects of the SiO2 shells on the phase transition of ZnO cores were discussed.

The structural stability of AlPO4-5 zeolite (AFI) has been studied as a function of pressure up to 34.4 GPa in a diamond anvil cell by using synchrotron x-ray diffraction. It is found that the AFI structural stability can be enhanced significantly when a mixture of silicone oil and liquid nitrogen is used as pressure transmission medium (PTM). In this case, the crystalline-to-amorphous transition pressure for AFI increased to be 15.9 GPa, much higher than that of 8.5 GPa observed in the experiment by using silicone oil as PTM. We found that the average distance of the interplanar crystal spacing along to most planes was expanded obviously when liquid nitrogen is used as one component in the PTM. The presence of liquid nitrogen in the PTM also affects the structural evolution of the AFI channel under pressure. The results demonstrated that nitrogen molecules can be inserted into the channels of porous zeolite AFI single crystals, exerting a supporting effect against the structure collapse of AFI and thus improving their structural stability.

The microstructure, crystal structure, and magnetic properties were studied for Nd-Fe-B nanoflakes prepared by surfactant-assisted high-energy ball milling (HEBM) with heptane and oleic acid as the milling medium. The microstructure, crystal structure, and magnetic properties of the nanoflakes were examined with scanning electronic microscopy, x ray diffraction, and vibrating sample magnetometer, respectively. Effect of ball-milling time on the c-axis crystallographic alignment and coercivity of Nd-Fe-B nanoflakes were systematically investigated. Microstructure observation shows that the Nd-Fe-B nanoflakes have an average thickness of tens of nanometers, an average diameter in the range of 500 ∼ 1000 nm, and an aspect ratio as high as 100. As the ball-milling time increases from 2 h to 12 h, the intensity ratio between (006) and (105) reflection peaks, which indicates the degree of c-axis crystal texture of the Nd2Fe14B phase, drops gradually, indicating that the long time-milling process undermines the ...

It has been found that in the low-frequency regime, the experimentally determined thermal conductivity of thin dielectric films deviates significantly from the theoretical expectation based on Cahi ...

In this paper, we illuminated the observation of asymmetric ferroelectric switching phenomena in fatigued P(VDF-TrFE) copolymer films. Two types of asymmetric switching were observed: one was the occurrence of switching dual peaks obtained from films with asymmetric electrode structure and the other was the jump of remanent polarization during fatigue process, as was obtained mostly from films with asymmetric electrodes and occasionally from films with symmetric electrodes. No evidence indicated the dependence between the switching dual peaks and the jump of remanent polarization. The possible mechanism of such asymmetric phenomena was also discussed.

High-temperature (∼90–150 °C) conduction mechanisms of metal-insulator-metal (MIM) capacitors with atomic-layer-deposited HfO2 dielectric are studied. In the low field range, the Schottky emission current is dominant, and the deduced dielectric constant is close to the static one of HfO2. In the high field range, the resulting leakage current complies with the Poole-Frenkel (PF) emission, which is demonstrated by the fact that the extracted dielectric constant equals the optical frequency one (i.e., square of refractive index) of HfO2. The underlying mechanisms are discussed based on carrier velocities under different electric fields. Further, the deduced Schottky barrier height is ∼0.251–0.274 eV in the low field range, which relates to the contributions from high density traps in the HfO2 film and the nonideal TaN/HfO2 interface, etc. The extracted trap potential well depth for the PF effect is ∼1.11–1.37 eV in the high field range.

The complex dielectric functions e(ω) from 0.75 to 6.6 eV of pseudomorphically strained Si1−xGex (0