Synthesis, characterization, growth mechanism, photoluminescence and field emission properties of novel dandelion-like gallium nitride

Dandelion-like gallium nitride (GaN) microstructures were successfully synthesized via Ni catalyst assisted chemical vapor deposition method at 1200 °C under NH₃ atmosphere by pre-treating precursors with aqueous ammonia. The as-synthesized product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). X-ray diffraction analysis revealed that as-synthesized dandelion-like GaN was pure and has hexagonal wurtzite structure. SEM results showed that the size of the dandelion-like GaN structure was in the range of 30-60 μm. Dandelion-like GaN microstructures exhibited reasonable field emission properties with the turn-on field of 9.65 V μm⁻¹ (0.01 mA cm⁻²) and threshold field of 11.35 V μm⁻¹ (1 mA cm⁻²) which is sufficient for applications of electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer. Photoluminescence (PL) measurements of dandelion-like GaN showed a strong near-band-edge emission at 370.2 nm (3.35 eV) with blue band emission at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) but with out yellow band emission. The room-temperature photoluminescence properties showed that it has also potential application in light-emitting devices. The tentative growth mechanism for the growth of dandelion-like GaN was also described.     

Effect of Excitation and Vibrational Temperature on the Dissociation of Nitrogen Molecules in Ar-N2 Mixture RF Discharge

Non-equilibrium argon-nitrogen mixture plasma generated at 13.56 MHz is characterized by optical emission spectroscopy and Langmuir probe techniques. The excitation and vibrational temperature are studied as a function of argon percentage in the mixture, at 30-Pa filling pressure and input RF powers of 200 and 300 watt, to find out their role in dissociation of N₂ molecules. In this work, the excitation temperature is determined from Ar-I emission line intensities by using the simple Boltzmann plot method and is found to increase with argon mixing in nitrogen plasma. In similar fashion, the vibrational temperature of second positive system has been determined and is found to also have increasing trend with argon addition. The effect of excitation and vibrational temperature on the nitrogen molecular dissociation level is also monitored. It is observed that N/N ₂ ratio increases with increase in excitation and vibrational temperature and falls slightly at the end.     

Influence of P<Superscript>+</Superscript>-ZnTe back surface contact on photovoltaic performance of ZnTe based solar cells

In order to improve photovoltaic performance of solar cells based on ZnTe thin films two device structures have been proposed and its photovoltaic parameters have been numerically simulated using Solar Cell Capacitance Simulator software. The first one is the ZnO/CdS/ZnTe conventional structure and the second one is the ZnO/CdS/ZnTe/P⁺-ZnTe structure with a P⁺-ZnTe layer inserted at the back surface of ZnTe active layer to produce a back surface field effect which could reduce back carrier recombination and thus increase the photovoltaic conversion efficiency of cells. The effect of ZnO, CdS and ZnTe layer thicknesses and the P⁺-ZnTe added layer and its thickness have been optimized for producing maximum working parameters such as: open-circuit voltage V_oc, short-circuit current density J_sc, fill factor FF, photovoltaic conversion efficiency η. The solar cell with ZnTe/P⁺-ZnTe junction showed remarkably higher conversion efficiency over the conventional solar cell based on ZnTe layer and the conversion efficiency of the ZnO/CdS/ZnTe/P⁺-ZnTe solar cell was found to be dependent on ZnTe and P⁺-ZnTe layer thicknesses. The optimization of ZnTe, CdS and ZnTe layers and the inserting of P⁺-ZnTe back surface layer results in an enhancement of the energy conversion efficiency since its maximum has increased from 10% for ZnO, CdS and ZnTe layer thicknesses of 0.05, 0.08 and 2 µm, respectively to 13.37% when ZnO, CdS, ZnTe and P⁺-ZnTe layer thicknesses are closed to 0.03, 0.03, 0.5 and 0.1 µm, respectively. Furthermore, the highest calculated output parameters have been J_sc?=?9.35 mA/cm², V_oc?=?1.81 V, η?=?13.37% and FF?=?79.05% achieved with ZnO, CdS, ZnTe, and P⁺-ZnTe layer thicknesses about 0.03, 0.03, 0.5 and 0.1 µm, respectively. Finally, the spectral response in the long-wavelength region for ZnO/CdS/ZnTe solar cells has decreased at the increase of back surface recombination velocity. However, it has exhibited a red shift and showed no dependence of back surface recombination velocity for ZnO/CdS/ZnTe/P?+?-ZnTe solar cells.     

Effect of neon mixing on vibrational temperature of molecular nitrogen plasma generated at 13.56 MHz

Optical emission spectroscopy is used to investigate the effect of neon mixing on the electron temperature and vibrational temperature of second positive and first negative system of nitrogen plasma generated by 13.56 MHZ RF generator. The electron temperature is determined from NeI lines intensities, using Boltzman's plot. The relative changes in vibrational population of N₂(C³Πu) and states with neon mixing are monitored by measuring the emission intensities of second positive and first negative system of nitrogen molecules. Vibrational temperature is calculated for the sequences Δν=0,1,−2, that follows the Boltzman's distribution. It is found that electron temperature as well as vibrational temperature of second positive and first negative system can be raised significantly by mixing neon in nitrogen plasma. Vibrational temperature at 250 watt RF power, of second positive system is raised up to 0.67 eV at 90% neon where as for first negative system it is raised up to 0.78 eV. It is found that vibrational temperature increases with the gas pressure.     

Synthesis of regenerated bacterial cellulose-zinc oxide nanocomposite films for biomedical applications

Regenerated bacterial cellulose (RBC) composites with zinc-oxide nanoparticles (ZnO) were prepared using a new strategy for enhanced biomedical applications of BC. Powdered BC was dissolved in N-methylmorpholine-N-oxide, and different concentrations of ZnO nanoparticles were mixed into the BC solution. RBC, RBC-ZnO1 (1 % ZnO) and ZnO-RBC2 (2 % ZnO) nanocomposite films were prepared by casting the solutions through an applicator. FE-SEM images confirmed the structural features and impregnation of the RBC films by nanoparticles. XRD analysis indicated the presence of specific peaks for RBC and ZnO in the composites. The RBC nanocomposites were found to have greatly enhanced thermal, mechanical and biological properties. Specifically, the degradation temperatures were improved from 334 °C for RBC to 339 and 344 °C for RBC-ZnO1 and RBC-ZnO2, respectively. The mechanical strength and Young’s modulus of the composites were also higher than those of pure RBC. The greatly improved antibacterial properties of the RBC-ZnO nanocomposites are the most striking feature of the present study. The bacterial growth inhibition measured for the RBC was zero, but reached up to 34 and 41 mm for RBC-ZnO1 and RBC-ZnO2, respectively. In addition to their antibacterial properties, the RBC-ZnO nanocomposites were found to be nontoxic and biocompatible with impressive cell adhesion capabilities. These RBC-ZnO nanocomposites can be used for different biomedical applications and have the potential for use in bioelectroanalysis.     

Evaluation of microbial loads,physical characteristics,chemical constituents and biological properties of radiation processed Fagonia arabica

Whole plant of Fagonia arabica with 3 different particle sizes (30, 50 and 70 mesh) were exposed to gamma radiation doses of 1–10 kGy from a Cobalt 60 source. A series of tests was performed in order to check the feasibility of irradiation processing of the plant. The applied radiation doses did not affect (P<0.05) pH and antimicrobial activities of the plant. The total weight of the dry extracts in methanol as well as water was found increased with irradiation. The irradiated samples showed significant increase in phenolic content and free radical scavenging activity using DPPH. Shortly after irradiation (on the day of radiation treatment) high amounts of free radicals were detected in the irradiated plant samples and the chemiluminescence measurements were generally found to be dose dependent. Maximum luminescence intensity was observed in case of samples with mesh size of 30 for all the radiation doses applied. After a period of one month the chemiluminescence signals of the irradiated samples approximated those of the controls. The study suggests that gamma irradiation treatment is effective for quality improvement and enhances certain beneficial biological properties of the treated materials.     

Local structure and redox state of copper in tellurite glasses

The local glass structure of tellurite glasses containing CuO with the nominal composition x(CuO) · (1−x)(TeO₂), where x=0.10, 0.20, 0.30, 0.40, and 0.50, as well as the valence state of the copper ions have been investigated by X-ray photoelectron spectroscopy (XPS) and magnetization measurements. The Te 3d core level spectra for all glass samples show symmetrical peaks (Te 3d_5/2 and Te 3d_3/2) at essentially the same binding energies as measured for TeO₂ indicating that the chemical environment of the Te atoms in the glasses does not vary significantly with the addition of CuO. The O 1s spectra, however, show slight asymmetry for all glass samples which results from two contributions, one from the presence of oxygen atoms in the Te-O-Te environment (bridging oxygen BO) and the other from oxygen atoms in an Te-O-Cu environment (non-bridging oxygen NBO). The ratio of NBO to total oxygen was found to increase with CuO content and to be in good agreement with calculated values for the TeO₄ trigonal bipyramid structure. Moreover, the appearance of a satellite peak in the Cu 2p spectra provides definitive evidence for the presence of Cu²⁺ ions in these glass samples where the asymmetry and broadening of the Cu 2p_3/2 and Cu 2p_1/2 peaks are indicative of the presence of both Cu²⁺ and Cu⁺ ions. The relative concentration Cu²⁺ determined from XPS is in good qualitative agreement with the determinations of Cu²⁺ from magnetic susceptibility measurements on the same glass samples. Furthermore the susceptibility data follow a Curie-Weiss temperature-dependent behavior (χ=C/(T−θ)) with negative Curie temperatures indicating that the predominant magnetic interactions between the Cu²⁺-Cu²⁺ exchange pairs are antiferromagnetic in nature.     

Effect of laser irradiation on the structure and valence states of copper in Cu-phosphate glass by XPS studies

The effect of laser irradiation using three different wavelengths (IR, visible and UV) generated from Nd:YAG laser on the local glass structure as well as on the valence state of the copper ions in copper phosphate glass containing CuO with the nominal composition 0.30(CuO)-(0.70)(P₂O₅), has been investigated by X-ray photoelectron spectroscopy (XPS). The presence of asymmetry and satellite peaks in the Cu 2p spectrum for the unirradiated sample is an indication of the presence of two different valence states, Cu²⁺ and Cu⁺. Hence, the Cu 2p_3/2 spectrum was fitted to two Gaussian-Lorentzian peaks and the corresponding ratio, Cu²⁺/Cu_total, determined from these relative areas clearly shows that copper ions exist predominately (>86%) in the Cu²⁺ state for the unirradiated glass sample under investigation. For the irradiated samples the symmetry and the absence of satellite peaks in the Cu 2p spectra indicate the existence of the copper ions mostly in Cu⁺ state. The O 1s spectra show slight asymmetry for the irradiated as well as unirradiated glass samples which result from two contributions, one from the presence of oxygen atoms in the P-O-P environment (bridging oxygen BO) and the other from oxygen in an P-O-Cu and PO environment (non-bridging oxygen NBO). The ratio of NBO to total oxygen was found to increase with laser power.     

Electronic structure and acid–base properties of Kojic acid and its dimers. A DFT and quantum topology study

Kojic acid is a polyfunctional heterocyclic compound, with several important reaction centres; it has a wide range of applications in the cosmetic, medicine, food, agriculture and chemical industries. The present study aims at better insight into its electronic structure and bonding characteristics. Thus, density functional theory at the M06-2x /6-311++G^** level of theory is used to investigate its ground state electronic and acid–base properties. Protonation and deprotonation enthalpies are computed and analysed. The ability of Kojic acid to form both water complexes and dimers is explored. Several different complexes and dimer structures were examined. Natural bond order and quantum topology features of the charge density were analysed. The origin of the stability of the studied complexes and dimer structures can be traced to hydrogen bonding, π-conjugative and non-covalent dispersive interactions.