Shailaja Mahamuni

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.

Spectroscopic and structural characterization of electrochemically grown ZnO quantum dots

We report a novel method for the synthesis of stable, OH free zinc oxide quantum dots, using an electrochemical route. The optical properties of these quantum dots were studied at room temperature, by taking the optical absorption and luminescencespectra. The band gapluminescence is predominant in ZnOquantum dots synthesized by the present technique, while the green defect induced luminescence, typical of ZnO, is strongly quenched. The role of defects in photoluminescence emission is discussed.

Synthesis and properties of Cu2O quantum particles

Cuprous oxide quantum particles as small as 2 nm (comparable to the Bohr exciton radius) were synthesized using an electrochemical route. Quantum confinement effects are evident from a blueshift in the optical absorption. The optical absorption spectra of Cu2O nanoparticles of different sizes are discussed. Structural analysis by x-ray diffraction as well as electron diffraction shows the nanoparticles to be cubic and single phased Cu2O. X-ray photoelectron spectroscopic studies indicate the presence of CuO on the surface of Cu2O core nanoparticles.

Photophysical properties of ZnS nanoclusters

Optical measurements on ZnS nanoclusters have been carried out to investigate surface effects along with quantum size effects. ZnS nanocrystals have been synthesized in the range of 1.5–2.5 nm, using different chemical methods as well as electronic passivating procedures. The size of nanoparticles has been estimated from empirical pseudopotential calculations. We have obtained a significantly narrower size distribution of ZnS nanocrystals than reported in earlier published results. We observed band gap luminescence in mercaptoethanol capped ZnS nanocrystals. Effects of various defect levels on the luminescent behavior of ZnS nanoparticles have been examined

Dopant induced modifications in the physical properties of sprayed ZnO : In films

Indium-doped zinc oxide (IZO) films were prepared by the spray pyrolysis technique. The effect of gradual incorporation of indium cations on the structural, electrical, and compositional properties of IZO films was studied in detail. It was observed that even a small addition of indium modifies the preferred growth of IZO film from the [002] direction to the [101] direction. Such a modification in growth pattern is a result of more nucleating centers created by indium doping. Indium dopant improves the electrical properties of the films. The carrier concentration depends mainly on the indium dopant level while the mobility is affected by the changes in crystal orientation that take place due to addition of dopants. X-ray photoelectron spectroscopy results show that indium doping does not lead to any stoichiometric changes in the IZO films and the dopant incorporation in the film is linearly proportional to that in the solution.

Luminescence behaviour of chemically grown ZnO quantum dots

The luminescent properties of undoped ZnO quantum dots encapsulated in various stabilizing agents have been investigated at room temperature. Stoichiometric as well as non-stochiometric ZnO quantum dots have been prepared by different routes. These studies indicate the importance of OH radical on the surface of ZnO quantum dots. In general, it has been observed that surface defects play an active role in the luminescence behaviour of quantum dots. ZnO quantum dots, doped with copper, have also been studied. Doping Cu in ZnO quantum dots, which have OH radical on its surface, is responsible for quenching green luminescence at 530 nm.

Effect of Sn incorporation on the growth mechanism of sprayed SnO2 films

In order to probe into the growth mechanism of sprayed SnO2 films, the films were deposited with different Sn concentrations in the precursor solution. The orientational properties were determined using grazing incidence x‐ray diffraction. The preferred growth changed from [110] to [200] direction as the Sn incorporation was increased. Such a change in growth can be anticipated from structure factor calculations. The compositional analysis was done using x‐ray photoelectron spectroscopy. The Hall effect measurements indicated that the carrier concentration and mobility are sensitively dependent on the orientation and composition of the films.

Formation of Single-phase CuO Quantum Particles

CuO nanocrystals of three different sizes 3.5, 6.5, and 10.0 nm were synthesized by a novel electrochemical route. Quantum size effects in these nanocrystals are evident from the optical absorption spectra. The optical absorption feature is observed to be blue shifted by about 1 eV with size reduction. The CuO q-particles were found to be single-phased and very stable as far as the size and phase is concerned.

Highly photoluminescent ZnSe/ZnS quantum dots

ZnSe quantum dots have been synthesized using a simple aqueous route. Coating ZnSe quantum dots with a ZnS monolayer, yields a remarkable enhancement in the PL quantum efficiency at room temperature without affecting the spectral distribution. The results suggest that passivation of surface states, along with an increased localization of the hole in core ZnSe layer, gives rise to high luminescence quantum yield.

Thiophenol-capped ZnS quantum dots

Free‐standing, stable, single‐sized, and highly pure ZnS nanosize particles or quantum dots are synthesized using a chemical route. The dimension of the quantum dots as estimated from x‐ray diffraction was about 7 and 15 A. The optical band gap for these particles was 5.2 and 4.8 eV, respectively, as compared with the bulk energy gap of 3.8 eV. These results are compared with the effective mass approximation. The observations indicate that only particular‐sized stable particles can be synthesized which should have minimum energy configuration. X‐ray photoelectron spectroscopy was utilized for examining stoichiometry and purity of the sample.