S. H. Tang

Raman spectra of CuO nanocrystals

CuO nanoparticles were successfully prepared by a one-step solid-state reaction under ambient conditions. Three pellet samples with different grain sizes were obtained by annealing at high temperature. The room temperature Raman spectra of these samples show that as the grain size decreases, the Raman peaks shift to lower wavenumber and become broader owing to size effects. The dependences of the Raman spectra on the wavelength of the excitation laser and temperature were also investigated. Copyright

Raman scattering investigations of the antiferroelectric–ferroelectric phase transition of NaNbO3

The antiferroelectric to ferroelectric phase transition in NaNbO3 was investigated by Raman scattering between 295 and 10 K. The transition occurs at 185 K on cooling and the antiferroelectric and ferroelectric phases coexist at low temperature. The transition occurs at 210 K on heating and the relatively large hysteresis is attributed to the difficulty in reversing the spontaneous polarization direction at the transition. The results agree well with low-temperature x-ray data which show that both phases coexist if the crystal samples are twinned. The transition is not considered to be of the displacive type, although two modes at 115 and 121 cm-1 were observed to soften at low temperature.

High temperature Raman study of phase transitions in antiferroelectric NaNbO3

Abstract The Raman spectra of antiferroelectric NaNbO3 have been studied at temperatures up to 480°C. Two phase transitions were observed, one at 376°C in consistence with the previous observations, and the other being a new phase transition at 190°C. There is no evident hysteresis for this transition, which indicates that the phase transition is of second order.

Temperature dependence of the raman scattering spectra of Zn/ZnO nanoparticles

By using the gas-evaporation technique with an induction heating method, Zn nanoparticles coated with ZnO were prepared in Ar. The Raman spectra of the Zn/ZnO nanoparticles were studied over a wide range from room temperature through liquid nitrogen temperature and up to 873 K. With heating in air, the transformations from the surface phonon mode to bulk vibrational mode in Zn/ZnO were successfully observed for the first time.

FT-IR Study of the Imidization Process of Photosensitive Polyimide PMDA/ODA

Fourier transform infrared spectroscopy (FT-IR) was used to study the cure reaction process of an ester-type photosensitive polyimide, PMDA/ODA films coated on silicon wafer substrates. In the in situ high-temperature FT-IR study carried out in vacuum, all the characteristic peaks of imide and the crosslinkable group were observed to decrease gradually with an increase in temperature. The completion of the imidization reaction is at ∼270 °C. Our result shows that the ester-type photosensitive polyimide can be cured at a much lower temperature than previously reported. The imide peak δas(C, C > NC) at 588 cm−1 was used to follow the imidization process, and the result agrees well with that obtained by using another imide peak v(C–N–C) at 1378 cm−1. The film shrinks by one third during the cure process, and the temperature dependence of the film thickness shows excellent agreement with the “imidization degree”.

Resonant optical nonlinearity of fullerenes in free‐standing polymethyl methacrylate films

We have prepared films of polymethyl methacrylate in which fullerenes (a mixture of C60 and C70) are uniformly embedded. By performing both degenerate four‐wave mixing experiments and irradiance‐dependent transmission measurements with nanosecond laser pulses of 608‐nm wavelength, the third‐order nonlinear optical susceptibility of these films is determined to be of the order of 10−10 esu. This susceptibility is dominated by its imaginary part. Excited‐state absorption is the main mechanism responsible for the observed nonlinear susceptibility.

Near infrared excited micro-Raman spectra of 4:1 methanol–ethanol mixture and ruby fluorescence at high pressure

Near infrared (NIR) lasers, as a new excitation source for Raman spectroscopy, has shown its unique advantages and is being increasingly used for some special samples, such as those emitting strong fluorescence in the visible region. This article focuses on some issues related to high-pressure micro-Raman spectroscopy using NIR excitation source. The Raman spectra of 4:1 methanol–ethanol mixture (4:1 M–E) show a linear variation in both Raman shifts and linewidths under pressure up to 18 GPa. This result is useful in distinguishing Raman scattering of samples from that of the alcohol mixture, an extensively used pressure-transmitting medium. The R1 fluorescence in the red region induced by two-photon absorption of the NIR laser is strong enough to be used as pressure scale. The frequency and line width of the R1 lines are very sensitive to pressure change and the glass transition of the pressure medium. Our results manifest that it is reliable and convenient to use NIR induced two-photon excited fluoresce...

Raman microspectroscopy of the brain tissues in the substantia nigra and MPTP-induced Parkinson's disease

The substantia nigra of the rostral mid-brain of a monkey was studied using Raman microspectroscopy. The white and grey matter could be clearly distinguished and their relative proportions evaluated from the Raman peaks in the 3000 cm−1 region. Raman spectra obtained from the substantia nigra for both treated and untreated samples showed that the process of staining the neuron nuclei affected the brain cells and the quality of the Raman spectra from the treated samples was poorer. The brain cells of a monkey suffering from Parkinsons disease was also studied. Raman spectra collected from the control cells and the diseased cells did not show a clear difference. Copyright

Terahertz carrier dynamics and dielectric properties of GaN epilayers with different carrier concentrations

Using terahertz time-domain spectroscopy, we measured the complex conductivity and dielectric function of n-type GaN with various carrier concentrations on sapphire substrate. The measured complex conductivity, which is due to the free carriers, is well fitted by simple Drude model. The contribution from the lattice vibration to the complex dielectric function increases with the decrease in free carrier concentration. A better fitting of the frequency-dependent complex dielectric response was obtained by considering both of the Drude and the classical damped oscillator model.

Ultrafast insulator–metal phase transition in vanadium dioxide studied using optical pump–terahertz probe spectroscopy.

We studied the ultrafast dynamic behavior of the photoinduced insulator–metal phase transition in VO2 thin film using optical pump–terahertz probe spectroscopy with different excitation fluences and at different temperatures. We observed two processes in the insulator–metal phase transition in VO2: a fast process and a slow process. The fast process is a nonthermal process, which is ascribed to the nucleation of the metal phase, while the slow process is strongly affected by temperature and is ascribed to the thermally driven growth and coalescence of metal domains in VO2. The transient complex conductivity spectra at different delay times are also investigated.