UV induced zener diode characteristic in a single n-ZnO/p++-Si nanoheterojunction.

Rectification is observed in a single n-ZnO/p++-Si nanoheterojunction using ultra high vacuum compatible scanning tunneling microscope. The nanohetrojunctions have been grown using catalyst free vapor-solid growth of ZnO nanorods on p++-Si substarte. A high rectification ratio approximately 100 at 2 V is observed in the current voltage measurements. Temperature dependent study in these nanohetero-junctions showed activation energy for carrier conduction approximately 66 meV, which is primarily associated to the presence of heterojunction induced interface states. Role of ultra violet excitation on these finite sized (approximately 500 nm) nanoheterojunction is also studied with photo-generated electron-hole pairs. A Zener breakdown is observed in this photo-excitation process. Increase in the concentration of minority carriers and corresponding decrease in barrier width and height at the junction have been identified for the observed tunneling behavior under UV illumination. The large carrier concentration in the finite sized device with large diffusion length of electron (approximately 2 microm) is made responsible for the observed voltage regulation.

Zinc oxide epitaxial thin film deposited over carbon on various substrate by pulsed laser deposition technique.

Zinc Oxide (ZnO) is a promising candidate material for optical and electronic devices due to its direct wide band gap (3.37 eV) and high exciton binding energy (60 meV). For applications in various fields such as light emitting diode (LED) and laser diodes, growth of p-type ZnO is a prerequisite. ZnO is an intrinsically n-type semiconductor. In this paper we report on the synthesis of Zinc Oxide-Carbon (ZnO:C) thin films using pulsed laser deposition technique (PLD). The deposition parameters were optimized to obtain high quality epitaxial ZnO films over a carbon layer. The structural and optical properties were studied by glazing index X-ray diffraction (GIXRD), photoluminescence (PL), optical absorption (OA), and Raman spectroscopy. Rutherford backscattering spectroscopy (RBS), scanning electron microscopy with energy dispersive spectroscopy (SEMEDS) and atomic force microscopy (AFM) were employed to determine the composition and surface morphology of these thin films. The GIXRD pattern of the synthesized films exhibited hexagonal wurtzite crystal structure with a preferred (002) orientation. PL spectroscopy results showed that the emission intensity was maximum at -380 nm at a deposition temperature of 573 K. In the Raman spectra, the E2 phonon frequency around at 438 cm(-1) is a characteristic peak of the wurtzite lattice and could be seen in all samples. Furthermore, the optical direct band gap of ZnO films was found to be in the visible region. The growth of the epitaxial layer is discussed in the light of carbon atoms from the buffer layer. Our work demonstrates that the carbon is a novel dopant in the group of doped ZnO semiconductor materials. The introduction of carbon impurities enhanced the visible emission of red-green luminescence. It is concluded that the carbon impurities promote the zinc related native defect in ZnO.

Stability of embedded indium nanoclusters in silica under thermal treatment and ion irradiation.

The stability of embedded Indium (In) nanoclusters (NCs) in silica under thermal annealing and ion irradiation was investigated. The In NCs were prepared by implantation of 890 keV indium ions in silica matrix at room temperature. Post implantation annealing resulted in the shifting of the size distribution to higher side. On the other hand 140 keV Nitrogen ion irradiation at elevated temperature resulted in the reduction of NCs size, with significant narrowing of the size distribution. The paper discusses the results of the study in the light of the models pertaining to the stability of NCs under ion irradiation conditions.

Intense ultraviolet-blue photoluminescence from SiO2 embedded ge nanocrystals prepared by different techniques.

We have made systematic studies on the ultraviolet-blue photoluminescence (PL) from Ge nanocrystals (NCs) grown embedded in SiO2 matrix. Embedded Ge NCs are grown by two different methods, namely, radio-frequency magnetron sputtering (SPT) and ion implantation (IMP). For comparison, Ar implanted SiO2 layer was processed similarly and studied to isolate the contribution of Ge atoms in the observed PL. X-ray diffraction, optical Raman and low frequency Raman scattering studies confirm the presence of Ge NCs in samples prepared by SPT and IMP methods, and Si nanoclusters in Ar implanted sample. Room temperature PL studies with 325 nm excitation show very strong UV-blue emission bands in the range 342-420 nm, and PL studies with 246 nm excitation show two strong UV emission bands at approximately 285 nm and approximately 393 nm in implanted samples. Deconvolution of UV-blue bands show that most of the emission peaks are not unique to the presence of Ge in the samples. Time resolved PL studies in the blue wavelength region show a fast decay dynamics (time constant of approximately 1.0 ns), irrespective of the NC size. PL excitation spectroscopy measurements show a large Stoke's shift for the UV emission bands. Our results indicate that contrary to the literature reports, the approximately 400 nm PL emission is band is not unique to the presence of Ge in the SiO2 matrix and it is likely to originate from a defective NC/SiO2 interface, irrespective of the species of NCs. Origin of various UV emission bands is discussed in the light of the experimental findings and literature reports.

Structure and photoluminescence properties of ion beam synthesized SiC nanoparticles in Si.

Silicon carbide nanoparticles were synthesized in Si(100) wafers by 300 keV C+ ion implantation at elevated substrate temperatures of 550, 650 and 700 degrees C. The implantation has been carried out upto a fluence of 2 x 10(17) ions/cm2 with a constant current density 1.2 microA/cm2. GIXRD analysis on the implanted sample confirms the formation of 3C-SiC. XTEM studies of sample implanted at 650 degrees C show that size of SiC nanoparticles is 6 nm at a depth 0.6 microm from the sample surface. PL spectrum of sample implanted at different temperatures showed a peak at 2.45 eV and 2.3 eV and the intensity of PL peak increases with implantation temperature. The peak at 2.45 eV corresponds to blue shifted emission from SiC nanoparticles having size 6 nm. The peak at 2.3 eV is assigned to the SiC nanoparticles with enhanced d-value.

MeV gold ion induced sputtered nanoparticles from gold nanostructures: dependence of incident flux and temperature.

The high-energy and heavy-ion induced sputtered particles from nanostructures under various conditions can result in variety of size distributions. 1.5 MeV Au2+ ions induced sputtering from isolated gold nanostructures deposited on silicon substrate have been studied as a function of incident ion flux (dose rate) and the sputter particle catcher at low temperature. At higher fluxes, a bimodal distribution of the sputtered particles has been observed. Cross-sectional transmission electron microscopy and Rutherford backscattering spectrometry measurements showed that the sputter particle size distribution depends on morphology at surface and interfaces. The results for the size distribution from a catcher at low temperature showed the less agglomeration of ejected clusters on the catcher grids, resulting in the lower-disperse size distribution.

Novel low temperature chemical synthesis and characterization of zinc oxide nanostructures.

We report a new and highly efficient method to synthesize zinc oxide (ZnO) nanostructures having a variety of sizes and shapes. A simple chemical reaction is followed that utilizes the oxidation of metallic zinc in the presence of an appropriate catalyst. This one-step method has advantages such as low temperature and atmospheric pressure synthesis, high yield of more than 90% and excellent optical and crystalline properties of the product. X-ray diffraction pattern of the samples shows hexagonal phase of ZnO with particles size in the range of 60-75 nm. Scanning electron microscope and transmission electron microscope images of the ZnO show hexagonal and rod-shaped nanoparticles. UV-visible spectra of the dispersed samples show strong absorption peaks at approximately 378 nm. The photoluminescence spectra show a strong emission peak at approximately 388 nm indicating good optical characteristics. The product formed is found to be dependent on the ratio of the starting materials and on other reaction conditions such as temperature, time etc. This method is suitable for large-scale production of nanosized ZnO and could be extended for the synthesis of other metal oxides, such as MgO etc.

Development of a universal serial bus interface circuit for ion beam current integrators.

A universal serial bus (USB) interface circuit has been developed to enable easy interfacing of commercial as well as custom-built ion beam current integrators to personal computer (PC) based automated experimental setups. Built using the popular PIC16F877A reduced instruction set computer and a USB-universal asynchronous receiver-transmitter/first in, first out controller, DLP2232, this USB interface circuit virtually emulates the ion beam current integrators on a host PC and uses USB 2.0 protocol to implement high speed bidirectional data transfer. Using this interface, many tedious and labor intensive ion beam irradiation and characterization experiments can be redesigned into PC based automated ones with advantages of improved accuracy, rapidity, and ease of use and control. This interface circuit was successfully used in carrying out online in situ resistivity measurement of 70 keV O(+) ion irradiated tin thin films using four probe method. In situ electrical resistance measurement showed the formation of SnO(2) phase during ion implantation.

Regional differences in constituents of gall stones.

The pathogenesis of pigment and mixed gall stone formation remains elusive. The elemental constituents of gall stones from southern states of Tamil Nadu, Kerala and Karnataka have been characterized. Our aim was to determine the elemental concentration of representative samples of pigment, mixed and cholesterol gall stones from Andhra Pradesh using proton-induced X-ray emission (PIXE) using a 3 MV horizontal pelletron accelerator. Pigment gall stones had significantly high concentrations of copper, iron and lead; chromium was absent. Except for iron all these elements were significantly low in cholesterol gall stones and intermediate levels were seen in mixed gall stones. Highest concentrations of chromium was seen in cholesterol and titanium in mixed gall stones respectively; latter similar to other southern states. Arsenic was distinctly absent in cholesterol and mixed gall stones. The study has identified differences in elemental components of the gall stones from Andhra Pradesh.

Trace Element Studies on Tinospora cordifolia (Menispermaceae), Ocimum sanctum (Lamiaceae), Moringa oleifera (Moringaceae), and Phyllanthus niruri (Euphorbiaceae) Using PIXE.

Traditionally, Tinospora cordifolia (Willd.) Hook. F. & Thomson (Menispermaceae), Ocimum sanctum L. (Lamiaceae), Moringa oleifera Lam. (Moringaceae), and Phyllanthus niruri L. (Euphorbiaceae) are some of the commonly used medicinal plants in India for curing ailments ranging from common cold, skin diseases, and dental infections to major disorders like diabetes, hypertension, jaundice, rheumatism, etc. To understand and correlate their medicinal use, trace element studies on the aqueous extract of these medicinal plants have been carried out using particle-induced X-ray emission technique. A 2-MeV proton beam was used to identify and characterize major and minor elements namely Cl, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, and Sr in them. Results have revealed that these elements are present in varying concentrations in the selected plants. Notable results include very high concentrations of Cl, K, and Ca in all the leaf samples, appreciable levels of Mn in all plants, high Zn content in T. cordifolia, and the aqueous extract of Moringa leaves compared to others and relative higher concentrations of Cr in all the plants.

Flux dependent MeV self-ion-induced effects on Au nanostructures: dramatic mass transport and nanosilicide formation.

We report a direct observation of dramatic mass transport due to 1.5 MeV Au(2+) ion impact on isolated Au nanostructures of average size ≈7.6 nm and height ≈6.9 nm that are deposited on Si(111) substrate under high flux (3.2 × 10(10)-6.3 × 10(12) ions cm(-2) s(-1)) conditions. The mass transport from nanostructures was found to extend up to a distance of about 60 nm into the substrate, much beyond their size. This forward mass transport is compared with the recoil implantation profiles using SRIM simulation. The observed anomalies with theory and simulations are discussed. At a given energy, the incident flux plays a major role in mass transport and its redistribution. The mass transport is explained on the basis of thermal effects and the creation of rapid diffusion paths in the nanoscale regime during the course of ion irradiation. The unusual mass transport is found to be associated with the formation of gold silicide nano-alloys at subsurfaces. The complexity of the ion-nanostructure interaction process is discussed with a direct observation of melting (in the form of spherical fragments on the surface) phenomena. Transmission electron microscopy, scanning transmission electron microscopy, and Rutherford backscattering spectroscopy methods have been used.

Mechanism of bright red emission in Si nanoclusters.

A bright photoluminescence around 1.7 eV is observed for post-annealed samples of 1 MeV Si(2+) implanted in an SiO(2) matrix. A super-linear power dependence of photoluminescence intensity accompanied by pulse shortening under continuous wave laser excitation is recorded without any spectral narrowing. An emission process comprised of an initial non-radiative recombination (time constant ∼280-315 ps) of excited carriers in the defect states in SiO(2) matrices to the conduction band minima of nc-Si, followed by a slower process of radiative recombination in the direct band transition for nc-Si along with a non-radiative Auger recombination (time constant ∼2.67 ns) is proposed.

Optical absorption and photoluminescence spectroscopy of the growth of silver nanoparticles.

Results obtained from the optical absorption and photoluminescence (PL) spectroscopy experiments have shown the formation of excitons in the silver-exchanged glass samples. These findings are reported here for the first time. Further, we investigate the dramatic changes in the photoemission properties of the silver-exchanged glass samples as a function of postannealing temperature. Observed changes are thought to be due to the structural rearrangements of silver and oxygen bonding during the heat treatments of the glass matrix. In fact, photoelectron spectroscopy does reveal these chemical transformations of silver-exchanged soda glass samples caused by the thermal effects of annealing in a high vacuum atmosphere. An important correlation between temperature-induced changes of the PL intensity and thermal growth of the silver nanoparticles has been established in this Letter through precise spectroscopic studies.

Blue luminescence of Au nanoclusters embedded in silica matrix.

Photoluminescence study using the 325 nm He-Cd excitation is reported for the Au nanoclusters embedded in SiO(2) matrix. Au clusters are grown by ion beam mixing with 100 KeV Ar(+) irradiation on Au [40 nm]/SiO(2) at various fluences and subsequent annealing at high temperature. The blue bands above approximately 3 eV match closely with reported values for colloidal Au nanoclusters and supported Au nanoislands. Radiative recombination of sp electrons above Fermi level to occupied d-band holes are assigned for observed luminescence peaks. Peaks at 3.1 and 3.4 eV are correlated to energy gaps at the X- and L-symmetry points, respectively, with possible involvement of relaxation mechanism. The blueshift of peak positions at 3.4 eV with decreasing cluster size is reported to be due to the compressive strain in small clusters. A first principle calculation based on density functional theory using the full potential linear augmented plane wave plus local orbitals formalism with generalized gradient approximation for the exchange correlation energy is used to estimate the band gaps at the X- and L-symmetry points by calculating the band structures and joint density of states for different strain values in order to explain the blueshift of approximately 0.1 eV with decreasing cluster size around L-symmetry point.