Fast Decay Time Research Articles

Analyzing dwell times with the Generalized Method of Moments.

The Generalized Method of Moments (GMM) is a statistical method for the analysis of samples from random processes. First developed for the analysis of econometric data, the method is here formulated to extract hidden kinetic parameters from measurements of single molecule dwell times. Our method is based on the analysis of cumulants of the measured dwell times. We develop a general form of an objective function whose minimization can return estimates of decay parameters for any number of intermediates directly from the data. We test the performance of our technique using both simulated and experimental data. We also compare the performance of our method to nonlinear least-squares minimization (NL-LSQM), a commonly-used technique for analysis of single molecule dwell times. Our findings indicate that the GMM performs comparably to NL-LSQM over most of the parameter range we explore. It offers some benefits compared with NL-LSQM in that it does not require binning, exhibits slightly lower bias and variance with small sample sizes (N<20), and is somewhat superior in identifying fast decay times with these same low count data sets. Additionally, a comparison with the Classical Method of Moments (CMM) shows that the CMM can fail in many cases, whereas the GMM always returns estimates. Our results show that the GMM can be a useful tool and complements standard approaches to analysis of single molecule dwell times.

Theoretical study of the ultrashort dynamics in graphene after fs-laser excitation

We report on the calculation of the full dynamics of graphene after ultrafast laser excitation using ab-initio molecular dynamics simulations. Our calculations show a biexponential decay of the (100) and (110) Bragg peaks in time, similar to experimental results. Furthermore, we calculate the time-evolution of the phonon energies averaged over sets of phonons, chosen by their phonon frequencies. Our results suggest that the fast decay time can be accounted by phonon-phonon interactions rather than the generation of SCOPs. The slower decay time is governed by the equilibration of the system with low frequency phonon modes.

Localization and transient emission properties in InGaN/GaN quantum wells of different polarities within core-shell nanorods.

Transient photoluminescence (PL) characteristics and localization phenomena in InGaN/GaN core-shell nanorods (NRs) were investigated from 6 K up to 285 K. The NRs exhibit three well-defined PL bands in the near-UV, blue, and green range ascribed to the emission of quantum well (QW) areas situated at the (1.00) sidewalls, (10.1) top facets, and (00.1) tip, respectively. At low temperature, time-resolved PL shows a fast decay time of about 0.5 ns for the semi- and non-polar QWs, while the polar QWs exhibit at least a twice-longer time. Rapid delocalization of carriers above 50 K indicates shallow potential fluctuations in the QWs. At room temperature, the characteristic fast PL decay time of the three QW bands stabilizes around 300 ps. The slow decaying PL components have different characteristic decay times that are explained by additional localization at basal stacking faults (BSFs), taking into account the quantum confined Stark effect. In addition, narrow excitonic luminescence lines are observed in the BSF-enriched polar QWs, providing direct evidence of the impact of the BSF/QW crossings on the optical properties of the NRs. A PL rise time of about 100 ps does not show any deviation between bands. These findings are suggestive of similar transport mechanisms in temperature equilibrium without inter-facet transport between different QWs. We believe that predictable transient characteristics can play a key role in creating uniform NR ensembles for device applications.

Surface functionalization-induced photoresponse characteristics of monolayer MoS2 for fast flexible photodetectors.

Monolayered, semiconducting molybdenum disulfide (MoS2) is of considerable interest for its potential applications in next-generation flexible, wearable, and transparent photodetectors because it has outstanding physical properties coupled with unique atomically thin dimensions. However, there is still a lack of understanding in terms of the underlying mechanisms responsible for the photoresponse dynamics, which makes it difficult to identify the appropriate device design strategy for achieving a fast photoresponse time in MoS2 photodetectors. In this study, we investigate the importance of surface functionalization on controlling the charge carrier densities in a MoS2 monolayer and in turn the corresponding behavior of the photoresponse in relation to the position of the Fermi-level and the energy band structure. We find that the p-doping and n-doping, which is achieved through the surface functionalization of the MoS2 monolayer, leads to devices with different photoresponse behavior. Specifically, the MoS2 devices with surface functional groups contributing to p-doping exhibited a faster response time as well as higher sensitivity compared to that observed for the MoS2 devices with surface functional groups contributing to n-doping. We attribute this difference to the degree of bending in the energy bands at the metal-semiconductor junction as a result of shifting in the Fermi-level position, which influences the optoelectronic transport properties as well as the recombination dynamics leading to a low dark and thus high detectivity and fast decay time. Based upon these findings, we have also demonstrated the broad applicability of surface functionalization by fabricating a flexible MoS2 photodetector that shows an outstanding decay time of 0.7 s, which is the fastest response time observed in flexible MoS2 detectors ever reported.

Synthesis of ternary oxide Zn2GeO4 nanowire networks and their deep ultraviolet detection properties.

Ternary oxide Zn2GeO4 with a wide bandgap of 4.84 eV, as a candidate for fourth generation semiconductors, has attracted a great deal of attention for deep ultraviolet (DUV) photodetector applications, because it is expected to be blind to the UV-A/B band (290–400 nm) and only responsive to the UV-C band (200–290 nm). Here, we report on the synthesis of Zn2GeO4 nanowire (NW) networks by lower pressure chemical vapor deposition and investigate their corresponding DUV detection properties. We find that pure Zn2GeO4 NWs could be obtained at a growth pressure of 1 kPa. The DUV detection tests reveal that growth pressure exerts a significant effect on DUV detection performance. The Zn2GeO4 NW networks produced under 1 kPa show an excellent solar-blind photoresponsivity with fast rise and decay times (trise ≈ 0.17 s and tdecay ≈ 0.14 s).

The Electromagnetic Calorimeter for the PANDA Target Spectrometer

The future experiment with a next generation detector will focus on hadron spectroscopy. It will use cooled anti-proton beams with a momentum between 1.5 GeV/c and 15 GeV/c interacting with various targets. This allows to directly form all states of all quantum numbers and measure their widths with an accuracy of a few tens of keV. The experiment will be located at the exceptional Facility for Anti-proton and Ion Research in Germany (FAIR), which is currently under construction. The electromagnetic target calorimeter of the experiment has the challenging aim to detect high energy photons with excellent energy resolution over the full dynamic range from 15 GeV down to a few tens of MeV within a 2 T solenoid. The target calorimeter itself is divided into a barrel and two endcaps. The individual crystals will be read out with two precisely matched large area avalanche photo diodes. In the very inner part of the forward endcap vacuum phototetrodes will be used instead. To reach the demands of the experiment, improved PbWO4 (PWO) scintillator crystals, cooled down to −25°C have been chosen. They provide a fast decay time for highest count rates, short radiation length for compactness, improved light yield for lowest thresholds and excellent radiation hardness [1]. The main part of the 15,740 crystals needed have been produced by the Bogoroditsk Plant of Technochemical Products (BTCP) in Russia. After stopping their business, a new potential producer for the missing 41% of crystals have been found. The company Crytur in Czech Republic provided 150 promising preproduction crystals so far. Except some of the very first produced crystals, all samples exceed the required high quality parameters. Most of them have already been used for the first major assembly stage of assembling one of the 16 barrel slice segments, which will be presented as well.

InGaN/GaN multiple quantum well for superfast scintillation application: Photoluminescence measurements of the picosecond rise time and excitation density effect

We report the study of the fast rise time and decay time in the ps time scale of the excitonic luminescence of a multiple quantum well (MQW) heterostructure of InGaN/GaN, including the excitation density effect. These structures were proposed as ultrafast scintillators for soft X-ray detectors and particle beam diagnostics. Measurements were carried out with a Hadland Imacon 500 streak camera following excitation of the sample by laser pulses of few tens of µJ at 266 nm and 400 nm, with pulse duration less than 200 fs. The rise time of the detected MWQ luminescence was less than 10 ps, with a possible contribution from the detection system and signal collection geometry. The calibration and the signal processing techniques employed to fully exploit the time resolution of the detection system are also described.

Novel highly efficient blue-emitting branched oligoarylsilanes

In this work synthesis and study of novel nanostructured organosilicon luminophores (NOLs) based on p-terphenyl and POPOP (1,4-bis(5-phenyloxazol-2-yl) benzene) moieties with unique optical properties is reported. NOLs are branched molecules composed from a donor-acceptor-donor framework with different optical bandgap luminescence moieties connected via silicon atoms. As a result effective intramolecular Förster resonance energy transfer can be observed. The main advantages of such materials in comparison with conventional organic luminophores are high molar absorption coefficient, excellent photoluminescence quantum yield, fast luminescence decay time, good processability and low toxicity. Using NOLs in plastic scintillators, widely utilized for radiation detection and in elementary particles discoveries, led to significant improving their efficiency, which combines both high light output and fast decay time. In the best samples the scintillation light yield was 20% higher in comparison with the standard commercially available plastic scintillator UPS-89.

Self-powered UV–visible photodetectors based on ZnO/graphene/CdS/electrolyte heterojunctions

A novel ZnO nanowire arrays (NAs)/graphene(G)/CdS/electrolyte heterojunction was successfully prepared via a simple three-step synthesis method when applied for photoelectrochemical type self-powered UV–visible photodetectors. Under zero bias, the responsivities of the self-powered UV–visible photodetectors on the basis of ZnO NAs/G/CdS/electrolyte heterojunctions are 27.3 mA/W and 4.3 mA/W for the UV and visible light, respectively, with fast rise and decay times (5 ms). Graphene improves the response of ZnO NAs to UV and visible light to a certain extent owing to its excellent charge-collecting ability and the acceleration of carrier separation and transference. After covering photosensitive CdS nanoparticles, the response to visible light is greatly increased because of the type II energy band structure of ZnO/CdS. It could be inferred from the evidence that the ZnO NAs/G/CdS/electrolyte heterojunction performs effectively in the aspect of self-powered UV–visible detection.

Plasma-assisted synthesis of ZnSe hollow microspheres with strong red emission

We demonstrate the growth of ZnSe microspheres in large quantities by using a mixture of Zn and Se as precursors via direct current arc discharge method without the addition of any catalyst or template. The as-obtained samples were characterized by X-ray diffraction, Raman, energy-dispersive X-ray spectrometry, scanning and transmission electron microscopy, and ultraviolet-visible reflectance spectroscopy. The majority of the ZnSe microspheres which were composed of ZnSe nanoparticles exhibited perfectly spherical morphology and a distinct hollow structure with a relatively rough surface. The growth mechanism of ZnSe hollow microspheres has been discussed. Room-temperature excitation and emission spectra as well as the fluorescence lifetime of ZnSe hollow microspheres were measured and the results showed ZnSe hollow microspheres exhibit an intensive wide broad emission bands from 550 to 750 nm upon 470 nm excitation with fast decay time 1.09 ms and CIE chromaticity coordinates (0.65, 0.34), which was ascribed to characteristic structure defects (vacancies and interstitials) in ZnSe. Additionally, the temperature dependent emission behavior was measured and discussed.

Temperature evolution of the photoexcited charge carriers dynamics in Ge/Si quantum dots

The kinetics of the photoinduced mid-infrared intraband hole absorption after the short interband excitation pulse has been studied in an dense array of surfactant-mediated Ge/Si(001) quantum dots. Absorption decay curves reveal two processes characterized by fast and slow decay times at low temperatures. A specific temperature evolution of the absorption transients was observed with the increase in temperature. A model of charge carrier kinetics has been developed based on the rate equation analysis. Measured transient characteristics of charge carriers are well described within this model. The energetic gap between two conduction band electron states within the quantum dots and the valence band energetic barrier at the quantum dot interface have been evaluated.

Nonpolar-Oriented Wurtzite InP Nanowires with Electron Mobility Approaching the Theoretical Limit.

As an important semiconductor nanomaterial, InP nanowires (NWs) grown with a typical vapor-liquid-solid mechanism are still restricted from their low electron mobility for practical applications. Here, nonpolar-oriented defect-free wurtzite InP NWs with electron mobility of as high as 2000 cm2 V-1 s-1 can be successfully synthesized via Pd-catalyzed vapor-solid-solid growth. Specifically, PdIn catalyst particles are involved and found to expose their PdIn{210} planes at the InP nucleation frontier due to their minimal lattice mismatch with nonpolar InP{2̅110} and {1̅100} planes. This appropriate lattice registration would then minimize the overall free energy and enable the highly crystalline InP NW growth epitaxially along the nonpolar directions. Because of the minimized crystal defects, the record-high electron mobility of InP NWs ( i.e., 2000 cm2 V-1 s-1 at an electron concentration of 1017 cm-3) results, being close to the theoretical limit of their bulk counterparts. Furthermore, once the top-gated device geometry is employed, the device subthreshold slopes can be impressively reduced down to 91 mV dec-1 at room temperature. In addition, these NWs exhibit a high photoresponsivity of 104 A W-1 with fast rise and decay times of 0.89 and 0.82 s, respectively, in photodetection. All these results evidently demonstrate the promise of nonpolar-oriented InP NWs for next-generation electronics and optoelectronics.

Scintillation Properties of Ce3+ Doped Silicon-Magnesium-Aluminum-Lithium Glass Scintillators by using Radiation Sources

Glass scintillators can be doped with different elements to improve their luminescence properties. In this study, we present 0.5 wt% Ce3+-doped (58−x)SiO2-4MgO-18Al2O3-20Li2O and (58−x)SiO2- 4MgO-18Al2O3-20LiF glass scintillators. We used a pulsed laser to measure the laser-induced emission spectrum and the decay time with decreasing temperature from 300 K to 10 K. The light intensity of both glass samples increased with decreasing temperature from 300 K to 10 K. Additionally, the two glass samples exhibited a fast decay time of approximately 25 ns. In the photo-induced spectrum, an excitation peak at 312 nm and two emission peaks, at 370 nm and 700 nm, were observed in both the glass samples. The X-ray-induced emission intensity of former is approximately six times higher than that of latter, and only former glass sample exhibits a proton-induced emission spectrum.

YAG:Ce-Phosphor Scintillators for Optical Fiber Radiation Sensors With High Temporal Resolution

A miniature optical fiber real-time X-ray radiation sensor based on plastic optical fiber coupled with a fast decay time trivalent cerium-activated yttrium aluminum garnet Y3Al5O12 (YAG:Ce) scintillator material is reported. The YAG material was tested in powder and ceramic form. Three separate sensors were irradiated three times on a Siemens Artiste linear accelerator (linac). The measured light output of the YAG:Ce ceramic to the detector at the distal end of the fiber is 4.52 times greater than the YAG:Ce powder, and its output intensity detected by photon counter is further increased 2.52 times using a two-layer highly reflective Teflon tape. The sensor made with YAG:Ce powder shows excellent repeatability, being a maximum of 0.36 %, but the value using YAG:Ce ceramic exhibits orientation sensitivity when located in orthogonal positions relative to the X-ray beam. To the best of our knowledge, the work described in this letter is the first instance of the direct high temporal resolution detection of the individual (circa 3.9 ms) output of X-ray light pulses of clinical linacs. The experimental results of pulsewidths are in good agreement with the manufacturer’s stated value of $3.45~\pm ~0.21~\mu \text{s}$ .

Studies on NiO-PVA Composite Films for Opto-Electronics and Optical Limiters

We report optical, excited-state lifetime, and nonlinear optical (NLO) properties of nickel oxide (NiO)/poly vinylalcohol (PVA) polymer free-standing films for opto-electronics and optical limiting applications. Optical absorption studies of NiO-loaded PVA films show that the exciton peaks are centered at 330 nm. Fluorescence studies reveal that the emission is due to exciton recombination and defects in NiO. Time-resolved fluorescence studies show that the recombination in PVA happens through NiO, where NiO has a faster decay time than the pure PVA. NLO measurements carried out using a 532 nm, 10-ns Nd: YAG laser, show that for a given energy, NiO/PVA shows good optical limiting, and the obtained NLO behavior is due to effective two-photon absorption.

Precursors effect: Photo conversion from UV to green/yellow in willemite nano phosphors

Manganese doped zinc silicate (Zn1.89SiO4:Mn0.11) green and yellow nanophosphors were prepared by solution combustion method using fumed silica and tetraethyl ortho silicate (TEOS) as sources of silica. The phosphors are calcined at 950 °C for 3 h. The formation of α and β- Zn2SiO4:Mn2+phases of the samples were confirmed by powder XRD measurements. The morphology and composition of the samples are investigated by SEM, TEM, FTIR and EDAX. FTIR shows that the main absorption bands located in the frequency region between 400 and 1200 cm−1. The effects of different sources of silica on the photoluminescence (PL) properties of Zn2SiO4:Mn2+ were studied. Among the silica sources, TEOS gave green and yellow emission at 527 and 581 nm, on excitation at 262 and 292 nm respectively. Fumed silica source gave only green emission at 527 nm on excitation at 262 nm. The emissions are originated from the 4T1 to 6A1 transitions of Mn2+ ions. Luminescence decay curve measurement show double exponential decay behavior and fast decay time has been observed.

Comparative Study of GdLu2Al2Ga3O12:Ce and GdY2Al2Ga3O12:Ce Scintillation Crystals for $\gamma$ -Ray Detection

The scintillation characteristics of Czochralski-grown GdLu2Al2Ga3O12:Ce and GdY2Al2Ga3O12:Ce single crystals were compared for $\gamma$ -ray detection. At 662-keV $\gamma$ -rays, light yield (LY) of 30700 photons/MeV obtained for GdLu2Al2Ga3O12:Ce is higher than that of 27200 photons/MeV obtained for GdY2Al2Ga3O12:Ce, while its energy resolution is inferior (14% versus 10.6%) due to a larger nonproportionality of LY. Scintillation decays were measured using the time-correlated single photon counting technique. A fast component decay time of 48 ns with relative intensity of 71% obtained for GdLu2Al2Ga3O12:Ce is superior to that of 106 ns (57%) for GdY2Al2Ga3O12:Ce. The coincidence time spectrum was measured in reference to a fast BaF2 scintillator, and coincidence time resolution was discussed.

The Effect of Intrinsic Radiation from a 3 × 3-in. LaBr3(Ce) Scintillation Detector on In Situ Artificial Radiation Measurements

In situ radiation measurements are commonly used to detect radioactive material in luggage; at border control checkpoints; for in-field monitoring; during the illicit transfer of nuclear material; and at radioactive contamination sites, e.g., the Fukushima nuclear accident site. In considering the high brightness, fast decay time, and good energy resolution of cerium-doped lanthanum bromide [LaBr3(Ce)] scintillation detectors, this work conducted an experimental analysis aimed at evaluating the potential for applying LaBr3(Ce) detectors to in situ artificial radiation measurements. The effect of the intrinsic radiation of the LaBr3(Ce) detector was investigated. In addition, the intrinsic radiation contribution to the background radiation of the region of interest (ROI) under full-energy peaks for several artificial point sources and the minimum detectable activity (MDA) values of a 3 × 3-in. LaBr3(Ce) detector for several artificial radioactive point sources under unshielded (in the natural background) and well-shielded (in a low background chamber) conditions were calculated. The results indicate that the intrinsic radiation has a significant effect on the background radiation of the ROI especially when the full-energy peaks of several artificial point sources are located in the low-energy region or near 789 and 1400 keV. In addition, the MDAs (the measured time is 300 s) of the LaBr3(Ce) detector for 152Eu (121.78 keV), 133Ba (356 keV), 137Cs (661.7 keV), and 60Co (1332.5 keV) were 218.2, 63.6, 61.3, and 59.6 Bq, respectively, under unshielded conditions and 111.4, 39.1, 46.1, and 38.6 Bq, respectively, under well-shielded conditions. The intrinsic radiation also has some effects on the MDA of the LaBr3(Ce) detector, especially in the low-energy region. Thus, the drawback of its intrinsic radiation limits its application to in situ weak artificial radiation measurements, but LaBr3(Ce) detectors have the potential for use in medium- and high-radiation measurements due to the better energy resolution of these detectors than NaI(Tl) detectors.

Nano-crystalline CdS thick films: a highly sensitive photo-detector

This work reports a simple method for fabrication of highly sensitive photo-detector based on uniform nano-crystalline CdS thick films deposited on glass substrate by spray pyrolysis technique. The presented photo-detector platform contains CdS thick films (3, 6, 9, 12 and 15 μm) and Ag electrode structure. In order to detect incident light signals, changes in electrical signals were measured (conductance and current). For photo-detection, the current response between the ‘ON’ and ‘OFF ’ states of incident light was measured when exposed to different wavelengths ranging from 450 to 700 nm. The fabricated device showed significant detection of low-power (∼5 mW cm−2) light for optimized film thickness of 12 μm. Time response measurements at different wavelength show fast response and decay time (25 ms) and high photo sensitivity (1123) at low bias voltage, indicating good quality of the deposited 12 μm thick CdS film.

Scintillation properties of Gd3(Al5-xGax)O12:Ce (x = 2.3, 2.6, 3.0) single crystals

Scintillation properties of Czochralski-grown Gd3(Al5-xGax)O12:Ce (x = 2.3, 2.6, 3.0) single crystals were investigated. With increasing Ga content in garnet host, the radioluminescence band due to 5d1 → 4f emission transition was blue-shifted due to a decrease in the crystal field splitting of the 5 d levels. At 662 keV γ rays, the LY value of 47,000, 55,100 and 51,900 ph/MeV was obtained for Gd3Al2.7Ga2.3O12:Ce, Gd3Al2.4Ga2.6O12:Ce and Gd3Al2Ga3O12:Ce, respectively. In spite of a lower LY value, Gd3Al2.7Ga2.3O12:Ce shows good energy resolution of 4.1% due to its proportionality of LY. The ratio of LY values under excitation with α- and γ-rays (α/γ ratio) was also determined. The fast component decay time with relative intensity of 146 ns (66%), 142 ns (76%) and 112 ns (76%) was measured in the scintillation decay of Gd3Al2.7Ga2.3O12:Ce, Gd3Al2.4Ga2.6O12:Ce and Gd3Al2Ga3O12:Ce, respectively. Good coincidence time resolution of Gd3Al2Ga3O12:Ce is due to a high initial photon emission rate.