Bingguo Liu

Temperature dependence of the Raman spectra of single-wall carbon nanotubes

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 E2g (GM) and that of the radial breathing mode in the SWCNT were determined to be ∼−0.038 and ∼−0.013 cm−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.

High-pressure Raman scattering and x-ray diffraction of phase transitions in MOO3

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=1...

Photoluminescence study of ZnO nanotubes under hydrostatic pressure

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 15Gpa, 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 29.6meV∕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 21.6meV∕GPa, about 30% smaller than that of the ZnO band gap, which suggests that it might originate from the ra...

In situ electrical impedance spectroscopy under high pressure on diamond anvil cell

The effect of grain boundary on electrical transportation properties is a basic physical problem and also a subject of material science and technology. In situ electrical measurement of powdered materials under high pressure provides a chance to figure out the electrical properties of grain boundaries. In this letter, the authors report an in situ impedance spectroscopy method used in conjunction with a diamond anvil cell for electrical property research of grain boundaries under high pressure. Powdered CdS was pressed up to 23GPa and an impedance arc corresponding to the grain boundary was detected. It was found that the electrical property of the grain boundary changed with pressure and could be determined by the resistance and the relaxation frequency. Pressure decreases the effective scattering section of the grain boundary to charge carriers, and finally leads to the decrease of resistance and activation energy of the grain boundary.

Pressure-dependent photoluminescence of ZnO nanosheets

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 ∼100nm. 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.

Study of the hydrostatic pressure dependence of the raman spectrum of W/WS2 fullerene-like nanosphere with core-shell structure

The structural behavior of a W/WS2 fullerene-like nanosphere with a core–shell structure has been studied in the hydrostatic pressure range from atmospheric pressure to 18 GPa by Raman spectroscopy using a methanol–ethanol–water mixture (16:3:1) as the pressure transmitting medium (PTM). We found that it is interesting that the intensity ratio of the LA+TA mode and the A1g mode changes with increasing pressure. We attribute this change to the shape transformation of an inorganic fullerene-like IF-W/WS2 nanosphere under high hydrostatic pressure. By comparing the Raman spectra of an IF-W/WS2 nanosphere released from high pressure with that of the original one, we found that the change in morphology is reversible. This indicates that the spherical shape of the IF-W/WS2 has excellent behavior in resisting compression.

Femtosecond time-resolved measurement of LDS698 molecular processes under high pressure

Femtosecond time-resolved measurements provide a means to understand high pressure effects on ultrafast molecular processes. We report here a method of transient absorption spectroscopy in conjunction with diamond anvil cells for investigating molecular energy relaxation of solidified LDS698 (pyridine, C19H23N2O4Cl) molecular solution. The pressure effect on the intermolecular energy relaxation is significant with increasing rate constant. The results are satisfactorily interpreted in terms of the intermolecular harmonic oscillator model with the anharmonic correction through surrounding thermal diffusion. This technique opens an approach to study molecular energy relaxation and other ultrafast processes of matter under high pressure.

Alternating current impedance spectroscopy measurement under high pressure.

A microcircuit was designed and fabricated on a diamond anvil cell for alternating current impedance spectroscopy measurement under high pressure. Sputtered molybdenum film on a diamond anvil was used as an electrode, maintained the contact between the sample and the electrode stable, and reduced the electrode effect on the impedance measurement. By the empty cell and short circuit tests, the parasitic capacitive impedance from the sample chamber wall was observed to be larger than 10(5) Ω at a frequency lower than 1.0 MHz and could be ignored for samples with higher conductivity. The wire inductance was only 1.0 μH and just appeared at frequency higher than 20 kHz, which could be subtracted from measured impedance for the samples with higher impedance than several hundred ohms. Using this apparatus, the impedances of the II-VI group cadmium sulfide were measured. The pressure dependence of the grain interior conductance of CdS crystal was obtained, which reflected that the phase transitions of CdS under high pressure are the same as the single crystal measurement results.

High pressure effect on the ultrafast energy relaxation rate of LDS698 (C 19 H 23 N 2 O 4 Cl) in a solution

Effects of high pressure in a range of up to 1.7 GPa on ultrafast energy relaxation of LDS698 (C(19)H(23)N(2)O(4)Cl) molecules in solution have experimentally been illustrated by a method of femtosecond time-resolved absorption spectroscopy. The rates of the intramolecular and intermolecular energy relaxations show quite different pressure dependences. The observed results are in good agreement with the theoretical interpretation based on the pressure influences on the molecular energy gaps, the intermolecular H-bond interaction, and the solution viscosity.

High pressure effect on the dielectric properties of rock-salt CdS

Using high pressure impedance spectroscopy measurement system, the dielectric property of rock-salt CdS was investigated up to 22.9 GPa. Below 10.0 GPa ɛ r′′ decreased with frequency linearly and ɛ r′ showed one-step decrease, which was due to the electron hopping transport between impurity levels. From 10.0 GPa to 11.6 GPa and from 15.8 GPa to 22.9 GPa, the one-step decrease of ɛ r′ with frequency was assigned to the electron transport along one path, from impurity level to Σ v and to L v, respectively, and the two-step decrease from 11.6 GPa to 15.8 GPa was caused by the electron transport along the two paths mentioned above. The dielectric constant of CdS grain was calculated and showed w-shape with pressure, which was affected by the pressure induced band structure change.