Biomolecule assisted hydrothermal synthesis of chainlike network of silver sulfide nanostructures.

A L-cysteine assisted hydrothermal route has been utilized for the growth of Ag2S nanostructures with chainlike network. It was observed that the experimental parameters such as the synthesis temperature and variation in the molar ratio of the anionic and cationic precursors play critical role in determining the morphology and crystal structure of the products. X-ray diffraction study revealed the formation of monoclinic acanthite Ag2S. L-cysteine was acting as complexing agent as well as sulfur source. The branching fractal morphology was explained through Cayley tree model and structure of L-cysteine.

ZnO hierarchical nanostructures: simple solvothermal synthesis and growth mechanism.

Hierarchical nano/micro structures of ZnO have been fabricated by solvothermal approach on sol-gel derived ZnO thin films. Paintbrush like nano/micro rod assembly, double-sided brush and windmill type architectures of ZnO are obtained when the ZnO thin film coated substrates were treated solvothermally in water at pH 10. Aligned nanorods are obtained at pH approximately 13.5 in water. In ethylenediamine-water solvent divergent micro/nanorod assemblies such as hemispherical dandelion, rice plant type bush of ZnO are obtained. Increase in the percentage of ethyelendiamine resulted in the formation of smaller assemblies of relatively thin nanorods. Initial slow reaction caused by the slow increase of the temperature inside the reaction medium and the different growth kinetics of the ZnO crystals are supposed to be the reason behind the architectural assemblies of the ZnO crystals.

Direct synthesis of ZnS nanoribbons, micro-sheets and tetrapods.

ZnS nano and micro structures such as nanoribbons, large sheets and tetrapod shaped crystals were fabricated by direct thermal evaporation of ZnS powder without using any catalyst. Formation of the one dimensional structures such as nanoribbons and micron order sheets was attributed to the vapor-solid growth mechanism. The formation of octahedron nucleus with cubic crystal structures was proposed as the growth unit of the wurtzite crystal structured tetrapods. Appearance of the periodic stacking faults or twining planes in between alternate cubic and hexagonal crystal structured zones along the growth direction of the ribbons provided secondary growth sites for the octahedron nucleus and subsequent crystal growth resulted in to the formation of the tetrapod arrays. These nano/micro structures of ZnS exhibited a green emission band at room temperature.

CdS:Mn nanorods: solvothermal synthesis and properties.

Manganese (0.05-9 mol.%) doped CdS nanorods were synthesized via solvothermal route using ethylenediamine (En) and a mixture of En and water as the solvents. The diameters and the lengths of the doped CdS nanorods varied from 40-100 nm and 600-2500 nm, respectively, with change in the composition of the solvents. The broad photoluminescence (PL) emission from the undoped CdS nanorods centered at approximately 535 nm is found to be blue shifted to 516 nm with the incorporation of Mn in the CdS crystal structure. Also increase in the intensity of the PL was noticed in the Mn doped CdS nanorods for both the solvent systems. Maximum PL intensity was observed for 1 mol.% Mn in case of En system and for 0.5 mol.% Mn in case of En/water system, above which quenching occurred as a result of Mn-Mn clustering. EPR study revealed six-line hyperfine splitting for low Mn concentration in both solvent systems. Increase in the Mn concentration caused EPR signal broadening due to Mn-Mn clustering.

Mn(2+)-induced substitutional structural changes in ZnS nanoparticles as observed from positron annihilation studies.

Zinc sulfide nanoparticles doped with different concentrations of manganese ions (Mn(2+)) were synthesized at various temperatures to investigate the effects of substitution and the associated defect evolution. Positron lifetime and Doppler broadening measurements were used as probes. The initial stage of defect recovery was dominated by the occupation of Zn(2+) vacancies by Mn(2+) ions, bringing in characteristic changes in the positron lifetimes, intensities and Doppler broadened lineshape parameters. Detailed analyses considering the presence of one and two types of defects were carried out to identify the type of defects which trap positrons at the different dopant concentrations. Electron paramagnetic resonance studies indicated increased Mn-Mn interaction and the formation of Mn clusters with further doping. The results are in striking contrast to those for nanorods, where vacancy recombination transformed their interior into regions free of defects.

Ga2O3 and GaN nanocrystalline film: reverse micelle assisted solvothermal synthesis and characterization.

Gallium oxide (beta-Ga2O3) nanoparticles were successfully deposited on quartz glass substrates using sodium bis(2-ethylhexyl) sulfosuccinate (AOT)/n-hexane/ethylene glycol monomethyl ether (EGME) reverse micelle-mediated solvothermal process with different omega values. The mean diameter of Ga2O3 particles was approximately 2-3 nm and found to be approximately independent of omega values of the reverse micelles. However, when the Ga2O3 nanocrystalline films were nitrided at 900 degrees C under flowing NH3 atmosphere for 1 h, the mean diameter of the resulted gallium nitride (wurtzite-GaN) nanoparticles varied from 3-9 nm. Both nanocrystalline films of Ga2O3 and GaN were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR) spectroscopy, UV-vis spectroscopy and photoluminescence in order to study their chemical and physical properties explicitly.

Single layer polycrystalline ZnO-Al2O3 nanocomposite thin films: optical and electrical properties.

Nanometric size dependent optical absorption coefficients, dispersive nature of the dielectric constants and ac conductivity are observed for the single layer ZnOx(Al2O3)1-x (x = 0.20 and 0.50) nanocomposites thin films. The sol-gel prepared thin films with thickness approximately 119 nm, contains randomly dispersed ZnO nanocrystallites in the A2O3, matrix. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) revealed homogeneous distributions of the nanoparticles in the matrix. The shifting and broadening of the optical absorption peaks to the lower wavelength region at higher annealing temperatures was analyzed in order to relate the particle-particle interactions and the sizes of the dispersed nanoparticles in the matrix. AC impedance spectroscopy in the frequency range of 500 Hz to 5 MHz was measured at room temperature (300 K), 325 K, 395 K, and 450 K. The dielectric relaxations in each case were found to be of Cole-Cole type. The semicircular pattern of the ac capacitance in the complex plane showed contributions from the grain and grain boundary subject to concentration of ZnO in the matrix. The appearance of the negative capacitance at higher frequency was also analyzed.

Growth of ZnO nanocrystals by a solvothermal technique and their photoluminescence properties.

ZnO nanocrystals with various morphologies such as nanorod arrays, flower like assemblies, spherical particles, hexagonal cones, and self assembled microstructures were prepared by a solvothermal approach. It was observed that morphology of the ZnO nanostructures were very much solvent dependent in solvothermal approach. Water, ethylenediamine, and ethylene glycol-water mixture favors the formation of nanorods. Flower like assemblies of ZnO were produced in benzene. Spherical as well as cone like nanoparticles and their assemblies were produced in ethylene glycol. The ZnO nanostructures were characterized by X-ray diffraction, scanning and transmission electron microscopy, and photoluminescence studies.

Solvothermal synthesis of CdS nanorods: role of basic experimental parameters.

CdS nanorods with varying dimensions were synthesized by solvothermal process. It was observed that the anions present with the Cd-salts play an important role in determining the dimensions of the CdS nanorods. The crystalline nature of the sources was found to play a crucial role in determining the phase of the products. The nature of the sulfur source, molar ratio of the precursors, filling fraction of the solvent, and the synthesis temperature play important role in defining the size and shape of the products. By controlling the experimental parameters it was possible to control the dimension of the CdS nanorods within a certain range (diameter of the nanorods could be varied within a wide range from approximately 7 to 100 nm by varying the temperature within 100-250 degrees C). Optical absorption, photoluminescence, and Raman studies of these samples were carried out to characterize the CdS nanorods.

Multicolor luminescence from transition metal ion (Mn2+ and Cu2+) doped ZnS nanoparticles.

Mn and Cu doped ZnS nanoparticles in powder form were prepared by a simple solvothermal route. Particle size and crystal structure of the products were investigated through X-ray diffraction study revealing the formation of cubic ZnS nanoparticles of average diameter 2.5 nm. Particle size was also verified by the high resolution transmission electron microscopic images. Blue emission at approximately 445 nm was observed from the undoped sample, which was attributed to the presence of large surface defects. With increasing doping concentration the defect related emission gradually quenches and subsequently the impurity related emissions appeared. Mn doped samples exhibited orange emission at approximately 580 nm which may be attributed to the transition between (4)T1 and (6)A1 energy levels of the Mn2+ 3d states. Whereas, the Cu doped ZnS nanoparticles exhibited a red shifted strong blue emission at approximately 466 nm which is attributed to the transition of the electrons from the surface states to the 't2' levels of Cu impurities.

Fabrication of SnS2 flower like nanoflake assemblies through thermal evaporation.

Uniform as well as flower like patterns of SnS2 nanoflakes were produced by a thermal evaporation process. Interpenetrating phenomenon was observed between the individual nanoflakes during the course of their lateral growth. The interpenetrating growth and controlled vapor concentration as well as the substrate temperature leads to the formation of flower like assemblies of SnS2 nanoflakes. Morphology and growth mechanism of the nanostructures were studied by scanning electron microscopic observations at different stages of the nanoflake growth. The produced nanoflakes were characterized by X-ray diffraction, scanning and transmission electron microscopy, and Raman spectroscopic measurements. SnS2 nanoflakes were perfectly single crystalline and growth direction of the nanoflakes was along the [101]-lattice plane.

Multipod znO nanoforms: low temperature synthesis and characterization.

ZnO nanotetrapods were synthesized by a simple thermal evaporation of Zn powder at a relatively low temperature approximately 600 degrees C. The tetrapods have four legs with hexagonal cross-section. Interpenetrating growth was observed in some of these nanotetrapods. Multipod ZnO nanoforms were produced at higher temperature. The optical characterizations such as optical absorbance, photoluminescence and Raman spectroscopy reveal excellent crystal qualities of these nanoforms. The field emission studies indicated that these nanoforms could be utilized in field emission based devices.

Silica encapsulated Ni nanoparticles: variation of optical and magnetic properties with particle size.

Ni nanoparticles embedded in SiO2 matrix were prepared by sol-gel process. The molar percentages of Ni were varied from 2 to 20% of total SiO2 present in the matrix. Transmission electron microscope (TEM) images revealed that particle sizes varied from 8.0-15.7 nm at an annealing temperature of 773 K with variation of concentration. The optical absorption spectra revealed that the surface plasmon resonance (SPR) peak in the UV region of the spectrum shifted with the particle diameter (D) from that at 247.3 nm for D = 8.0 nm to 250.7 nm for D = 15.7 nm. In hysteresis loop measurements the magnetizations (M) of the nanocomposites also increased with higher Ni content in the matrix and did not saturate in the measuring limit of the magnetic filed (H) of 4 KOe. The anhysteric curves for different samples were analyzed with the law of approach to saturation (LAS). The zero field cooled (ZFC) and field cooled (FC) magnetization measurements at 50 Oe showed increasing broadening of the ZFC curve with the higher Ni content. To calculate the average blocking temperature ((T(B a distribution of the blocking temperatures (T(B)) was assumed to initiate theoretical fittings and it was found to be increasing with the Ni concentration in the matrix.

Chelating ligand-mediated synthesis of hollow ZnS microspheres and its optical properties.

Monodispersed hollow ZnS microspheres have been successfully synthesized by a facile ethylenediamine tetraacetic acid (EDTA) mediated hydrothermal route. The sizes of the hollow spheres vary from 1.5 to 3.5 microm when the reaction temperature varied from 130 to 230 degrees C. The formation of these hollow spheres is attributed to the oriented aggregation of ZnS nanocrystals around the gas-liquid interface between H(2)S and water. EDTA plays important role as chelating ligand and capping reagent, which regulates the release of Zn(2+) ions for the formation of ZnS hollow spheres. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis spectroscopy, photoluminescence, and Raman spectroscopy. The obtained ZnS hollow spheres show a sharp and photostable UV emission approximately 370 nm, which is attributed to the recombination process associated with interstitial sulfur vacancy.

Shape selective growth of CdS one-dimensional nanostructures by a thermal evaporation process.

CdS one-dimensional nanoforms such as nanowires, nanoribbons, network-like nanowires, pearl necklace type nanowires, helical-like nanowires, and nanowire arrays were formed on Si substrates by a simple thermal evaporation route. The shapes of the one-dimensional CdS nanoforms were controlled by varying the experimental parameters such as temperature and position of the substrates. Formation of the CdS one-dimensional nanoforms was initiated by the Au catalyzed vapor-liquid-solid technique, whereas the vapor-solid process played a crucial role in defining the shapes of the nanoforms. Different optical characterizations such as optical absorbance, photoluminescence, and Raman spectroscopy were adopted to explore the physical and structural quality of these CdS nanoforms.

Simple solvothermal route to synthesize ZnO nanosheets, nanonails, and well-aligned nanorod arrays.

ZnO nanosheets, nanonails, and well-aligned nanorods were fabricated on Zn foils by a solvothermal approach using ethanol as the solvent. A lower synthesis temperature and a shorter time period favor the formation of nanosheets. By optimizing the synthesis temperature and time period, ZnO nanonails with a hexagonal cap and a long stem could be produced. A higher temperature was not favorable to produce uniform and smooth nanorods. Well-aligned ZnO nanorod arrays were produced with diameters within 100-250 nm and lengths up to approximately 6 microm when NaOH was added to the solvent. By optimizing the reaction parameters, the morphology, size, and orientation of the nanoforms could be tailored. The ZnO nanorods exhibit an excitonic strong UV emission and a defect-related broad green emission at room temperature. The defect-related green emission band decreased with the improvement of the degree of alignment of the nanorods.

One-dimensional ZnO nanostructure arrays: synthesis and characterization.

One-dimensional ZnO nanostructure arrays such as nanowires, nanonails, and nanotrees, have been synthesized by oxygen assisted thermal evaporation of metallic zinc on a quartz substrate over a large area. Morphological evolution of ZnO nanostructures at different time scales and different positions of the substrates have been studied by electron microscopy. A self-catalyzed vapor-liquid-solid (VLS) process is believed to be responsible for the nucleation and subsequently a vapor-solid process is operative for further longitudinal growth. The photoluminescence spectrum showed a weak UV and a broad green emission peak at 3.25 and 2.49 eV, respectively. The latter was attributed to the presence of zinc interstitial defects. Electrical resistivity as a function of temperature showed activated mechanisms to be present. The electrical response of the ZnO nanonail arrays to different gases (CO, NO2, and H2S) indicated that there could be possible application as gas sensors for this material.

Surfactant-assisted route to synthesize well-aligned ZnO nanorod arrays on sol-gel-derived ZnO thin films.

Anisotropic growth of ZnO nanorod arrays on ZnO thin films was achieved at a temperature of 90 degrees C by a surfactant-assisted soft chemical approach with control over size and orientation. ZnO thin films with c-axis preferred orientation had been achieved by the sol-gel technique. Lengths, diameters, and the degree of alignment of the ZnO nanorods were controlled by changing the experimental parameters. It was observed that the surfactant was essential to restrict the lateral growth of the nanorods, whereas the pH level of the reaction medium controlled the length of the nanorods. On the other hand, the orientation of the nanorods depended on the crystalline orientation of the film as well as the pH of the reaction medium. Room-temperature photoluminescence studies revealed that the ZnO nanorods with the best alignment exhibited the best emission property. The ZnO nanorods exhibited a strong UV emission peak at approximately 3.22 eV, ascribed to the band-edge emission. The field emission studies of the well-aligned nanorod arrays exhibited a low turn-on field of 1.7 V/microm to get an emission current density of 0.1 microA/cm(2).

Potential of cadmium sulphide nanorods as an optical microscopic probe to the folding state of cytochrome C.

The folding behavior of cytochrome C (Cyt-C) conjugated with CdS nanorods (CdSnr) is amenable to monitoring by bright field microscopy, the porosity and percolating behavior of such protein conjugated nanoclusters depending on the folding history prior to the conjugation. The method has been used to predict the thermal melting behavior as well as guanidine hydrochloride induced unfolding of Cyt-C. Dynamic light scattering studies indicate that the size distribution of the nanoforms widens in presence of the protein. Furthermore, there is emergence of clusters with higher conductivity and altered zeta potential. Increase of second virial coefficient of CdS nanoforms in the presence of Cyt-C (obtained from static light scattering experiments) implies presence of protein coat over the hydrophobic nanosurface. The results are supported by morphological changes observed through scanning electron microscopy (SEM). Accordingly, the X-ray diffraction pattern shows a change of crystallographic orientations of CdSnr in presence of Cyt-C.

Synthesis and characterization of one-dimensional MgO nanostructures.

Magnesium oxide (MgO) nanowire arrays, nanoribbons, two- and three-dimensional network like nanostructures were prepared by the simple thermal evaporation of Mg powder with and without using catalyst at a relatively low temperature. The non-catalytic approaches favor the formation of network like nanoforms whereas the catalytic approaches favors the formation of one-dimensional nanowire arrays and quasi one-dimensional nanoribbons depending on the temperature and vapor concentrations of the growth site. The diameter and length of the MgO network like columns varied within 40-50 nm and approximately 200 nm respectively. The MgO nanowires produced by the catalytic approach had diameter within 20-30 nm and length approximately 2 microm. Whereas the widths of the nanoribbons varied within 50-100 nm and their length were of the order of a few hundred micrometers. The nanoforms were single crystalline and cubic in phase. The products were characterized by the X-ray diffraction study, energy dispersive analysis of X-ray study, scanning and transmission electron microscopy, and photoluminescence measurements to explore the structural, compositional, morphological, and physical properties of the MgO nanoforms.