Time-resolved Rise Research Articles

Effect of Annealing Temperature on Structural, Photoluminescence and Photoconductivity Properties of ZnO Thin Film Deposited on Glass Substrate by Sol–Gel Spin Coating Method

In this work, zinc oxide (ZnO) thin films have been deposited on glass substrate using a simple and inexpensive multi-step sol–gel spin coating method at annealing temperatures of 300, 400, 500 and 600 °C in an open atmosphere. The influence of annealing on structural, photoluminescence and photoconductivity properties of ZnO thin films has been systematically investigated. X-ray diffraction pattern reveals that all ZnO thin films are polycrystalline with hexagonal wurtzite structure. Scanning electron micrograph depicted the formation of ZnO nanofibrous structure. Photoluminescence properties of ZnO thin films have been investigated by photoluminescence spectroscopy at room temperature. Photoconductivity properties have been investigated in terms of several parameters such as voltage dependence of photocurrent and dark current as well as time-resolved rise and decay of photocurrent. The rise and decay spectra under periodic illumination show reproducible and stable photoresponse. The synthesized ZnO thin film seems to be having potential use in UV–Vis photodetectors.

Characterization of the Mercury pulsed power x-ray source spectrum using multichannel density aerogel Cherenkov detectors.

The Aerogel Cherenkov Detector for Cygnus (ACD/C) is a time-dependent, x-ray spectral detector that uses SiO2 aerogels spanning an index of refraction (n = 1.02-1.07) corresponding to a 1.1-2.3 MeV x-ray energy threshold. The ACD/C was developed for pulsed power x-ray sources like Cygnus located at the Nevada National Site and Mercury located at the Naval Research Laboratory (NRL). Aerogels sit between the measurement capabilities of gas (>2 MeV) and solids such as fused silica (>0.3 MeV). The detector uses an aluminum converter to Compton scatter incoming x-rays and create relativistic electrons, which produce Cherenkov light in an aerogel or a fused silica medium. The ACD/C was fielded at the NRL when Mercury was tuned to produce up to 4.8 MeV endpoint bremsstrahlung. Despite a high radiation and electromagnetic interference background, the ACD/C was able to achieve high signal over noise across five aerogel densities and fused silica, including a signal to noise for a 1.1 MeV aerogel threshold. Previous experiments at Cygnus observed a signal that was comparable to the noise (1×) at the same threshold. The ACD/C observed time-resolved rise and fall times for different energy thresholds of the photon spectrum. Monte Carlo simulations of the ACD/C's aerogel response curves were folded with a simulation of Mercury's photon energy spectrum and agree within the error to the observed result.

ZnO nanoparticles: Structural, optical and photoconductivity characteristics

Zinc acetate dihydrate, Zn(CH3COO)2·2H2O has been used as a precursor to prepare zinc oxide nanoparticles (NPs) by thermal decomposition in air at 400°C. Two samples with reaction time of 3 and 12h are studied. Structural study of ZnO NPs has been analyzed by X-ray diffraction (XRD). The synthesized ZnO NPs are found to possess wurtzite structure. The average crystallite size determined by XRD is found as 29.66 and 32.68nm corresponding to reaction time of 3 and 12h, respectively. Optical properties of the ZnO NPs have been investigated by UV–visible spectroscopy and photoluminescence (PL) spectroscopy at room temperature. Blue shift in band gap is observed due to nano size. PL spectra exhibit a UV emission peak at ∼396nm (∼3.11eV), and broad visible emission peaks including violet emission at ∼418nm (∼2.96eV), blue emission at ∼441nm (∼2.79eV), blue–green emission ∼481nm (∼2.56eV) and green emission ∼522nm (∼2.37eV). Photoconductivity properties have been investigated with several parameters such as voltage dependence of photocurrent and darkcurrent, time-resolved rise and decay of photocurrent and spectral response of photocurrent. Longer reaction time of 12h manifests in improved photosensitivity as compared to that for sample with reaction time of 3h.

Photoluminescence and ultraviolet photoresponse in ZnO nanophorsphors prepared by thermal decomposition of zinc acetate

In the present work, simple, low-cost, and direct route is used for the UV- photodetection and photoluminescent zinc-oxide nanoparticles (NPs) by decomposing zinc acetate in air at 400 0 C for 12 hrs. The X-ray diffraction (XRD) result indicates that the synthesized ZnO NPs is pure and single crystalline structure with wurtzite type. The crystallite size of the ZnO nanoparticles is in the range of 20-50 nm and average crystallite size of synthesized nanoparticles is found to be ~33 nm. The synthesized ZnO NPs exhibits several photoluminescence peaks centered at 396 nm, 418 nm, 441 nm, 481 nm and 522 nm. The time-resolved rise and decay of photocurrent spectrum shows initial significant increase in photocurrent and, subsequently falls gradually under UV-illumination. The photocurrent abruptly falls when illumination is turned off. The variation of photo and dark-current with applied field is found to follow power law i.e I α V. At low voltage the behavior is sub-linear which becomes super-linear at high voltages. The ZnO NPs is found to have double traps of 0.59 eV and 0.67 eV. Copyright © 2011 VBRI press.

Structural, photoconductivity and photoluminescence characterization of cadmium sulfide quantum dots prepared by a co-precipitation method

In this paper, cadmium sulfide (CdS) quantum dots (QDs) are synthesized by a simple co-precipitation method. X-ray diffraction (XRD) confirmed the formation of a cubical zinc blend structure of CdS nanoparticles. Transmission Electron Microscopy (TEM) images revealed that the CdS QDs are of 2–5 nm in size. The UV-vis absorption spectra showed an absorption peak at 427 nm (∼2.90 eV) indicating a blue shift of 0.48 eV as compared to bulk CdS. We estimated the particle sizes with the help of X-ray diffraction (XRD) patterns (3.665 nm) and the shift of the band gap absorption in the UV-vis spectrum (4.276 nm), which is very close to the TEM micrograph result. The photoluminescence spectrum shows three major emission peaks centered at 453 nm (∼2.73 eV), 526 nm (∼2.35 eV) and 551 nm (∼2.24 eV) at room temperature, which may be attributed to excitonic transitions, donor-acceptor (D-A) pairs recombination and the sulphur interstitial defects (Is) present in the band gap. To study the photoconductivity, the field dependence of the photocurrent and the dark-current was assessed, as was the time-resolved rise and decay photocurrent spectrum and wavelength-dependence photocurrent spectrum assessment of the CdS QDs. The time-resolved rise and decay photocurrent spectra exhibited negative photoconductivity (NPC) behavior when the CdS QDs were illuminated with 490 nm light. Such anomalous NPC may be attributed to the light-induced desorption of water molecules. The wavelength-dependence of the photocurrent was found to be close to the absorption and PL spectrum. The photoconductivity properties of the CdS QDs were measured using a thick film of powder without any binder. These CdS QDs can find potential application in optoelectronic devices and photodetectors.

Photoluminescence and photoconductivity of ZnS:Mn 2+ nanoparticles synthesized via co-precipitation method

Mn 2+ doped ZnS nanoparticles are characterized using UV–vis, photoluminescence and photoconductivity studies. The size of Mn 2+ doped ZnS NPs is estimated to be 2–4 nm by X-ray diffraction. UV–vis spectra show a blue shift in absorption edge as compared to bulk counterpart. Photoluminescence spectra indicate that orange luminescence varies with Mn 2+ concentration. The Mn 2+ doped ZnS nanoparticles are found to be photosensitive. The doping of Mn 2+ ions improves the photosensitivity of the ZnS nanoparticles system. The time-resolved rise and decay of photocurrent indicate anomalous behavior during steady state illumination.

Time-resolved rise of iodine molecule (1-) upon oxidation of iodide at aqueous titania colloid

The rise of the one-electron oxidation intermediate I[sub 2] formed upon band-gap excitation of bare TiO[sub 2] colloidal particles in aqueous iodide solution has been resolved for the first time. These measurements are based on a very sensitive transient absorption technique that allows monitoring of I[sub 2][sup [minus]] at iodide concentrations as low as 10[sup [minus]4] M. The linear dependence of the pseudo-first-order rise constant on iodide concentration (slope = (7.9 [+-] 1.2) [times] 10[sup 9] L mol[sup [minus]1] s[sup [minus]1]) suggests that the observed step reflects the reaction of surface-adsorbed I atoms with iodide from the surrounding solution phase. The adsorbed I atoms are formed by the reaction of surface-adsorbed iodide with valence band holes within the 5-ns duration of the 355 band-gap excitation pulse. Analysis of the decay kinetics of I[sub 2][sup [minus]] and measurement of the concurrent I[sub 3][sup [minus]] yield revealed that interparticle reduction by trapped conduction band electrons is the main decay path of I[sub 2][sup [minus]] These results are compared with a previous time-resolved study of iodide photooxidation at dye-sensitized TiO[sub 2] colloids. 44 refs., 6 figs.