Effects of growth conditions on properties of CBD synthesized ZnO nanorods grown on ultrasonic spray pyrolysis deposited ZnO seed layers.

ZnO nanorods were synthesized on a seed layer coated glass substrate using chemical bath deposition (CBD). Prior to growth, a seed layer had been prepared via ultrasonic spray pyrolysis method. The aim was to explore the influence of varying the chemical bath deposition conditions namely: growth time, bath temperature and concentration levels of the precursor on the orientation, structural, optical and vibrational properties of the subsequently grown nanorods. The presence of ZnO nanorods resembling the hexagonal-wurtzite structure having preference of orientation along the c-axis and varying crystallinity under different growth parameters was confirmed by X-ray diffraction (XRD). Scanning Electron Microscopy (SEM) acquired images of uniformly arranged and vertically oriented ZnO nanorods grown at a relatively higher bath temperature of 90 °C and shorter growth period of 2 h. UV/Vis/NIR spectrophotometer measurements revealed an optical transmittance of between 50 - 70 % for the nanorods. Raman spectroscopy results confirmed the presence of Raman active E2(low) and E2(high) modes corresponding to 98 cm-1 and 478 cm-1 belonging to the hexagonal ZnO phase. This work shows that the orientation, structural, optical and vibrational properties of the grown nanorod structures are controlled via alteration of the growth parameters.

Superficial and Fundamental Correspondences in the Terahertz/IR (6-15 THz) Absorption Spectra of Aspirin and Benzoic Acid.

The terahertz absorption spectra of aspirin and benzoic acid have been measured in the range 200-500 cm-1 (6-15 THz). Density-functional theory (DFT) modeling has assigned fundamental vibrational modes to the observed absorption bands. Hydrogen bonds between the crystalline planes of aspirin resulted in better agreement between the experimental and modeled spectra than for benzoic acid. The similar structure of these two molecules suggests a similar absorption spectrum, which indeed was obtained experimentally. However, the detailed crystal structure and molecular differences result in some of the apparently common absorption bands being assigned to different vibrational modes through the DFT modeling. Thus, our study importantly reveals that even though crystalline forms of two similar molecules may have similar experimental terahertz spectra, the resemblance may be superficial rather than fundamental.

Collective librations of water molecules in the crystal lattice of rubidium bromide: experiment and simulation.

Terahertz spectroscopy of RbBr reveals four prominent absorption lines at room temperature and a further 15 lines at 10 K. Via density-functional-theory (DFT) numerical modelling using the PBE0 hybrid GGA functional, all the absorptions are identified as correlated librations of water molecules in the RbBr lattice. Each libration mode is a combination of rocking, wagging and twisting motions of the water molecules. The number of libration lines and numerical modelling show that the C2v symmetry of water in RbBr is broken. Our modelling shows that the distribution of libration amplitudes and phases for different water molecules in the RbBr unit cell varies greatly between the different modes. All librational lines red-shift with increasing temperature. The rate of change for most lines is in the range 60-90 MHz K(-1) (or (2-3) × 10(-3) cm(-1) K(-1)). Two lines shift more rapidly with temperature, at rates of 240 and 300 MHz K(-1) (or (8 and 10) × 10(-3) cm(-1) K(-1)), respectively. Furthermore, the temperature dependence of the linewidth distinguishes two groups of lines. For one group, with weak linear temperature dependence of linewidth, cubic anharmonic terms in the RbBr crystal field are significant. This group is mainly associated with fully symmetric correlated librations. For the second group, with strong non-linear temperature dependence of the linewidth, quartic anharmonic terms in the RbBr crystal field are significant. However, the distribution of libration amplitudes, as well as the type of libration modes, influence the temperature dependence of the red shift, the linewidth, and the intensity, as well. Our combined experimental and theoretical investigation confirms the necessity of obtaining low-temperature data to observe all the calculated modes; moreover, the richness of detail in the temperature dependence of the data invites further modelling spanning a range of temperatures.