Electron Recombination Time Research Articles

Efficient Photocatalytic Oxidative Coupling of Amines under Visible Light Using a Trioporphyrins-Based Covalent Triazine Framework.

Porphyrins-based porous organic polymers (POP) were widely used in photocatalytic oxidation under visible light owing to their superiority in the activation of oxygen. In contrast, the efficiency is usually limited due to the fast recombination and slow electron transfer. Herein, we report the use of a trioporphyrins-based covalent triazine framework (Por-CTF) as visible-light-active photocatalyst for the coupling oxidative of amines to imines at room temperature. By incorporating the π-conjugated porphyrin building block led to the enhanced electron transport between molecules, and the extended recombination time of excited electrons. The photocatalytic efficiency of Por-CTF is superior to that of polymer in absence of triazine framework (POP-TSP), which was prepared by radical polymerization using tetra-(4-vinylphenyl) porphyrin (TSP) as monomer. Por-CTF catalyst presented excellent efficiency for various primary amines and stability. This work provides a reasonable guidance of catalyst molecular structure design for enhancing efficiency in the photocatalytic oxidation.

Enhancement of the vortex critical velocity in superconducting/normal metal bilayers

This study investigates the influence of normal metal thickness on the Larkin-Ovchinnikov (LO) instability in superconductor/normal metal heterostructures. Bilayers were fabricated using a 22 nm-thick Mo2N layer, with varying Pt, W, and Al thicknesses. The samples exhibited a critical temperature of approximately 7.5 K. The LO instability was characterized through current–voltage curves, which relate the maximum vortex velocity to the recombination time of normal electrons into Cooper pairs (τ). The results reveal a significant enhancement in vortex velocity at moderate and high magnetic fields when a metallic layer with thickness exceeding the coherence length is added. Furthermore, a direct correlation is observed between velocity values and the thermal conductance of the metal, indicating a contribution from phonons and heat dissipation to the maximum achievable velocity in the heterostructures.

In-situ grown carbon as charge transfer medium for enhanced photoinduced electrons extraction from polymer carbon nitride toward TiO2

Interfacial charge transfer resistance is one of the main limiting factors for realizing high photocatalytic efficiency of heterostructures system. Herein, an activated carbon layer is successfully introduced between the interface of polymer carbon nitride (CN) and TiO2 heterostructure (CNP-x) as charge transfer medium by in situ pyrolysis carbonization method. Because of the lower spatial resistance of the crystalline/amorphous interface and the fast carrier transportation character of activated carbon, the efficiency of TiO2 in extracting photoinduced electrons from CN was significantly improved. That is, the separation/transport of photocarriers in CNP-x heterostructure is accelerated, and the recombination time of photogenerated electrons and holes is prolonged. The CNP-1 exhibits a H2 evolution rate of 1298.5 μmol h−1 with apparent quantum yield (AQY) of 34.5 %, 20.3 % and 12.6 % at 365 nm, 380 nm and 400 nm, respectively. This work offers a novel and unique strategy to promote interface charge separation and transport of CN-based heterostructures by accurately introduction charge transfer medium.

In-situ electropolymerization of congo red-doped polypyrrole and gold nanoparticle nanocomposites and its electrocatalytic application

A one-step electrochemical approach has been utilized to synthesize congo-red doped polypyrrole film embedded with gold nanoparticles (PPy-CR-AuNP). This study discusses for the first time the electrocatalytic properties of PPy-CR and PPy-CR-AuNP as counter-electrode in dye-sensitized solar cell (DSSC). Physicochemical characterizations of the films with and without AuNP have been presented. Both films/electrodes displayed excellent charge transfer abilities, as demonstrated by cyclic voltammetry analysis. Notably, PPy-CR-AuNP exhibited enhanced electron transfer kinetics with a 17% increase in electroactive surface area and improved efficiency in the I−/I3− redox pair activity compared to PPy-CR. Following extended storage under ambient conditions, PPy-CR-AuNP maintained a better mass transport and electron transfer at the electrode-electrolyte interface. The DSSC employing PPy-CR-AuNP as counter electrode exhibited a 12.5% enhancement in efficiency, lower electron transport time, higher electron recombination time and electron collection efficiency of up to 89%. Moreover, the electron decay rate was found to be 12.5% lower compared to the DSSC with PPy-CR electrode. These results demonstrated that PPy-CR-AuNP is an appropriate candidate for DSSC.

In-situ prepare graphene/g-C3N4 D-π-A in-plane heterojunctions for high-performance photocatalytic hydrogen production

In this study, graphene/g-C3N4 in-plane heterojunctions (SGCN-x) with D-π-A structure are prepared by hydrothermal and pyrolysis carbonization methods. The interfacial bonded hydrothermal precursor can assist the synthesis of graphite carbon and g-C3N4 at mild thermodynamic conditions. The characterizations indicated that graphene was stitched onto the edge of carbon nitride. The interface of the in-plane heterojunction is carbon-nitrogen heterocyclic rings with π conjugate property, which can be regarded as the transmission channel of electrons. That is, the separation/transport of photocarriers in SGCN-x heterostructure is accelerated, and the recombination time of photogenerated electrons and holes is prolonged. The highest yield of hydrogen production performance has reached up to 7446 μmol/(g·h) for the optimization of the photocatalyst, almost 6.73 times higher compared with pure g-C3N4. This work opens new opportunities for the significant strategy for the in-situ preparation of graphene-based materials and construction of efficient photoactive material for catalysis.

Effect of carrier (hole) temperature on performance of optical amplifiers quantum dot structure

Abstract Carriers temperature in quantum dot (QD) optical amplifiers has been modelled theoretically taken into account hole contribution, which is not considered early. The contributions of wetting layer (WL), first excited state (ES1), and second excited state (ES2) have also been considered. Effect of WL−ES2 recombination time of both electrons and holes, carrier heating (CH) time of electrons and holes, in addition to electron hole recombination time are examined. The results show that there is a gap between electron and hole heating temperature due to the difference between there recovery times. It is found that fast hole (long electron) recombination time yield a high carrier temperature.

Facile preparation of porous g-C3N4/MoS2 heterojunction for hydrogen production under simulated sunlight

The limited surface areas and the high recombination rate of photogenerated carriers have always been two sticky problems of graphite-like carbon nitride (g-C3N4) for the application of solar hydrogen production. In this article, the porous g-C3N4 was prepared by the improved thermal polymerization method to enlarge its specific surface areas and to facilitate the transfer of internal electric charges. Then, heterojunction formed by MoS2 deposited on the g-C3N4 was built to extend the recombination time of photoproduction electrons and holes. Several techniques were used to research their structures as well as the physical and chemistry properties, such as the specific surface area analysis (BET), Fourier transform infrared spectroscopy, UV–vis diffuse reflectance spectra, transmission electron microscopy, X-ray photoelectron spectroscopy, and so on. Finally, their performance of photocatalytic hydrogen evolution was studied under simulated sunlight irradiation. As a result, the optimal ratio of melamine to urea is 1:3 for preparing the porous g-C3N4, and the g-C3N4/MoS2 has a high efficiency of photocatalytic hydrogen evolution, ca. 1913 μmol·g–1·h–1. The results showed that the composites had higher hydrogen evolution than the pure ones, which the activity increasing first, then decreasing as the contents of MoS2 growing in quantity. Furthermore, the synergy effects are elaborated by electrochemical impedance spectroscopy and Mott-Schottky analyses.

Cage-shaped [P(W3O10)4]3- nanoclusters deposited carbon nitride to construct a dual-functional catalyst for photocatalytic mineralization wastewater and dye detector

In this work, [P(W3O10)4]-g-C3N4 composite catalyst is prepared from phosphotungstic acid and thin-layer carbon nitride at high temperature, which can efficiently remove Rhodamine B (RhB) organic wastewater by photocatalysis. The results of UV–vis, TPC, PL and EIS tests show that the introduction of [P(W3O10)4]3- increases the light absorption range of the g-C3N4 and prolongs the recombination time of photogenerated electrons and holes. The degradation efficiency of g-C3N4 and the [P(W3O10)4]-g-C3N4 catalyst are compared and analyzed, and the corresponding active species (O2·-) is proposed. The new catalyst can decolor and mineralize the RhB solution in a short time, and its degradation rate and mineralization rate are 98.0% and 81.0%, respectively. Interestingly, [P(W3O10)4]-g-C3N4 catalyst and RhB solution mixture can be used as a chili powder dye detector. Its special performance can be attributed to the dissociation of the composite catalyst under acid conditions, and the newly formed H3PW12O40 combines with the RhB solution to change color. In short, a dual-function catalyst is synthesized to remove water pollution, and its advantages such as high efficiency, rapidity and strong practicability are worthy of attention.

Linear bounded potential model for semiconductor band bending

We propose a simple but unexplored model for the semiconductor band bending with the aim to obtain a relatively simple expression to calculate the energy spectrum for the confined levels and the analytical expressions for wave-functions. This model consists of a linear potential but it is bounded or trimmed in energy unlike the well known wedge potential (WP) model. We present exact solutions for this potential in the frame of the effective mass approximation and they are valid for electron or hole confinement potential. This model provides a more adequate physical scenario than the WP since it takes into account the charge balance involved in the band bending potential. These results allow to treat confined potential problems as in the case of a two-dimensional electron gas in a simplified way. We discuss the application of this approximation to the recombination time of electrons an holes and for the Franz–Keldysh effect.

Far-IR to deep-UV adaptive supercontinuum generation using semiconductor nano-antennas via carrier injection rate modulation

Supercontinuum generating sources, which incorporate a non-linear medium that can generate a wideband intensity spectrum under high-power excitation, are ideal for many applications of photonics such as spectroscopy and imaging. Supercontinuum generation using ultra-miniaturized devices is of great interest for on-chip imaging, on-chip measurement, and for future integrated photonic devices. In this study, semiconductor nano-antennas are proposed for ultra-broadband supercontinuum generation via analytical and numerical investigation of the electric field wave equation and the Lorentz dispersion model, incorporating semiconductor electron dynamics under optical excitation. It is shown that by a rapid modulation of the carrier injection rate for a semiconductor nano-antenna, one can generate an ultra-wideband supercontinuum that extends from the far-infrared (Far-IR) range to the deep-ultraviolet (Deep-UV) range for an infrared excitation of arbitrary intensity level. The modulation of the injection rate is achieved by high-intensity pulsed-pump irradiation of the nano-antenna, which has a fast nonradiative electron recombination mechanism that is on the order of sub-picoseconds. It is shown that when the pulse period of the pump irradiation is of the same order with the electron recombination time, rapid modulation of the free electron density occurs and electric energy accumulates in the nano-antenna, allowing for the generation of a broad supercontinuum. The numerical results are compared with the semiempirical second harmonic generation efficiency results for validation and a mean accuracy of 99.7% is observed. The aim of the study is to demonstrate that semiconductor nano-antennas can be employed to achieve superior supercontinuum generation performance at the nanoscale and the process can be programmed in an adaptive manner for continuous spectral shaping via tuning the pulse period of the pump irradiation.

The Basic Category and Application of Graphene-based Hybrid Photodetector

A photodetector is an optoelectronic device that can be divided into inorganic photodetector, which consists of traditional semiconductors, and organic photodetector, the recent direction of researching that made of organic semiconductors with the help of organic dye molecular or other organic compounds. The core of the improvement of photodetector is graphene, a single layer of the two-dimensional (2D) carbon crystalline structure, which provides the property with wide spectral bandwidth and considerably high response speed. This review focuses on the basic knowledge about graphene-based hybrid photodetector, because the hybrid photodetectors have the advantages of breaking the limitation of light absorption and shortening the recombination time of electrons and holes to increase photoresponsivity. Photodetectors can also behave differently when lights are in the infrared, ultraviolet, and terahertz region, so that the selection of materials based on graphene to detect the specific wavelengths of light becomes a challenge. In order to explain the complicated advanced technology briefly, this review can be regarded as two main parts. The first one is the basic properties of inorganic and organic photodetectors with the addition of perovskite, a new material that has been extensively studied recently, and the second part is about how photodetectors can be applied in a different wavelength of lights.

Comparison of full-wave models of terahertz photoconductive antenna based on ordinary differential equation and Monte Carlo method

The investigation and comparison of two full-wave models of photoconductive terahertz antenna, made from low-temperature grown gallium arsenide, is performed. One model solves simple approximation of drift-diffusion equations another uses Monte Carlo simulation for estimation of the electrical current in the active region of antenna. Simulation results revealed that the simple model can be useful in the cases when the duration of photoexcitation is relatively long (FWHM $$\ge $$ 250 fs). In a case of shorter laser pulses (FWHM $$\ge $$ 130 fs), electron recombination (or trapping) time should be in the order of 0.1 ps, which is characteristic of highly compensated semiconductors. In other cases, transient dynamics of electron drift velocity at sub-picosecond timescales makes significant impact to the growth speed of the photocurrent. This leads to the overestimation of electric field amplitude in the high-frequency range when the simple model is used. Full-wave simulation shows good agreement with experimental results when the detectors’ response is included in the calculation. The calculated results were confirmed experimentally which increases the reliability of the full-wave model presented in the paper.

Impact of tetrabutylammonium, iodide and triiodide ions conductivity in polyacrylonitrile based electrolyte on DSSC performance

Gel polymer electrolytes (GPEs) with polyacrylonitrile (PAN)-based polymer, ethylene carbonate (EC) and propylene carbonate (PC) plasticizers, and different amounts of tetrabutylammonium iodide (TBAI) salt and iodine (I2) have been prepared and used in dye-sensitized solar cells (DSSCs). The maximum room temperature conductivity of 5.14 mS cm−1 is obtained for electrolyte with a composition of 8 wt% PAN-30 wt% EC-30 wt% PC-30 wt% TBAI-2 wt% I2 (S3 electrolyte) which influenced by the highest charge carrier density of 7.93 × 1020 cm−3 estimated from fitting the impedance Nyquist plot. The DSSC fabricated with S3 electrolyte revealed the highest power conversion efficiency of 3.45% with open-circuit voltage (Voc) of 582 mV and short-circuit current density (Jsc) of 12.9 mA cm−2. The incident photon-to-current conversion efficiency of the DSSC with highest efficiency is 54.01%. The electrical impedance spectroscopy of the same cell shows the lowest series resistance indicating the superiority of electrolyte charge transport characteristics in DSSC. In addition, electron transfer time constant τtr and electron recombination time τrec, charge collection efficiency ηcc, electron diffusion coefficient D and diffusion length LD of DSSC fabricated with GPEs prepared have been estimated by intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy techniques. The DSSC with highest efficiency shows lowest τtr of 34.46 ms and highest τrec of 90.41 ms due to the huge amount of TBA+ ions that covered the surface area of mesoporous TiO2. The ηcc of 0.62, D of 4.00 × 10−5 cm2 s−1 and LD of 19.02 μm further support the photovoltaic efficiency of DSSC.

Hierarchical flower-like TiO2 microspheres with improved dye-sensitized solar cell performance

Hierarchical flower-like TiO2 microspheres (FMS) and TiO2 nanorice (NR) were obtained, respectively, by controlling the dosage of Ti precursor via a simple hydrothermal process. Flower-like TiO2 microspheres consist of nanopetals grown from the center radially, the nanopetals are about several nm in average thickness, and each nanopetal has a thinned tip with an average size of 15 nm. The unique hierarchical TiO2 microspheres with large surface area (118.6 m2 g−1) suggested its potential application in dye-sensitized solar cells (DSSCs). The power conversion efficiency of FMS-based DSSCs (9.58%) is much higher than that of NR-based DSSCs (7.13%), which could be ascribed to its excellent light-scattering and dye absorption ability, shorter electron transport pathway and longer electron recombination time derived from the thin thickness and large specific surface area of nanopetals.

Insights into the synergetic effect for enhanced UV/visible-light activated photodegradation activity via Cu-ZnO photocatalyst

A series of Cu-doped ZnO (Cu-ZnO) photocatalysts have been synthesized by a simple hydrothermal method. X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (UV–vis DRS), Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) confirmed that Cu ions have been effectively incorporated into the ZnO lattice, and as the doped Cu amounts increased to 1.5%, the Cu2O will be formed. Under the effect of Cu ions and the Cu2O/ZnO heterojunction, the prepared 1.5% Cu-ZnO nanocomposites exhibited superior catalytic activity in photodegradation of Rhodamine B (RhB) dye under UV light irradiation. RhB could be degraded 99.9% after the irradiation time of 21 min. The photodegradation rate constant achieved 0.2429 min−1, which was 2 times than that of pure ZnO. The results of photoluminescence (PL) and photoacoustics (PA) spectroscopy studies indicated that the recombination efficiency of photogenerated electrons and holes was decreased for 1.5% Cu-ZnO nanocomposites as compared to that of samples. The surface photovoltage (SPV) and the transient photovoltage (TPV) measurements directly demonstrated that separation efficiency was improved, and the recombination time of electrons and holes in 1.5% Cu-ZnO nanocomposites was prolonged due to the synergistic effect of the impurity level and p-n heterojunction. Finally, the possible mechanism is proposed to illuminate the photogenerated charge behavior under UV light irradiation，and the roles of doped Cu and precipitated Cu2O in the photodegradation process were discussed. This work opens a novel avenue to preparing the metal doping photocatalysts with efficiently charge separation and charge transfer.

Aqueous synthesis of Mn-doped CuInSe2 quantum dots to enhance the performance of quantum dot sensitized solar cells.

Herein, we present the direct aqueous synthesis of manganese (Mn) doped CuInSe2 (Mn-CISe) quantum dots (QDs) under microwave irradiation to improve the photochemical properties of solar cells. As a result of Mn doping, the narrower bandgap energy of Mn-CISe leads to higher visible light absorption. The Mn-CISe QDs are therefore used as photosensitizers in quantum dot sensitized solar cells (QDSSCs), exhibiting enhanced performance which is dependent on Mn concentration. To the best of our knowledge, this is the first time to construct an Mn-CISe sensitized-TiO2 photoanode to boost the photovoltaic performance of QDSSCs. The incorporation of Mn into CISe increases short-circuit current, which is ascribed to the effective injection of the excited electrons from QDs into TiO2 and the consequent higher electron lifetime, likely through a newly formed Mn midgap in the CISe band structure. Compared to the undoped QDs, Mn-CISe QDSSCs show a shorter electron transport time (τt) and a longer electron recombination time (τr) which are studied by intensity-modulated photocurrent spectroscopy and intensity-modulated photovoltage spectroscopy, respectively. In fact, a combination of higher light-harvesting efficiency, slower charge recombination, and a longer electron lifetime gives rise to a maximum photovoltaic performance of 6.28%.

Fabrication of Fe3O4/ZnO magnetite core shell and its application in photocatalysis using sunlight

Three chore-shell magnetite ZnO catalysts (Fe3O4/ZnO: M1, Fe3O4/SiO2/ZnO: M2, Fe3O4/SiO2-APTS/ZnO: M3) were prepared, characterized and tested for their photocatalysis in the degradation of methylene blue (MB) dye under sunlight irradiation. M2 and M3 catalysts exhibited better kinetics and higher activity. The silica coating of Fe3O4 in M2 and M3 catalysts enhanced efficient transport of MB dye to the ZnO site and thus enhanced photocatalysis. Photocatalytic reactivity was also related to the defects present in the samples under the synthesized conditions. Photoluminescence studies indicated the order of the defects in ZnO catalyst as M2>M3>M1. The presence of ZnO defects in the samples (M2 & M3) slowed electron recombination time, enhanced hydroxyl radical formation and hence performed faster degradation of MB. The significant enhancement in photoactivity could be attributed to the synergistic effect of adsorption by silica and ZnO catalysis. The recycle experiments using external magnetic showed the remained activity which demonstrated the stable and efficient magnetic separable catalysts.

One-dimension carbon self-doping g-C3N4 nanotubes: Synthesis and application in dye-sensitized solar cells

One-dimension carbon self-doping g-C3N4 nanotubes (CNT) with abundant communicating pores were synthesized via thermal polymerization of saturated or supersaturated urea inside the framework of a melamine sponge for the first time. A ∼16% improvement in photoelectric conversion efficiency (η) is observed for the devices fabricated with a binary hybrid composite of the obtained CNT and TiO2 compared to pure TiO2 device. The result of EIS analysis reveals that the interfacial resistance of the TiO2-dye|I3−/I− electrolyte interface of TiO2-CNT composite cell is much lower than that of pure TiO2 cell. In addition, the TiO2-CNT composite cell exhibits longer electron recombination time, shorter electron transport time, and higher charge collection efficiency than those of pure TiO2 cell. Systematic investigations reveal that the CNT boosts the light harvesting ability of the photovoltaic devices by enhancing not only the visible light absorption but also the charge separation and transfer.

A combined experimental and theoretical investigation of donor and acceptor interface in efficient aqueous-processed polymer/nanocrystal hybrid solar cells

As a route to improving the energy conversion of organic-inorganic hybrid-solar cells, we have tested the performance of poly (phenylene vinylene) (PPV), poly(2,5-thienylene vinylene) (PWTV) polymers and CdTe nanocrystal devices produced via aqueous-processing. It is found that small differences in the conformation of the sensitizer lead to dramatic effects on the solar cell efficiency. Using a combination of UV-Vis absorption spectroscopy and first-principles non-adiabatic molecular dynamics (NAMD) based on time-dependent density-functional theory (TDDFT), PPV is found to have a longer electron injection and recombination time despite seeming to have a better energy alignment with the substrate, which leads to a higher devices performance than PWTV. The present results shed new light on the understanding of organic-inorganic hybrid-solar cells and will trigger further experimental and theoretical investigations.

Nano-plasmonic near field phase matching of attosecond pulses

Nano-structures excited by light can enhance locally the electric field when tuned to plasmonic resonances. This phenomenon can be used to boost non-linear processes such as harmonic generation in crystals or in gases, Raman excitation, and four wave mixing. Here we present a theoretical investigation of the near-field phase matching of attosecond pulses emitted by high-order harmonic generation (HHG) of an atom immersed in a multi-cycle femtosecond infrared laser field and a spatially inhomogeneous plasmonic field. We demonstrate that the spatial inhomogeneity factor of the plasmonic field strongly affects the electron trajectory and recombination time which can be used to control the attosecond emission. For further insight into the plasmonic field effect, we monitor the phase of each quantum path as a function of the inhomogeneity strength. Moreover, we investigate the attosecond emission as a function of near-field phase matching effects. This is achieved by calculating the coherent field superposition of attosecond pulses emitted from various intensities or field inhomogeneities. Finally, far-field and near-field phase matching effects are combined to modulate the harmonic spectral phase towards the emission of a single attosecond pulse.