V. P. Mahadevan Pillai

Growth and characterization of molybdenum oxide nanorods by RF magnetron sputtering and subsequent annealing

High-densely packed and uniformly distributed molybdenum oxide nanorods have been grown onto glass substrates by RF magnetron sputtering and subsequent annealing in an oxygen atmosphere. A two-step growth mechanism (sputtering redeposition and enhanced rearrangement during annealing) for the formation of MoO3 nanorods has been proposed. The morphological, structural, optical and electrical properties of the nanorods have been investigated systematically using atomic force microscopy, scanning electron microscopy, x-ray diffraction, micro-Raman, UV–visible, photoluminescence (PL) spectroscopy and dc resistivity studies. The nanorods in the as-sputtered film and the film annealed at 473 K are amorphous in nature. However, the nanorods in the films annealed at 573 and 673 K exhibit the presence of monoclinic Mo8O23 and orthorhombic MoO3, respectively. Vibrational analysis of the molybdenum and oxygen atoms in the nanorods is carried out by micro-Raman spectra. The nanorods show room temperature PL in the UV–visible region. The PL emission is found to be strongly enhanced by post-deposition annealing. The low temperature resistivity measurement is done on the as-deposited film; the activation energy and polaron hopping energy for electrical conduction are calculated. The MoO3 nanorods are expected to exhibit enhanced functionality, particularly in nanoscale, photochromic and gas sensing applications.

Bandgap renormalization in titania modified nanostructured tungsten oxide thin films prepared by pulsed laser deposition technique for solar cell applications

Pure and titania modified WO3 films are prepared using pulsed laser ablation technique in an oxygen ambient of 0.12mbar at a substrate temperature of 873K. Titania incorporation effects on the microstructure, optical, and electrical properties on the tungsten oxide films are systematically investigated using techniques such as x-ray diffraction, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, micro-Raman spectroscopy, and UV-visible absorption spectroscopy measurements. The resistivity measurements of the pure and titania modified WO3 films are done at room temperature. The variation of resistivity with temperature for the range of 170–450K is also investigated. The microstructural analysis indicates that titania addition strongly perturbs the tungsten oxide lattice and suppresses the grain growth. Optical measurements revealed a bandgap renormalization in tungsten oxide films for higher titania concentrations. Bandgap values of the films decrease from 3.17eV f...

Enhanced ethanol sensing response from nanostructured MoO3:ZnO thin films and their mechanism of sensing

A novel ZnO incorporated MoO3 nanostructured thin film system exhibiting high sensitivity and selectivity to ethanol has been developed. The MoO3:ZnO nanostructures exhibit enhanced ethanol sensing performance in non-humid and humid (75% r.H. at 21 °C) atmospheres compared to the pure MoO3 layer; with increase in ZnO concentrations, the sensitivity and stability increased, and the response/recovery time decreased. The response (Gethanol/Gair) of the 25% MoO3:ZnO sensor at an operating temperature of 300 °C against 500 ppm ethanol is up to 171 under non-humid and 117 under humid (75% r.H.) conditions. By comparing the response of the 25% ZnO added MoO3 sensor toward various gases (H2, CO, C3H6, CH4 and C2H5OH), distinctive selectivity to ethanol is observed. The ethanol sensitivity action over MoO3 nanostructures can be ascribed to the catalytic oxidation of ethanol to acetaldehyde, and the enhancement of gas sensing response of the MoO3:ZnO system can be attributed to more active centers that are obtained from the enhanced oxygen vacancy defects induced by ZnO. The presence of a humid atmosphere has a dramatic influence on the sensor performance towards ethanol; the sensitivity diminishes drastically due to the partial site precluding nature of the adsorbed hydroxyl groups to the analyte. The ZnO incorporated MoO3 nanostructure based sensing layers in the present work show significantly superior ethanol sensing performance to the works previously reported for various metal oxide systems.

Effect of substrate roughness on photoluminescence spectra of silicon nanocrystals grown by off axis pulsed laser deposition

Silicon nanoparticles were prepared by off axis pulsed laser deposition (PLD) technique. The optical properties of Si nanoparticles grown on quartz substrate have been characterized by optical absorption, photoluminescence, Raman, and transmission electron microscopy. TEM has demonstrated that the radius of nanocrystals decreases from 4to0.8nm as the off axis target to substrate distance increases. A blueshift up to 4.2eV is observed in the optical absorption spectra of the Si quantum dots. The slope of log(hν) vs log(α) graph shows that the optical transitions in Si quantum dots are allowed direct, allowed indirect, and direct forbidden transitions which depend on the nature of the substrate used for the growth of silicon. Relaxation of k selection rule is observed in these samples. Photoluminescence (PL) emission consists of an intense broad emission extending over visible to ultraviolet region. The photoluminescence peak energy and intensity are found to be sensitive to the nature of substrate. Possibl...

Structural, spectroscopic and electrical studies of nanostructured porous ZnO thin films prepared by pulsed laser deposition

ZnO thin films are grown on quartz substrates at various substrate temperatures (ranging from 573 to 973 K) under an oxygen ambience of 0.02 mbar by using pulsed laser ablation. Influence of substrate temperature on the structural, morphological, optical and electrical properties of the ZnO thin films are investigated. The XRD and micro-Raman spectra reveal the presence of hexagonal wurtzite structure of ZnO with preferred orientation (002). The particle size is calculated using Debye-Scherer equation and the average size of the crystallites are found to be in the range 17-29 nm. The AFM study reveals that the surface morphology of the film depends strongly on the substrate temperature. UV-Visible transmittance spectra show highly transparent nature of the films in visible region. The calculated optical band gap energy is found to be decrease with increase in substrate temperatures. The complex dielectric constant, the loss factor and the distribution of the volume and surface energy loss of the ZnO thin films prepared at different substrate temperatures are calculated. All the films are found to be highly porous in nature. The PL spectra show very strong emission in the blue region for all the films. The dc electrical resistivity of the film decreases with increase in substrate temperature. The temperature dependent electrical measurements done on the film prepared at substrate temperature 573 K reveals that the electric conduction is thermally activated and the activation energy is found to be 0.03911 eV which is less than the reported values for ZnO films.

Effect of silver incorporation in phase formation and band gap tuning of tungsten oxide thin films

Silver incorporated tungsten oxide thin films are prepared by RF magnetron sputtering technique. The effect of silver incorporation in micro structure evolution, phase enhancement, band gap tuning and other optical properties are investigated using techniques such as x-ray diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, and UV-Visible spectroscopy. Effect of silver addition in phase formation and band gap tuning of tungsten oxide thin films are investigated. It is found that the texturing and phase formation improves with enhancement in silver content. It is also found that as the silver incorporation enhances the thickness of the films increases at the same time the strain in the film decreases. Even without annealing the desired phase can be achieved by doping with silver. A broad band centered at the wavelength 437 nm is observed in the absorption spectra of tungsten oxide films of higher silver incorporation and this c...

Vibrational spectroscopic studies of FeClMoO4, Na2MoO4 and Na2MoO4·2H2O/D2O

Abstract FTIR and Raman spectra of FeClMoO4 single crystal and polycrystalline Na2MoO4, Na2MoO4·2H2O and Na2MoO4·2D2O are recorded and analysed. The band positions for different modes suggest that MoO4 tetrahedron is more distorted in FeClMoO4. The larger splitting observed for the bending modes and partial retention of degeneracy of the asymmetric stretching mode indicate that angular distortion is greater than liner distortion in MoO42− ion in FeClMoO4 confirming x-ray data. The non-appearance of the ν1 and ν2 modes in the IR and partial retention of the degeneracies of various modes show that MoO42− ion retains Td symmetry in Na2MoO4. Wavenumber values of the ν1 mode indicate that the distortion of MoO4 tetrahedra in the four crystals are in the order FeClMoO4> Na2MoO4·2H2O>Na2MoO4·2D2O>Na2MoO4. The water bands suggest the presence of two crystallographically distinct water molecules in Na2MoO4·2H2O. They form strong hydrogen bonds.

INFRARED AND RAMAN SPECTRA OF CS2VOP2O7 AND SINGLE CRYSTAL RB2(VO)3(P2O7)2

Abstract Infrared (IR) and Raman spectra of Cs 2 VOP 2 O 7 and single crystal Rb 2 (VO) 3 (P 2 O 7 ) 2 have been recorded and analysed. The spectra are interpreted on the basis of P 2 O 7 4− and VO vibrations. The appearance of ν s P-O-P in both Raman and IR spectra suggests a bent P–O–P bridge for the P 2 O 7 4− ions in both compounds, in agreement with X-ray data. The observed bands point to an eclipsed configuration for the P 2 O 7 4− ions with C 2 ν symmetry in Rb 2 (VO) 3 (P 2 O 7 ) 2 . The appearance of P–O stretching modes in Cs 2 VOP 2 O 7 at higher wavemumbers than those in Rb 2 (VO) 3 (P 2 O 7 ) 2 indicates that the P–O bonds in the Cs compound is stronger than in the Rb compound. VO modes are also interpreted.

Optimum Wavelength for the Differentiation of Brain Tumor Tissue Using Autofluorescence Spectroscopy

OBJECTIVE The role of autofluorescence spectroscopy in the detection and staging of benign and malignant brain tumors is being investigated in this study, with an additional aim of determining an optimum excitation wavelength for the spectroscopic identification of brain tumors. MATERIALS AND METHODS The present study involves in-vitro autofluorescence monitoring of different human brain tumor samples to assess their spectroscopic properties. The autofluorescence measurement at four different excitation wavelengths 320, 370, 410, and 470 nm, were carried out for five different brain tumor types: glioma, astrocytoma, meningioma, pituitary adenoma, and schwannoma. RESULTS The fluorescence spectra of tumor tissues showed significant differences, both in intensity and in spectral profile, from those of adjacent normal brain tissues at all four excitation wavelengths. The data were then subjected to multivariate statistical analysis and the sensitivities and specificities were calculated for each group. Of the four excitation wavelengths being considered, 470 nm appeared to be the optimal wavelength for detecting tissue fluorescence of brain tumor tissues. CONCLUSIONS In conclusion, the spectroscopic luminescence measurements carried out in this study revealed significant differences between tumor tissue and adjacent normal tissue of human brains for all the tumor types tested, except for pituitary adenoma. From the results of this study we conclude that excitation wavelengths ranging from 410-470 nm are most suitable for the detection of brain tumor tissue. Moreover, in this particular study, only excitation at 470 nm indicated that samples we considered to be normal tissue were not normal, and that these were indeed pituitary adenoma tissues. This distinction was not clear at other excitation wavelengths.

Infrared and polarized Raman spectra of (CH3)2NH2Al(SO4)2 · 6H2O

FTIR and single crystal Raman spectra of (CH3)2NH2Al(SO4)2 x 6H2O have been recorded at 300 and 90 K and analysed. The shifting of nu1 mode to higher wavenumber and its appearance in Bg species contributing to the alpha(xz) and alpha(yz) polarizability tensor components indicate the distortion of SO4 tetrahedra. The presence of nu1 and nu2 modes in the IR spectrum and the lifting of degeneracies of nu2, nu3, and nu4 modes are attributed to the lowering of the symmetry of the SO4(2-) ion. Coincidence of the IR and Raman bands for different modes suggest that DMA+ ion is orientationally disordered. One of the H atoms of the NH2 group of the DMA+ ion forms moderate hydrogen bonds with the SO4(2-) anion. Al(H2O)6(3+) ion is also distorted in the crystal. The shifting of the stretching modes to lower wavenumbers and the bending mode to higher wavenumber suggest that H2O molecules form strong hydrogen bonds with SO4(2-) anion. The intensity enhancement and the narrowing of nu1SO4, deltaC2N and Al(H2O)6(3+) modes at 90 K confirm the settling down of the protons in the hydrogen bonds formed with H2O molecules and NH2 groups. This may be one of the reasons for the phase transition observed in the crystal.