Vijayan C

Stable silver nanoparticles were generated in aqueous medium by amine, hydroxy and carboxylate terminated poly(amidoamine) (PAMAM) dendrimers in the absence of conventional reducing agents and light irradiation. Mechanistic studies suggest that, in spite of their lower pKa values, tertiary amines present in the dendritic structures act as the reducing agent. Dynamic light scattering and electron microscopic studies reveal that PAMAM dendrimers aggregate in presence of silver ions and the aggregation propensity plays a pivotal role in altering the size and shape of the nanoparticles generated in the dendritic milieu. The formation of metal nanoparticles with uniform size and shape by amine terminated PAMAM dendrimer is attributed to the unique aggregation propensity of amine terminated PAMAM. ©2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This paper is on the study of diffraction efficiency of photorefractive gratings recorded in two-beam coupling configuration in crystals of BaTiO3 and BaTiO3:Ce at multiple wavelengths. We study the distortive effects in a thick PR hologram due to the angle between the two interfering beams, modulation and the orientation of the crystal and measure the diffraction efficiency as a function of spatial frequency, modulation and the angle between the c-axis and the grating vector. The behaviour of both the crystals is compared at 441.6 nm, 632.8 nm and 780 nm using He-Cd, He-Ne and semiconductor diode lasers respectively. Value of diffraction efficiency of the undoped crystal is higher at 441.6 nm compared to other wavelengths whereas it is higher at 632.8 nm for the doped crystal. The difference in the spectral variation of optical absorption of the two crystals is found to affect the diffraction efficiency considerably. Diffraction efficiency is also estimated using Kukhtarev's theory and the results are compared with the experimental data.

The technique of ionic thermocurrents (ITC) is used to study the field‐induced reorientation of electric dipoles in crystals of KCl:Gd2+. Besides the peak at 240 K due to impurity‐vacancy (I–V) dipoles, a new peak is observed at 213 K which can be attributed to Z1‐centres. This supports a model of Z1‐centre involving an F‐centre coupled to an I–V dipole. No new ITC peak is observed in crystals containing Z1‐centres. Copyright © 1988 WILEY‐VCH Verlag GmbH & Co. KGaA

Photorefractive beam coupling in barium titanate crystals is characterised experimentally by measuring the signal beam gain and the exponential gain coefficient at 440 nm, 632.8 nm and 780 nm. The figure of merit parameters such as the change in refractive index, the space charge field and the trap density are estimated using Kukhtarev's theory. A comparative study at multiple wavelengths is presented. The signal beam gain is found to be maximum at 440 nm.

In this work we probe the photothermal conversion properties of individual metal (Ag) nanoparticle, semiconductor (β-In2S3) microflowers, dielectric (SiO2) nanostructures and Ag nanoparticles embedded in β-In2S3 and SiO2 matrix. The heat generated from the Ag nanoparticles is much higher when embedded inside polymer encapsulated SiO2 and β-In2S3 matrix than the individual nanoparticle assembly. The heat generation mechanism is shown to be an ultrafast process (picoseconds) when the Ag nanoparticles are embedded in a β-In2S3 complex, while for individual Ag nanoparticles and Ag: SiO2 matrix the process is shown to be time delayed. The change is attributed to the resonant heat transfer mechanism. The measurements were carried out by illuminating the samples with pump beam of 445 nm. The refractive index gradient produced in the surrounding air due to thermal waves emanating from the sample is measured using a probe beam of wavelength 546 nm and a position sensitive quadrant cell detector.

A new scheme for photonic information storage based on photothermal F to Z2 colour centre conversion is presented. Z2 centres are formed by F-band bleaching in the temperature range 300 to 500 K in additively coloured crystals of KCl doped with gadolinium. The variation of the optical density in the F-band is studied as a function of the logarithm of exposure in order to obtain the Hurter-Driffield characteristics of the storage medium. The ideal parameters of storage are arrived at and the writing sensitivity of the medium for the proposed process is determined. Photonic information storage using a helium-neon laser is demonstrated and the results are compared with other reported schemes. © 1991.

A simple technique is presented to obtain normalized photoacoustic (PA) spectra corrected for the spectral variation of the source in a single scan. The input light beam is passed through the center of a dual slot chopper, which splits it into two chopped output beams at two different frequencies at a fixed ratio. The beams fall on the sample and the reference kept side-by-side in the same sample chamber. The PA signals are detected by a microphone and processed by two lock-in amplifiers tuned at two different frequencies. The technique is tested by recording the PA spectra of standard samples. © 2007 American Institute of Physics.

CdS nanocrystals having mean diameters in the range 1.4-6 nm are synthesised in the polymer matrix Nafion. X-ray diffraction (XRD) patterns of the samples show broadened peaks corresponding to the cubic phase of CdS. A detailed analysis of the XRD profiles using a profile fitting and separation programme (PROFIT) revealed the broadening to be arising from the size effects. The mean diameters of the nanocrystals are determined from the full width at half maxima (FWHM) of the XRD peaks. © 2001 Elsevier Science B.V.

Nanocrystals of CdS are synthesized in the proton exchange membrane nafion in different sizes in the range 1.6 to 6 nm. To understand the process leading to the formation of these quantum dots, we have probed the proton exchange by ac conductance measurements in the frequency range 100 Hz to 13 MHz. Nafion shows good electrical conductivity due to proton transport probably via the Grothus mechanism. Incorporation of cadmium ions by replacement of the hydrogen ions in the sulphonic acid group resulted in a large decrease in conductance indicating the reduction of the mobile carrier density. The conductivity plots all show strong frequency dependence with higher conductance towards the higher frequencies where a near-flat frequency response is seen. After the formation of CdS clusters, there is a partial recovery of conductance corresponding to the reinstatement of the protonic carriers on the side groups. The conductivity of the nafion films embedded with the semiconductor quantum dots exhibits a size-dependence with the highest conductivity obtained for the largest clusters. These findings lend clear experimental evidence for the model of synthesis of quantum dots in nafion by the exchange mechanism.

Nanoclusters of CdS and PbS were prepared using two different organic solvents as stabilizers in order to understand the factors affecting their formation and stabilization. Growth of the nanoclusters was monitored by optical absorption spectroscopy at regular intervals of time. Mean cluster size was characterized by X-ray diffraction (XRD). The surface structure of nanoclusters was analyzed using infrared (IR) spectroscopy. Spectroscopic studies under identical experimental conditions reveal interesting correlations between the stability of the nanoclusters formed, the nature of the solvent and the size of metal ion involved, leading to a better understanding of nanocluster formation. © 2004 Elsevier B.V. All rihgts reserved.