Vacancy States Research Articles

Synthesis, tailoring the optical properties of insulating alkaline-earth metal oxide (MgO) with dopants of transition metal (Cu) and vibrating sample magnetometer measurements

X-ray diffraction studies on MgO with copper dopants show the solubility limit of copper up to 3% preferably with f.c.c. (face centered cubic) structures. With doped of copper exceeding 3% in MgO. XRD showed phase changes with dopants of copper at 5% and exceeding 5% in MgO. Composites of two different crystalline phases are expected to appear (may be f.c.c cubic and f.c. triclinic). This confirmed to the fact that substitution or replacement of magnesium ions with copper ions would result into composite structures. Secondary phase appeared with exceeding 3% copper doping in MgO in the form of CuO as spherical nanoparticles and the same could be for MgO as spherical nanoparticles. EDX confirmed this behavior for copper dopants in MgO.With UV–visible absorption spectra, the quantum confinement effect is attributed to defects produced with dopants of copper ions as interstials in MgO within a limit of one to three percent. The observed size of MgO with SEM is 90.85 nm and the band gap is 4.68eV which are indicative of the transitions from insulating to dielectric behavior or to topological insulator.Experimental results of a.c. magnetic susceptibility with VSM on magnesium oxide and varying dopants of copper in MgO even with composite structures and phase changes show an antiferromagnetic behavior. This antiferromagnetic behavior is a manifestation of ‘topological insulator’ with enhanced dielectricity. Spin fluctuations due to nonlinear spin dynamics, triggered gyroscopic behavior in the nanomaterials, thereby producing quasiparticles of electrons and holes (vacant quantum states of electrons) as Majorana Fermions. Step transitions with upward and downward peaks of magnetization in MgO, with 3% copper ions concentrations and MgO with 8% copper ions concentration show ‘skyrmion like’ excitations with decreasing trends due to phase changes and composite structures but accompanied with negative slopes thereby maintaining ‘antiferromagnetic behavior’ as manifestations of topological insulators. Other samples of MgO with 1%, 5% and 10% copper ions concentrations showed antiferromagnetic behavior. With proper selection of copper dopants in MgO, various kinks of ‘spin fluctuations’ as manifestations of magnetic excitations can be produced to making these materials useful for diverse categories of devices.Mathematical result is proposed for eigenfunctions (shape profiles) and energy eigenvalues by considering the quasiparticles of energy fields for electrons and holes as Majorana Fermions with twisting and twigging in a braided configuration with QED behavior.

Tailoring S-vacancy concentration changes the type of the defect and photocatalytic activity in ZFS

Defect engineering is crucial in the development of semiconductor catalyst activity. However, the influence of defect/vacancy density and states on catalysis remains vague. Thus, the optimized sulfur vacancy (SV) state is achieved among Fe-ZnS models (ZFS) via a chemical etching strategy for photocatalytic degradation (PD). As the SV concentration (ρSV) increases, the predominant state of vacancies changes from isolated defects－a state to a combination of a state and vacancy clusters－e state, as verified by positron annihilation and X-ray absorption fine structure spectra. However, the two types of defect states activated the intrinsic activity of the crystal via radically different mechanisms and exerted different degrees of influence on PD activity, as revealed by first-principles calculations and quantitative structure-activity relationship. Our results suggest that the SV activity is strongly influenced by its concentration in the ZFS crystal, while the vacancy concentration is not a control parameter for the PD activity, but a defect form. The underlying essence of atomic defects behavior affecting crystal catalytic activity at the atomic level is also revealed in this paper. Uncovering these structural relationships provide a theoretical basis for designing effective catalysts.

Field ion microscopy images of multilayered graphene and graphene oxide

Field emission and field ion microscopy (FIM) of graphene edges exhibit exotic images with enigmatic fine structures, which are not expected in ordinary metal emitters such as tungsten and molybdenum. Graphene emitters were prepared by three methods; scratching highly oriented pyrolytic graphite by a tungsten needle, electrophoretic deposition of graphene oxide (GO), and putting droplets of GO solution on the tip of a tungsten needle. FIM of graphene show arrayed sharp streaks, split-double spots, and hollow “race-track” shaped spots, running transverse to a graphene plane. It is speculated that these patterns originate from vacant electronic states and reflect the symmetry and distribution of π*-orbitals at graphene edges.

Probing Photoexcited Charge Carrier Trapping and Defect Formation in Synergistic Doping of SrTiO3

Strontium titanate (SrTiO3) is widely used as a promising photocatalyst due to its unique band edge alignment with respect to the oxidation and reduction potential corresponding to oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). However, further enhancement of the photocatalytic activity in this material could be envisaged through the effective control of oxygen vacancy states. This could substantially tune the photoexcited charge carrier trapping under the influence of elemental functionalization in SrTiO3, corresponding to the defect formation energy. The charge trapping states in SrTiO3 decrease through the substitutional doping in Ti sites with p-block elements like Aluminium (Al) with respect to the relative oxygen vacancies. With the help of electronic structure calculations based on density functional theory (DFT) formalism, we have explored the synergistic effect of doping with both Al and Iridium (Ir) in SrTiO3 from the perspective of defect formation energy, band edge alignment and the corresponding charge carrier recombination probability to probe the photoexcited charge carrier trapping that primarily governs the photocatalytic water splitting process. We have also systematically investigated the ratio-effect of Ir:Al functionalization on the position of acceptor levels lying between Fermi and conduction band in oxygen deficient SrTiO3, which governs the charge carrier recombination and therefore the corresponding photocatalytic efficiency.

Ultrafast energy transfer between π-stacked aromatic rings upon inner-valence ionization.

Non-covalently bound aromatic systems are ubiquitous and govern the physicochemical properties of various organic materials. They are important to many phenomena of biological and technological relevance, such as protein folding, base-pair stacking in nucleic acids, molecular recognition and self-assembly, DNA-drug interactions, crystal engineering and organic electronics. Nevertheless, their molecular dynamics and chemical reactivity, particularly in electronic excited states, are not fully understood. Here, we observe intermolecular Coulombic decay in benzene dimers, (C6H6)2-the simplest prototypes of noncovalent π-π interactions between aromatic systems. Intermolecular Coulombic decay is initiated by a carbon 2s vacancy state produced by electron-impact ionization and proceeds through ultrafast energy transfer between the benzene molecules. As a result, the dimer relaxes with the emission of a further low-energy electron (<10 eV) and a pair of C6H6+ cations undergoing Coulomb explosion. Coincident fragment-ion and electron momentum spectroscopy, accompanied by ab initio calculations, enables us to elucidate the dynamical details of this ultrafast relaxation process.

Vacancy Defects in Ga2O3: First-Principles Calculations of Electronic Structure.

First-principles density functional theory (DFT) is employed to study the electronic structure of oxygen and gallium vacancies in monoclinic bulk β-Ga2O3 crystals. Hybrid exchange–correlation functional B3LYP within the density functional theory and supercell approach were successfully used to simulate isolated point defects in β-Ga2O3. Based on the results of our calculations, we predict that an oxygen vacancy in β-Ga2O3 is a deep donor defect which cannot be an effective source of electrons and, thus, is not responsible for n-type conductivity in β-Ga2O3. On the other hand, all types of charge states of gallium vacancies are sufficiently deep acceptors with transition levels more than 1.5 eV above the valence band of the crystal. Due to high formation energy of above 10 eV, they cannot be considered as a source of p-type conductivity in β-Ga2O3.

Influence of zinc on the multifunctional properties of ferrites [formula omitted] (M = Cu, Mg, Ni, x = 0, 0.35)

We study the magnetic and gas sensing properties of M1-xZnxFe2O4 where M = Cu, Mg, Ni, and x = 0, 0.35, focussing on the influence of zinc on their application as a soft magnetic material and a sensor as well. Ferrites are synthesized by thermal decomposition of the glyoxylate precursor obtained from the redox reaction between metal nitrate and ethylene glycol. The phase formation and microstructure were investigated using X-ray diffraction and electron microscopic methods. X-ray diffraction analysis confirmed the cation redistribution in the spinel lattice due to zinc doping. The porous structure was studied using nitrogen adsorption-desorption isotherms and confirmed the role of zinc on the surface structure. The magnetic properties of the ferrites under the influence of zinc were measured using a vibration sample magnetometer. The oxidation state of cations and oxygen vacancies in the spinel structure has been examined using X-ray photoelectron spectroscopy (XPS) which confirmed a strong interplay between the oxidation state of cations, adsorbed oxygen level, and electrical conductivity. The n-type semiconducting property has been correlated to the adsorbed oxygen abundance and its interaction with gas molecules. The study revealed enhanced soft magnetic properties of ferrite due to zinc doping. In addition, this study presents a stable and sensitive material for detecting a low concentration of carbon monoxide gas.

Superfluidity of vacancies in solid 4He

Under the adiabatic approximation, we present a superfluid state of vacancy motion in solid 4He. The vacancies in solid 4He can sense an attractive effective interaction by exchange of virtual longitudinal acoustic phonons. Such an interaction leads to a Bardeen-Cooper-Schrieffer (BCS) condensation state, in which the vacancies with opposite wave vectors are bound into pairs and single, unpaired bare vacancies are transformed into a new kind of quasiparticle, the dressed vacancy. A dressed vacancy is a vacancy clothed with a cloud of virtual longitudinal-acoustic phonons. The vacancy system undergoes a first-order phase transition from the normal state to the BCS condensation state. The critical temperature may be the transition temperature of the supersolid state. We derive the low-temperature expressions of vacancy heat capacity and vacancy number density in the BCS condensation state. The BCS condensation state of vacancies is the superfluid state of vacancies, which possesses potentially important and novel effects in condensed matter physics.

Ag@CuO@Cu(OH)2: A Synergistic Catalyst for H2O2 Detection with Peroxidase‐Mimic Activity without Interference of O2

AbstractHerein, we present silver nanoparticle anchored on copper oxide and copper hydroxide (Ag@CuO@Cu(OH)2) by green synthetic approach. Interestingly, as‐obtained Ag@CuO@Cu(OH)2 not only enhances the peroxidase‐like activity but also eliminates oxidase‐like activity. The peroxidase‐like activity was enhanced mainly due to the abundance of Ag+ and Cu2+ vacant states, which populate reactive oxygen species (ROS). This prolonged peroxidase‐like activity of Ag@CuO@Cu(OH)2 was successfully recognized H2O2 and also have long stability and repeatability. The proposed strategy, finding nanozymes with single activity will open a new opportunity to enhance the performance of colorimetric sensors.

Enhanced reliability and uniformity for Ge pMOSFET with low temperature supercritical fluid treatment

Significant improvement on uniformity and reliability characteristics in Ge pMOSFET are achieved with a novel low temperature supercritical phase CO 2 fluid treatment with H 2 O cosolvent. Devices with the proposed treatment also exhibit a lower subthreshold swing, a higher on/off current ratio, and a lower interface trap density. The improvement can be attributed to the reduction of oxygen vacancy and low oxidation states in the interfacial layer (IL), and the quality enhancement on both IL and high-k gate stack. • A low temperature supercritical phase CO 2 fluid with H 2 O cosolvent treatment was proposed. • The uniformity and reliability characteristics of p-channel Ge transistor can be improved. • Improvement can be attributed to reduction of oxygen vacancy and low oxidation states. • The process is promising for the mass production of chip on large-size wafers.

Effect of O2 plasma exposure time during atomic layer deposition of amorphous gallium oxide

Amorphous gallium oxide thin films were grown by plasma-enhanced atomic layer deposition on (100) silicon substrates from trimethylgallium Ga(CH3)3 precursor and oxygen plasma. At 200 °C, the growth per cycle is in the range of 0.65–0.70 Å for O2 plasma exposure times ranging from 3 up to 30 s during each cycle. The effect of O2 plasma exposure times on the interfacial SiOx regrowth and the electrical properties was investigated. In situ spectroscopic ellipsometry shows that the SiOx regrowth occurs during the first three cycles and is limited to 0.27 nm for plasma times as long as 30 s. Increasing the O2 plasma exposure during each ALD cycle leads to a drastic decrease in the leakage current density (more than 5 orders of magnitude for 30 nm films), which is linked to the suppression of oxygen vacancy states as evidenced by spectroscopic ellipsometry. Interestingly, an increase in the dielectric constant with increasing O2 plasma exposure time is observed, reaching a value of εr∼14.2, larger than that of single crystalline β-Ga2O3. This study highlights the crucial role of oxygen plasma exposure time in the control and tuning of the electrical properties of amorphous gallium oxide films.

Origin of light instability in amorphous IGZO thin-film transistors and its suppression

Radiating amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with deep ultraviolet light (λ = 175 nm) is found to induce rigid negative threshold-voltage shift, as well as a subthreshold hump and an increase in subthreshold-voltage slope. These changes are attributed to the photo creation and ionization of oxygen vacancy states (VO), which are confined mainly to the top surface of the a-IGZO film (backchannel). Photoionization of these states generates free electrons and the transition from the neutral to the ionized VO is accompanied by lattice relaxation, which raises the energy of the ionized VO. This and the possibility of atomic exchange with weakly bonded hydrogen leads to metastability of the ionized VO, consistent with the rigid threshold-voltage shift and increase in subthreshold-voltage slope. The hump is thus a manifestation of the highly conductive backchannel and its formation can be suppressed by reduction of the a-IGZO film thickness or application of a back bias after radiation. These results support photo creation and ionization of VO as the main cause of light instability in a-IGZO TFTs and provide some insights on how to minimize the effect.

Probing charge transport in manganite film through switching parameters

We have investigated the bipolar resistive switching of Y0.95Ca0.05MnO3 (YCMO) thin film on Si substrate using pulsed laser deposition. Simulation of Mn L3,2 near-edge X-ray absorption fine structure has been executed by CTM4XAS to corroborate the presence of a mixed-valence state of Mn ions and oxygen vacancies. The charge transport in the film is described by the space charge limited mechanism. Murgatroyd and space charge limited mechanism relations are used to calculate the mobility and other switching parameters at high resistance state. With a decrease in the switching layer (near to positively biased electrode) thickness, better resistive switching was observed. This work indicates that the localized switching thickness and temperature strongly affect the resistive switching of the YCMO film.

Electrical characterization of gate stack charge traps in floating body gate-all-around field-effect-transistors

Individual charge traps in the gate stack of gate-all-around field-effect-transistors have been identified from their random telegraph noise (RTN) characteristics in the time and frequency domains. The energy level and depth location of the corresponding charge traps were extracted from capture/emission time constant and corner frequency. The charge traps were determined to be the excited states of oxygen vacancies in the dielectric located 3 nm away from the interface. Both the time domain and frequency domain RTN measurements lead to an identical result.

Chlorine-passivation of ozone-treated SnO2 thin films: Occurrence of oxygen vacancies to manipulate conducting states and bipolarities in resistive switching

Highly conductive SnO2 thin films are of interest in transparent conducting electrodes for displays and n-type transport layer for perovskite solar cells. We fabricated SnO2 thin films by spin coating with ozone exposure. This process compensates for the oxygen vacancy states of the films. A local current–voltage measurement by conductive atomic force microscopy revealed the conducting characteristics of the films. Under different ambient conditions, the films exhibited an exotic cycling bipolarity in their resistive switching phenomena, which is likely dependent on the distinct formation of the oxygen vacancies. After application of a chlorine-based surface coating, the SnO2 surface showed an identical bipolar resistive switching. In particular, the chlorine-passivation was simple and robust to control oxygen vacancies and conducting states, which yields a low power operation below 1–2 V and a huge difference between low- and high-resistant states of a 108–1010 order of magnitude. Ozone-treated and chlorine-passivated oxide beyond SnO2 will provide guidance for optoelectronic device improvement.

Enhanced Electrical Characteristics of Ge nMOSFET by Supercritical Fluid CO2 Treatment With H2O2Cosolvent

Significant improvements on Ge nMOSFET can be achieved by a novel low temperature supercritical phase fluid treatment with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> cosolvent. Thanks to the reduction of oxygen vacancy and unstable oxidation states, devices with the proposed treatment exhibit a low equivalent oxide thickness of 0.67 nm, 2-order reduction on gate leakage current, 7 times improvement on on-current, very low subthreshold swing of 79 mV/dev, high on/off current ratio, small hysteresis of 43.3 mV, low interface trap density, excellent uniformity and reliability characteristics.

Bandgap Engineering in a Staggered-Type Oxide Perovskite Heterojunction for Efficient Visible Light-Driven Photocatalytic Dye Degradation.

Oxide perovskite materials with ABO3 structure have been widely employed for photocatalytic applications. However, owing to the disadvantageous electron-hole recombination process and wide bandgap of some materials, the photocatalytic performance is seemingly restricted. Coupling two catalysts together through the formation of a heterojunction ensures effective charge carrier separation. The intimate interaction between the materials is propitiously useful for charge transfer, thereby increasing the efficacy. In this study, the photocatalytic activity of a KxNa(1-x)NbO3-BaBiO3 (KNN-BBO) heterojunction material for the degradation of Rhodamine 6G organic dye was investigated. The materials were extensively characterized by X-ray diffraction, UV-Vis diffused reflectance spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and N2 adsorption isotherms. The degradation efficiency of the organic contaminant under 1 sun simulated sunlight is monitored by spectral analysis from UV-Vis absorption spectroscopy. The resistance to charge transfer was also observed by electrochemical impedance spectroscopy. The effect of the sintering temperature on the photoinduced degradation activity was also included in our study. An unsintered KNN-BBO (UKB) composite material is found to be the most efficient catalyst with 84% removal efficiency as compared to the sintered one (SKB). This is attributed to the reduced bandgap with staggered-type band alignment, increased surface area, and surface oxygen vacancy states. Together with the crucial findings of this work, a probable mechanism for enhanced photocatalytic activity has been proposed here.

Layer spacing gradient (NaLi)1−xCoO2 for electrochemical Li extraction

Layer spacing gradient (NaLi)1−xCoO2 for electrochemical Li extraction

Vacancy Engineering of Ultrathin 2D Materials for Photocatalytic CO2Reduction

AbstractPhotocatalytic carbon dioxide (CO2) reduction is a sustainable and green strategy for the conversion of CO2into hydrocarbon solar fuels, whereas its large‐scale application is severely restricted by lack of highly effective photocatalysts. Ultrathin 2D materials with tunable electronic structure display great potential towards photocatalytic CO2reduction. However, the photocatalytic performance still remains unsatisfied due to high activation energy of CO2molecules on catalytic sites. To this end, surface vacancy engineering can endow coordinately unsaturated sites as actively catalytic sites for CO2molecules chemisorption and activation. In this review, we focus on vacancy‐engineered ultrathin materials for CO2photoreduction. Different vacancies with classified anion vacancies, cation vacancies, vacancy pairs, voids, and their corresponding role in CO2photoreduction are proposed. The different strategies based on vacancy engineering, including direct modulation of vacancy concentrations, refining vacancy states by heteroatom, and vacancy‐engineered heterostructure, are presented. Finally, the future developments and their associated challenges concerning defective ultrathin 2D materials are discussed.

Photoinduced trapping of charge at sulfur vacancies and copper ions in photorefractive Sn2P2S6 crystals

Electron paramagnetic resonance (EPR) is used to monitor photoinduced changes in the charge states of sulfur vacancies and Cu ions in tin hypothiodiphosphate. A Sn2P2S6 crystal containing Cu+ (3d10) ions at Sn2+ sites was grown by the chemical vapor transport method. Doubly ionized sulfur vacancies (VS2+) are also present in the as-grown crystal (where they serve as charge compensators for the Cu+ ions). For temperatures below 70 K, exposure to 532 or 633 nm laser light produces stable Cu2+ (3d9) ions, as electrons move from Cu+ ions to sulfur vacancies. A g matrix and a 63,65Cu hyperfine matrix are obtained from the angular dependence of the Cu2+ EPR spectrum. Paramagnetic singly ionized (VS+) and nonparamagnetic neutral (VS0) charge states of the sulfur vacancies, with one and two trapped electrons, respectively, are formed during the illumination. Above 70 K, the neutral vacancies (VS0) are thermally unstable and convert to VS+ vacancies by releasing an electron to the conduction band. These released electrons move back to Cu2+ ions and restore Cu+ ions. Analysis of isothermal decay curves acquired by monitoring the intensity of the Cu2+ EPR spectrum between 74 and 82 K, after removing the light, gives an activation energy of 194 meV for the release of an electron from a VS0 vacancy. Warming above 120 K destroys the VS+ vacancies and the remaining Cu2+ ions. The photoinduced EPR spectrum from a small concentration of unintentionally present Ni+ ions at Sn2+ sites is observed near 40 K in the Sn2P2S6 crystal.