A.C. Pandey

OXYGEN VACANCY INDUCED STRUCTURAL PHASE TRANSFORMATION IN TIO2 NANOPARTICLES

Nanosize TiO2 powders prepared by the sol–gel technique at pH of precipitation 4.5 and 6.5 show the anatase phase after calcining at 500 °C. Anatase to rutile phase transformation (ART), however, occurs at 650 °C in the case of pH 6.5 while 850 °C is found to be the ART temperature for the lower pH sample. pH of precipitation dependent ART temperature has not been reported so far to the best of our knowledge. It is known that the smaller the particle size, the lower the ART temperature, and vice versa. The observation of higher crystallite size and lower ART temperature in the case of the higher pH sample contradicts the reported result. We realized from x-ray photoelectron spectroscopic studies that oxygen vacancy concentration drives the ART temperature to lower values in the higher pH sample compared with the sample synthesized at lower pH; even the particle size is found to be higher in the former one.

Zinc Oxide (1% Cu) Nanoparticle in Nematic Liquid Crystal: Dielectric and Electro-Optical Study

The present study reports the dielectric and electro-optical study of inorganic ZnO (Cu 1%) nanoparticles doped nematic liquid crystal (5CB). The dielectric permittivity is found to decrease in the nano doped sample. Nanoparticle doped liquid crystal has good response under various conditions to evaluate their potential as electro-optical materials for various purposes. From electro-optical study we have found Freedricksz transition voltage Vth and response time τ0 and hence K11 and rotational viscosity γ are also calculated. It is found that K11 and Vth increases for nano-doped sample. Rotational viscosity γ also increases for the nanoparticle doped sample. These changes have been explained in this article according to Osipov and Ternentjev theory of rotational viscosity of nematic liquid crystal.

Tunable Visible Emission of Ag-Doped CdZnS Alloy Quantum Dots

Highly luminescent Ag-ion-doped Cd1−xZnxS (0 ≤ x ≤ 1) alloy nanocrystals were successfully synthesized by a novel wet chemical precipitation method. Influence of dopant concentration and the Zn/Cd stoichiometric variations in doped alloy nanocrystals have been investigated. The samples were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscope (HRTEM) to investigate the size and structure of the as prepared nanocrystals. A shift in LO phonon modes from micro-Raman investigations and the elemental analysis from the energy dispersive X-ray analysis (EDAX) confirms the stoichiometry of the final product. The average crystallite size was found increasing from 1.0 to 1.4 nm with gradual increase in Ag doping. It was observed that photoluminescence (PL) intensity corresponding to Ag impurity (570 nm), relative to the other two bands 480 and 520 nm that originates due to native defects, enhanced and showed slight red shift with increasing silver doping. In addition, decrease in the band gap energy of the doped nanocrystals indicates that the introduction of dopant ion in the host material influence the particle size of the nanocrystals. The composition dependent bandgap engineering in CdZnS:Ag was achieved to attain the deliberate color tunability and demonstrated successfully, which are potentially important for white light generation.

Size effect on electronic sputtering of LiF thin films

Electronic sputtering in polycrystalline LiF thin film by 120MeV Ag25+ is investigated. The sputter yields of Li and F for the different thicknesses (10–265nm) of films are measured with online elastic recoil detection analysis technique. A reduction in sputter yield, from ∼2.3×106 to 2.2×104 atoms/ion, is observed with increase in the film thickness. The trend in the experimental results can be explained in terms of size effect in thin film following inelastic thermal spike model. The confinement of energy in the film having smaller grains and lower thickness results in higher temperature causing higher sputtering yield.

Fractal and multifractal characteristics of swift heavy ion induced self-affine nanostructured BaF2 thin film surfaces.

Fractal and multifractal characteristics of self-affine surfaces of BaF2 thin films, deposited on crystalline Si ⟨1 1 1⟩ substrate at room temperature, were studied. Self-affine surfaces were prepared by irradiation of 120 MeV Ag(9+) ions which modified the surface morphology at nanometer scale. The surface morphology of virgin thin film and those irradiated with different ion fluences are characterized by atomic force microscopy technique. The surface roughness (interface width) shows monotonic decrease with ion fluences, while the other parameters, such as lateral correlation length, roughness exponent, and fractal dimension, did not show either monotonic decrease or increase in nature. The self-affine nature of the films is further confirmed by autocorrelation function. The power spectral density of thin films surfaces exhibits inverse power law variation with spatial frequency, suggesting the existence of fractal component in surface morphology. The multifractal detrended fluctuation analysis based on the partition function approach is also performed on virgin and irradiated thin films. It is found that the partition function exhibits the power law behavior with the segment size. Moreover, it is also seen that the scaling exponents vary nonlinearly with the moment, thereby exhibiting the multifractal nature.

Studies of swift heavy ion induced colour centres in LIF thin films deposited on silica substrates

Swift heavy ion induced colour centres in polycrystalline LiF thin films deposited on silica substrate were studied. LiF thin films of 200 nm thickness, deposited by the electron beam evaporation method, and silica substrates were irradiated with 120 MeV Au9+ ions in the fluence range 5 × 1010–1 × 1013 ions cm−2. Optical absorption, photoluminescence (PL) and glancing angle x-ray diffraction (GAXRD) techniques were used for the characterization of the irradiated films. Optical absorption spectroscopy was performed on irradiated LiF films by using irradiated silica substrates as reference sample. The resulting spectra show three bands at 260 nm, 380 nm and 445 nm corresponding to F, F3 and F2 colour centres, respectively. Absorbance of colour centres increases with fluence and seems to saturate at higher fluences. PL spectra, performed by an excitation source of 442 nm, show two broad bands of F2 and colour centres. The intensity of both the centres increases upto 3 × 1011 ions cm−2 followed by some decrease. The GAXRD results show that the average grain size decreases systematically from 35 nm for the pristine film to 20 nm for the sample irradiated at a fluence of 1 × 1013 ions cm−2.

Guest–host interaction in ferroelectric liquid crystal–nanoparticle composite system

The present paper deals with the characterization of a ferroelectric liquid crystal–nanoparticle (FLC–NP) composite system. The dielectric, electrical and polarization property of the FLC–NP composite system have been studied as a function of temperature and frequency. Ferroelectric Cu-doped ZnO (Cu–ZnO) nanoparticles have been added to the pure ferroelectric liquid crystal (FLC) Felix 17/100. The nanoparticles are bigger in size as compared to FLC molecules; therefore, they distort the existing geometry of FLC matrix and set up an antiparallel correlation with the dipole moments of the host FLC molecules. This antiparallel correlation of guest–host geometry reduces the net ferroelectricity of the composite system and modifies all the physical properties of the pure FLC. The change in properties has been analysed and explained in the light of guest–host interaction.

Rapid thermal and swift heavy ion induced annealing of Co ion implanted GaN films

Thin epitaxial GaN films grown on 6H-SiC(0001) substrates were implanted with 180keV Co ions at three different fluences. As-implanted samples were characterized with secondary ion mass spectrometry and Rutherford backscattering spectrometry to obtain the Co depth profiles and the maximum Co concentrations. As-implanted samples were annealed applying two different techniques: rapid thermal annealing and annealing by swift heavy ion irradiation. Rapid thermal annealing was done at two temperatures: 1150°C for 20s and 700°C for 5min. 200MeV Ag ions at two fluences were used for annealing by irradiation. Crystalline structure of the pristine, as-implanted, and annealed samples was investigated using x-ray diffraction, and the results were compared. Improvement of the crystalline quality was observed for rapid thermal annealed samples at the higher annealing temperature as confirmed with rocking curve measurements. The results indicate the presence of Co clusters in these annealed samples. Swift heavy ion irr...

Europium activated gadolinium sulfide nanoparticles

In the present work, we report the synthesis and characterization of Eu3+ doped Gd2S3 nanoparticles, a potential candidate for T1-weighted MRI contrast agents and molecular markers. Eu3+ doped Gd2(1−x)S3:Eux (x = 0.0, 0.02, 0.04 and 0.06) were synthesized by a chemical route by varying the Eu concentration. X-ray diffraction and scanning electron microscopy were performed to extract the information about structure and surface morphology of prepared nanoparticles, respectively. The oxidation state of the Eu ions was elucidated from X-ray absorption near edge structure (XANES) spectra, which indicate the presence of only Eu3+ ions without any signature of Eu2+ ions. 120 MeV Ag9+ ions were used for ionoluminescence (IL) measurements to study the optical properties. The IL results show that the luminescence intensity increases with increasing Eu doping and no saturation or degradation of the luminescence were observed for the as-prepared nanoparticles. The nanoparticles doped with 2 and 4% Eu showed an increase in the IL intensity initially before decreasing to saturation at higher fluences. On the other hand, IL intensity from 6% doped samples decreased exponentially and saturated at higher fluences.