The Statistics of Electron Trapping Processes in Microcrystals of Silver Halides*

The probability for the direct trapping of electrons from the conduction band of microcrystals of silver halides is calculated as a function of volume, temperature, and depth and number of traps per microcrystal. For grains with a volume of 4 µm³ at 298 K, and a small number of trapping stales, it is shown that the trap depth must be greater than 0.4.1 eV for there to be a significant trapping probability. The condition for electron trapping is thatE_t> kT In Z, where E_t is the trap depth and Z the partition function for the conduction electrons. Individual sites with trap depths less than 0.05eV have a negligible probability for trapping electrons at room temperature. The implications of these results for theories of latent image formation and of spectral sensitization and desensitization are discussed.     

Conduction mechanism of metal-free phthalocyanine single crystals as a function of temperature

Metal-free phthalocyanine (H₂Pc) single crystals grown by vacuum sublimation were investigated for their conductivity (both in dark and light). The investigations consisted of dark- and photo-current variations with (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·8 kV/cm to 6 kV/cm. The temperature range was from 300°K to around 570°K. The crystals were found to be photoconductive. Based on activation energies calculated from photoconductivity due to temperature dependence, an energy level scheme for H₂Pc single crystals is proposed. The model consists of two trapping levels within the forbidden gap — one at 0·4 eV below the conduction band edge from which electrons are thermally excited into the conduction band and the other acts as recombination centre at 0·3 eV above the valence band edge. The band gap is calculated to be 1·4 eV. Comparative study of the proposed model with that of earlier investigations on the same crystals of the H₂Pc is in good agreement, thereby indicating that H₂Pc is thermally stable even at relatively higher temperature as semiconductor.     

Conduction mechanism in anthracene as a function of temperature

Conduction mechanism in anthracene single crystal grown by Bridgman method was carried out. The investigations consisted of dark- and photo-current variation with respect to (i) applied electric field and (ii) temperature. The applied electric field ranged from 0·5 to 2·5 kV/cm and the temperature range was between 300 K and 450 K. Photo and dark current variations with temperature indicate, based on activation energy determination, that a band model can be applied to the conduction process. The band gap is calculated to be 1·6 eV. The band model consists of a recombination centre 0·37 eV above the valence band edge and a trap level 0·55 eV below the conduction band edge to which electrons are first thermo-optically excited and then they are thermally excited into the conduction band.     

Spectroscopic studies of pr(3+) ions in silver halide crystals

The luminescence of AgBr and AgBrCl crystals doped with Pr(3+) rare-earth ions was investigated in the visible and infrared spectral ranges. We measured the excitation, emission, and absorption spectra as well as the kinetic parameters over a broad temperature range. The strong influence of the silver halide composition on Pr(3+) ions spectroscopic properties was revealed. We calculated useful optical parameters for Pr:AgBr crystals by using the Judd-Ofelt approximation. Good agreement between the theoretical and the measured parameters was obtained.     

The optical properties of CuInS2 thin films

The optical absorption in flash-evaporated CuInS₂ thin films was studied in the photon energy range from 0.5 to about 4.2 eV. CuInS₂ was found to be a direct gap semiconductor with a gap energy of 1.524±0.005 eV at room temperature. The ground state energy of the free exciton was found to be about 8 meV. An indirect allowed transition was observed at 1.565±0.005 eV and was ascribed to an optical transition from the valence band maxima at the boundary of the Brillouin zone to the lowest conduction band minimum at the zone centre. Three further optical transitions which were probably due to the copper d states in the valence band were found at energies well above the fundamental edge.     

Chemical vapor growth and properties of CuA¢lS2

CuA¢lS₂ single crystals up to 1 cm in length have been grown by iodine transport by vertically pulling the pre-reacted elements through a temperature gradient, and various optical and electrical properties studied. We find CuA¢lS₂ can be a highly transparent p-type semiconductor with a bandgap near 3.53 eV at 77°K.     

Cathodoluminescence study of h– and c–GaN single crystals grown by a Na or K flux

Luminescence properties of hexagonal (h-) and cubic (c-) GaN freestanding single crystals were studied by means of cathodoluminescence spectroscopy. The h-GaN crystals of about 0.2–2 mm in dimension were synthesized at 750 °C by the reaction of Ga and N2 in a Na flux, while c-GaN crystals of about 0.3 mm or less in a K flux. The h-GaN showed rather strong band edge emission at room temperature compared with the crystal grown by using NaN3 as a nitrogen source. At 20 K, the band edge emission of h-GaN was split into four peaks. The main energy peak position was 3.478 eV, which was assigned as the A-free exciton emission. The energy position of the main peak of c-GaN was 3.268 eV. Assuming the binding energies of excitons in h- and c-GaN as 25 and 26 meV, respectively, the energy difference of bandgap between h-and c-GaN is estimated to be 209 meV. Since these crystals are free from strain from the substrates, the peak energies are reliable for the intrinsic GaN crystals. The full widths at half maximum of the emission of c-GaN were much broader than those of h-GaN, suggesting that the cubic phase is much defective compared with the hexagonal one.     

Synthesis, structural and optical characterization of ZnO crystals grown in the presence of silver

Zinc oxide microcrystals grown at atmospheric pressure by the laser assisted flow deposition method were characterized by morphological, structural and optical techniques. A set of samples with 0, 0.8, 2.0, 3.0 and 5.0 mol% nominal AgNO₃ content were studied in order to analyse the influence of silver amount on the near band edge recombination. A detailed study of the samples' photoluminescence evidence that at room temperature the free exciton recombination occurs at ~ 3.334 eV for crystals grown in the presence of silver instead of the broad peak observed at ~ 3.28 eV for the undoped ones.     

Enhanced optoelectronic,thermal, mechanical and third order nonlinear optical properties of dichlorobis(thiourea)zinc(II) crystal: an effect of Phenol red dye

For the first time the growth of dichlorobis (thiourea) zinc(II) (ZTC) monocrystal in presence of Phenol red dye of considerably good size (~13 mm?×?5 mm) has been achieved from aqueous solution using slow evaporation solution technique at room temperature. The solubility was calculated at different temperatures. The crystal structure was confirmed through X-ray diffraction analysis. The crystallinity of the dyed crystals has been enhanced revealed by Powder X-ray diffraction study. The presence of dye was inveterate by FT-IR and FT-Raman spectroscopic studies. The scanning electron microscopy analyses show that the grown crystals with dye are better than pure. Diffused reflectance was measured and optical band gap was calculated found to be enhanced from 4.5 to 4.65 eV for dyed crystals. The enhancement in the PL intensity has been observed in the dyed crystals. DSC study shows that the thermal stability has been remarkably enhanced from 163?°C (pure) to 203?°C (dyed) and various other thermal parameters are calculated. The surface study shows that the grown crystals with dye possess less dislocation than pure. Mechanical strength of the dyed crystal is found to be remarkably enhanced. Third-order nonlinear optical susceptibility (χ3) of PRZTC was found to be 2 times higher than pure. All results suggest that the properties of ZTC crystals grown in presence of dye are enhanced and can be considered as better contender in various nonlinear devices.     

Preparation of hierarchical CdS structures and effect of sodium dodecyl benzene sulfonate (SDBS) on the morphologies

Dendritic-like CdS and flower-like CdS structures have been prepared by hydrothermal method. The as-prepared CdS have been analyzed by X-ray diffraction, field emission scanning electron microscopy (FESEM), ultraviolet–visible and room temperature photoluminescence (PL). And the photocatalytic activities also have been investigated. FESEM results indicate the role of SDBS is to make the CdS crystals assemble together. The optical energy band gap of dendritic-like CdS is 2.46 eV, and flower-like CdS is 2.48 eV. PL results show that dendritic-like CdS and flower-like CdS emit both blue and green fluorescence. Photocatalysis results show that the catalytic efficiency of dendritic-like CdS is better than flower-like CdS.     

Influence of Fe dopant concentration and annealing temperature on the structural and optical properties of ZnO thin films deposited by sol–gel method

Nanostructured Fe doped ZnO thin films were deposited onto glass substrates by sol–gel spin coating method. Influence of Fe doping concentration and annealing temperature on the structural, compositional, morphological and optical properties were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–Vis spectroscopy and photoluminescence (PL) measurements. XRD analysis showed that all the films prepared in this work possessed a hexagonal wurtzite structure and were preferentially oriented along the c-axis. Pure ZnO thin films possessed extensive strain, whereas Fe doped films possessed compressive strain. In the doped films, least value of stress and strain was observed in the 0.5 at.% Fe doped thin film, annealed at 873 K. Average crystallite size was not significantly affected by Fe doping, but it increased from 15.57 to 17.79 nm with increase in annealing temperature from 673 to 873 K. Fe ions are present in +3 oxidation state as revealed by XPS analysis of the 0.5 at.% Fe doped film. Surface morphology is greatly affected by changes in Fe doping concentration and annealing temperature which is evident in the SEM images. The increase in optical band gap from 3.21 to 3.25 eV, with increase in dopant concentration was attributed to Moss–Burstein shift. But increase in annealing temperature from 673 to 873 K caused a decrease in band gap from 3.22 to 3.20 eV. PL spectra showed emissions due to excitonic combinations in the UV region and defect related emissions in the visible region in all the investigated films.     

Photovoltaic properties and photoconductivity in multilayer Ge/Si heterostructures with Ge nanoislands

Interband optical transitions in multilayer heterostructures with SiGe nanoislands were investigated using photocurrent spectroscopy and photo-emf. The n-p heterostructures containing Ge nanoislands in the area of the potential barrier were prepared by molecular-beam epitaxy at the temperature about 500 °C. It was shown that electron transitions from the ground state of the valence band in a nanoislands to the conduction band of Si surrounding made the main contribution into the vertical photo-emf in the range 0.75–1.05 eV, which is below the interband absorption edge of Si. The lateral photoconductivity observed in the range 0.63–0.8 eV at 77 K can be attributed to indirect interband transitions from the ground state of a nanoisland to L-state of the conduction band of a nanoisland. Analysis of Raman scattering spectra revealed that the Ge composition x in a nanoisland is about 0.87, while elastic deformation value amounts to ε _xx  = −0.016. The calculated energies of interband transitions from the ground state of a nanoisland to the conduction band of Si surrounding (0.63 eV) and to L-state of the conduction band of a nanoisland (0.81 eV) fit the experimental data with a rather good accuracy.     

Optical and electrical properties of ZrSe3 single crystals grown by chemical vapour transport technique

Single crystals of the lamellar compound, ZrSe₃, were grown by chemical vapour transport technique using iodine as a transporting agent. The grown crystals were characterized with the help of energy dispersive analysis by X-ray (EDAX), which gave confirmation about the stoichiometry. The optical band gap measurement of as grown crystals was carried out with the help of optical absorption spectra in the range 700–1450 nm. The indirect as well as direct band gap of ZrSe₃ were found to be 1.1 eV and 1.47 eV, respectively. The resistivity of the as grown crystals was measured using van der Pauw method. The Hall parameters of the grown crystals were determined at room temperature from Hall effect measurements. Electrical resistivity measurements were performed on this crystal in the temperature range 303–423 K. The crystals were found to exhibit semiconducting nature in this range. The activation energy and anisotropy measurements were carried out for this crystal. Pressure dependence of electrical resistance was studied using Bridgman opposed anvils set up up to 8 GPa. The semiconducting nature of ZrSe₃ single crystal was inferred from the graph of resistance vs pressure. The results obtained are discussed in detail.     

Complex oxide structures formed by oxidation of Ag(100) and Ag(111) by hyperthermal atomic oxygen

Abstract

Silver (100) and (111) single crystals were exposed to a unique hyperthermal atomic oxygen source, which produces a high flux of 5.1eV atomic oxygen, for seven hours at 220°C. The resultant oxide and oxide–metal interfaces were characterized by optical, scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM). The oxide scale was more than 10 µm thick and very weakly attached to the Ag substrate. The silver oxides were complex and surprising, differ in their thickness and the oxide phases due to the orientation of the Ag single crystals. The cross-section TEM studies revealed complex microstructures with many defects, such as micro-twins, porosity and irregular shaped grains.     

Crystal growth and characterization of Pt0.97S2

Large, well-formed plates of Pt_0.97 S₂ were grown by chemical vapor transport. A combination of phosphorus and chlorine was used as the transport agent. The best crystals were grown when the charge zone was maintained at 800°C and the growth zone at 740°C. The crystals were found to be diamagnetic with a susceptibility of ?31(2) × 10^?6 emu/mole at 77°K and showed semiconducting behavior with a band gap of 0.20(2)eV. From Hall voltage measurements, the sign of the carriers was ascertained to be positive. Pt_0.97S₂ particles less than 63μ underwent thermal decomposition in air at 270(10)°C.     

Effects of annealing temperature on the optical, bonding, structural and electrical properties of nitrogenated amorphous carbon thin films grown by surface wave microwave plasma chemical vapor deposition

We have studied the effects of annealing temperature (AT) on the properties of nitrogenated amorphous carbon (a-C:N) films grown at room temperature (RT) on quartz substrates by surface wave microwave plasma chemical vapor deposition (SWMP-CVD) using camphor alcohol gas as carbon plasma sources. The thickness, optical, bonding, structural and electrical properties of the as-grown (RT) and anneal-treated in range from 100 to 500°C of a-C:N films were measured and compared. The film thickness is decreased rapidly with increasing AT above 350°C. The wide range of optical absorption characteristics is observed depending on the AT. The optical band gap of as-grown a-C:N films is approximately 2.8 eV, gradually decreased to 2.5 eV for the films anneal-treated at 300°C and beyond that it decreased rapidly up to 0.9 eV at 500°C . Visible-Raman Spectroscopy (Raman) revealed the amorphous structure of as-grown a-C:N films and, the growth of nanocrystallinity of a-C:N films upon increase of AT. Raman and Fourier transform infrared spectroscopy (FTIR) analyses respectively shown the structural and composition of the films can be tuned by optimizing the AT. The change of optical, bonding, structural and electrical properties of SWMP-CVD grown a-C:N films with higher AT was attributed due to the fundamental changes in the bonding and band structure of the a-C:N films.     

Optical properties of nanostructured ruthenium dioxide thin films via sol–gel approach

High purity ruthenium dioxide (RuO₂) nanoparticles with the average size is about 9 nm in diameter are readily synthesized through a low cost sol–gel method. RuO₂ thin films have been deposited on SiO₂ substrates by sol–gel spin coating techniques at room temperature, followed by annealing at 500 °C for 2 h. The result of X-ray diffraction indicates that the RuO₂ nanoparticles are well crystallized with a rutile tetragonal structure. Morphological of RuO₂ films were characterized using atomic force microscopy (AFM), transmission electron microscopy and high resolution transmission electron microscopy. The AFM images confirmed a spherical-shape nanoparticles with diameter of 9 nm and surface roughness of 12 nm of the films. The optical absorption studies showed the presence of direct band transition with band gap equal to 1.87 eV. Refractive index and dielectric properties of the films were estimated from optical measurements. Room temperature photoluminescence of RuO₂ film showed an emission band at 432 nm.     

Space-charge-limited currents and carrier trapping in plasma-polymerized malononitrile films

The dark electrical conductivity of plasma-polymerized malononitrile (PPMa) films in an Ag/PPMa/Ag sandwich structure was investigated over the temperature range 300–525 K at a reduced pressure of 10^-5 Torr. The room temperature current-voltage characteristics indicate space-charge-limited currents. The results for the conductivity as a function of inverse temperature are in accordance with the band model proposed by Barbe and Westgate, and the thermal energy gap between the top of the valence band and the bottom of the conduction band was determined to be 0.86±0.01 eV. At temperatures below about 380±5 K the conduction process is consistent with the presence of an electron trapping level situated 0.34±0.05 eV below the conduction band edge with a density of (5.02±0.05) × 10¹⁶ cm^-3. It is assumed that above about 435±5 K the conduction process is intrinsic. Experiments on the chemisorption of oxygen suggest that electrons are the majority carriers in PPMa films.     

Influence of pyrolytic temperature on the properties of ZnO films optimized for H2 sensing application

In this work, we report the influence of pyrolytic temperature on the properties of ZnO films deposited by a novel spray pyrolysis deposition route. XRD results revealed an improvement in crystal quality of the films with increase in growth temperature. The optical measurements of the films show a maximum transmittance of ~85 % and the band gap of ~3.5 eV. Photoluminescence spectra revealed that the UV emission peaks at 385 nm is improved with increase in growth temperature upto 300 °C, which corresponds to the increase of optical quality and decrease of Zn interstitial defect in the films. Gold ohmic contacts were evaporated on the optimized ZnO film prepared at the substrate temperature of 300 °C, and response of the film to different concentrations of hydrogen (150–500 ppm) at room temperature was investigated. The ZnO sensor showed significant sensitivity to hydrogen for concentration as low as 150 ppm at room temperature, and the sensor response was observed to increase with increase in hydrogen concentration. The increased sensitivity of the film was attributed to the large roughness of the film revealed from AFM analysis. The results ensure the application of our novel sensor, to detect H₂ at low concentration and at room temperature.     

Optical properties of nanocrystalline ZnS:Mn thin films prepared by chemical bath deposition method

Nanocrystalline ZnS:Mn thin films were fabricated by a chemical bath deposition route on glass, silicon, and quartz substrates using a weak acidic bath, in which citrate ions acts as a nontoxic complexing agent for zinc ions and thioacetamide acts as a source of sulfide ions at 60 °C. The composition of films were characterized by energy-dispersive X-ray spectrometer, inductively coupled plasma atomic emission spectroscopy, Rutherford backscattering, and attenuated total reflection-Fourier transform infrared spectroscopy. X-ray diffraction pattern and transmission electron microscopy image confirm that the films have nanocrystalline nature. The band gap energy of ZnS:Mn films is blue-shifted by about 0.3 eV with respect to the bulk value (3.67 eV), probably due to the quantum size effect as expected from the nanocrystalline nature of the ZnS:Mn thin films. The dispersion and optical constants of the films were determined. These parameters changed with the deposition time.