Surface Recombination Processes Research Articles

Microstructure of tungsten coatings effect on deuterium plasma-driven permeation through RAFM steel

Tungsten (W) coatings with different morphologies were deposited on CLF-1 steel by changing the sputtering time and discharge power using the magnetron sputtering method. Deuterium (D) plasma-driven permeation (PDP) experiments through W-coated CLF-1 were then carried out in the temperature range of 583–833 K. The obtained results showed that dense W coatings reduced steady-state permeation flux and prolonged the time of permeation flux reaching steady-state. Due to the existence of high-density intrinsic defects, D effective diffusivities of the coating are lower than that of bulk W. The increase of coating thickness reduced permeation flux. In addition, by changing the bias voltage and studying the influence of incident ion energy on D-PDP, it is found that the introduction of W coating affected the overall D transport by re-balancing the bulk diffusion and surface recombination processes. The thickness and microstructure of W coatings played a key role in determining the D steady-state permeation flux at varying temperatures.

(Invited) Molecular Reaction Imaging of Charge Transport in MoS2 Nanoflake Photoelectrodes

Transition metal dichalcogenides (TMD) such as WSe2 and MoS2 are highly efficient and stable light absorbers in TMD∣I−,I3 −∣Pt liquid junction solar cells. It is generally accepted that TMD crystals with a large fraction of exposed edge sites exhibit lower power conversion efficiencies (PCEs) than apparently smooth crystals. However, one open question is why does the PCE vary significantly from one crystal to another? Answering this critical question could lead to robust syntheses for high quality and uniform TMD samples. In this work, we apply nanoscale photoelectrochemical microscopy techniques to study n-type TMD nano flake∣I−,I3 −∣Pt cells. Using a combination of near-diffraction-limited photocurrent mapping and molecular reaction imaging techniques, we reveal a previously hidden surface recombination process: photogenerated holes in hidden p-type domains travel micron-scale distances parallel to the solid/liquid interface and preferentially react with iodide at step-edges. The overall efficiency of the nanoflake, as evidenced from whole nanoflake-level photoelectrochemical measurements, is dictated by the size, efficiency, and location of n- and p-type domains. These results provide a unifying view of efficiency losses in smooth TMD photoelectrodes and open the possibility to design electrode architectures that leverage the long-range lateral charge transport property for photoelectrocatalysis.

4H-SiC Auger recombination coefficient under the high injection condition

The on-resistance of bipolar devices depends on the carrier lifetime, which is determined by Shockley–Read–Hall, surface, radiation, and Auger recombination processes. Values for the Auger recombination coefficient have been previously reported, but the values were constant in each report. However, the Auger recombination coefficient should depend on the concentration of excited carriers and the presence of traps. In this study, we observed excited carrier recombination in 4H-SiC under the high injection condition using time-resolved free carrier absorption measurements. Consequently, we discovered that the Auger recombination coefficient is dependent on the excited carrier concentration and that the traps have negligible effects on the coefficient.

Quantum Dots, Passivation Layer and Cocatalysts for Enhanced Photoelectrochemical Hydrogen Production.

Solar-driven photoelectrochemical (PEC) hydrogen production is one potential pathway to establish a carbon-neutral society. Nowadays, quantum dots (QDs)-sensitized semiconductors have emerged as promising materials for PEC hydrogen production due to their tunable bandgap by size or morphology control, displaying excellent optical and electrical properties. Nevertheless, they still suffer from anodic corrosion during long-term cycling, offering poor stability. This Review discussed advancements to improve long-term stability of QDs particularly in terms of cocatalysts and passivation layers. The working principle of PEC cells was reviewed, along with all important configurations adopted over recent years. The equations to assess PEC performance were also described. A greater emphasized was placed on QDs and incorporation of cocatalysts or passivation layers that could enhance the PEC performance by influencing the charge transfer and surface recombination processes.

Molecular Reaction Imaging of a Surface Recombination Process Explains Performance Variation Among Smooth MoS2 Photoelectrodes

Transition metal dichalcogenides (TMD) such as WSe2 and MoS2 are highly efficient and stable light absorbers in TMD∣I−,I3 −∣Pt liquid junction solar cells. It is generally accepted that TMD crystals with a large fraction of exposed edge sites exhibit lower power conversion efficiencies (PCEs) than apparently smooth crystals. However, one open question is why does the PCE vary significantly from one crystal to another? Answering this critical question could lead to robust syntheses for high quality and uniform TMD samples. In this work, we apply nanoscale photoelectrochemical microscopy techniques to study n-type TMD nanoflake∣I−,I3 −∣Pt cells. Using a combination of near-diffraction-limited photocurrent mapping and molecular reaction imaging techniques, we reveal a previously hidden surface recombination process: photogenerated holes in hidden p-type domains travel micron-scale distances parallel to the solid/liquid interface and preferentially react with iodide at step-edges. The overall efficiency of the nanoflake, as evidenced from whole nanoflake-level photoelectrochemical measurements, is dictated by the size, efficiency, and location of n- and p-type domains. These results provide a unifying view of efficiency losses in smooth TMD photoelectrodes and open the possibility to design electrode architectures that leverage the long-range lateral charge transport property for photoelectrocatalysis.

Synthesis of titanium-doped CdIn2S4 thin film and its photoelectric properties for intermediate band solar cells

The series Titanium-doped CdIn2S4 (Ti-CIS) thin films, with a spinel structure, were successfully synthesized by a typical technology of magnetron sputtering. The prepared Ti-CIS thin films were characterized by X-ray diffraction patterns, Raman spectrometer, energy dispersive X-ray analysis, transient photocurrent response and UV–vis–NIR spectrophotometer. The experimental results demonstrated that the optical absorption spectra were widened in Ti-CIS films. Due to Ti doping, two additional sub-band edge responses were observed at about 760 nm (1.63 eV) and 1150 nm (1.07 eV), which indicated a partially filled intermediate band (IB) was formed in the forbidden band of Ti-CGS3 film by the spin-up t2 g states, and leaded to lower-energy photon absorption. Whereas, no additional sub-band edge responses were obviously observed in the optical absorption spectra of the Ti-CIS1 and Ti-CIS2 films, probably because of the Ti-CIS films inferior in quality or less doped-Ti content. The result analysis of the transient photocurrent response further verified that a larger number of carriers are generated in the Ti-CIS films, the surface recombination process rarely occurs. In general, the good photoelectric properties of Ti-CIS films prepared by magnetron sputtering expected to be a promising candidate IB material for the high-efficiency thin film solar cells.

Optothermal characterization on thermal diffusivity, carrier diffusion and recombination properties of annealed thin Au film deposited on Si substrate

In this work, an optothermal wave characterization based on the photoacoustic (PA) technique was first implemented to investigate the thermal and carrier transport properties of Au thin film layer deposited on Si wafers (Au/Si) annealed at different temperatures. The XRD pattern showed that a metastable gold (Au) silicide of Au81Si19 phase formation was traced at annealing temperature of 330 oC and this structure disappeared when the temperature was further elevated up to 370 oC. The result showed that the PA signal obtained for Au/Si structure was lower than the pure Si wafer. The thermal and carrier transport properties of Si and Au/Si were elucidated from the fitting of PA signalphase relation. From the result, the thermal diffusivity and surface recombination velocity of Au/Si increased with shorter recombination lifetime as the raise of annealing temperature. Nonetheless, as the temperature approaches 370 oC, the surface recombination and thermal transport process diminishes, which may be ascribed to the rupture of silicide clusters.

Carrier recombination in SrTiO3 single crystals: impacts of crystal faces and Nb doping

We observed the carrier recombination in SrTiO3 single crystals with several crystal faces and Nd doping concentrations using time-resolved photoluminescence and microwave photoconductivity decay methods. The shapes of the decay curves were independent of the excited carrier concentrations, and thus the carriers recombined through the Shockley–Read–Hall and surface recombination processes. From the measurements for different crystal faces, we found that, although the surface recombination is significant for the (100), (110), and (111) faces, they have similar surface recombination velocities of ∼106 cm s−1. Therefore, as photocatalysts, the shape of the materials has a minor effect on the energy conversion efficiency. Based on the dependence of the Nb doping concentration, a high doping concentration significantly enhances the carrier recombination, whereas a moderate Nb doping induces only a slight reduction in the time constants of the carrier decay. In addition, the time constant increases with temperature, and thus moderate Nb doping will have a positive effect on the energy conversion efficiency of SrTiO3 photocatalysts at high temperatures.

Temperature and spatial dependence of carrier lifetime and luminescence intensity in Ge0.95Sn0.05 layer

In this work we present a study of spatial and temperature dependencies of carrier lifetime and photoluminescence in pseudomorphic Ge0.95Sn0.05 layer on silicon by applying contactless time-resolved differential absorption and photoluminescence techniques. The observed small lifetime and photoluminescence intensity spatial variations (~10%) indicate for good material homogeneity. Differential transmission kinetics exhibit fast bleaching and slow absorption components related to thermalization (few hundreds of picoseconds) and surface recombination (few tens of nanoseconds) processes. Temperature dependent photo-luminescence measurements indicate activation of carriers from L to Γ valley, governing dominant direct photoluminescence emission band at 1850 nm at 300 K. These direct transitions explain more efficient and blue-shifted by 150 nm photoluminescence emission at room temperature. At low temperatures direct emission is related to non-thermalized carriers, while indirect one is caused by depopulation of light hole band with temperature.

Protection Mechanism against Photocorrosion of GaN Photoanodes Provided by NiO Thin Layers

The photoelectrochemical properties of n‐type Ga‐polar GaN photoelectrodes covered with NiO layers of different thicknesses in the range 0–20 nm are investigated for aqueous solution. To obtain layers of well‐defined thickness and high crystal quality, NiO is grown by plasma‐assisted molecular‐beam epitaxy. Stability tests reveal that the NiO layers suppress photocorrosion. With increasing NiO thickness, the onset of the photocurrent is shifted to more positive voltages and the photocurrent is reduced, especially for low bias potentials, indicating that hole transfer to the electrolyte interface is hindered by thicker NiO layers. Furthermore, cathodic transient spikes are observed under intermittent illumination, which hints at surface recombination processes. These results are inconsistent with the common explanation of the protection mechanism that the band alignment of GaN/NiO enables efficient hole‐injection, thus preventing hole accumulation at the GaN surface that would lead to anodic photocorrosion. Interestingly, the morphology of the etch pits as well as further experiments involving the photodeposition of Ag indicate that photocorrosion of GaN photoanodes is related to reductive processes at threading dislocations. Therefore, it is concluded that the NiO layers block the transfer of photogenerated electrons from GaN to the electrolyte interface, which prevents the cathodic photocorrosion.

Effect of argon gas in oxygen catalytic recombination on a silica surface: A reactive molecular dynamics study

Molecular dynamics analysis using a reactive force field (ReaxFF) potential is carried out to investigate microscopically the effect of argon gas in atomic oxygen recombination on a silica surface. The effect of argon atoms is studied by computing both the potential energy surface of incoming atoms and the oxygen recombination efficiency. Potential energy surfaces for oxygen adsorption show that incoming oxygen atoms exhibit more tendency to make a chemical bond with the outer silicon atomic layer. Additionally, the potential energy curves for both oxygen and argon atoms are computed at three different points of interest on the silica surface. The atomic surface recombination process is modeled by imposing a fixed influx boundary condition on the incoming oxygen gas at a partial pressure of 10 atm and temperature of 1000 K. Four different cases of argon gas partial pressure near the surfaces, ranging from 0 to 15 atm, are considered to study the influence of argon atoms on the recombination mechanism. It is observed that the presence of argon atoms has a substantially minor influence on the overall oxygen recombination efficiency on the silica surface.

Understanding Surface Recombination Processes Using Intensity‐Modulated Photovoltage Spectroscopy on Hematite Photoanodes for Solar Water Splitting

AbstractConverting solar energy into hydrogen through photoelectrochemical (PEC) water splitting offers a promising route towards a fully renewable energy economy. A fundamental understanding of photogenerated charge recombination processes in semiconducting photoelectrodes is a key consideration in optimizing solar water splitting cells and intensity‐modulated photovoltage spectroscopy (IMVS) has recently emerged as a promising technique for gaining new insight. However, the interpretation of IMVS data under various conditions (that is, when photoelectrodes are in sacrificial electrolytes or employ catalytic overlayers) is not complete. Using IMVS data we present herein an analysis of charge recombination processes under open circuit conditions on a model photoanode system: nanostructured hematite. Employing two sacrificial oxidation conditions compared to standard water oxidation conditions, our IMVS results establish a direct correlation between surface recombination processes and the low frequency IMVS response. We found that surface intermediate states for water oxidation under open circuit condition exhibit a lifetime of 229 ms under standard illumination conditions. Applying a nickel iron oxide overlayer also gives insight into surface passivation effects through IMVS analysis of photoanode system.

UV-induced photosensing characteristics of SiC and GaN nanowires

PurposeThis study aims to investigate photosensing characteristics of SiC and GaN nanowire-based devices through exposure to UV light. The photocurrent transients have been modeled to determine rise and decay process time constants. The 1D-semiconductor nanowires can exhibit higher light sensitivity compared to bulk materials because of their large surface area to volume ratio and the quantum size effects.Design/methodology/approachNanowire devices have been fabricated through dielectrophoresis for integrating nanowires onto pre-patterned electrodes (10 nm Ti/ 90 nm Au) with a spacing about 3 µm onto SiO2/Si (doped) substrate. The photocurrent measurements were carried out under room temperature conditions with UV light of 254 nm wavelength.FindingsSiCNWs yield very short rise and decay time constants of 1.3 and 2.35 s, respectively. This fast response indicates an enhanced surface recombination of photoexcited electron-hole pairs. Conversely, GaNNWs yield longer rise and decay time constants of 10.3 and 15.4 s, respectively. This persistent photocurrent suggests a reduced surface recombination process for the GaNNWs.Originality/valueHigh selective UV light sensitivity, small size, very short response time, low power consumption and high efficiency are the most important features of nanowire-based devices for new and superior applications in photodetectors, photovoltaics, optical switches, image sensors and biological and chemical sensing.

Excitation wavelength-dependent carrier dynamics in n-type and semi-insulating 6H-SiC using ultrafast transient absorption spectroscopy

Transient absorption spectroscopy was utilized to monitor ultrafast carrier dynamics in conductive and semi-insulating 6H-SiC wafers. By tuning the excitation wavelength around the bandgap of 6H-SiC (365–400 nm), the surface and bulk recombination processes could be accordingly distinguished. Simultaneously, the locations of surface and defect states in the bandgap were inferred by investigating the evolution of transient absorption spectra for different photoexcited carrier distributions. Vanadium (V) doping had a notable modulation on transient absorption in semi-insulating 6H-SiC, resulting from an additional decay process induced by carrier capturing of V deep levels. The carrier lifetimes induced by various recombination mechanisms were determined and interpreted unequivocally by global analysis and simplified model. The time constant via bulk capturing (∼400 ps) was three orders of magnitude faster than that via inherent defects. Controlling the lifetime by excitation wavelength and doping conditions is essential for fabricating 6H-SiC-based ultrafast photonic devices.

Radiative and Non-Radiative Recombination Pathways in Mixed-Phase TiO2 Nanotubes for PEC Water-Splitting

Anatase and rutile mixed-phase TiO2 with an ideal ratio has been proven to significantly enhance photoelectrochemical (PEC) activity in water-splitting applications due to suppressing the electron–hole recombination. However, the mechanism of this improvement has not been satisfactory described yet. The PEC water oxidation (oxygen evolution) at the interface of TiO2 photoanode and electrolyte solution is determined by the fraction of the photogenerated holes that reach the solution and it is defined as the hole transfer efficiency. The surface and bulk recombination processes in semiconductor photoanodes majorly influence the hole transfer efficiency. In this work, we study the hole transfer process involved in mixed-phase TiO2 nanotube arrays/solution junction using intensity-modulated photocurrent and photovoltage spectroscopy (IMPS and IMVS); then, we correlate the obtained hole transfer rate constants to (photo)electrochemical impedance spectroscopy (PEIS) measurements. The results suggest that the enhanced performance of the TiO2 mixed-phase is due to the improved hole transfer rate across the TiO2/liquid interface as well as to the decrease in the surface trap recombination of the holes.

Water Oxidation and Electron Extraction Kinetics in Nanostructured Tungsten Trioxide Photoanodes.

A thorough understanding of the kinetic competition between desired water oxidation/electron extraction processes and any detrimental surface recombination is required to achieve high water oxidation efficiencies in transition-metal oxide systems. The kinetics of these processes in high Faradaic efficiency tungsten trioxide (WO3) photoanodes (>85%) are monitored herein by transient diffuse reflectance spectroscopy and correlated with transient photocurrent data for electron extraction. Under anodic bias, efficient hole transfer to the aqueous electrolyte is observed within a millisecond. In contrast, electron extraction is found to be comparatively slow (∼10 ms), increasing in duration with nanoneedle length. The relative rates of these water oxidation and electron extraction kinetics are shown to be reversed in comparison to other commonly examined metal oxides (e.g., TiO2, α-Fe2O3, and BiVO4). Studies conducted as a function of applied bias and film processing to modulate oxygen vacancy density indicate that slow electron extraction kinetics result from electron trapping in shallow WO3 trap states associated with oxygen vacancies. Despite these slow electron extraction kinetics, charge recombination losses on the microsecond to second time scales are observed to be modest compared to other oxides studied. We propose that the relative absence of such recombination losses, and the observation of a photocurrent onset potential close to flat-band, result directly from the faster water oxidation kinetics of WO3. We attribute these fast water oxidation kinetics to the highly oxidizing valence band position of WO3, thus highlighting the potential importance of thermodynamic driving force for catalysis in outcompeting detrimental surface recombination processes.

Generation of femtosecond extreme ultraviolet pulses using low-energy electron beams for a pump-probe experiment

The surface recombination process and molecular dynamics are generally on the order of tens of femtoseconds; therefore, the research and development of an accelerator-based intense and short-pulse generation scheme are needed for pump-probe experiments, which are widely utilized tools for investigating fast molecular dynamics. Here, we propose an echo-enabled harmonic generation (EEHG)-technique-based free electron laser (FEL) scheme that uses a low-energy beam ( KE∼200MeV). The proposed scheme is designed to generate short-pulse soft extreme ultraviolet radiation at ∼80nm, with a pulse duration of 3 fs for the full width at half maximum. An electron injector consisting of a photo-cathode-based S-band radio frequency electron-gun, solenoid magnets, and three S-band accelerating columns was designed and optimized using a multiobjective particle swarm optimization method. For the EEHG-FEL section, the narrow bands of electrons produced by a second modulator and a few-cycle laser pulse with a linear momentum compaction at the second chicane had a perfect upright position at the top of the current modulation produced by the first modulator, which enhanced the peak current by a factor of approximately 30 %. In this scheme, two conventional lasers with wavelengths of 5.2μm and 800 nm were adopted to enhance the high bunching factors by generating microbunching structures. The saturated output power of the proposed FEL was approximately 4.97 MW.

Calculation of Quantum Efficiency of Photodetectors Based on A3B5 Metal-Solid Solutions Contacts

In our investigation, we have modelled the quantum efficiency of the photodetecting structures based on the p-n junction and metal-semiconductor contacts. The calculation has been performed based on the solution of the continuity equation, taking into account the impact of the surface recombination. In most cases, this process has an adverse effect on the parameters of instruments; in particular, in photodetectors, this effect should reduce sensitivity. However, in our investigation it is shown that the effect of the surface recombination may be used as a useful factor for developing selective photodetectors. The effect of the surface recombination process will significantly increase with an increase of the absorption coefficient, since in this case more photons will be absorbed at the surface. Since there is a significant dependence of the absorption coefficient on the incident photon energy in semiconductors, the effect of the surface recombination on the photodetector sensitivity in the long-wave and short-wave regions of the spectral range will be significantly different.

Tungsten-doped TiO2/reduced Graphene Oxide nano-composite photocatalyst for degradation of phenol: A system to reduce surface and bulk electron-hole recombination

Recombination of photogenerated charges is the main factor affecting the photocatalytic activity of TiO2. Here, we report a combined strategy of suppressing both the bulk as well as the surface recombination processes by doping TiO2 with tungsten and forming a nanocomposite with reduced graphene oxide (rGO), respectively. Sol-gel method was used to dope and optimize the concentration of W in TiO2 powder. UV-Vis, XPS, PL and time resolved PL spectra along with DFT calculations indicate that W6+ in TiO2 lattice creates an impurity level just below the conduction band of TiO2 to act as a trapping site of electrons, which causes to improve the lifetime of the photo-generated charges. Maximum reduction in the PL intensity and the improvement in charge carrier lifetime was observed for TiO2 doped with 1 at.% W (1W-TiO2), which also displayed the highest photo-activity for the degradation of p-nitro phenol pollutant in water. Tuning of rGO/TiO2 ratio (weight) disclosed that the highest activity can be achieved with the composite formed by taking equal amounts of TiO2 and rGO (1:1), in which the strong interaction between TiO2 and rGO causes an effective charge transfer via bonds formed near the interface as indicated by XPS. Both these optimized concentrations were utilized to form the composite rGO/1W-TiO2, which showed the highest activity in photo-degradation of p-nitro phenol (87%) as compared to rGO/TiO2 (42%), 1W-TiO2 (62%) and pure TiO2 (29%) in 180 min. XPS and PL results revealed that in the present nanocomposite, tungsten species traps the excited electron to reduce the interband recombination in the bulk, while the interaction between TiO2 and rGO creates a channel for fast transfer of excited electrons towards the latter before being recombined on the surface defect sites.

Probing the charge recombination in rGO decorated mixed phase (anatase-rutile) TiO2 multi-leg nanotubes

Recombination of photo-generated charges is one of the most significant challenges in designing efficient photo-anode for photo electrochemical water oxidation. In the case of TiO2, mixed phase (anatase-rutile) junctions often shown to be more effective in suppressing electron-hole recombination compared to a single (anatase or rutile) phase. Here, we report the study of bulk and surface recombination process in TiO2 multi-leg nanotube (MLNTs) anatase-rutile (A-R) junctions decorated with reduced graphene oxide (rGO) layers, through an analysis of the photo-current and impedance characteristics. To quantify the charge transport/transfer process involved in these junctions, holes arriving at the interface of semiconductor/electrolyte were collected by adding H2O2 to the electrolyte. This enabled us to interpret the bulk and surface recombination process involved in anatase/rutile/rGO junctions for photo-electrochemical water oxidation. We correlated this quantification to the electrochemical impedance spectroscopy (EIS) measurements, and showed that in anatase/rutile junction the increase in PEC performance was due to suppression in electron-hole recombination rate at the surface states that effectively enhances the hole transfer rate to the electrolyte. On the other hand, in rGO wrapped A-R MLNTs junction it was due to both phenomenon i.e decrease in bulk recombination rate as well as increase in hole transfer rate to the electrolyte at the semiconductor/electrolyte interface.