Xiaozhou Liu

Phonon mode study of Si nanocrystals using micro‐Raman spectroscopy

First‐order Raman spectra of hydrogenated nanocrystalline silicon (nc:Si:H) films show unexpected features in their optical vibrational modes for crystallites with sizes ranging from 2 to 6 nm. Two size‐dependent spectral regions, one with the stronger intensity peaking at 505–509 cm−1 and another a shoulder‐like band between 512 and 517 cm−1, are clearly identified using a detailed line‐shape analysis and the strong phonon confinement model. The strong size dependence of the relative integrated intensities of the two bands suggests that the modification of the vibrational spectra can be attributed to an effect induced by the atomic vibrations from the near‐surface region of the nanocrystals.

Complete set of material constants of Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystal with morphotropic phase boundary composition

Using combined resonance and ultrasonic methods, a full set of material constants has been measured for morphotropic phase boundary (MPB) composition xPb(In1/2Nb1/2)O3–(1−x−y)Pb(Mg1/3Nb2/3)O3–yPbTiO3 (PIN-PMN-PT) single crystals poled along [001]c. Compared with the MPB composition (1−x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN-PT) single crystals, the PIN-PMN-PT single crystals have smaller anisotropy, higher Curie temperature (Tc≈197 °C), and higher rhombohedral to tetragonal phase transition temperature (TR-T≈96 °C). The electromechanical properties obtained here are the best found so far for this ternary system with d33≈2742 pC/N, d31≈−1337 pC/N, k33≈95%, and k31≈65%.

Raman scattering of alternating nanocrystalline silicon/amorphous silicon multilayers

Nanocrystallite size distribution and structural properties in alternating hydrogenated nanocrystalline silicon/amorphous silicon multilayers were investigated by means of Raman scattering. The obtained Raman spectra show a broad peak at ∼480 cm−1 from amorphous Si and some small peaks superposed on the broad peak. According to the positions of the crystallite peak, the mean crystallite size and volume fraction of the crystalline were calculated. Since these small peaks have strong size dependence of their relative intensities, an effect induced by the atomic vibrations from the near‐surface region of nanocrystals is considered to be responsible for the modification of the vibrational properties and the stable photoluminescence from our samples.

A complete set of material properties of single domain 0.26Pb(In1/2Nb1/2)O3–0.46Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 single crystals

Pb(In(12)Nb(12))O(3)-Pb(Mg(13)Nb(23))O(3)-PbTiO(3) (PIN-PMN-PT) single crystals have been developed recently, which can increase the operating temperature by at least 20 degrees C compared to PMN-PT crystals. We have measured a complete set of material properties of single domain PIN-PMN-PT crystal, which is urgently needed in theoretical studies and electromechanical device designs using this crystal. Because the rotated values of d33*=1122 pCN and k33*=89% along [001](c) calculated using the single domain data obtained here are in good agreement with the [001](c) poled multidomain PIN-PMN-PT crystals, one may conclude that the physical origin of the ultrahigh piezoelectric properties mainly come from orientation effect.

X-RAY DIFFRACTION STUDY OF ALTERNATING NANOCRYSTALLINE SILICON/AMORPHOUS SILICON MULTILAYERS

Structural properties of alternating nanocrystalline silicon/amorphous silicon multilayers with visible light emission at room temperature were examined by means of x-ray diffraction. According to the linewidths and intensities of the diffraction peaks in the low- and high-angle ranges, we have determined the effective interface thickness, the mean crystallite sizes, and the internal strains, which are closely related to the photoluminescence in this material. In addition, the existence of the voids or holes was also observed, indicating that the improved electrical properties of this kind of hydrogenated nanocrystalline materials are due to the inhomogeneous structure of the material.

Field effect on thermal emission from the 0.40 eV electron level in InGaP

Results are reported of electric‐field dependence on thermal emission of electrons from the 0.40 eV level at various temperatures in InGaP by means of deep‐level transient spectroscopy. The data are analyzed according to the Poole–Frankel emission from the potentials which are assumed to be Coulombic, square well, and Gaussian, respectively. The emission rate from this level is strongly field dependent. It is found that the Gaussian potential model is more reasonable to describe the phosphorus‐vacancy‐induced potential in InGaP than the Coulombic and square‐well ones.

Relationship between the temperature and the acoustic nonlinearity parameter in biological tissues

Recently with the rapid development of the high-intensity focused ultrasound (HIFU) in biomedical ultrasound, much attention has been paid to the noninvasive temperature estimation in biological tissue in order to determine the region and degree of the ultrasound-induced lesions. In ultrasound hyperthermal therapy it is highly desirable to study the real-time noninvasive monitoring of temperature distribution in biological tissue. In this paper, the relationship between the nonlinearity parameterB/A and the temperature in biological tissue is studied and compared with the theoretical model as well as the experimental results from the thermocouple. Results indicated thatB/A could be used as an effective tool to monitor the temperature distribution in biological media.

Investigations of electron spin resonance on light emitting nano-crystallites embedded in a-Si:H films☆

Abstract We report here the studies of electron spin resonance (ESR) on light emitting nano-crystallites embedded in a-Si:H matrix, which are prepared by plasma enhanced CVD on quartz substrates without any post-processing. The ESR spectra contains three components: (1) A pair of hyperfine lines of axial symmetry with g  =1.9967 and g ⊥ =2.0016, and thehyperfine constant 120G; (2) an isotropic line with g=2.0052 and line-width ΔH pp =10G; (3) an axial symmetric line with g  =2.0042 and g ⊥ =2.0057. The dependence of these paramagnetic defects on sample preparation conditions and light emission intensity are presented.

Vibrational properties of near-surface regions of silicon nanocrystallites

Raman spectra of hydrogenated nanocrystalline silicon films clearly exhibit two size- and crystalline-fraction-dependent spectral regions related to the structural properties of nanocrystals. One is the crystalline band from crystallite core consisting of two neighbouring peaks at 513 and 519 cm−1. The appearance of the two peaks is attributed to the contribution of both LO and TO phonons at phonon wavevector q ≠ 0 due to phonon confinement. The other consists of several small peaks superposed on the broad band at ∼480 cm−1. Based on a three-region model of the nanocrystallites, the existence of these small peaks is proposed to be due to atomic vibrations of the near-surface regions in silicon nanocrystallites, sandwiched between the crystalline core and disordered outer surface.

Observation of folded acoustic phonons in nanocrystalline silicon/amorphous silicon multilayers

We report in this letter the observation of folded acoustic phonons in hydrogenated nanocrystalline silicon/amorphous silicon multilayers with visible emission, which are prepared in a plasma enhanced chemical vapor deposition system. In the low‐frequency range of 10–100 cm−1, the obtained Raman spectra clearly show some folded doublets from longitudinal acoustic phonons. Using the elastic continuum model, we calculated their frequencies and the obtained results were in agreement with the experimental ones. In addition, some broad folded doublets and additional peaks were clearly observed in the sample with thin nancorystalline sublayers. We attributed them to the mixing of longitudinal and transverse acoustic phonons due to the layered structure. A confined acoustic mode was also proposed to be responsible for the strongly folded longitudinal acoustic phonon peak at 61 cm−1.