Recombination Of Free Electrons Research Articles

Double-pulse-laser volumetric modification of fused silica: the effect of pulse delay on light propagation and energy deposition.

Volumetric modification of dielectrics by ultrashort laser pulses is a complex dynamic phenomenon involving material photoexcitation and associated nonlinear processes. To achieve control over modification, it is necessary to gain a deep insight into the dynamics of laser-excited processes that can be realized using double-laser-pulse experiments with different time separations supported by numerical simulations. In this paper, we apply this approach to investigate fused silica modification with femtosecond laser pulses that provides time-resolved information about the dynamic behavior of the laser-excited bandgap material. It is shown that the laser-generated free-electron plasma causes a shielding effect for the following pulse with a characteristic duration of ∼600 fs after the pulse action. Within this time interval, the second pulse produces a reduced modification as compared to a longer time separation between pulses. For double pulses with different energies, it was found that the volumetric modification is stronger when a lower-energy pulse couples with material first. This is explained by the combination of the effects of the re-excitation of self-trapped excitons, which are generated as a result of free electron recombination and associated light shielding. Experimental results are supported by numerical simulations of double laser pulse propagation in nonlinear media based on Maxwell's equations. Our findings offer a route for better controlling the inscription of 3D photonic structures in bulk optical materials.

Sonication to remove organic dyes with the aid of NH2-MIL-125(Ti) and carbonization derivatives

Sonication provides a green and sustainable strategy to remove the organic dye pollutants in the environment. In this study, the NH2-MIL-125(Ti) and carbonization derivatives were prepared successfully, aiming to assist the sonochemical removal of organic dyes. By comparison, the NH2-MIL-125(Ti) had a structured cylinder while the carbonization derivatives were irregular in shape, and the NH2-MIL-125(Ti) had much larger specific surface area than the carbonization derivatives, even though the carbonization lowered the band gap. Moreover, the high-resolution XPS spectra showed that the NH2-MIL-125(Ti) had abundant Ti-Ox bonds and vigorous electron transfer, and the PL spectra suggested that the carbonization derivatives with narrow band gap caused the fast recombination of free electrons and holes. According to the observations on the dye removal experiments, the NH2-MIL-125(Ti) showed better catalytic activity under ultrasound irradiation, and the removal rate to Rhodamine B could reach about 80 % within 60 min, even almost 100 % in the presence of peroxydisulfate. Through the qualitative and quantitative analysis, an adsorption/degradation mechanism was proposed and demonstrated for the NH2-MIL-125(Ti)-mediated sonication. Thus, the catalyst-aided sonochemical processes will have attractive application prospect in the removal of industrial organic dyes.

Cathodoluminescence Study of m‐Plane α‐Ga2O3 Grown by Mist Chemical Vapor Deposition

The cathodoluminescence (CL) properties of m‐plane α‐Ga2O3 grown by mist chemical vapor deposition are investigated. The m‐plane α‐Ga2O3 is found to have two types of characteristic luminescence bands: blue luminescence (BL) band (centered at 2.8 eV) and UV luminescence (UVL) band (centered at 3.7 eV). To characterize these bands, the Sn concentration and temperature dependences of the CL spectra are investigated using samples with varying Sn concentrations from 1 × 1017 to 1 × 1019 cm−3. The Sn concentration‐dependent CL spectra show that the luminescence from the UVL band is dominant in the low Sn concentration region, while that from the BL band is dominant in the high Sn concentration region. The temperature‐dependent CL spectra indicate that the peak intensity of the UVL band depending on temperature is larger than that of the BL band. The activation energies in the UVL band of the undoped and lightly Sn‐doped samples are determined as 110 meV. In addition, the parameters of the Huang–Rhys factor in the UVL band are obtained as 28.6 and 18.8, respectively. These results strongly suggest that the UVL band is derived from self‐trapped excitons, which is the recombination of free electrons and self‐trapped holes.

Irradiation of Cu(In, Ga)Se-=SUB=-2-=/SUB=- Thin Films by 10 MeV Electrons at 77 K: Effect on Photoluminescence Spectra

Thin films of Cu(In, Ga)Se2 on Mo/glass were irradiated by 10 MeV electrons at 77 K and examined by photoluminescence at 77 K before and after irradiation without warming the samples as well as after warming to 300 K. The photoluminescence spectra revealed a broad band constituting 3 merged peaks (P1, P2, P3) assigned to: band-to-band recombination (P1) and recombination of free electrons with holes localised at acceptors influenced by the valence band tail (P2, P3). Irradiation reduced the intensity of the peaks due to deep traps generated by electrons and anomalously reduced the degree of compensation of the material. Keywords: thin films, irradiation, photoluminescence, recombination, band-to-band.

Irradiation of Cu(In, Ga)Se-=SUB=-2-=/SUB=- Thin Films by 10 MeV Electrons at 77 K: Effect on Photoluminescence Spectra

Thin films of Cu(In,Ga)Se2 on Mo/glass were irradiated by 10 MeV electrons at 77 K and examined by photoluminescence at 77 K before and after irradiation without warming the samples as well as after warming to 300 K. The photoluminescence spectra revealed a broad band constituting 3 merged peaks (P1, P2, P3) assigned to: band-to-band recombination (P1) and recombination of free electrons with holes localised at acceptors influenced by the valence band tail (P2, P3). Irradiation reduced the intensity of the peaks due to deep traps generated by electrons and anomalously reduced the degree of compensation of the material. Keywords: thin films, irradiation, photoluminescence, recombination, band-to-band.

Electron transfer kinetics in CdS/Pt heterojunction photocatalyst during water splitting

Noble metal cocatalysts have shown great potential in boosting the performance of CdS in photocatalytic water splitting. However, the mechanism and kinetics of electron transfer in noble-metal-decorated CdS during practical hydrogen evolution is not clearly elucidated. Herein, Pt-nanoparticle-decorated CdS nanorods (CdS/Pt) are utilized as the model system to analyze the electron transfer kinetics in CdS/Pt heterojunction. Through femtosecond transient absorption spectroscopy, three dominating exciton quenching pathways are observed and assigned to the trapping of photogenerated electrons at shallow states, recombination of free electrons and trapped holes, and radiative recombination of locally photogenerated electron-hole pairs. The introduction of Pt cocatalyst can release the electrons trapped at the shallow states and construct an ultrafast electron transfer tunnel at the CdS/Pt interface. When CdS/Pt is dispersed in acetonitrile, the lifetime and rate for interfacial electron transfer are respectively calculated to be ~5.5 ps and ~3.5 × 1010 s−1. The CdS/Pt is again dispersed in water to simulate photocatalytic water splitting. The lifetime of the interfacial electron transfer decreases to ~5.1 ps and the electron transfer rate increases to ~4.9 × 1010 s−1, confirming that Pt nanoparticles serve as the main active sites of hydrogen evolution. This work reveals the role of Pt cocatalysts in enhancing the photocatalytic performance of CdS from the perspective of electron transfer kinetics.

Enhanced Photocatalytic Activity Using Near Visible Light of ZnO Nanorods by Doping with Mn2+ Ions

ZnO is a promising photocatalyst for photocatalytic oxidation of organic compounds under the influence of sunlight that provides clean energy and decomposes sustainable organic pollutants substances. ZnO is found to have non-toxic properties, long-term stability, high carrier mobility, low cost and biocompatibility. However, some disadvantages of ZnO limit its use in photocatalysis. Due to its wide bandgap, ZnO can only be activated under UV illumination. On the other hand, the photo-excited electron-hole pairs that recombine quickly on ZnO surface, suppress its photocatalytic properties. To improve its properties and performance, doping with transition metals was used to improve the optical properties of ZnO. Among the transition metal ions, Manganese (Mn) was commonly used to improve and tune the optical, electrical, diameter, height, and the number of nanorods (NRs) per unit area. Introduction of Mn into ZnO could enhance the photocatalytic activity due to the increase in the defect sites that effectively decreased the recombination of free electrons and holes. This study successfully synthesized ZnO nanorod arrays generated on glass substrates with different concentrations of doping Mn (0, 0.5, 1, 1.5 and 2%) at 100 °C by a simple hydrothermal method. To investigate the structure, morphology and optical properties, ultraviolet-visible spectroscopy (UV-Vis), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were conducted. With the range of Mn doping ≤ 2% mol, the band gap reduced slightly, and the most optimized Mn doping concentration was of 0.5%. Overall, this work shows that the most effective way to increase ZnO’s photocatalytic activity in the visible region by reducing its band gap was the reduction in the size of the material or denaturation of ZnO by certain metals or non-metals.

Излучательная рекомбинация на ионно-индуцированных дефектах в тонких пленках твердых растворов Cu(In, Ga)Se-=SUB=-2-=/SUB=-

In thin films of Cu(In,Ga)Se2 solid solutions radiation-induced effects after irradiation of hydrogen ions with an energy of 2.5, 5 and 10 keV with dose of ~ 3∙10^15 cm-2 were studied. A comparative analysis of the optical characteristics of non-implanted and implanted Cu(In,Ga)Se2 thin films was carried out based on the measurements of photoluminescence spectra and the luminescence excitation spectra at liquid helium temperature of ~ 4.2 K. The bandgap of Cu(In,Ga)Se2 solid solutions determined from the data of mathematical processing of the luminescence excitation spectra was ~ 1.171 eV. An intense band with a maximum of ~ 1.089 eV was found in the photoluminescence spectra of non-implanted and hydrogen-implanted Cu(In,Ga)Se2 thin films caused by the recombination of free electrons with holes localized in the tails of the valence band. It was established that appearance of intense broad bands in the photoluminescence spectra with maxima in the energy range of ~ 0.92 eV and ~ 0.77 eV is due to radiative recombination of nonequilibrium charge carriers at deep energy levels of acceptor type ion-induced defects formed in the bandgap of Cu(In,Ga)Se2 solid solutions. The conditions for the appearance of the ionic passivation effect of dangling electronic bonds on the surface and in the bulk of Cu(In,Ga)Se2 polycrystalline films, possible nature of point defects in the structure and the mechanisms of radiative recombination are discussed.

Makemake + Sedna: A Continuum Radiation Transport and Photoionization Framework for Astrophysical Newtonian Fluid Dynamics

Astrophysical fluid flow studies often encompass a wide range of physical processes to account for the complexity of the system under consideration. In addition to gravity, a proper treatment of thermodynamic processes via continuum radiation transport and/or photoionization is becoming the state of the art. We present a major update of our continuum radiation transport module, Makemake, and a newly developed module for photoionization, Sedna, coupled to the magnetohydrodynamics code PLUTO. These extensions are currently not publicly available; access can be granted on a case-by-case basis. We explain the theoretical background of the equations solved, elaborate on the numerical layout, and present a comprehensive test suite for radiation–ionization hydrodynamics. The grid-based radiation and ionization modules support static one-dimensional, two-dimensional, and three-dimensional grids in Cartesian, cylindrical, and spherical coordinates. Each module splits the radiation field into two components, one originating directly from a point source—solved using a ray-tracing scheme—and a diffuse component—solved with a three-dimensional flux-limited diffusion (FLD) solver. The FLD solver for the continuum radiation transport makes use of either the equilibrium one-temperature approach or the linearization two-temperature approach. The FLD solver for the photoionization module enables accounting for the temporal evolution of the radiation field from direct recombination of free electrons into hydrogen's ground state as an alternative to on-the-spot approximation. A brief overview of completed and ongoing scientific studies is given to explicitly illustrate the multipurpose nature of the numerical framework presented.

Effects of irradiation of ZnO/CdS/Cu2ZnSnSe4/Mo/glass solar cells by 10 MeV electrons on photoluminescence spectra

Solar cells with the structure ZnO/CdS/Cu2ZnSnSe4/Mo/glass were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 1015 cm−2 and then 5.4 × 1015 cm−2 of 10 MeV electrons carried out at 77 K in liquid nitrogen bath. The low temperature PL spectra before irradiation revealed two bands, a broad and asymmetrical dominant band at 0.94 eV from the CZTSe layer and a lower intensity high energy band (HEB) at 1.3 eV, generated by defects in the CdS buffer layer. Analysis of the excitation intensity and temperature dependencies suggested that the dominant band is free-to-bound (FB): the recombination of free electrons with holes localised at acceptors whose energy levels are affected by potential fluctuations of the valence band due to high concentrations of randomly distributed charged defects. Irradiation did not induce any new band in the examined spectral range (from 0.5 μm to 1.65 μm) but reduced the intensity of both bands in the PL spectra measured at 77 K without warming the cells. The higher the dose the greater was the reduction. After this the cells were warmed to 300 K and moved to a variable temperature cryostat to measure temperature dependencies of the PL spectra. After irradiation the red shift rate of the FB band with temperature rise was found to increase. Electrons displace atoms in the lattice creating primary defects: interstitials and vacancies. These defects recombine during and shortly after irradiation forming secondary defect complexes which work as deep non-radiative traps of charge carriers reducing the PL intensity and increasing the rate of the temperature red shift. Irradiation did not affect the mean depth of the band tails estimated from the shape of the low energy side of the dominant PL band.

Exciton Quenching due to Hole Trap Formation in Aged Polymer Light‐Emitting Diodes

AbstractPolymer light‐emitting diodes based on two poly(p‐phenylene vinylene) derivatives are aged at a constant current density, leading to the formation of hole traps. Time‐resolved photoluminescence spectroscopy (TRPL) measurements show that the degraded polymer light‐emitting diodes (PLEDs) also demonstrate a decrease in exciton lifetime. The amount of nonradiative exciton quenching sites in the aged devices is quantified by Monte Carlo simulations. It is found that the number of hole traps obtained from electrical charge transport measurements matches the number of newly formed nonradiative quenching sites determined from the TRPL experiments. The results reveal the origin for the apparent different behavior of the electroluminescence and photoluminescence upon PLED degradation. The decrease of the electroluminescence is governed by recombination of free electrons with trapped holes, whereas the photoluminescence is reduced by nonradiative quenching processes between excitons and hole traps.

Photocurrent transient measurements in MAPbI3 thin films

Methylammonium lead iodide (CH3NH3PbI3 or MAPbI3) is one of the hybrid perovskite materials most widely used in high performing perovskite solar cells. In this work, the charge carrier recombination process of MAPbI3 perovskite thin-film fabricated on corning 1737 glass substrate is studied by means of transient photocurrent measurements and the current decay curves after turning off the lights. The influences of temperature (25–70 °C), illumination time (30–90 s) and intensity of illumination (100–1000 W m−2) on the photocurrent response are studied. The photocurrent measurements are taken in vacuum to avoid the effect of atmospheric air and moisture, at an applied field of 100 V cm−1. The MAPbI3 film shows good photosensitivity with two orders of magnitude change in current when exposed to full intensity of light. As soon as the light is turned on, the current increases rapidly; however, a small decrease is observed before a stabilized current is achieved. The stabilized value of current does not depend upon exposure time. The current falls abruptly as soon as the light is turned off and then slowly reaches back the dark current value. The fast decay of photocurrent can be attributed to fast recombination of free electrons and hole having decay time constant approximately 100 ms and the slow decay corresponds to slow re-emission of trapped carriers which has at least 3–4 times higher decay time constants compared to the fast decay. Intensity-dependent photocurrent follows a power law (Iph ~ Φ γ), indicating predominantly bimolecular recombination process. The activation energy (Ea) estimated from the temperature-dependent dark current measurement is found to be ~ 210 meV.

Theoretical analysis for transmission of Gaussian and sine time irradiance of electromagnetic beam in collisional dusty plasmas

AbstractA theoretical model of propagation of Gaussian and Sine time irradiance of an electromagnetic beam in collisional dusty plasma has been done in the present analysis. It contains equilibrium of dust charge, particle density, and energy of plasma ingredients having charge neutrality. Ionization of neutral particles, recombination of free electrons with ions, adsorption and emission of electrons from dust grain surface, and binary collisions between plasma components are also considered in this treatment. Time varying behaviour of modified electron temperature and collision frequency has been illustrated numerically as a function of dust densities. Also, the comparative analyses of variation of beam waist parameter with the dimensionless length of transmission for both the Gaussian and Sine time irradiance are involved in this model as a function of distinguishable time width, collision frequencies, and dust densities under the condition that the size of dust nebulous is greater than the electrons mean free path for the adsorption on the dust grain surface. The observed results are significant for the applications in industry and astrophysics.

Gate-Voltage Control of Quantum Yield in Monolayer Transition-Metal Dichalcogenide

Two-dimensional transition metal dichalcogenide (2D-TMD) monolayers, which reveal remarkable semiconductor properties, are the subject of active experimental research.Recently it has been shown experimentally that quantum yield in MoS2 and WSe2 monoatomic layers can reach values close to unity when electrostatic doping makes them intrinsic semiconductors. However, the available theoretical description does not give an understanding of the physical mechanisms underlying in the gate voltage control of quantum yield.This work is an attempt to propose a consistent semi-phenomenological theory of photo-induced charge carriers relaxation in 2D-TMDs, which allows obtaining an analytical dependence of the quantum yield on the voltage applied to the FET gate. We consider a standard experimental situation, when the 2D-TMD monolayer and the metal gate are plates of a flat capacitor, and the capacitor charge is proportional to the gate voltage. The dependences of the TMD monolayers quantum yield on the gate voltage and the carrier generation rate have been calculated for the cases of the prevailing recombination of free electrons and holes (radiative and non-radiative Auger recombination) and recombination of excitons (radiative and Auger recombination). In both cases analytical expressions were derived for the dependence of quantum yield on the gate voltage and photo-induced carriers generation rate at a fixed gate voltage. Quantitative agreement with experiment allows concluding about the relevance of the proposed theoretical model for the description of carriers photo-generation and recombination in 2D-TMD monolayers. Obtained results demonstrate the possibilities of 2D-TMD quantum yield control by the gate voltage and indicate that 2D-TMDs are promising candidates for modern optoelectronics devices.

Observation of bandgap renormalization in mesoscopic zinc oxide particles

Deep UV photoluminescence (PL) spectroscopy was employed to study the optical properties of mesoscopic zinc oxide (ZnO) particles using the third harmonic laser (λ ~ 260 nm) generated from a Ti: sapphire pulse laser for optical excitation. PL spectrum measured at 15 K exhibits excitonic emissions at 3.362 and 3.375 eV due to the donor bound exciton (DoX) and free exciton (FX) transitions, respectively. The dominant emission peak is observed at around 3.308 eV due to the recombination of free electrons from the conduction band to the holes bound to an acceptor state (FA) along with longitudinal optical (LO) phonon replicas. From the power dependent PL measurement, we observed a redshift of FX and DoX emission peaks with increasing excitation laser power, showing a bandgap renormalization effect. Temperature dependent PL measurements were also performed to explore the bandgap renormalization effect. Fitting of our power dependent PL data with Vashishta, and Kalia model shows the clear trend in a good agreement with the model.

A simple fluorescence method detection levofloxacin in milk based on GSH-CdTe QDs

Abstract A simple and sensitive method for detecting LFO was developed based on electron transfer mechanism quenched the fluorescence of GSH-CdTe QDs. LFO, as an effective electron acceptor, is added into the QDs solution for electron transfer, the transfer of electrons from the conduction band of QDs to LFO prevents the recombination of free electrons and valence band holes of QDs, thus causing the fluorescence quenching of QDs. Under the optimal experimental conditions, the fluorescence quenching degree of QDs has a good linear relationship with the concentration of LFO within the range of 0.73–7.30 μg mL−1, the correlation coefficient is 0.9978, and the detection limit is 1.53 ng mL−1. In addition, the reaction mechanism and experimental conditions were investigated, and the method was successfully applied to the detection of LFO in milk and serum samples.

Photoluminescence and photoelectric properties of CdTe crystals doped with Mo

The low-temperature photoluminescence (PL) and photodiffusion current (PDC) spectra of CdTe crystals doped with Mo atoms of different impurity concentrations N = (1018-1020) cm−3 were studied. It was shown that the most intense PL band for the crystal with N = 1018 cm−3 is caused by the recombination of free electrons and acceptor centers associated with a cadmium vacancy as well as Cu, Na or Li residual impurities. An increase in the concentration of Mo atoms up to N = 1019 cm−3 results in a significant decrease in cadmium vacancies and increase of optical quality of the crystals. Here, PL spectrum contains only the A1oX-line. An increase in the concentration of Mo atoms (N = 1020 cm−3) causes a decrease in optical quality that may be due to the formation of MoTe2 compound in CdTe crystal. The results of the PDC measurements are in accordance with the data of PL studies of CdTe:Mo crystals.

Temperature-dependence of X-ray excited luminescence of β -Ga2O3 single crystals

We investigated the temperature-dependence of X-ray excited luminescence of β-Ga2O3 single crystals. The recombination of free-electrons with self-trapped holes (STHs) responsible for the ultraviolet luminescence (UVL) band was observed. For the as-grown sample with a high electron concentration, the Auger recombination becomes important, which accelerates the decay dynamics and reduces the ultraviolet luminescence band. The ultraviolet luminescence band is controlled by temperature quenching due to the migration of self-trapped holes to VGa3− centers. Annealing in an oxygen environment can increase the VGa3− concentration, which contributes to the blue-green luminescence band when VGa3− captures a self-trapped hole forming a VGa2− center at rising temperature.

Effect of precursor concentration of microstructured titanium-di-oxide (TiO2) thin films and their photocatalytic activity

TiO2 thin films were synthesised by hydrothermal method on ITO coated glass substrate at a hydrothermal temperature of 160 °C for 24 h with different titanium precursor amount (0.3 ml, 0.5 ml, 0.7 ml, 1 ml). The band gap energy of prepared thin films varies from 3.2 eV to 3.6 eV. A sharp peak was observed at about 280 nm in the absorption spectrum, revealing a large band gap in the UV region. Reflectance spectra show high value with decreasing wavelength. XRD analysis confirms the presence of a TiO2 thin film phase. FESEM images of the thin films show sieve-like porous layered morphology. On the increasing amount of precursor, the porosity of the thin films increases. We calculated porosity from the transmittance data and the refractive index from the reflectance data. Particle sizes of the thin films were roughly found to lie between 10 nm–40 nm. Sharp emission at 440 nm is due to electron transition mediated by defect states. Small peaks between 450 nm–500 nm are assigned to free electron recombination from conduction band to the ground state. The visible light photocatalytic degradation rate of Methylene Blue (MB) is highest for the TiO2 thin film whose precursor amount is 0.5 ml. The kinetic rate of degradation and porosity is also greater for the same sample.

Unusual properties of the RY3 center in GaN

The investigation and identification of point defects in GaN is crucial for improving the reliability of light-emitting and high-power electronic devices. The RY3 defect with a characteristic emission band at about 1.8 eV is often observed in photoluminescence (PL) spectra of $n$-type GaN grown by hydride vapor phase epitaxy, and it exhibits unusual properties. Its emission band consists of two components: a fast (10-ns lifetime) RL3 with a maximum at 1.8 eV and a slow (100--300 \ensuremath{\mu}s lifetime) YL3 with a maximum at 2.1 eV and zero-phonon line at 2.36 eV. In steady-state PL measurements, the YL3 component emerges with increasing temperature from 90 to 180 K, concurrently with a decrease in the RL3 intensity. The activation energy of both processes is about 0.06 eV. In time-resolved PL, the YL3 intensity abruptly rises when the RL3 intensity begins to saturate. These and other phenomena can be explained using a model of an acceptor with two excited states. A delocalized, effective-mass state at about 0.2 eV above the valence band captures photogenerated holes. These holes transition to the ground state, which produces the RL3 component with a lifetime of \ensuremath{\sim}10 ns. Alternatively, they may nonradiatively transition over a 0.06 eV-high barrier to a localized excited state with a level at 1.13 eV above the valence band. Recombination of free electrons or electrons at shallow donors with the holes at this localized excited state is responsible for the YL3 component. The relative intensities of the RL3 and YL3 components are dictated by the probabilities of holes at the shallow excited state to transition to the ground or to the localized excited states. Transition metals and complex defects are considered as the main candidates for the RY3 center.