Akhilesh K. Arora

Optical phonon confinement in zinc oxide nanoparticles

Effect of confinement is investigated on optical phonons of different symmetries in the nanoparticles of zinc oxide with wurtzite structure using Raman spectroscopy. An optical phonon confinement model is used for calculating the theoretical line shapes, which exhibit different asymmetric broadening and shifts, depending on the symmetries of phonon and their dispersion curves. The best fit to the data is found for particle diameters consistent with those estimated using x-ray diffraction.

Modifying the nanocrystalline characteristics—structure, size, and surface states of copper oxide thin films by high-energy heavy-ion irradiation

In the present study, x-ray diffraction, Raman spectroscopy, spectroscopic ellipsometry, photoluminescence, and x-ray photoelectron spectroscopy techniques were used to study the effect of 120 MeV 107Ag9+ ion irradiation on nanocrystalline Cu2O thin films grown by the activated reactive evaporation technique. The influence of dense electronic excitations during ion irradiation on the structural and optical properties of the Cu2O thin films was studied. Experimental results demonstrate that the phase and the size of nanocrystallites in the Cu2O thin films as well as associated surface states can be tailored by controlling ion fluence. The Cu2O higher symmetry cubic phase is observed to be quite stable under a higher temperature and irradiation-induced thermal spikes, which accompanies ion irradiation.

Inhomogeneous broadening in the photoluminescence spectrum of CdS nanoparticles

The effect of particle size distribution of CdS nanoparticles on the band-edge photoluminescence (PL) spectrum is investigated. The resulting inhomogeneously broadened lineshape is found to be asymmetric and depend on the width of particle size distribution. The calculated lineshape is fitted to the measured PL spectrum of 3.6 nm diameter CdS nanoparticles, which showed the band-edge PL peak around 3.00 eV and that due to recombination at defects at 2.56 eV. The parameters of the PL spectrum of a bulk CdS film are used as inputs for the fitting. The average particle size and the standard deviation of the size distribution are obtained from the analysis. The particle size estimated from the PL data is compared with that obtained from x-ray diffraction measurements.

Interactions, structural ordering and phase transitions in colloidal dispersions

Abstract The structural ordering in colloidal dispersions is found to be very similar to that of atomic systems, such as crystalline solids, atomic liquids and even glasses. A number of intrinsic as well as extrinsic parameters influence the stability of colloids and induce transitions between different phases. It is the richness of the phase behavior that makes the colloids interesting also from the fundamental point of view. This article reviews the recent advances in the area of ordering and phase transitions brought about by parameters such as particle volume fraction, surface charge density, polydispersity, added electrolyte and external fields, such as shear, electric, magnetic and laser optical fields. Some of the recent experimental techniques that provide insight into the ordering phenomena are also covered. Microscopic investigations of suspensions under confined geometries and their implications on current understanding of the effective interparticle interaction are discussed. Finally, recent efforts in the direction of epitaxial growth of ordered structures on specially designed templates and their applications in synthesizing advanced materials are also briefly reviewed.

Raman spectra of CdS nanocrystals in Nafion: longitudinal optical and confined acoustic phonon modes

Abstract Quantum confinement effects on the longitudinal optical and acoustic phonons in CdS nanocrystals in the strongly confined regime in the polymer matrix Nafion are studied using Raman spectroscopy. The LO-phonon modes show size-dependent asymmetric broadening though the broadening and asymmetry are less than those predicted by the phonon confinement models. Two types of confined acoustic modes corresponding to n =1, l =0 and n =1, l =2 spheroidal vibrations are observed. Softening of the spheroidal modes is observed in the strongly confined regime.

OPTICAL PROPERTIES OF SELENIUM NANOPARTICLES DISPERSED IN POLYMER

Nanoparticles of indirect gap semiconductor selenium dispersed in a polymer host are synthesized by the precipitation in a viscous polymer solution and by subsequent evaporation of the solvent. Optical absorption and photoluminescence investigations are carried out on this system. Present investigations show a blue shift of the interband transition energies suggesting confinement of charge carriers within the nanoparticles. The particle size is estimated to be 43 A. The enhancement factor of the radiative recombination rate relative to that of a bulk monoclinic-Se film is estimated to be of the order of 6.

High-pressure Raman spectroscopic study of zirconium tungstate

Raman spectroscopic measurements on negative-thermal-expansion (NTE) material zirconium tungstate Zr(WO4)2 at 20 K over the complete range of phonon frequencies yield 39 out of the predicted 54 optical phonons. The modes are assigned as lattice modes, and translational, librational and internal modes of the WO4 ion. High-pressure measurements in a diamond anvil cell (DAC) have revealed that in addition to the low-frequency rigid-unit modes (RUMs) several other phonons, including the bending modes of the WO4 ion, also exhibit negative Gruneisen parameter in the cubic phase. In the high-pressure orthorhombic phase above 0.3 GPa splitting of phonon modes is found to be consistent with the lowering of symmetry. Pressure-induced amorphization in this system at 2.2±0.3 GPa is argued to arise because a pressure-induced decomposition of the compound into a mixture of ZrO2 and WO3 is kinetically constrained. The temperature dependence of specific heat and thermal expansion coefficient are calculated and compared with reported results. In contrast to earlier models and calculations, which considered only the phonons below 8 meV (64 cm-1) to explain the NTE, it is shown that modes much higher than 8 meV also contribute significantly to NTE in this material.

Pressure-induced amorphization versus decomposition

Abstract Pressure-induced amorphization of crystalline compounds is believed to arise due to kinetic hindrance of equilibrium phase transitions, while the final state remains speculative/unknown in most instances. Here, we examine an alternate route to amorphization that it could arise in many complex systems if equilibrium decomposition to more dense-packed daughter compounds is constrained kinetically. It is shown that many systems that exhibit pressure-induced amorphization are also good candidates for pressure-induced decomposition leading to a disordered assemblage, forming equilibrium crystalline materials when subjected to elevated temperatures. The route explains the results in Fe2SiO4 and CaAl2Si2O8 and predicts decomposition of several other compounds at high pressure.

The effect of annealing on the properties of nanoparticles dispersed in oxide glass containing Zn

The optical and vibrational properties of nanoparticles dispersed in oxide glass containing Zn are investigated as functions of the annealing temperature using photoluminescence, optical absorption and Raman spectroscopy. Upon annealing the samples, a blue-shift of the optical absorption and photoluminescence is observed, in contrast to the expected red-shift due to particle growth. This can be understood if one invokes the inclusion of Zn into the particle which more than compensates the red-shift due to particle growth. The observed increase in the LO phonon frequency upon annealing also confirms this. The particle size, obtained from the analysis of low-frequency Raman spectra in terms of confined acoustic phonons, shows considerable growth upon annealing above . Overtones up to 3-LO are identified. Furthermore, the LO phonon and its overtones are found to exhibit strong resonance enhancement as a function of exciting wavelength. However, upon annealing, the Raman intensities reduce dramatically due to the shift of the electronic transition energy away from that of the incident photon.

Structural phase transition in indium tungstate

Structural phase transition in In2W3O12 has been studied using differential scanning calorimetry, dilatometry, and Raman spectroscopy. The phase transition at 523 K has been found to be of first order with a heat of transition of 1.22 kJ/mole. In contrast to the earlier reports, the present dilatometric studies suggest that high temperature phase could have a small positive thermal expansion. Phonon frequencies in the monoclinic phase of this network structure were obtained from the Raman spectrum at 10 K. Large discontinuous changes in phonon frequencies are not found across the phase transition; however, temperature coefficients of phonon frequencies exhibit changes. Present results indicate that the local environment of tungstate ions in the high temperature phase is not significantly different from that in the monoclinic phase. Furthermore, weak temperature dependence of the phonon frequencies suggests that their anharmonicities are small in the high temperature phase and this could be responsible for...