Pushan Ayyub

Effect of the size-induced structural transformation on the band gap in CdS nanoparticles

The interrelation between particle size, crystal structure and optical properties in semiconductor quantum dots has elicited widespread interest. We report the first attempt at relating the size-induced transformation from a hexagonal to a cubic structure in CdS nanoparticles to a change in the band gap. CdS nanoparticles with particle size in the 0.7-10 nm range were prepared by chemical precipitation using thiophenol as a capping agent. Whereas the band gap for bulk hexagonal CdS is about 2.5 eV, that for 1 nm cubic CdS nanoparticles was found to be almost 3.9 eV. We also suggest a simple mechanism (based on the periodic insertion of stacking faults) for the transformation from the cubic zinc blende structure to the hexagonal wurtzite structure.

PREPARATION OF NANOPARTICLES OF SILVER HALIDES, SUPERCONDUCTORS AND MAGNETIC MATERIALS USING WATER-IN-OIL MICROEMULSIONS AS NANO-REACTORS

I. II. III. IV. V. ABSTRACT Water-in-oil microemulsions have been used for the synthesis of a variety of nano- particles since the technique was first introduced in 1982. In this paper we have reviewed several articles pertaining to the synthesis of nanoparticles in microemulsions and described in some detail our research efforts in the past decade in the field of synthesis of nanoparticles of silver halides, superconductors and magnetic materials using water- in-oil microemulsions as nano-reactors.

Preparation of acicular γ-Fe2O3 particles from a microemulsion-mediated reaction

A microemulsion-mediated chemical reaction was used to synthesize acicular particles of γ-Fe2O3 with an equivalent spherical diameter (ESD) of 7–8 run. From a comparison of the measured values of the hydrodynamic diameter of the water-in-oil microemulsion droplets and the ESD of the resulting γ-Fe2O3 particles, we show that each microemulsion droplet gives birth to a single particle of γ-Fe2O3.

SPIN-POLARIZED TUNNELING IN THE HALF-METALLIC FERROMAGNETS LA0.7-XHOXSR0.3MNO3 (X=0 AND 0.15) : EXPERIMENT AND THEORY

The magnetoresistance (MR) in polycrystalline colossal magnetoresistive compounds follows a behavior different from single crystals below the ferromagnetic transition temperature. This difference is usually attributed to spin polarized tunneling at the grain boundaries of the polycrystalline sample. Here we derive a theoretical expression for the contribution of spin polarized tunneling to the magnetoresistance in granular ferromagnetic systems under the mean field approximation. We apply this model to our experimental data on the half metallic ferromagnet La0.7Sr0.3MnO3, and find that the theoretical predictions agree quite well with the observed dependence of the spin polarized MR on the spontaneous magnetization.

Influence of the sputtering gas on the preferred orientation of nanocrystalline titanium nitride thin films

Abstract Nanocrystalline titanium nitride thin films have been deposited by high pressure reactive magnetron sputtering from an elemental titanium target using a mixture of an inert gas and nitrogen. The mean crystallite or grain size in these films is in the range 8–12 nm as measured from X-ray line broadening. Interestingly, the type of inert gas used in the sputtering gas mixture significantly influences the microstructure and preferred orientation in these films. Thus, using a 70% He+30% N2 gas mixture results in a strongly (002) oriented film whereas using a 70% Ar+30% N2 gas mixture results in a strongly (111) oriented film with a similar grain size. In addition, films have also been deposited using pure nitrogen as the sputtering gas. These films exhibited a strong (002) orientation and had a significantly larger grain size as compared with those deposited using a mixture of an inert gas and nitrogen. Details of the microstructure in these films have been investigated by transmission electron microscopy. The ability to tailor the size and preferred orientation of grains in the TiN thin films (by proper choice of sputtering gas) is expected to have a significant impact on the properties of these films in a variety of technological applications.

Lattice expansion in nanocrystalline niobium thin films

High-purity nanocrystalline niobium (Nb) thin films have been deposited using high-pressure magnetron sputter deposition. Increasing the pressure of the sputtering gas during deposition has systematically led to reduced crystallite sizes in these films. Based on x-ray and electron diffraction results, it is observed that the nanocrystalline Nb films exhibit a significantly large lattice expansion with reduction in crystallite size. There is however, no change in the bcc crystal structure on reduction in crystallite size to below 5 nm. The lattice expansion in nanocrystalline Nb has been simulated by employing a recently proposed model based on linear elasticity and by appropriately modifying it to incorporate a crystallite-size-dependent width of the grain boundary.

Mechanism of the Size Dependence of the Superconducting Transition of Nanostructured Nb

In nanocrystalline Nb films, the superconducting Tc decreases with a reduction in the average particle size below 20nm. We correlate the decrease in Tc with a reduction in the superconducting energy gap measured by point contact spectroscopy. Consistent with the Anderson criterion, no superconducting transition was observed for sizes below 8 nm. We show that the size-dependence of the superconducting properties in this intermediate coupling Type II superconductor is governed by changes in the electronic density of states rather than by phonon softening.

Synthesis of crystalline carbon nitride thin films by laser processing at a liquid–solid interface

Pulsed laser induced reactive quenching at a liquid–solid interface was used for the synthesis of tetrahedrally coordinated crystalline carbon nitride on a tungsten substrate. The crystalline phase was identified by transmission electron diffraction. X‐ray photoelectron spectroscopy indicated that the carbon atoms are coordinated only tetrahedrally with nitrogen—as expected for C3N4. The atomic percentage of N (considering only those atoms coordinated with C) is about 35%.

Preparation and characterization of ultrafine TiO 2 particles in reverse micelles by hydrolysis of titanium di-ethylhexyl sulfosuccinate

We describe the synthesis and characterization of ultrafine TiO 2 particles (in both anatase as well as rutile form) produced by a chemical reaction within the aqueous core of a water-in-oil microemulsion. The microemulsion was stabilized and the Ti 4+ ions provided by a functionalized surfactant derived from the commercially available Aerosol-OT, i.e., sodium bis (2-ethylhexyl) sulfosuccinate (Na-DEHSS). The Na + ions in Aerosol-OT were completely replaced by Ti 4+ through an ion-exchange reaction in nonaqueous solvents. Ultrafine TiO 2 particles were produced by the hydrolysis of the Ti-containing surfactant in the water droplets. The dependence of the size of the precipitated TiO 2 · x H 2 O particles on various structure parameters of the microemulsion was studied in detail.

Optical properties of transparent nanocrystalline Cu2O thin films synthesized by high pressure gas sputtering

Abstract We report the controlled synthesis of nanocrystalline thin films of Cu 2 O as well as metallic Cu on glass substrates at room temperature by the versatile high pressure magnetron sputtering technique. Deposition of nanophase materials takes place in a relatively high pressure of inert gas (2–600 mTorr) at 300 K. The average primary grain size of the materials thus synthesized was found to lie in the 4–14 nm range and could be controlled by proper choice of process parameters (nature and the pressure of the sputtering gas, applied power, substrate temperature, and system geometry). In general, nanocrystalline thin films were formed at 300K, while loosely aggregated nanoparticles deposited at 77K. We define the conditions under which it is possible to deposit optically transparent, nanocrystalline thin films of semiconducting, single phase Cu 2 O on glass or quartz.