Recombination Centers Of Charge Carriers Research Articles

TiO2 nanotube arrays as a photoelctrochemical platform for sensitive Ag+ ion detection based on a chemical replacement reaction

Silver ions (Ag+) are essential for living organisms; however, they are toxic at high concentrations. Herein, a photoelectrochemical (PEC) platform is constructed using TiO2 nanotube arrays (TiNTs) prepared by electrochemical anodization of electrode substrate, with high photocurrent response in visible-light region by coating PbS nanocrystals (NCs) through continuous chemical bath deposition (CBD) technique. Combining the low band-gap of PbS with the improved photogenerated electron-hole separation at PbS/TiO2 heterojunctions, the resulting PbS/TiNTs electrode possesses a highly improved PEC response under visible light. Owing to an efficient chemical replacement of Pb(II) in PbS by targeted Ag+ ions, the formed Ag2S components become the new recombination centers for charge carriers, leading to decreased PEC response. Based on photocurrent reduction, the PEC electrode demonstrated an excellent sensing ability for trace Ag+ in solution with a linear range of 5.0 × 10−3 ∼ 5.0 × 10−8 mol L−1 and a limit of detection reaching 1.5 × 10−8 M (S/N = 3). The semiconductor hybrid nanotubes-based photoelectrochemical platform provides a promising route to design highly sensitive and selective sensors for directly detecting ions or molecules.

Vacancy-Regulated Charge Carrier Dynamics and Suppressed Nonradiative Recombination in Two-Dimensional ReX2 (X = S, Se).

Point defects in semiconductors usually act as nonradiative charge carrier recombination centers, which severely limit the performance of optoelectronic devices. In this work, by combining time-domain density functional theory with nonadiabatic molecular dynamics simulations, we demonstrate suppressed nonradiative charge carrier recombination and prolonged carrier lifetime in two-dimensional (2D) ReX2 (X = S, Se) with S/Se vacancies. In particular, a S vacancy introduces a shallow hole trap state in ReS2, while a Se vacancy introduces both hole and electron trap states in ReSe2. Photoexcited electrons and holes can be rapidly captured by these defect states, while the release process is slow, which contributes to an elongated photocarrier lifetime. The suppressed charge carrier recombination lies in the vacancy-induced low-frequency phonon modes that weaken electron-phonon coupling, as well as the reduced overlap between electron and hole wave functions that decreases nonadiabatic coupling. This work provides physical insights into the charge carrier dynamics of 2D ReX2, which may stimulate considerable interest in using defect engineering for future optoelectronic nanodevices.

Fully Condensed Poly (Triazine Imide) Crystals: Extended π‐Conjugation and Structural Defects for Overall Water Splitting

AbstractConventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine‐based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π‐conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π‐conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one‐step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.

Fully Condensed Poly (Triazine Imide) Crystals: Extended π‐Conjugation and Structural Defects for Overall Water Splitting

AbstractConventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine‐based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π‐conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π‐conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one‐step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.

Structural Features Dictate the Photoelectrochemical Activities of Two-Dimensional MoSe2 and WSe2 Nanostructures.

The exfoliation of layered materials into two-dimensional (2D) semiconductors creates new structural domains, for example, basal planes, defect-rich in-planes, and edge sites. These surface species affect the photoelectrochemical (PEC) performance, which in turn determines their applicability in solar energy conversion technologies. In this study, a custom-designed microdroplet cell-based spatially resolved PEC approach was employed to identify the structural parts and to measure the PEC activity of the mechanically exfoliated MoSe2 and WSe2 nanosheets for bulk, few-layer, and monolayer specimens. The PEC performance decreased with the decreasing thickness of nanoflakes, and the relative PEC activity (photo/total current) reduced by introducing more defects to the 2D flakes: 1–3% loss was found for in-plane defects and 30–40% for edge defects. While edge sites act as charge carrier recombination centers, their electrocatalytic activity is higher than that of the basal planes. The comparison of PEC activity of micromechanically and liquid phase exfoliated bulk and few-layer MoSe2 and WSe2 flakes further confirmed that the PEC performance of 2D flakes decreases with an increasing number of edge sites.

Influence of Fe3+ ion doping on the luminescence emission behavior and photocatalytic activity of Fe3+, Eu3+-codoped TiO2 thin films

Abstract Thin films of Fe3+ doped TiO2:0.01Eu3+ nanoparticles were coated by sol-gel spin coating technique on glass substrates. The studies carried out by using X-ray Diffractometer, SEM-EDX, HRTEM, spectroscopic ellipsometer, Raman, EPR, UV–Vis, and fluorescence spectrometer techniques. The crystalline structure and phase formation of Fe doped TiO2:0.01Eu3+ nanoparticles were investigated using X-ray Diffractometer. The HRTEM pictures of 3%, and 5% Fe doped TiO2:0.01Eu3+ films confirmed that the particle size is about 11 nm and 9.5 nm respectively. Raman spectroscopy was used to study the phase identification and vibrational modes present in Fe, Eu codoped TiO2 films. The band gap declined with cumulative amount of Fe3+. The EPR analysis endorses that the Fe3+ ions are successfully doped in the lattice of Titania by replacing Ti4+ ions and formation of oxygen vacancies and Ti3+ sites. With 393 nm excitation, the Fe doped TiO2:0.01Eu3+ nanoparticles show characteristic photoluminescence in the visible range. The strong luminescence emission at 614 nm is related to the 5D0 → 7F2 ED transition. The emission peak intensity declined with an increase in the amount of Fe3+. J–O and radiative parameters of Fe doped TiO2:0.01Eu3+ films obtained from the emission spectra. These nanophosphor films having significant CIE chromaticity coordinate (x, y) values with high color purity might have potential applications in solid-state display devices. The photocatalytic activity (PCA) of Fe, Eu codoped TiO2 nanostructured films were estimated using decolorization of methylene blue (MB) and methyl orange (MO). Fe, Eu codoped TiO2 films exhibit strong photocatalytic performance than undoped and single doped TiO2 films. The degradation efficiencies significantly increase with an increase in Fe3+ doping concentration reached maximum at 3%Fe doping and then reduces gradually at high concentrations due to the formation of recombination centers for charge carriers at a higher amount of Fe doping.

Near-unity photoluminescence quantum yield in inorganic perovskite nanocrystals by metal-ion doping.

The luminescence and charge transport properties of inorganic CsPbX3 perovskite nanocrystals (NCs) make them attractive candidates for various optoelectronic applications, such as lasing, X-ray imaging, light communication, and light-emitting diodes (LEDs). However, to realize cutting-edge device performance, high-quality NCs with high photoluminescence quantum yields (PLQYs) are essential. Therefore, substantial efforts and progress have been made to attain superior design/engineering and optimization of the inorganic NCs with a focus on surface quality, reduced nonradiative charge carrier recombination centers, and improved colloidal stabilities. Metal-ion doping has been proven to have a robust influence on the electronic band structure, PL behavior, and charge carrier recombination dynamics. Thus, in this perspective, we summarize the recent progress of the significant impact of metal cation doping on the optical properties, including the PL enhancement of CsPbCl3, CsPbBr3, and CsPbI3 perovskite NCs. Moreover, we shed light on the mechanism behind such improved properties. We conclude by recommending possible aspects and strategies to be further explored and considered for better utilization of these doped NCs in thin-film optoelectronic and energy conversion devices.

In-situ creating elastic lattice O O bonds over semicrystalline yellow TiO2 nanoparticles for significantly enhanced photocatalytic H2 production

Atomic defects (e.g., Ti3+, oxygen vacancies) have been intenstively investigated for modifying TiO2 in order to reach visible light active photocatalytic H2 production. However, the atomic defects within TiO2 could easily act as photo-generated charge-carrier recombination centers, resulting in relatively low H2 conversion efficiency. In this paper, semicrystalline yellow TiO2 nanoparticles rich of superoxide ions are new synthesized by a simple aqueous solution method. Instead of introducing atomic defects, we show for the first time that catalytic performance can also be significantly improved via in-situ creating lattice OO bonds within metastable semicrystalline TiO2. The synthesized semicrystalline yellow TiO2 exhibits significantly enhanced photocatalytic activity for H2 production after cycle tests. The formaldehyde in aqueous solution is used as target pollutant to simulate industrial wastewater. In-situ created elastic lattice OO bonds are proposed to improve catalytic performance through facilitating the breakage of CH bonds of HCHO. A series of internally consistent reaction equations is proposed that describes the role of in-situ created lattice OO bonds for improving the catalytic performance. This is strongly supported by that the H2 production rate at the end of the fourth cycle test is significantly more than that of the beginning of the first cycle test.

Tunable whole visible region color emission, enhancing emission intensity and persistent performance of a self-activated phosphor:Na2CaSn2Ge3O12

The composition and tunable luminescence properties of garnet phosphors are flexible because of their unique A3B2C3O12 structure. Garnet phosphors with tailored luminescence properties can be designed by varying in the A, B, and C cation sublattices. In this research, the self-activated long-persistent phosphor (LPP) Na2CaSn2Ge3O12 (NCSG) was successfully synthesized by a solid-state method. A series of dopants (Mn2+, Pr3+, Tb3+, and Dy3+) were selected as single-component emitters to tailor the emission color of the NCSG phosphor. NCSG emits in yellow at ~ 560 nm and in the near-infrared at ~ 790 nm with long-persistent luminescence (PersL). By doping with these ions, the PersL of the NCSG was significantly improved, and various colors covering the entire visible region from violet to the near-infrared were obtained. The trap types and depths were examined by electron paramagnetic resonance (EPR) and the thermo-luminescence (TL) glow curves. The EPR signals revealed that oxygen vacancies are the trap centers. The TL glow curves showed that the intrinsic and extrinsic luminescence centers both act as charge carrier recombination centers. Furthermore, the PersL mechanism of the NCSG and NCSG:R (R = Mn2+, Pr3+, Tb3+ and Dy3+) phosphors was discussed in detail. The strategies employed in this work can be an effective way to fabricate new multi-wavelength, garnet-based LPPs with potential applications.

Electronic and optical properties study on Fe[sbnd]B co-doped anatase TiO2

We investigate the density of states and optical properties for Fe, 2B and (Fe, 2B) doped TiO2 with DFT calculations. The calculated results reveal mono-doping introduces midgap states which are half-occupied and easy to become the recombination centers of charge carriers, thus inhibiting the enhancement of photocatalystic activity. The coupling of 2p-3d states in the (Fe, 2B) compensated co-doped TiO2 makes gap states couple with the valence bands edge, thus greatly causing the band gap narrowing and higher visible light absorption. Moreover, the gap states cannot become recombination centers of the photoexcited carriers, thus promoting the separation of electron-hole pairs, prolonging the lifetime of carriers. The analysis of electron density indicates more electrons from Fe transfer to adjacent B, realizing the charge compensation and forming a stronger FeB bond. Therefore, the (Fe, 2B) compensated co-doped TiO2 exhibits the higher visible-light photocatalystic activity than those of pure and solely doped TiO2.

Period-doubling reconstructions of semiconductor partial dislocations

Atomic-scale understanding and control of dislocation cores is of great technological importance, because they act as recombination centers for charge carriers in optoelectronic devices. Using hybrid density-functional calculations, we present period-doubling reconstructions of a 90° partial dislocation in GaAs, for which the periodicity of like-atom dimers along the dislocation line varies from one to two, to four dimers. The electronic properties of a dislocation change drastically with each period doubling. The dimers in the single-period dislocation are able to interact, to form a dispersive one-dimensional band with deep-gap states. However, the inter-dimer interaction for the double-period dislocation becomes significantly reduced; hence, it is free of mid-gap states. The Ga core undergoes a further period-doubling transition to a quadruple-period reconstruction induced by the formation of small hole polarons. The competition between these dislocation phases suggests a new passivation strategy via population manipulation of the detrimental single-period phase. Theoretical simulations suggest a pathway toward reducing the population of efficiency-draining defects in gallium arsenide (GaAs) solar cells. Modern, multilayered light-harvesting devices can have lattice mismatches at interfaces where the crystals meet, and this can create dislocations - two-dimensional ribbons of atoms that disrupt the bulk crystal structure. Joongoo Kang from DGIST in Daegu, Korea, and collaborators at the USA's National Renewable Energy Laboratory have used sophisticated density-functional calculations to analyse GaAs dislocations that had different patterns of partnered atomic dimers. Repetitive ‘single-period’ dimers had deep gap states that degrade the optoelectronic performance. But less dense ‘double-period’ and newly discovered ‘quadruple-period’ structures can compete with the single-period phase and limit its influence — a passivation strategy that may be realized through shallow p-type doping of GaAs, suggest the researchers. Using hybrid density-functional calculations, we present period-doubling reconstructions of a 90° partial dislocation in GaAs, for which the periodicity of like-atom dimers along the dislocation line varies from one to two, to four dimers. The electronic properties of a dislocation change drastically with each period doubling, suggesting a new passivation strategy by biasing the competition between these phases through the tuning of the carrier density in the dislocation.

On the generation of charge-carrier recombination centers in the sapphire substrates of silicon-on-sapphire structures

A method for the production of high-quality radiation-resistant silicon-on-sapphire structures through the fabrication of a layer of nanopores in sapphire by helium ion implantation, i.e., by creating charge-carrier recombination centers, is proposed. In this case, the quality of the silicon layer is simultaneously improved. The problem of the thermal stability of the pores is discussed with the aim of analyzing the possibility of producing a microcircuit on the resultant modified silicon-on-sapphire sample. The layer of pores possesses a large total surface area and, hence, decreases the lifetime of charge carriers generated during irradiation of the operating microcircuit. This effect reduces the charge at the silicon-sapphire interface and improves radiation resistance.

Exploring the effect of boron and tantalum codoping on the enhanced photocatalytic activity of TiO2

Aiming to improve the photocatalytic activity, boron and tantalum co-doped TiO2 nanocrystals were prepared by a sol–gel method. Pure and doped titania samples were characterized by X-ray diffraction (XRD) measurements, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectroscopy. The results revealed that all the samples have highly crystalline anatase structure, and no any other second phase was observed. It is found that the incorporation of dopants inhibits the growth of TiO2 nanocrystal. Moreover, XPS observations showed that Ta are in the form of Ta5+ while the majority of B species are present at interstitial sites. Finally, photocatalytic activities of all the samples were evaluated by degradation of methylene blue (MB) solutions under simulated solar light irradiation. Ta-doped and (B, Ta)-codoped catalysts exhibit enhanced photocatalytic activities in comparison with pure TiO2, with the codoped sample exhibiting the best photodegradation performance. This indicates that B and Ta co-doping is an effective way to improve photocatalytic performance of TiO2. The catalytic activity of B-doped TiO2 becomes slightly worse than pure TiO2 nanocrystals, which is plausible due to increased charge carrier recombination centers induced by incorporation of boron species.

Improvement of Hematite as Photocatalyst by Doping with Tantalum

The use of tantalum as a highly effective dopant for hematite photoelectrochemistry (PEC) has shown contradictory results in previous reports. We show here through screening of different compositions by scanning electrochemical microscopy that Ta doping significantly improves the PEC performance of dropcast films on fluorine-doped tin oxide (FTO). In studies with larger electrodes, a 2% Ta-doped hematite photoanode fabricated at 500 °C shows the highest improvement of photoactivity, which is ∼32 times higher than pure hematite even under visible light. At fabrication temperature higher than 500 °C (e.g., 600, 680 °C), the substrate FTO becomes more resistive and the dopant Ta prefers to segregate from the bulk phase (α-Fe2O3) and forms tantalum fluoride oxide (TaO2F), which may act as charge-carrier recombination centers, and the corresponding Ta-doped samples show much lower photoactivities. Ta-doped hematite samples show stronger (110) diffraction as compared with the pure α-Fe2O3. We show that the doping of Ta induced a preferential growth along the {001} basal plane, which has been reported to have good conductivity. We found the conductivity of the Ta-doped hematite was improved up to at least about one order of magnitude after the incorporation of Ta, with the improved carrier mobility decreasing recombination of the photogenerated holes and electrons.

Synthesis of Ag/ZnO nanorods array with enhanced photocatalytic performance

Silver-modified ZnO nanorods array has been prepared and the effect of silver modification has been studied. ZnO nanorods array were fabricated through a wet chemical route and a photo deposition method was taken to fabricate silver nano particulate on the ZnO nanorods. The structural and optical properties were characterized by field emission scanning electron microscope, high resolution transmission electron microscope, X-ray photoelectron spectroscopy, Raman, UV–vis and photoluminescence (PL) spectra. The UV photocatalytic activity of these materials was studied by analyzing the degradation of methylene blue (MB) in aqueous solution. The photocatalytic performance indicated that Ag deposit acted as not only electron sinks to enhance the separation of photoexcited electrons from holes, but also charge carrier recombination centers, so the optimized amount of Ag deposit was investigated.

Radiation defects in SrB 4O 7:Eu 2+ crystals

Radiation-induced defects in SrB 4O 7:Eu 2+ (0.033 at.%) single crystal irradiated with γ and X-ray quanta has been studied. The induced optical absorption in the 400–700 nm region has been ascribed to F + centers. The Eu 2+ ions have been shown to act simultaneously as traps and as radiative recombination centers of charge carriers. Basing on the thermally stimulated luminescence (TSL), optical absorption and photoluminescence studies of SrB 4O 7:Eu 2+ crystals, a TSL mechanism has been proposed associated with the decay of F + centers being in non-equivalent crystallographic positions followed by radiative recombination of charge carriers on europium ions. Various positions of localization of the radiation-induced defects in the SrB 4O 7 crystal structure have been discussed.

Photovoltaic properties of SnCl 2Pc films and SnCl 2Pc/pentacene heterostructures

The optical and photovoltaic properties of dichlorotin phthalocyanine (SnCl 2Pc) films and SnCl 2Pc/pentacene (Pn) heterostructures (HS) have been studied. Weak bands at 1.35, 1.52 and 2.05 eV have been found in absorption and modulated photoreflectance spectra of SnCl 2Pc films. These bands can be caused by the formation of charge transfer states. The low concentration of recombination centers of charge carriers has been formed on a free surface of SnCl 2Pc films. This concentration essentially decreases at air evacuation before vacuum deposition of a Pn layer. Therefore, interface with an insignificant recombination rate of charge carriers is formed for SnCl 2Pc/Pn HS.

Passivation of defects at the SrTiO3/Si interface with H and H2

A silicon surface covered with ∼150 Å of strontium titanium oxide was exposed to both atomic and molecular hydrogen at temperatures between 20 and 300 °C. A rf probe was used to continuously monitor changes in charge-carrier recombination centers at the SrTiO3/Si interface by following the steady-state photogenerated carrier concentration in the silicon. Independent passivation of interfacial defects was observed by both atomic and molecular hydrogen.

Rate constants for the reaction of H2 with defects at the SiO2/Si(111) interface

A radio frequency probe has been used to monitor changes in charge-carrier recombination centers at a SiO2/Si interface by following the steady-state photogenerated carrier concentration in the silicon. A silicon surface covered with ∼200 Å of thermal oxide was exposed to gaseous molecular hydrogen at temperatures between 135 and 300 °C. The reaction was found to occur with a nonexponential rate law that could be described by a cluster of rate constants governed by a Gaussian distribution of activation energies. At high temperatures, the dominant reaction was found to be that of “Pb” centers with a rate law that is consistent with that previously obtained by electron paramagnetic resonance. At low temperatures the reaction is dominated by the passivation of a carrier recombination center with a much lower activation energy. This new species makes up about 25% of the total recombination center concentration at the Si/SiO2 interface. Almost 20% of the recombination centers could not be passivated by H2.

Charge carrier recombination centers in high-purity, dislocation-free, float-zoned silicon due to growth-induced microdefects

Swirl defects (A- or B-type) and frozen-in defects are found to be carrier recombination centers in dislocation-free, as-grown, float-zoned Si crystals. The A-type defect has an effective carrier capture range of ∼ 40 μm, as determined by EBIC analysis. The activation energy for forming the fast cooling frozen-in defects was estimated to be about 0.31 eV. Hydrogen doping can suppress swirl defect formation but causes other recombination centers. Defect formation can be changed by growth conditions, and long minority charge-carrier lifetimes are achieved through moderately high growth speeds and low thermal gradients during crystal growth, by which both swirl-defects and frozen-in defects are avoided.