Photoelectron Lifetime Research Articles

Supersolidity of undercoordinated and hydrating water.

Supersolidity of ice, which was proposed in 2013 and intensively verified since then [C. Q. Sun et al., Density, Elasticity, and Stability Anomalies of Water Molecules with Fewer than Four Neighbors, J. Phys. Chem. Lett., 2013, 4, 2565-2570; C. Q. Sun et al., Density and phonon-stiffness anomalies of water and ice in the full temperature range, J. Phys. Chem. Lett., 2013, 4, 3238-3244], refers to the water molecules being polarized by molecular undercoordination, which is associated with the skin of bulk ice, nanobubbles, and nanodroplets (often called confinement), or by the electrostatic field of ions in salt solutions [X. Zhang et al., Mediating relaxation and polarization of hydrogen-bonds in water by NaCl salting and heating, Phys. Chem. Chem. Phys., 2014, 16(45), 24666-24671; C. Q. Sun et al., (H, Li)Br and LiOH solvation bonding dynamics: molecular nonbond interactions and solute extraordinary capabilities, J. Phys. Chem. B, 2018, 122(3), 1228-1238]. From the perspective of hydrogen bond (O:H-O or HB with ":" representing the lone pairs on O2-) cooperative relaxation and polarization, this review features the recent progress and recommends future trends in understanding the bond-electron-phonon correlation in the supersolid phase. Supersolidity is characterized by a shorter and stiffer H-O bond, longer and softer O:H nonbond, deeper O 1s energy band, and longer photoelectron and phonon lifetimes. The supersolid phase is less dense, viscoelastic, and mechanically and thermally more stable. Furthermore, O:H-O bond cooperative relaxation offsets the boundaries of structural phases and increases the melting point while lowering the freezing temperature of ice, which is known as supercooling and superheating.

Selective Doping of Quantum Dot Nanomaterials for Managing Intersubband Absorption, Dark Current, and Photoelectron Lifetime

ABSTRACTNovel approach to optimize quantum dot (QD) materials for specific optoelectronic applications is based on engineering of nanoscale potential profile, which is created by charged QDs. The nanoscale barriers prevent capture of photocarriers and drastically increase the photoelectron lifetime, which in turn strongly improves the photoconductive gain, responsivity, and sensitivity of photodetectors and decreases the nonradiative recombination losses of photovoltaic devices. QD charging may be created by various types of selective doping. To investigate effects of selective doping, we model, fabricated, and characterized AlGaAs/InAs QD structures with n-doping of QD layers, doping of interdot layers, and bipolar doping, which combines p-doping of QD layers with strong n-doping of the interdot space. We have measured spectral characteristics of photoresponse, photocurrent and dark current. The experimental data show that providing the same electron population of QDs, the bipolar doping creates the most contrasting nanoscale profile with the highest barriers around dots.

Speciation of uranium and doping induced defects in Gd1.98U0.02Zr2O7: Photoluminescence, X-ray photoelectron and positron annihilation lifetime spectroscopy

Based on photoluminescence spectroscopy it was inferred that uranium stabilizes as both U(IV) as well as U(VI) in Gd2Zr2O7 which was also corroborated using X-ray photo electron spectroscopy (XPS). Absence of equidistant vibronic structure in emission spectrum of Gd1.98U0.02Zr2O7 confirmed that U(VI) stabilizes in the form of UO66−. Based on luminescence lifetime it was inferred that majority of UO66− stabilizes at both Gd3+/Zr4+ whereas U4+ stabilizes only at Zr4+ sites. The positron lifetime doesn’t change on uranium doping indicating the formation of antisite defect. Infact it is this antisite defect in Gd1.98U0.02Zr2O7 which favours the stabilization of its fluorite phase.

Light-Induced Peroxide Formation in ZnO: Origin of Persistent Photoconductivity.

The persistent photoconductivity (PPC) in ZnO has been a critical problem in opto-electrical devices employing ZnO such as ultraviolet sensors and thin film transistors for the transparent display. While the metastable state of oxygen vacancy (VO) is widely accepted as the microscopic origin of PPC, recent experiments on the influence of temperature and oxygen environments are at variance with the VO model. In this study, using the density-functional theory calculations, we propose a novel mechanism of PPC that involves the hydrogen-zinc vacancy defect complex (2H-VZn). We show that a substantial amount of 2H-VZn can exist during the growth process due to its low formation energy. The light absorption of 2H-VZn leads to the metastable state that is characterized by the formation of (peroxide) around the defect, leaving the free carriers in the conduction band. Furthermore, we estimate the lifetime of photo-electrons to be ~20 secs, which is similar to the experimental observation. Our model also explains the experimental results showing that PPC is enhanced (suppressed) in oxygen-rich (low-temperature) conditions. By revealing a convincing origin of PPC in ZnO, we expect that the present work will pave the way for optimizing optoelectronic properties of ZnO.

Direct time-domain observation of attosecond final-state lifetimes in photoemission from solids.

Attosecond spectroscopic techniques have made it possible to measure differences in transport times for photoelectrons from localized core levels and delocalized valence bands in solids. We report the application of attosecond pulse trains to directly and unambiguously measure the difference in lifetimes between photoelectrons born into free electron-like states and those excited into unoccupied excited states in the band structure of nickel (111). An enormous increase in lifetime of 212 ± 30 attoseconds occurs when the final state coincides with a short-lived excited state. Moreover, a strong dependence of this lifetime on emission angle is directly related to the final-state band dispersion as a function of electron transverse momentum. This finding underscores the importance of the material band structure in determining photoelectron lifetimes and corresponding electron escape depths.

Relationship between Photoelectron Lifetime and Dielectric Constant and Its Effect on the Photocatalysis

Relationship between Photoelectron Lifetime and Dielectric Constant and Its Effect on the Photocatalysis

Improved photovoltaic performance of mesoporous perovskite solar cells with hydrogenated TiO2: prolonged photoelectron lifetime and high separation efficiency of photoinduced charge

Hydrogenated titanium dioxide (H-TiO2) nanocrystals and nanorods (H-TNRs) are successfully synthesized and employed as electron transfer materials in mesoscopic perovskite solar cells (PSCs).

High visible light photocatalytic property of Co2+-doped TiO2 nanoparticles with mixed phases

Mixed phases Co2+-doped TiO2 nanoparticles have been prepared by a novel method combined with sol–gel and hydrothermal methods. The section of sol–gel method, sol, provides an unstable colloidal reaction system for the next reaction process. The hydrothermal method is to treat the above reaction system to prepare undoped and doped samples. The as-prepared samples have been characterized by XRD, SEM, TEM, HRTEM and UV–vis spectroscopy. The results show that the as-prepared samples contain three titania polymorphs: brookite, rutile and anatase phases. These titania polymorphs probably form polymorph-junctions that can extend the lifetime of photogenerated electron–hole pairs. The photocatalytic activity has been evaluated by the photocatalytic degradation of Rhodamine B in air under visible-light irradiation. The degradation results indicate that the photocatalytic activity of as-prepared samples is higher than that of Degussa P25, especially the doped sample. This is ascribed to the fact that the phases with smaller band gap can enhance visible-light photocatalytic activity, the polymorph-junctions effectively extend the photoelectron lifetime and the nano size effect and Co-doping induce the shift of the absorption edge into the visible-light region. Furthermore, the XRD, SEM, and TEM data indicate that Co2+-doping results in the decrease of particle size.

Ultra-long electron lifetime induced efficient solar energy storage by an all-vanadium photoelectrochemical storage cell using methanesulfonic acid

An all-vanadium photoelectrochemical storage cell (PESC) using methanesulfonic acid (MSA) shows superior photoelectrochemical performance in solar energy storage.

Effects of CH3NH3I on fabricating CH3NH3PbI(3-x)Clx perovskite solar cells

Perovskite solar cell, which is prepared by using the organic-inorganic hybrid halide CH3NH3PbX3 (X = I, Cl and Br), receives widespread attention because of its solution processability and high photon-to-electron conversion efficiency. The highest reported photon-to-electron conversion efficiency is that using CH3NH3PbI(3-x)Clx as an absorber. It is reported that the diffusion length is greater than 1 micrometer in this mixed halide perovskite. The method most commonly used in preparing CH3NH3PbI(3-x)Clx film is the one-step pyrolysis method, which has a complex reaction mechanism. In this paper, we review the work about CH3NH3PbI(3-x)Clx perovskite, in which emphasis is put on the importance of the preparation process, and analyze the role of CH3NH3I in the one-step pyrolysis method for fabricating the CH3NH3PbI(3-x)Clxperovskite layer. Scanning electron microscope images show that CH3NH3I can improve the coverage and crystallinity of the perovskite layer for precursors in low CH3NH3I concentrations (CH3NH3I/PbCl2=2.0 and 2.5). For precursors in high CH3NH3I concentrations (CH3NH3I/PbCl2=2.75 and 3), this change is not obvious. X-ray photoelectron spectroscopy confirms the change of coverage, and indicates that the content of Cl in CH3NH3PbI(3-x)Clx will be less than 5% for precursors with high CH3NH3I concentrations (CH3NH3I/PbCl2>2.5). Perovskite solar cells based on CH3NH3PbI(3-x)Clx with different Cl dopant concentrations are studied by photoelectric measurements. Photocurrent density-photovoltage curves show that the performance of the devices increases with the increase of CH3NH3I concentration in precursors. And the incident-photon-to-current conversion efficiency (IPCE) measurements indicate that the devices fabricated by using precursors with high CH3NH3I concentration have a relatively high external quantum efficiency. These results imply that only CH3NH3PbI(3-x)Clx with very low Cl dopant concentration will be effective material for photovoltaic application. To further understand the difference between these devices during working condition, transient photovoltage/photocurrent measurements are carried out to investigate the carrier dynamics in the device. Transient photovoltage decay curves indicate that high Cl dopant concentration may decrease the photoelectron lifetime in CH3NH3PbI(3-x)Clx, and results in a relative low open-circuit photovoltage in the corresponding photovoltaic devices. The charge transport time in the devices of various Cl concentrations are studied by transient photocurrent decay method. CH3NH3PbI(3-x)Clx with low Cl dopant concentration has relative short transport time, which can avoid the recombination process and increase the charge collection efficiency.

Wide-Band Excited YTiTaO6: Eu3+/Er3+ Phosphors: Structure Refinement, Luminescence Properties, and Energy Transfer Mechanisms

Eu3+-/Er3+-activated YTiTaO6 phosphors have been prepared via conventional solid state reaction process. X-ray diffraction (XRD) and structure refinement, Raman spectra, X-ray photoelectron Spectrum (XPS), photoluminescence (PL) spectra, cathodoluminescence (CL) spectra, and lifetimes were utilized to characterize the synthesized samples. Under UV light excitation, the YTiTaO6 sample shows broad band emission centered near 505 nm due to the Ta(Ti)O6 polyhedron. Eu3+ and Er3+ ions doped YTiTaO6 samples show strong line emissions coming from the characteristic f–f transitions. The energy transfer from the Ta(Ti)O6 group of the host to Eu3+ and Er3+ in YTiTaO6 phosphors has been demonstrated to be a resonant type via a dipole–dipole mechanism, and the critical distance (RC) for host emission to Eu3+ and Er3+ calculated by concentration quenching method are 10.02 and 18.86 A, respectively. Under the low voltage electron beam excitation, the CL spectra of YTiTaO6, YTiTaO6: Eu3+, and YTiTaO6: Er3+ samples are s...

Construction of efficient CdS–TiO2 heterojunction for enhanced photocurrent, photostability, and photoelectron lifetimes

The photoefficiency of CdS/TiO2 electrodes can be enhanced by employing efficient method of CdS sensitization from which, the contact area, thickness of CdS layer, and the recombination of photoelectrons with electrolyte can be controlled. Here, we demonstrate a simple solvothermal approach of CdS quantum dots (QDs) sensitization on TiO2 nanoparticle (NP) film coated on FTO. Our new approach prevents the clogging of CdS QDs and promotes uniform deposition of QDs throughout the mesoporous TiO2 NP film. The sensitization of CdS can be controlled by the reaction time and the concentration of the precursors. The solvothermally sensitized photoanodes exhibit enhanced photocurrents and fill factors and improved photostability in aqueous solution compared to the one prepared by a conventional SILAR method. Open-circuit potential decay measurement under shutting off illumination shows that the lifetime of photoelectron is extended with solvothermally prepared CdS layer, indicating efficient suppression of recombination of the accumulated electron in TiO2 to the electrolyte. This methodology can be applied in making more efficient heterojunctions consisting of CdS and other wide band gap oxide semiconductors which could improve charge separation and mitigate charge recombination for photoelectrochemical applications.

Facile formation of mixed phase porous TiO2 nanotubes and enhanced visible-light photocatalytic activity

A simple and stable procedure for preparing mixed phase porous TiO2 nanotubes via an emulsion electrospinning is reported. The obtained porous TiO2 nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction pattern (XRD), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), photoluminescence spectrum (PL) and lifetime. These TiO2 nanotubes have a highly porous structure with an average diameter of 200nm. The results show that the small grain size of the rutile crystallites in the porous TiO2 nanotubes would make more catalytic “hot spots” at the rutile/anatase interface than that at the mixed phase TiO2 nanofibers and Degauss P25 under visible light irradiation. According to the decay curve, it implies these “hot spots” would restrict electron/hole recombination and leading a longer life time of 3.11ns in this porous nanotube compared to nanofibers and Degauss P25. Thus, these “hot spots” would stabilize the charge separation and enhance the photocatalytic activity of methyl orange (MO) under visible light irradiation. This method could be extended to the fabrication of various types of highly porous nanotubes.

Enhanced photoanode properties of CdS nanoparticle sensitized TiO2 nanotube arrays by solvothermal synthesis

Abstract We demonstrate the synthesis of CdS-sensitized TiO2 (CdS/TiO2) nanotube arrays via the solvothermal method and describe their enhanced photoelectrochemical properties. Our new approach prevents the clogging of CdS quantum dots (QDs) at the TiO2 nanotube mouth and promotes the uniform deposition of a polycrystalline CdS on the densely aligned TiO2 nanotube arrays (TiO2 NTAs). The deposition of CdS onto the TiO2 NTA can be controlled by the deposition time and the concentration of the precursors. Photoelectrochemical measurements indicate that the electrode synthesized via the solvothermal method can achieve a stable photocurrent density of 5.7 mA/cm2 at 0-V versus Ag/AgCl under AM 1.5 G illumination, which is approximately 10% higher than that prepared using the chemical bath deposition (CBD) method. Solvothermally prepared CdS/TiO2 NTAs exhibit 125-fold enhancement in their photocurrent under visible light compared with bare TiO2 NTAs, indicating facile photogenerated electron transfer from CdS to TiO2. Solvothermally prepared CdS/TiO2 NTAs shows better photostability and longer lifetime of photoelectrons compared to those prepared by CBD method suggesting more favorable CdS–TiO2 NTA interfacial contact. In general, we propose that this methodology can be useful in designing multijunction semiconductor configuration such as coating nanostructures.

Improvement of QDIP performance due to quantum dots with built-in charge

The charging of quantum dots provides two strong effects which improve Quantum Dot Infrared Photodetector (QDIP) performance. First, electrons placed in the quantum dots enhance IR-induced transitions and increase electron coupling to IR radiation. Second, the built-in-dot charge creates potential barriers around dots and these barriers strongly suppress the photoelectron capture and exponentially increase the photoelectron lifetime. Both effects enhance the IR photoresponse. Long photoelectron lifetime decreases the generation–recombination noise and increases the device sensitivity. To investigate the potential profiles around charged dots, we used the nextnano3 software which allows for simulation of multilayer structures combined with realistic geometries in one, two, and three spatial dimensions. In weak electric fields the photoelectron kinetics and transport in the potential created by charged dots have been studied analytically. In strong fields the results were based on Monte-Carlo modeling. The effects of dot charging have been investigated in QD structures which were fabricated using molecular beam epitaxy. InAs quantum dots were grown on AlGaAs surfaces by deposition of approximately 2.1 monolayers of InAs. In the obtained structures the dot charging is realized via intra-dot and inter-dot doping. The increase in photoresponse due to dot charging is in good agreement with the model which takes into account anisotropy of potential barriers around QDs in QD layers.

Effect of Ag nanoparticle size on the photoelectrochemical properties of Ag decorated TiO2 nanotube arrays

Silver (Ag) nanoparticle decorated titanium dioxide nanotube arrays (TiO2 NTAs) were fabricated using a two-step method based on an electrochemical anodization procedure followed by a photochemical reduction process. UV–visible diffuse reflectance spectroscopy and photoluminescence emission spectroscopy demonstrated that the Ag decorating significantly enhanced the light absorption of TiO2 NTAs in the visible spectral range and improved the separation of photo-induced charge carriers in the TiO2 NTAs. The photoelectrochemical properties of prepared samples were investigated by the transient photocurrent measurement and the potential dependence of photocurrent under visible light irradiation. The results show that the photoelectrochemical behavior of Ag decorated TiO2 NTAs is dependent on the size of the Ag particles. The maximum photoconversion efficiency of Ag decorated TiO2 NTAs is about 3.3times greater than that of bare TiO2 NTAs. Electrochemical impedance spectroscopy measurements of the subject materials revealed that the photoelectron lifetime of Ag decorated TiO2 NTAs increases as the Ag nanoparticle’s size decreases. We propose that the enhanced photoelectrochemical properties of Ag decorated TiO2 NTAs results from different ratios of light absorption to scattering caused by the localized surface plasmon resonance of the various sizes of Ag nanoparticles. The tunable photoelectrochemical properties of Ag decorated TiO2 NTAs are expected to have promising applications in photoelectrochemical solar cells and other light-harvesting devices.

Green Preparation of TiO2-ZnO Nanocomposite Photoanodes by Aqueous Electrophoretic Deposition

Aqueous-based electrophoretic deposition (EPD) of titania nanoparticles (Evonic's P25) was successfully applied to the fabrication of mesoporous electrodes for dye-sensitized solar cells (DSSC). Poor film adhesion and cracking arising from the electrolysis of water and interstitial water evaporation during drying were largely overcome via the selection of low EPD operating current density/DC voltage (0.1 mA cm−2/2.5-3.1 V) but more importantly the addition of zinc nitrate in a 5 vol.% isopropanol-water suspension. As a result, a composite nanostructured TiO2-ZnO film was fabricated. Photoelectrochemical characterization indicates that in-situ formation of ZnO causes suppression of charge recombination at the electrode/electrolyte interface thus prolonging photoelectron lifetime. A 5.01% conversion efficiency was obtained under 1 sun illumination (100 mW·cm−2) of a 13 μm thick single transparent film. The efficiency was further improved to 6.19% upon further optimization via multi-layer construction of the transparent anatase film (18 μm thick) by repeated cycles of EPD.

Effective harvesting, detection, and conversion of IR radiation due to quantum dots with built-in charge

We analyze the effect of doping on photoelectron kinetics in quantum dot [QD] structures and find two strong effects of the built-in-dot charge. First, the built-in-dot charge enhances the infrared [IR] transitions in QD structures. This effect significantly increases electron coupling to IR radiation and improves harvesting of the IR power in QD solar cells. Second, the built-in charge creates potential barriers around dots, and these barriers strongly suppress capture processes for photocarriers of the same sign as the built-in-dot charge. The second effect exponentially increases the photoelectron lifetime in unipolar devices, such as IR photodetectors. In bipolar devices, such as solar cells, the solar radiation creates the built-in-dot charge that equates the electron and hole capture rates. By providing additional charge to QDs, the appropriate doping can significantly suppress the capture and recombination processes via QDs. These improvements of IR absorption and photocarrier kinetics radically increase the responsivity of IR photodetectors and photovoltaic efficiency of QD solar cells.

Preparation of DSSC Nanotitania Thin Film Photoanodes by Electrophoretic Deposition in an Aqueous Suspension

Aqueous-based electrophoretic deposition (EPD) of titania nanoparticles (Evonic's P25) for the fabrication of mesoporous electrodes for dye-sensitized solar cells is investigated. Poor film adhesion and cracking arising from the electrolysis of water and interstitial water evaporation during drying were largely overcome via the judicious selection of EPD operating current but more importantly the addition of zinc nitrate in a 5 vol.% isopropanol-water suspension. As a result, a composite nanostructured TiO2-ZnO film was fabricated. Photoelectrochemical characterization indicates that formation of ZnO causes suppression of electron leakage from substrate to electrolyte and additionally prolongs photoelectron lifetime. A 5.01% conversion efficiency was obtained under 1 sun illumination (100 mW•cm−2) of a 13 µm thick single transparent layer. Fabrication of multilayer film by repeated cycles of EPD found to increase the conversion efficiency to 5.52% for the same total film thickness due to better overall film construction.

ELECTRON HEATING IN QUANTUM-DOT STRUCTURES WITH COLLECTIVE POTENTIAL BARRIERS

Here we report our research on quantum-dot structures with collective barriers surrounding groups of quantum dots (planes, clusters etc) and preventing photoelectron capture. Employing Monte-Carlo simulations, we investigate photoelectron kinetics and calculate the photoelectron lifetime as a function of geometrical parameters of the structures, dot occupation, and electric field. Results of our simulations demonstrate that the capture processes are substantially suppressed by the potential barriers and enhanced in strong electric fields. Detailed analysis shows that the effects of the electric field can be explained by electron heating, i.e. field effects become significant, when the shift of the electron temperature due to electron heating reaches the barrier height. Optimized photoelectron kinetics in quantum-dot structures with collective barriers allows for significant improvements in the photoconductive gain, detectivity, and responsivity of photodetectors based on these structures.