Reduction Of Oxygen Vacancies Research Articles

High-Performance X-ray Detector Based on Single-Crystal β-Ga2O3:Mg.

X-ray detection plays an important role in medical imaging, scientific research, and security inspection. Recently, the β-Ga2O3 single-crystal-based X-ray detector has attracted extensive attention due to its excellent intrinsic properties such as good absorption for X-ray photons, a high breakdown electric field, high stability, and low cost. However, developing a high-performance β-Ga2O3-based X-ray detector remains a challenge because of the large dark current and the high oxygen vacancy concentration in the crystals. In this paper, we report a high-performance Mg-doped β-Ga2O3 single-crystal-based X-ray detector with a sandwich structure. The reduced dark current enables the detector to have a high sensitivity of 338.9 μC Gy-1 cm-2 under 50 keV X-ray irradiation with a dose rate of 69.5 μGy/s. The sensitivity is 16-fold higher than that of the commercial amorphous selenium detector. Furthermore, the reduced oxygen vacancy concentration can improve the response speed (<0.2 s) of the detector. The present studies provide a promising method to obtain the high performances for the X-ray detector based on β-Ga2O3 single crystals.

Current-induced thermal tunneling electroluminescence via multiple donor–acceptor-pair recombination

Current-induced thermal tunneling electroluminescence from a highly-compensated ZnO:Ga microrod via multiple donor–acceptor-pair radiative recombination.

Microstructure boosting the cycling stability of LiNi0.6Co0.2Mn0.2O2 cathode through Zr-based dual modification

Ni-rich NCM (LiNixCoyMnzO2, 0.6≤x<1, 0<y<1, 0<z<1) are up-and-coming cathode materials with high working voltage (~3.8 V) for usage in lithium ion batteries. In order to achieve higher capacity through lithium extraction, higher cutoff voltages (>4.3 V) are applied on these materials. Consequently, the batteries will suffer from aggravated side reactions at the cathode/electrolyte interface and the inherent instabilities of Ni-rich NCM cathodes, and thus exhibit rapid capacity fading and voltage decay. Herein, we report a facile and scalable strategy that employs Zr doping concurrent with LixZryOz surface coating, contributing to outstanding capacity retentions of 97.8% at 0.2C and 91.6% at 2C after 100 cycles over 2.8-4.5 V. The improved cycling stability can be attributed to the enhanced bulk stability and suppressed nonequilibrium diffusion reactions, which are further ascribed to the reduced oxygen vacancies and optimized microstructure.

Large domain-wall current in BiFeO3 epitaxial thin films

Large domain wall conductivity in insulating ferroelectric thin films provides a new idea for non-destructive readout of domain information of a nonvolatile ferroelectric domain wall memory. However, there are several challenges hindering its development, including the excessive leakage current and insufficient on-state read current. A lot of experimental and theoretical data have referred to the accumulation of oxygen vacancies at the domain wall regions of ferroelectric thin films as the extrinsic contribution of the wall conduction. In this work, we found that the wall current varied with the angle between the applied electric field and the initial polarization, and that the wall current could be increased to 1.9 μA for a bismuth ferrite thin-film device with the reduced oxygen vacancies in contrast to the previous value of 2 nA for the device with rich oxygen vacancies. The oxygen vacancy concentration can be controlled through the thermal annealing of the sample at two atmospheres of 5% hydrogen mixed with argon gas and pure oxygen gas. For the former, the enhanced oxygen vacancies within the annealed sample at the reduced atmosphere decreased the domain wall current by over a few orders of magnitude, in contrast to the significant enhancement of the wall current for the sample in the latter annealed at the oxidizing atmosphere. The change of oxygen vacancy concentration was implied from the Fe2+⇔Fe3+ transition by the X-ray photoelectron spectroscopy analysis. This change confirmed the rigid domain wall conduction mechanism as well as the extrinsic adverse defect compensation of the wall current, paving the way to the exploration of high-power domain-wall nanodevices with sufficient output currents.

TiO2 surface oxygen vacancy passivation towards mitigated interfacial lattice distortion and efficient perovskite solar cell

It is known that there exist inherent oxygen vacancies which is reported to be negative to PSCs performance on the TiO2 film surface where contacts the gaseous phase while the manufacturing process. In this work, we clarified the role of surface oxygen vacancy on TiO2/perovskite interface structure, which would not only serve as trap states to capture generated electrons, but also cause perovskite crystal distortion and PbI bond broken due to the interfacial interaction, generating additional interfacial defects. In addition, by analyzing the interfacial carrier transport routes, the common reason behind the different carrier loss pathways was ascertained to be the oxygen vacancy. Hence, for reducing oxygen vacancies and its negative impact on the device performance, the surface passivation method with gaseous fluorine was proposed, both the DFT (density functional theory) calculation results and experimental approaches proved that the surface passivation method could effectively reduce the traps states, mitigate interface structure distortion and enhance the interfacial charge transfer. As a consequence, MAPbI3-based PSCs with surface passivation obtained a champion PCE of 20.43%, 7.7% higher than that (18.98%) of control device, and FA0.15MA0.85PbI3-based PSCs with surface passivation obtained a champion PCE of 21.16%, 6.2% higher than that (19.92%) of control device.

High energy storage density of NBT-0.10BFO solid solution films

To reduce the leakage current of the Na0.5Bi0.5TiO3 film and improve its energy storage properties, the sol-gel method was implemented to prepare (1-x)Na0.5Bi0.5TiO3-xBiFeO3 (NBT-xBFO) (x = 0.00, 0.05, 0.10, 0.15) lead-free perovskite-phase solid solution films. The point defects formed by Fe3+ occupying Ti4+ and oxygen vacancies formed composite defects FeTi′−Vo··−FeTi′, which reduced oxygen vacancies, cut down the domain wall pinning, and improved the polarization, thereby enhancing the recoverable energy density. Among all the films, the NBT-0.10BFO film exhibited a high recoverable energy density of 38.5 J/cm3 and an energy storage efficiency of 52.0% under an electric field of 2000 kV/cm. In addition, this film had fatigue resistance up to 1 × 109 cycles and suitable thermal stability within a wide temperature range of 20~110 °C. The NBT-0.10BFO film is expected to have broad application prospects in the field of energy storage.

Effect of annealing on electrical characteristics of graphene/Al-Zr co-doped ZnO Schottky contact

In this paper, the effect of annealing on the electrical properties of graphene/Al-Zr ZnO Schottky contacts was studied in detail. The results showed that the grain size of Al-Zr doped ZnO films grew with the increasing of annealing temperature. Meanwhile, the defective oxygen in the films decreased. In addition, the leakage current of graphene/Al-Zr ZnO Schottky contact decreased and barrier height increased. This phenomenon can be explained by the reduction of oxygen vacancies and the weakening of Fermi level pinning.

Effect of inorganic interfacial modification layer on the performance of quantum-dots light-emitting diodes

Efficiency and operational durability are crucial characteristics for the high-performance quantum-dots light-emitting diodes (QLEDs). In order to improve the efficiency and the stability of the QLEDs, we have inserted an inorganic Al2O3 interfacial modification layer (IML) next to the electron transport layer (ETL) using atomic layer deposition (ALD). We conducted a comparative analysis of two different positions of Al2O3 IML inside the device, inserting IML before ETL and after ETL, to find an optimized structure for the efficient QLEDs. As a result, when Al2O3 IML was located after ETL, reduction of oxygen vacancy in the ZnO layer was occurred due to the reaction of O2 plasma reactant that used during the ALD process. It can have the effects of decreasing the exciton quenching phenomenon and balancing the charge injection. Therefore, improved device performances were observed, and the maximum luminance was 56 108 cd m−2 when 1 nm Al2O3 IML was deposited after ZnO ETL. These results suggest Al2O3 IML deposition would provide a useful way to improve the performance of QLEDs.

(Invited) A Practical Model of Oxygen Exchange Reaction on a Mixed Conducting Electrode for SOFC and SOEC

Oxygen exchange on mixed conducting electrodes for SOFC/SOEC have been investigated by many researchers, and the reaction mechanism has been made clearer in these decades. In a practical modeling works, however, a Butler-Volmer equation with unproven parameters is still used to describe current-voltage (I-V) relationships due to the complexity of realistic reaction models. Recently, we have collaborated with Muramatsu and Terada to develop a simulation code “SIMUDEL” that can calculate oxygen potential distribution and the resulting chemical strain in SOFC/SOEC components [1]. In this code, porous electrode is described as a homogeneous medium in which oxygen exchange reaction is given as the source term in the continuity equation. In this code, the surface reaction rate is given as a function of oxygen potentials in the gas phase and at the solid surface. This work aimed to provide a simple but plausible model of oxygen exchange at a mixed conducting electrode surface.The work started with modeling I-V characteristics of a dense film electrode. Steady state dc measurements were made with La0.6Sr0.4CoO3-δ (LSC) and La0.6Sr0.4Co0.8Fe0.2O3-δ (LSCF) films deposited by pulsed laser deposition (PLD) on doped ceria substrates. Oxygen vacancy concentration of the film was estimated from the chemical capacitance by ac impedance measurements and was used as a parameter to represent the I-V relationships. The reaction rate on a dense electrode was compared with the volumetric surface reaction rate in a porous electrode assuming one-dimensional transport approximation. Several porous electrodes with micro- and nano- meter-sized particles were fabricated by slurry coating or metal organic decomposition. A plausible reaction equation obtained for a film electrode was applied to estimate I-V curves of a porous electrodes, and the result was compared with the experiments to test the model.The surface reaction resistance on a dense film electrode showed an activation energy in the range of 1.5 to 2.0 eV. It was much larger than that of a porous electrode when it was obtained as the volumetric reaction resistance of the porous medium and normalized to the actual surface area. The magnitude of the reaction resistance on a porous electrode was close to that observed for a sample powder by pulse isotope exchange experiments. No remarkable difference was observed between the micro and nano porous electrodes. The large activation energy on the dense film electrode is partly due to the reduced oxygen vacancy concentration in a PLD film electrode. As many other researchers pointed out, oxygen exchange reaction on a mixed conductor is strongly affected by the concentration and mobility of oxygen vacancy. The apparent activation energy of surface reaction is the sum of activation energies for vacancy formation and reaction. The difference between the dense and porous electrode, however, was not fully explained only with the difference in vacancy concentration. Furthermore, the reaction rate on a dense film electrode was found to be strongly affected by the coexisting chemicals on the surface. As we reported earlier, applying LaSrCoO4 (R-P phase) particles on the surface dramatically enhanced the surface reaction rate. It also decreased the activation energy. Existence of R-P phase was found to clean the Sr enriched segregates on the surface. On the contrary, significant degradation was observed for reaction on a sintered ceramic surface due to the segregation of Sr rich layer. These observations suggested the existence of two series reaction barriers for oxygen exchange; i.e. the surface reaction and sub-surface transport. The I-V curve of a dense film electrode was modelled on the basis of the above consideration. The surface process was described to be proportional to the oxygen vacancy concentration, and the sub-surface process was given to overcome some energy barrier. For a porous electrode, the sub-surface process was assumed to be negligible. The difference in oxygen vacancy concentration and lack of sub-surface process explained fairly well the oxygen partial pressure dependence of I-V curves of the porous electrodes. The obtained model is to be applied to the description of the surface reaction in SIMDEL code. Acknowledgements This work was partly supported by Japan Science and Technology Agency for the financial support during JST-CREST program (JPMJCR11C1, 2011-2016). Development of SIMDEL was made under the support by NEDO.

Effect of V2O5 promoter on characteristics and performance of NiO/CeO2 catalyst in methane bi-reforming

In this work, a series of 10 wt% NiO/CeO2 catalysts (Ni/Ce) promoted by V2O5 with content varying in the range of 0–0.5 wt% was prepared by the co-impregnation method. The characteristics of the catalysts were investigated by several techniques including N2 physisorption (BET), X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), EDS mapping, carbon dioxide temperature-programmed desorption (CO2-TPD), and hydrogen temperature-programmed reduction (H2-TPR). The activity of the catalyst was studied in the micro-flow system in a temperature range of 550 °C–800 °C, the feedstock composition CH4/CO2/H2O of 3.0/1.2/2.4 and the weight hourly space velocity (WHSV) of 30,000 ml.h−1.g−1. Adding V2O5 additive, on the one hand increases the reducibility and basicity of Ni/Ce catalyst, on the other hand reduces oxygen vacancies and increases the crystal size of CeO2, leading to various effects on catalyst activity depending on its content. Ni/Ce catalyst promoted with 0.3 wt% of V2O5 was the best among tested ones, on which at reaction temperature of 700 °C, the conversion of CH4 and CO2 reached 97% and 77% respectively, and the molar ratio of H2/CO was 2.1. Meanwhile, on non-promoted Ni/Ce catalyst, the corresponding quantities were 83%, 62% and 1.9, respectively. It is important to note that performance of both was stable for more than 30 h thanks to the better resistance to coke deposition and structural stability.

Extremely Stable Dual Gate Coplanar Amorphous InGaZnO Thin Film Transistor With Split Active Layer by N2O Annealing

We report the effect of N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O post-fabrication annealing (PFA) on the electrical stability of dual gate, active split amorphous indium gallium zinc oxide (a-IGZO) thin-film transistor (TFT). Optimized N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O PFA at 360 °C for 3 h results in threshold voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> ) close to zero and an increase in the on-current almost 2 times than as-fabricated TFT. N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O annealed TFT exhibited robust V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> stability under positive bias temperature stress at 60 °C and negative bias illumination stress, exhibiting ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> of 0.1 V ,and0.1 V, respectively, whereas as-fabricated TFT showed large ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> of ~ 10 V and -3 V. The X-ray photoelectron spectroscopy depth profile reveals the origin of such improvement as the increasing metal-oxide bonds and the reduction in oxygen vacancy/ OH related defects.

Compositional induced dielectric relaxation and electrical conduction behavior of samarium modified bismuth sodium titanate ceramic

The present manuscript analyzes the dielectric relaxation and conduction behavior of samarium modified sodium bismuth titanate ((Bi(1-x)Smx)0.5Na0.5TiO3) ceramic prepared by solid-state reaction process. The X-Ray Diffraction acknowledges the presence of rhombohedral (R3c) phase in all composition with existence of small secondary phase for composition more than 2% of Sm doping. The dielectric relaxation and conduction mechanism of the compositions were studied through the impedance spectroscopy. The impedance values initially decrease up to doping level of x = 2% and increase with higher doping concentration. The Cole-Cole plot indicates the existence of grain and grain boundary resistance in all compositions. The conductivity enhances up to x = 0.02 and then reduces with doping concentration, and the conductivity spectra fitted with the modified Jonscher's power law indicates that small polaron hopping is the dominant phenomena for conduction in all compositions. The activation energy related to relaxation and conduction was obtained using the Arrhenius equation, and activation energy obtained specifies a reduction in oxygen vacancies with doping. The activation energy suggests that the oxygen vacancies associated with the short-range hopping is responsible for relaxation and doubly ionized oxygen vacancies are responsible for conduction.

ZnGa2O4 deep-ultraviolet photodetector based on Si substrate

Deep-ultraviolet photodetectors with fast response speed are urgently required in flame detection and atmospheric monitoring. A method for reducing oxygen vacancies in ZnGa2O4 films by high-temperature and oxygen thermodynamic conditions is proposed, to increase the response speed of ZnGa2O4/Si heterojunction deep-ultraviolet photodetectors. Here, a ZnGa2O4/Si heterojunction deep-ultraviolet photodetector with excellent performance is fabricated, exhibiting a decay time of ~67 ms, a dark current of 0.027 nA and a photo-to-dark current ratio of ~490. These results indicate that the high-temperature and oxygen-rich annealing method proposed here can provide a reference for the fabrication of ZnGa2O4 deep-ultraviolet photodetectors with excellent performance.

Tuning conduction mechanism via surface termination and point defects in n-type LaAlO3/SrTiO3 interfaces

Based on first-principles calculations, the electronic properties of n-type LaAlO3/SrTiO3 (LAO/STO) interfaces with LaO and AlO2 terminations were investigated. Thermodynamic analysis shows that the AlO2-terminated and LaO-terminated LaAlO3 surfaces are preferably formed under O-rich and O-poor conditions, respectively. We found that there are two polarization fields in heterostrucutres: one is the build-in polarization field induced by alternating LaO+1 and AlO2−1 layers and another is the strain-induced additional polarization field only existing in relaxed structures. The two polarization fields have a distinct influence on the electronic structures of interfaces. In this work, the influences of oxygen vacancies and cation defects at different sites on the conduction mechanism were analyzed in detail. The results indicate that point defects affect the electronic states of interfaces by two ways: providing or accepting electrons, and disturbing the internal polarization field. Approaching the interface, the defects tend to play the former role. Moreover, the calculations show that under O-rich condition, the reduced oxygen vacancies and the improved cation vacancies spontaneously formed at the interface deteriorate the n-type conductivity. The present results provide a theoretical guidance to utilize the effects of surface termination and point defects to tune the conductivity of perovskite heterostructures.

Whole Metal Oxide p-i-n Deep Ultraviolet Light-Emitting Diodes Using i-Ga₂O₃ Active Emissive Film

In this work, whole transparent p-NiO/i-Ga2O3/n-Ga2O3:Si deep ultraviolet light-emitting diodes (DUV-LEDs) were fabricated and studied. The p-NiO film was deposited using a radio frequency magnetron sputtering system. The i-Ga2O3 film and the n-Ga2O3:Si film were deposited using a vapor cooling condensation system at about 80 K and then annealed in an oxygen ambience at 900 °C for 60 minutes. Due to the significantly reduced oxygen vacancies and intrinsic defects resided in the i-Ga2O3 active emissive film, UV-C LEDs were obtained. By measuring the electroluminescence spectrum of the p-i-n DUV-LEDs, the peak wavelength of 240.0 nm and the half width at half maximum of 20.2 nm were obtained.

Study of impedance and magnetoelectric property of lead-free xLCNZFO+(1-x)BGTDO multiferroic composites

Multiferroic xLCNZFO+(1-x)BGTDO composites are prepared by the conventional solid-state reaction method. The phase identification of ferrite and perovskite structure is executed by the XRD analysis. The initial permeability enhances with an increment in ferrite content because of high magnetic LNCZFO and a reduction of oxygen vacancies. The dispersive dielectric nature is observed in the low-frequency region due to Maxwell-Wagner interfacial polarization. The sample of x = 0.60 is comprised of the maximum dielectric constant of about 1.02 × 104 at 1 kHz. The ac conductivity results from a small polaron hopping mechanism. The complex impedance spectra of some samples exhibit a single semi-circular arc due to the dominance of grain boundary resistance, whereas other samples show double arcs representing the dominance of grain and grain boundary effect on the conduction mechanism. There is a strong coupling between the two ferroic orders and the optimum magnetoelectric coupling voltage of 172 × 103 Vm−1T−1 is obtained for the x = 0.10 sample.

Radiation damage induced by reactor neutrons in nano-anatase TiO2 thin film

Reactor neutrons damage induced in nanocrystalline anatase TiO2 thin film, deposited by sol-gel on silicon substrate, was investigated by grazing angle X-ray diffraction (GAXRD), Raman spectroscopy, four-point probe and UV–visible spectrophotometry. After irradiation, the crystallinity of TiO2 films improves and the electrical resistivity increases, due to an annealing effect associated with the reduction in oxygen vacancies in material. The observed change in the response of irradiated material at higher fluence, demonstrated by a slight reduction in the degree of crystallinity and electrical resistivity, testifies to the simultaneous presence of an annealing effect and a damaging effect during neutron irradiation. Moreover, the difference in the response of anatase and rutile phases of TiO2 under neutron irradiation is explained by the difference in their crystalline structures and kinetic properties of defects. The optical band gap remains unchanged after irradiation meaning that no energy level has been produced within the forbidden region.

Synthesis and Magnon Thermal Transport Properties of Spin Ladder Sr14Cu24O41 Microstructures

AbstractThe spin ladder compound (Sr,Ca,La)14Cu24O41 exhibits an incommensurate layered structure with strong antiferromagnetic coupling. Besides intriguing superconducting behavior, recent experiments on bulk Sr14Cu24O41 single crystals have revealed a remarkable magnon thermal conductivity, which is the largest above 100 K among all known quantum magnets. Although bulk (Sr,Ca,La)14Cu24O41 crystals have been synthesized and studied extensively, there have been few reports on the synthesis and magnon thermal transport investigation of their microstructures. Here, the synthesis and thermal transport properties of Sr14Cu24O41 microrods are reported. Electron microscopy studies indicate that these microrods synthesized by a coprecipitation method are single crystals grown preferentially along the ladder axis. Based on a four‐probe thermal transport measurement, the thermal conductivity of the microrods reveals appreciable magnon transport in the microstructures. According to a kinetic model analysis, magnon transport in the microrods is suppressed mainly by increased point defect scattering compared to the bulk crystals, whereas surface scattering is negligible for anisotropic 1D magnon transport along the ladder. Moreover, the thermal conductivity is enhanced after annealing as a result of reduced oxygen vacancies. These results help to build the foundation for future heterogeneous integration of magnetic microstructures in microscale devices for the transport of energy and quantum information.

Oxygen vacancy and grain boundary resistance regulate the intrinsic ferroelectric properties of Bi0.96Sr0.04Fe0.98−xMnxCo0.02O3 thin film

Bi0.96Sr0.04Fe0.98−xMnxCo0.02O3 (BSFMxCO, x = 0.00–0.05) thin films with ferroelectric properties were successfully prepared by chemical solution deposition (CSD). It is found that BSFMxCO has a stable trigonal structure. The conversion from Fe3+ to Fe2+ inhibiting Mn3+ and the synergistic effects of Mn3+ and Co3+ reduce the oxygen vacancies, which prevents the formation of defect complexes. The results show that the leakage current of BSFM0.04CO is dropped to 0.011 A/cm2. Simultaneously, the weakened grain boundaries resistance and the pinning effect by doping Mn3+ reduce the built-in electric field effectively, which enhances the polarization reversal current and the capacitance. Under the applied electric field, we have obtained the intrinsic residual polarization value of 151 μC/cm2, thanks to the easily reversed domains of BSFM0.04CO and the enhanced intrinsic polarization. Therefore, ion doping can reduce oxygen vacancies and the grain boundary resistance. As a result, the intrinsic ferroelectricity in BiFeO3 is enhanced by the improvement of intrinsic polarization.

Effects of lithium doping and ultraviolet photo-patterning on electrical properties of InGaZnO thin film transistors

Alkali metals, such as Li and Na, have been added in solution-processed oxide thin-film transistors (TFTs) because they improved electrical characteristics of the oxide TFTs. In previous research, however, fine-patterning of the alkali metal-doped oxide layers was not performed due to potential performance degradation during photolithography process. In this work, we demonstrate ultraviolet (UV) photo-patterned solution-processed indium gallium zinc oxide (IGZO) TFTs containing various amount of lithium (Li). For patterning of active layer, we applied deep UV (184 nm, 10% and 254 nm, 90%) photo-patterning process. We also investigated the chemical composition and bonding structures of the Li doped IGZO (Li-IGZO) films. It was found that appropriate Li doping on IGZO films can reduce oxygen vacancies and improved the coordination of the metal-oxygen bonding. In addition, Li-IGZO TFTs showed improved mobility and bias stability compared to un-doped counterparts. We believe that this simple patterning process and Li doping can be applied to other types of the high-performance solution-processed oxide TFTs.