Density Of Vacancies Research Articles

Three-species drift-diffusion models for memristors

A system of drift-diffusion equations for the electron, hole, and oxygen vacancy densities in a semiconductor, coupled to the Poisson equation for the electric potential, is analyzed in a bounded domain with mixed Dirichlet–Neumann boundary conditions. This system describes the dynamics of charge carriers in a memristor device. Memristors can be seen as nonlinear resistors with memory, mimicking the conductance response of biological synapses. In the fast-relaxation limit, the system reduces to a drift-diffusion system for the oxygen vacancy density and electric potential, which is often used in neuromorphic applications. The following results are proved: the global existence of weak solutions to the full system in any space dimension; the uniform-in-time boundedness of the solutions to the full system and the fast-relaxation limit in two space dimensions; the global existence and weak–strong uniqueness analysis of the reduced system. Numerical experiments in one space dimension illustrate the behavior of the solutions and reproduce hysteresis effects in the current–voltage characteristics.

Phases of the hard-plate lattice gas on a three-dimensional cubic lattice.

We study the phase diagram of a lattice gas of 2×2×1 hard plates on the three-dimensional cubic lattice. Each plate covers an elementary plaquette of the cubic lattice, with the constraint that a site can belong to utmost one plate. We focus on the isotropic system, with equal fugacities for the three orientations of plates. We show, using grand canonical Monte Carlo simulations, that the system undergoes two phase transitions when the density of plates is increased: the first from a disordered fluid phase to a layered phase, and the second from the layered phase to a sublattice-ordered phase. In the layered phase, the system breaks up into disjoint slabs of thickness two along one spontaneously chosen Cartesian direction, corresponding to a twofold (Z_{2}) symmetry breaking of translation symmetry along the layering direction. Plates with normals perpendicular to this layering direction are preferentially contained entirely within these slabs, while plates straddling two adjacent slabs have a lower density, thus breaking the symmetry between the three types of plates. We show that the slabs exhibit two-dimensional power-law columnar order even in the presence of a nonzero density of vacancies. In contrast, interslab correlations of the two-dimensional columnar order parameter decay exponentially with the separation between the slabs. In the sublattice-ordered phase, there is twofold symmetry breaking of lattice translation symmetry along all three Cartesian directions. We present numerical evidence that the disordered to layered transition is continuous and consistent with universality class of the three-dimensional O(3) model with cubic anisotropy, while the layered to sublattice transition is first-order in nature.

Plasma-catalytic synthesis of ammonia over Ru/BaTiO3-based bimetallic catalysts: Synergistic effect from dual-metal active sites

Plasma-catalytic synthesis of ammonia (NH3) was carried out using BaTiO3 supported Ru-M bimetallic catalysts (Ru-M/BaTiO3, M = Fe, Co and Ni) in a dielectric barrier discharge (DBD) reactor. The NH3 synthesis performance followed the order of Ru-Ni/BaTiO3 > Ru/BaTiO3 > Ru-Co/BaTiO3 > Ru-Fe/BaTiO3, with the highest NH3 concentration (3895 ppm) and energy yield (0.39 g kWh−1) achieved over Ru-Ni/BaTiO3 at 25 W and 10 W, respectively. To gain insights into the physio-chemical properties of the Ru-M/BaTiO3 catalysts, comprehensive catalyst characterizations were performed, including X-ray diffraction, N2 physisorption measurements, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), and temperature-programmed desorption of CO2 and N2 (CO2 and N2-TPD). The results indicated that the loading of Ni enhanced the basicity and N2 adsorption capacity of the catalyst, as well as the density of oxygen vacancy (OV) on the BaTiO3 surface, which facilitated the adsorption and activation of N2 on catalyst surface. These effects led to the enhanced NH3 synthesis, as excited N2 could be adsorbed on Ru-Ni/BaTiO3 from plasma region and stepwise hydrogenated to form NHx species and ultimately NH3.

Simple and efficient strategy for α-MnO2/C by in-situ synthesis and its performance for degrading urea process wastewater

A simple and low-cost method for the synthesis of α-MnO2/C catalyst was proposed, in which KMnO4 was reduced in-situ by cellulose in urea solution. The heterogeneous catalytic degradation of urea wastewater were carried out over α-MnO2/C. The effect of KMnO4-urea solution concentration on the structure and performance of the catalysts was investigated. The results indicate that δ-MnO2 tends to curl and collapse into tunnel structure of α-MnO2 as the concentration of KMnO4-urea solution increases, leading to α-MnO2 is evenly distributed on carbon carrier. When the concentration of KMnO4-urea solution is appropriate, the catalyst has not only a large specific surface area but also high oxygen vacancy density, which is the key to be able to completely degrade the urea process wastewater. The reaction kinetics show that the catalytic hydrolysis of urea behaves as a pseudo-first-order reaction. A possible catalytic mechanism of urea hydrolysis over α-MnO2/C was proposed. In addition, the reusability experiments show that the α-MnO2/C catalyst has a good recycling ability.

Iodine-assisted ultrafast growth of high-quality monolayer MoS<sub>2</sub> with sulfur-terminated edges

Two-dimensional (2D) semiconductors have attracted great attention to extend Moore’s law, which motivates the quest for fast growth of high-quality materials. However, taking MoS2 as an example, current methods yield 2D MoS2 with low growth rate and poor quality with vacancy concentrations three to five orders of magnitude higher than silicon and other commercial semiconductors. Here, we develop a strategy of using intermediate product of iodine as a transport agent to carry metal precursors efficiently for ultrafast growth of high-quality MoS2. The grown MoS2 has the lowest density of sulfur vacancies (~ 1.41 × 1012 cm-2) reported so far and excellent electrical properties with high on/off current ratios of 108 and carrier mobility of 175 cm2V-1s-1. Theoretical calculations show that by incorporating iodine, the nucleation barrier of MoS2 growth with sulfur-terminated edges reduces dramatically. The sufficient supply of precursor and low nucleation energy together boost the ultrafast growth of sub-millimeter MoS2 domains within seconds. This work provides an effective method for ultrafast growth of 2D semiconductors with high quality, which will promote their applications.

Investigation of deep defects and their effects on the properties of NiO/β-Ga2O3 heterojuncion diodes

In this study, the effect of rapid thermal annealing (RTA) on the electrical and optical properties of NiO/ β-Ga2O3 heterojunction diodes was investigated using capacitance-voltage, current-voltage, Deep Level Transient Spectroscopy (DLTS), Laplace DLTS, photoluminescence and micro-Raman spectroscopy techniques, and SILVACO-TCAD numerical simulator. The NiO is designed to be lowly-doped, allowing for the NiO full depletion at zero bias and the study of properties of β-Ga2O3 and its interface with NiO. Micro-Raman results revealed good agreement with the theoretical and experimental results reported in the literature. The photoluminescence intensity of the sample after RTA is five times higher than the fresh sample due to a rise in the density of gallium and oxygen vacancies (VGa + VO) in the annealed β-Ga2O3 samples. The current-voltage characteristics showed that annealed devices exhibited a lower ideality factor at room temperature and higher barrier height compared with fresh samples. The DLTS measurements demonstrated that the number of electrically active traps were different for the two samples. In particular, three and one electron traps were detected in fresh samples and annealed samples, respectively. SILVACO-TCAD was used to understand the distribution of the detected electron E2 trap (Ec-0.15 eV) in the fresh sample and the dominant transport mechanisms. A fairly good agreement between simulation and measurements was achieved considering a surface NiO acceptor density of about 1 × 1019 cm−3 and E2 trap depth into the surface of β-Ga2O3 layer of about 0.220 µm and the effect of the most observed Ec-0.75 eV trap level in β-Ga2O3. These results unveil comprehensive physics in NiO/β-Ga2O3heterojunction and suggest that RTA is an essential process for realizing high-performance NiO/β-Ga2O3devices.

Boosting Methanol-Mediated CO2 Hydrogenation into Aromatics by Synergistically Tailoring Oxygen Vacancy and Acid Site Properties of Multifunctional Catalyst.

Even though the direct hydrogenation of CO2 into aromatics has been realized via a methanol-mediated pathway and multifunctional catalyst, few works have been focused on the simultaneously rational design of each component in multifunctional catalyst to improve the performance. Also, the structure-function relationship between aromatics synthesis performance and the different catalytic components (reducible metal oxide and acidic zeolite) has been rarely investigated. Herein, we increase the oxygen vacancy (Ov ) density in reducible Cr2 O3 by sequential carbonization and oxidation (SCO) treatments of Cr-based metal-organic frameworks. Thanks to the enriched Ov , Cr2 O3 -based catalyst affords high methanol selectivity of 98.1 % (without CO) at a CO2 conversion of 16.8 % under high reaction temperature (350 °C). Furthermore, after combining with the acidic zeolite H-ZSM-5, the multifunctional catalyst realizes the direct conversion of CO2 into aromatics with conversion and selectivity as high as 25.4 % and 80.1 % (without CO), respectively. The property of acid site in H-ZSM-5, especially the Al species that located at the intersection of straight and sinusoidal channels, plays a vital role in enhancing the aromatics selectivity, which can be precisely controlled by varying the hydrothermal synthesis conditions. Our work provides a synergistic strategy to boost the aromatics synthesis performance from CO2 hydrogenation.

Highly Efficient and Stable Perovskite Solar Cells Incorporating Chlorinated-Tin Oxide Electron Transport Layers Prepared via Coordination of Diacyl-Metal Cations

The surface passivation of metal oxide electron-transport layers (ETLs) is a powerful strategy for realizing high-performance perovskite solar cells. The surface properties of ETLs strongly influence carrier injection and transfer dynamics; therefore, control of the carrier trap density is crucial. Semiconducting small molecules are considered suitable materials for surface passivation. However, they are expensive and vulnerable to humid atmospheres. Ultrathin polymer layers can have poor surface coverage owing to their spinodal dewetting. In this study, we employ adipoyl chloride as an organic ligand for the chemically robust and efficient surface passivation of SnO2 ETLs. Through the strong coordination of diacyl-metal cations, acyl groups are adsorbed onto the SnO2 surfaces, and the density of oxygen vacancies is significantly reduced. Furthermore, the changing surface properties of the SnO2 ETLs also contribute to the improvement of perovskite morphologies. The deeper energy levels and reduced defect density of the ETLs promote electron injection and transfer at the perovskite and ETL interface. The enlarged perovskite grains are accompanied by improved electron mobility and reduced grain-boundary density. The resulting power conversion efficiency (PCE) increases from 19.36 to 21.41%. The normalized PCE is retained at 90.4% of the initial value for 720 h under 1 sun illumination without the encapsulation of the devices.

Tuning the microstructure and hydrogen permeability of VC/V composite membranes by radio frequency magnetron sputtering with applied substrate bias

Our recent work has shown that vanadium carbide (VC) films prepared by radio frequency magnetron sputtering (RFMS) can be used as H2 dissociation/recombination catalysts on vanadium (V) for high temperature hydrogen separation. During the deposition of VC on V by RFMS, substrate bias was further applied to tune the microstructure and hydrogen permeability of the VC/V composite membranes. It is found that moderate substrate bias during RFMS generates nanocrystalline VC films with hexagonal close packed V2C (hcp-V2C) phase, which exhibits finer grain size and higher carbon vacancy density compared to the films with face centered cubic VC (fcc-VC) phase prepared by RFMS without bias. The increase in carbon vacancy density results in an increase in the electron density around the Fermi level (EF) and a lowering of the d-band center for the VC, which contributes to its catalytic activity. The resulted VC/V composite membranes show significantly enhanced hydrogen permeability, particularly at all temperatures evaluated, above that of industrially used Pd. This work demonstrates that RFMS with applied substrate bias as microstructural engineering is an efficient method to prepare highly hydrogen-permeable VC/V composite membranes.

Construction of Ultrasensitive Surface‐Enhanced Raman Scattering Substates Based on TiO2 Aerogels

AbstractRecent advances in surface‐enhanced Raman scattering (SERS) on semiconductor substrates offer this technology improved selectivity on top of other advantages, such as cost efficiency. However, the enhancement factor (EF) based on the semiconductors is still low compared with the noble metal substrates. Here, a new strategy of developing the semiconductor substrates based on aerogels is proposed for the first time. According to the modified Herzberg–Teller coupling rule, TiO2 aerogels are selected as the control object because of their large tunability. The surface area, amorphousness, and surface oxygen vacancy densities of TiO2 aerogels are regulated synergically. Due to the tuning of band structure, including band gap and defect band, multiresonant interband charge transfer (CT) pathways are generated and enhanced CT efficiency. A strong, intrinsically activated SERS effect is generated. Amorphous TiO2 aerogel with the highest surface oxygen vacancies shows a significant EF of 2.42 × 107, and TiO2 aerogels afford the large surface area and more active sites, which is conducive to promoting the adsorption of molecules. The aerogel‐based SERS is demonstrated to have wide applicability for ultrasensitive detection of explosives and organic dyes. The aerogel nanomaterials demonstrated here open a way for the construction of low‐cost and high‐sensitivity SERS substrate materials.

In–Ga–Zn–O memristor with double layers of different oxygen vacancy densities and long-term memory towards neuromorphic applications

An In–Ga–Zn–O (IGZO) memristor with double layers of different oxygen vacancy (VO) densities has been developed, and long-term memory towards neuromorphic applications has been confirmed. The IGZO layer of the higher VO density functions as a pseudo electrode to avoid the Schottky behavior, whereas that of the lower VO density functions as a conductance change layer. The long-term potentiation and long-term depression are observed based on the memristor characteristic by applying pulse voltages, which demonstrates the future possibility towards neuromorphic applications.

High platinum utilization for proton exchange membrane fuel cells via low-temperature substrate sputtering on acid-treated carbon nanotube sheet

Low-temperature substrate sputtering on an acid-treated carbon nanotube (ATCNT) sheet result in high platinum (Pt) utilization. A thin catalyst layer (CL, ∼300 nm) with a high density of Pt and vacancies was formed on the ATCNT sheet (Pt-deposited ATCNT). It replaced the commercial carbon-supported Pt and microporous layers of the proton exchange membrane fuel cells. The substrate temperature (Tsub: 20, 5, −10, −30, and −50 ℃) contributed to crack formation in the porous Pt thin film and restrained the growth of Pt grain (average grain size from 8.60 to 3.97 nm). Below −10 ℃, the crack and grain size changed significantly. The increased crack and smaller grain size enlarged the Pt surface area while forming vacancies; this was confirmed via physical and electrochemical characterizations. When compared to Pt-deposited ATCNT (Tsub = 20 ℃, 47.61 m2 g-1Pt), the electrochemical surface area of Pt-deposited ATCNT (Tsub = -50 ℃, 72.06 m2 g-1Pt) increased by 51.35%. We present a high Pt mass-specific peak power density (5.93 kW g-1Pt), focusing on a strategy to increase Pt utilization.

Tuning the Surface Electron Accumulation Layer of In2 O3 by Adsorption of Molecular Electron Donors and Acceptors.

In2 O3 , an n-type semiconducting transparent transition metal oxide, possesses a surface electron accumulation layer (SEAL) resulting from downward surface band bending due to the presence of ubiquitous oxygen vacancies. Upon annealing In2 O3 in ultrahigh vacuum or in the presence of oxygen, the SEAL can be enhanced or depleted, as governed by the resulting density of oxygen vacancies at the surface. In this work, an alternative route to tune the SEAL by adsorption of strong molecular electron donors (specifically here ruthenium pentamethylcyclopentadienyl mesitylene dimer, [RuCp*mes]2 ) and acceptors (here 2,2'-(1,3,4,5,7,8-hexafluoro-2,6-naphthalene-diylidene)bis-propanedinitrile, F6 TCNNQ) is demonstrated. Starting from an electron-depleted In2 O3 surface after annealing in oxygen, the deposition of [RuCp*mes]2 restores the accumulation layer as a result of electron transfer from the donor molecules to In2 O3 , as evidenced by the observation of (partially) filled conduction sub-bands near the Fermi level via angle-resolved photoemission spectroscopy, indicating the formation of a 2D electron gas due to the SEAL. In contrast, when F6 TCNNQ is deposited on a surface annealed without oxygen, the electron accumulation layer vanishes and an upward band bending is generated at the In2 O3 surface due to electron depletion by the acceptor molecules. Hence, further opportunities to expand the application of In2 O3 in electronic devices are revealed.

Nitrogen vacancies in GaN templates and their critical role on the luminescence efficiency of blue quantum wells.

In InGaN-based LEDs, an InGaN layer underlying active region has been widely used to improve the luminescence efficiency of the quantum wells (QWs). It has been reported recently that the role of InGaN underlayer (UL) is to block the diffusion of point defects or surface defects in n-GaN into QWs. The type and the source of the point defects need further investigations. In this paper, using temperature-dependent photoluminescence (PL) measurements, we observe emission peak related to nitrogen vacancies (VN) in n-GaN. In combination with secondary ion mass spectroscopy (SIMS) measurement and theoretical calculation, it is found that VN concentration in n-GaN is as high as about 3 × 1018 cm-3 in n-GaN grown with low growth V/III ratio and can be suppressed to about 1.5 × 1016 cm-3 by increasing growth V/III ratio. Luminescence efficiency of QWs grown on n-GaN under high V/III ratio is greatly improved. These results indicate high density of nitrogen vacancies are formed in n-GaN layer grown under low V/III ratio and diffuse into quantum wells during epitaxial growth and reduce the luminescence efficiency of the QWs.

Optimizing Li+ transport in Li7La3Zr2O12 solid electrolytes

The Li + conductivity of Li7La3Zr2O12 (LLZO) solid electrolytes was optimized by controlling the addition of the dopant (Al) into the crystal structure. A solution-based synthesis method was used to minimize Al segregation and ensure a homogeneous substitution into the garnet framework. Neutron and x-ray diffraction were used to monitor the structural transition from tetragonal to cubic LLZO. It was found that the critical dopant concentration (xc) required to stabilize the cubic phase was 0.18 mol pfu. The maximum ionic conductivity of 5.54 × 10−4 S/cm was obtained at the critical composition. The enhanced conductivity was achieved by accurately controlling the vacancy density at the active Li octahedral sites. Neutron structure refinements were used to validate the changes in Li vacancy distribution at different dopant concentrations. Impedance measurements reveal a strong dependence of Li+ transport properties on Al concentration, specifically around the critical dopant concentration xc = 0.18 mol pfu. These findings demonstrate the importance of fine-tuning composition and provide guidance for engineering improvements in ionic conductivity.

Oxygen vacancy and hydrogen in amorphous HfO2

Amorphous hafnium dioxide (a-HfO2) is widely used in electronic devices, such as ultra-scaled field-effect transistors and resistive memory cells. The density of oxygen vacancy (OV) defects in a-HfO2 strongly influences the conductivity of the amorphous material. Ultimately, OV defects are responsible for the formation and rupture of conductive filament paths which are exploited in novel resistive switching devices. In this work, we studied neutral OV in a-HfO2 using methods. We investigated the formation energy of OV, the binding energy of di-OVs, the OV migration, unperturbed and in the close presence of a hydrogen atom, as well as the migration of hydrogen atom towards OV. A shallow and short-range OV migration barrier (0.6 eV) exists in a-HfO2 in contrast to the barrier (2.4 eV) in crystalline HfO2. Nearby hydrogen has a limited impact on the OV migration; however, hydrogen can diffuse easily by hopping among OVs.

Deposition of titanium carbide catalytic films on vanadium foils by ion beam sputtering for hydrogen separation and purification

Titanium carbide (TiC) catalytic films were deposited on vanadium (V) foils by ion beam sputtering (IBS) technique for hydrogen purification. The effect of sputtering temperature in the IBS process on the phase structure and catalytic property of the TiC catalytic films and the hydrogen permeation performance of TiC/V composite membranes was investigated. The carbon vacancy density of TiC films increases with the sputtering temperature, resulting in increasing catalytic property of TiC films and hydrogen permeation performance of TiC/V composite membranes. At sputtering temperature of 400 °C, the TiC/V composite membranes exhibit highest hydrogen permeability (∼9.5 × 10−8 mol H2 m−1 s−1 Pa−0.5) at 650 °C and excellent stability during the thermal cycles at 450–650 °C.

Visible light sensitive Au–TiO2 nanocomposites formed by effective attachment of Au with TiO2 nanoparticles using liquid-phase pulsed-laser ablation

Liquid-phase pulsed-laser ablation was utilized to grow visible-light-sensitive Au–TiO2 nanocomposites. We performed the nanosecond pulsed-laser ablation of a Ti target in aqueous solution of HAuCl4 of different concentrations varying from 0 mM to 0.24 mM. Structural characterization studies using transmission electron microscopy (TEM) and X ray diffraction (XRD) analyses confirmed formation of the nanoparticles of TiO2 and Au–TiO2 in respective solutions. In Au–TiO2 samples, nanocomposites formation by efficient attachment between Au and TiO2 nanoparticles was confirmed by absorption and X-ray photoelectron spectroscopy (XPS) analyses. Optical absorption analysis revealed that with the attachment of Au, there is a red shift in the onset of the absorption band of TiO2 from the ultraviolet to visible range. This red shift is found to increase from 3.5 to 2.7 eV with respect to the increase in the amount of Au present in Au–TiO2 nanocomposites. While detailed XPS analysis suggested that when the amount of Au attached with TiO2 increased, the number density of oxygen vacancy or Ti3+ states increased. Therefore the attachment of Au nanoparticles and oxygen vacancy or Ti3+ states generations in TiO2 were found to be responsible for the large red shift in the onset of the TiO2 absorption band. Consequently our study showed that single step liquid phase laser ablation method and its subsequent nonequilibrium growth conditions are useful in synthesizing Au and TiO2 nanoparticles simultaneously and also in providing efficient attachment between Au and TiO2 which is responsible for visible range sensitization of Au-TiO2 nanocomposites .

Conductometric sensor for gaseous sulfur-mustard simulant by gold nanoparticles anchored on ZnO nanosheets prepared via microwave irradiation

We report the microwave-assisted synthesis of Au nanoparticles (NPs) anchored porous ZnO nanosheets (Au-ZnO NSs) and their application in the sensitive detection of 2-chloroethyl ethyl sulfide (2-CEES) vapors, which are a simulant of sulfur mustard chemical weapon. Upon microwave irradiation for < 1 min with an annealing, highly crystalline Au NPs with a narrow particle-size distribution (2.32 ± 0.40 nm) are densely formed on the surface of porous ZnO NSs, which increases the density of oxygen vacancies in the ZnO. Under the optimal working temperature (450 °C), the response of the Au-ZnO NS sensor was measured to be 787 for 10 ppm 2-CEES, which is ∼14 times higher than that observed for the bare porous ZnO NSs-based sensor. Moreover, Au-ZnO NSs can detect 2-CEES gas even under high humidity (∼80 %) benefiting from its high sensitivity. The highly reproducible sensing performance was verified by repeated sensing test (20 times for 12 h). According to gas screening data, the Au-ZnO NSs exhibited outstanding selectivity toward sulfide compounds due to the high Au-S affinity. In summary, we have successfully demonstrated a simple and facile approach to form the Au-ZnO heterostructure by microwave irradiation and enhanced the gas-sensing performance by inducing catalytic activity.

Electrospun TiO2 Nanofibers Featuring Surface Oxygen Vacancies as a Multifunctional Interlayer for High-Performance Lithium-Sulfur Batteries in a Wide Temperature Range.

Despite great achievements having been made in lithium-sulfur batteries (LSBs), further improvements regarding rate performance, cycle life, and operating temperature are needed for realistic applications. Herein, we developed a simple electrospun method for the preparation of TiO2 coaxial nanofiber (TCNFs)-modified Celgard separators to suppress the polysulfide shuttling. LSBs with a TCNF/Celgard separator display excellent electrochemical performance. For an areal sulfur loading of 2.5 mg cm-2, the cells exhibited a capacity of 1279 mA h g-1 at 0.5 A g-1, remained 798 mA h g-1 at 2.5 A g-1, and low-capacity decay of 0.057% per cycle within 1000 cycles. At 50 and -10 °C, the capacity of the cells is maintained at 932 and 931 mA h g-1 after 80 cycles at 0.5 A g-1, respectively. Detailed structural analysis and theoretical calculations revealed that the hollow-structured TCNFs offer high density of accessible electropositive Ti sites and oxygen vacancies and thus enables efficient trapping of polysulfides and facilitates Li+ transfer, leading to excellent performance. The simplicity of this strategy and the diversity of hollow-structured metal oxides holds great promise to design separators for high-performance LSBs.