Vacancy States Research Articles

Angular dependence of L X-rays emission for Ag by 10 keV electron-impact

Abstract The characteristic X-ray intensities of Ag-L α , L β1 , L β2 and L γ 1 are measured in electron-impact ionization at energy of 10 keV. The emission angle in this work ranges from 0° to 20° at interval of 5°. The angular dependence of L X-ray intensity ratios has been investigated for L α / L β1 , L β2 / L β1 and L γ1 / L β1 . It is found from the experimental results that the emissions of L β1 , L β2 and L γ 1 X-rays are spatially isotropic, while the L α X-rays exhibit anisotropic emission. Consequently, the alignment behavior of vacancy states is discussed with thorough analysis of vacancy transfer process.

Resonant valence bond states in zinc vacancies induce the ferromagnetism of ZnO

A theoretical model was proposed to investigate the mechanism of ferromagnetism in ZnO as well as to simulate the experimental result that the ferromagnetism can be enhanced by UV irradiation as UV photon energy is equivalent to the band gap. In the model, the spin moments arise from the trapped electrons in oxygen vacancy states and coexist with the itinerant electrons which reside in zinc vacancy states and fall into resonant valence bond states. Charge exchange between the conduction band of ZnO and both vacancy states makes electrons on both vacancy states delocalized and results in a decrease of the ferromagnetism as well.

Spin-polarized annihilation lifetime of positron of d0 ferromagnetism in gallium nitride: A two-component density functional theory simulation

The spin-polarized annihilation lifetime of a positron (SPALP) trapped by the Ga vacancies in GaN was investigated using two-component density functional theory calculations with geometry optimization. SPALP clearly depends on the charged states of the Ga vacancies and the induced magnetization. This dependence is attributed to the overlap between the positron and electron densities around the defect. Thus, the present study proves the useful role of SPALP for probing and revealing the mechanism of d0 ferromagnetism.

Structural-relaxation-driven electron doping of amorphous oxide semiconductors by increasing the concentration of oxygen vacancies in shallow-donor states

The electronic states of oxygen vacancies (VO s) in amorphous oxide semiconductors are shallow donors, deep donors or electron traps; these are determined by the local atomic structure. Because the amorphous phase is metastable compared with the crystalline phase, the degree of structural disorder is likely to decrease, which is referred to as structural relaxation (SR). Thus SR can affect the VO electronic state by changing the local atomic conditions. In this study, we demonstrated that electron doping is possible through the SR of amorphous oxides without redox reactions using a novel device structure that prevents extrinsic reactions with electrodes and ambient atmosphere during annealing. The concentration of VO s in the shallow-donor state in amorphous In-Ga-Zn-O (a-IGZO) increases from ~1016 to ~1019 cm−3 with increasing annealing temperatures between 300 and 450 °C. The SR-driven doping effect is strongly dependent on the annealing temperature but not on the annealing time. The Arrhenius activation energy of the SR-driven doping effect is 1.76 eV, which is similar to the bonding energies in a-IGZO. Our findings suggest that the free volume in a-IGZO decreases during SR, and the VO s in either deep-donor or electron-trap states are consequently transformed into shallow-donor states. A chemical-free method for enhancing the conductivity of transparent transistor materials could benefit flat-panel and flexible displays. Amorphous indium–gallium–zinc oxide (a-IGZO) is a semiconductor that can withstand bending better than silicon technology thanks to its disordered internal structure. However, a process known as structural relaxation slowly reduces the randomness of a-IGZO atoms, which alters the electrical characteristics of a-IGZO. To initiate controlled structural relaxations, Young-Chang Joo from Seoul National University and colleagues annealed a stacked novel metal/a-IGZO/metal device at temperatures below the glass transition point. Electrical measurements revealed the relaxations increased the concentration of shallow-donor-state oxygen vacancies, enabling electron dopants to be added to a-IGZO without using redox chemistry. The temperature-dependent doping can be sustained for prolonged annealing times, making it a promising approach for enhancing the thermal stress resistance of flexible electronics. The effects of structural relaxation (SR) on the electronic state of oxygen vacancies (VO s) in amorphous oxide semiconductors is investigated. Without redox reactions, the concentration of VO s in the shallow-donor state (NDS) increases about 103 times with increases in the annealing temperature from 300 to 450 °C. The reduction in the free volume size and transformation of VO s in either deep-donor or electron-trap states into the shallow-donor state during SR is the primary mechanism responsible for the increase in NDS.

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (EF) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO2)Ge). The concentration of oxide on the (GeO2)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO2)Ge surface to 6 nm was studied. At energies below 9 eV above EF, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO2)Ge surface is accompanied by an increase in the work function of the surface, Evac–EF, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.

Bipolar resistive switching behaviors of ITO nanowire networks

We have fabricated indium tin oxide (ITO) nanowire (NW) networks on aluminum electrodes using electron beam evaporation. The Ag/ITO-NW networks/Al capacitor exhibits bipolar resistive switching behavior. The resistive switching characteristics of ITO-NW networks are related to the morphology of NWs. The x-ray photoelectron spectroscopy was used to obtain the chemical nature from the NWs surface, investigating the oxygen vacancy state. A stable switching voltages and a clear memory window were observed in needle-shaped NWs. The ITO-NW networks can be used as a new two-dimensional metal oxide material for the fabrication of high-density memory devices.

Fabrication of Wide-Range-Visible Photocatalyst Bi2WO6-x nanoplates via Surface Oxygen Vacancies.

Bi2WO6 as a high visible-light-driven catalyst has been aroused broad interest. However, it can only be excitated by the light with λ < 450 nm and the solar energy utilization need to be improved. Here, the wide–range–visible photoresponse Bi2WO6−x nanoplates were fabricated by introducing surface oxygen vacancies through the controllable hydrogen reduction method. The visible photoresponse wavelength range is extended from 450 nm to more than 600 nm. In addition, the photocatalytic activity of Bi2WO6−x is also increased and is 2.1 times as high as that of pristine Bi2WO6. The extending of the photoresponse range and the enhancement of the photoactivity both can be attributed to the surface-oxygen-vacancy states. This is because surface-oxygen–vacancy states generated above and partly overlapping of with the valence band (VB) will result in the rising of valence band maximum (VBM), thus broadening the VB width. This approach is proposed to develop many types of wide–range–visible optical materials and to be applicable to many narrow and wide bandgap materials.

Stress Induced Vacancy Clustering Mechanism of Resistive Switching in Hafnium Oxides

ABSTRACTReversible changes in the conductivity of HfO2 dielectric film between high and low resistive states of a metal-insulator-metal memory cell were attributed to the formation of oxygen vacancies and their clustering across the insulator layer. In this study we present an innovative model which includes generation of two-charged states of oxygen vacancies at the anode, their diffusion to the cathode, transformation to one-charged state, and then to neutral vacancies. Vacancy clusters in the insulator layer are built from only neutral vacancies, while the kinetics of the clustering process is controlled by diffusion of mobile one-charged state vacancies. Resistive switching is treated as the formation of critical size vacancy cluster which provides continuous conductive path through the dielectric layer. Good agreement between the experimental data and the theoretical bias and temperature dependences for the delay time was obtained.

The instability of S vacancies in Cu2ZnSnS4

The electronic structures of the possible charge states of vacancies in the earth-abundant solar cell absorber material Cu2ZnSnS4 (CZTS) are investigated using screened-exchange hybrid density functional theory.

The nature of chemical bonding in actinide and lanthanide ferrocyanides determined by X-ray absorption spectroscopy and density functional theory.

The electronic properties of actinide cations are of fundamental interest to describe intramolecular interactions and chemical bonding in the context of nuclear waste reprocessing or direct storage. The 5f and 6d orbitals are the first partially or totally vacant states in these elements, and the nature of the actinide ligand bonds is related to their ability to overlap with ligand orbitals. Because of its chemical and orbital selectivities, X-ray absorption spectroscopy (XAS) is an effective probe of actinide species frontier orbitals and for understanding actinide cation reactivity toward chelating ligands. The soft X-ray probes of the light elements provide better resolution than actinide L3-edges to obtain electronic information from the ligand. Thus coupling simulations to experimental soft X-ray spectral measurements and complementary quantum chemical calculations yields quantitative information on chemical bonding. In this study, soft X-ray XAS at the K-edges of C and N, and the L2,3-edges of Fe was used to investigate the electronic structures of the well-known ferrocyanide complexes K4Fe(II)(CN)6, thorium hexacyanoferrate Th(IV)Fe(II)(CN)6, and neodymium hexacyanoferrate KNd(III)Fe(II)(CN)6. The soft X-ray spectra were simulated based on quantum chemical calculations. Our results highlight the orbital overlapping effects and atomic effective charges in the Fe(II)(CN)6 building block. In addition to providing a detailed description of the electronic structure of the ferrocyanide complex (K4Fe(II)(CN)6), the results strongly contribute to confirming the actinide 5f and 6d orbital oddity in comparison to lanthanide 4f and 5d.

Structure of the Ru, Ag and Te L X-ray emission spectra

The emission of X-rays in atomic transitions from L-shell vacancy states of Ru, Ag and Te induced by electron incidence was studied.

An oxygen-vacancy rich 3D novel hierarchical MoS2/BiOI/AgI ternary nanocomposite: enhanced photocatalytic activity through photogenerated electron shuttling in a Z-scheme manner.

An oxygen-vacancy rich, bismuth oxyiodide-based Z-scheme 3D hierarchical MoS2/BiOI/AgI ternary nanocomposite photocatalyst was fabricated using a simple precipitation process in ethylene glycol and water. The presence of oxygen-vacancies in BiOI and the two-dimensional nature of molybdenum disulfides in the composite prolongs the charge carrier lifetime through a Z-scheme system and enhances the performance of the photocatalyst for the degradation of rhodamine B. On the basis of efficient separation of photoexcited electron-hole pairs, a mechanism is proposed whereby MoS2 and oxygen vacancy states increase charge carrier lifetimes and improve the photocatalytic activity. The Z-scheme mechanism of the photocatalysis is consistent with the results of static and time-resolved photoluminescence, scavenging, and terephthalic acid photoluminescence experiments. Among the as-synthesized photocatalysts, the one containing 2 wt% of MoS2 in a composite of MoS2/BiOI/AgI exhibited the highest photocatalytic activity towards rhodamine B degradation, and its activity was 7 and 16 times higher than that of BiOI/AgI and BiOI, respectively. Degradation of phenol, the colorless model pollutant, was studied to confirm the visible-light photocatalytic performance of the MoS2/BiOI/AgI composite. This easily fabricated Z-scheme based MoS2/BiOI/AgI composite exhibits promising photocatalytic activity and will be useful for potential applications in energy and environmental areas.

Magnetic susceptibility of carbon nanotubes

For (n0) and (nn) nanotubes in a magnetic field parallel to the nanotube axis, energy level formulas have been derived and energy level diagrams have been calculated. It has been shown that, for metallic nanotubes in a magnetic field, an energy gap between the occupied and vacant states appears, the gap width depending on n. For semiconducting nanotubes in a magnetic field, the gap width decreases. The diagrams of the gap width versus n are reported. Theoretical study of the magnetic susceptibility of carbon nanotubes in a magnetic field parallel to the nanotube axis has been carried out. Nontrivial temperature dependences of magnetic susceptibility typical of quasi-one-dimensional nanostructures have been obtained. Depending on the chemical potential, magnetic susceptibility spectra have been obtained as sharp peaks with the amplitudes increasing with a decrease in temperature. Dependences of magnetic susceptibility on (n0) and (nn) nanotube diameter have been studied.

Evidence of oxygen vacancy and possible intermediate gap state in layered α-MoO3 single-crystal nanobelts

Multilayered meso-structured MoO3 nanobelts have been synthesized by thermally oxidizing a molybdenum chip in a reduced oxygen atmosphere, with a view to disclosing the existence of oxygen vacancy and understanding the mechanism behind the influence of oxygen vacancy on the electronic structure of molybdenum oxides. Based on the measurements from X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM) and transmission electron microscope (TEM), it is found that the as-grown sample is single-crystal α-MoO3 with a (001) preferred orientation, which shows an irregular belt-like morphology being composed of some ~20nm single-crystal thin layers. The present sample includes a lot of oxygen vacancies in the lattice, as evidenced by the considerably reduced coordination number of the central Mo atoms from X-ray absorption spectra (XAS) as well as the red shift of the main Raman peaks. The existence of the oxygen vacancies are further tested by the photoluminescence (PL) results as the main emission peak shows an obvious red shift with the corresponding optical band gap reduced to 2.3eV. Very importantly, an extra emission positioned at 738nm (1.68eV) is believed to originate from the recombination of the electrons from the intermediate band (IB) to the valence band (VB), and the formation of the IB in the gap is also caused by oxygen-ion vacancies.

Optical properties in Mn-doped ZnS thin films: Photoluminescence quenching

Mn-doped ZnS thin films are synthesized on soda-lime glass substrates using magnetron co-sputtering technique. X-ray diffraction and atomic force microscopy measurements indicate that of the as-obtained films including the highest Mn (~11% relative to the Zn concentration) in the lattice of ZnS are amorphous with a granular morphology. X-ray photoelectron spectroscopy reveals the presence of Zn2+, Mn2+ and S2− chemical states in the films. The undoped ZnS film exhibits photoluminescence (PL) peaks at energies around 3.26eV (wavelength ~379nm) and 2.95eV (~420nm), which originate from the interplay between excited electron, defect (sulfur vacancy) states and the valence band. For the Mn-doped ZnS films, the band-to-band emission peak is quenched and shifts toward to higher energies at a rate of 11.7±2meV/Mn%. We propose that Mn dopant-mediated structural phases and non-radiative deep traps in ZnS cause the modification in the optical transition.

Vacancy and Doping States in Monolayer and bulk Black Phosphorus.

The atomic geometries and transition levels of point defects and substitutional dopants in few-layer and bulk black phosphorus are calculated. The vacancy is found to reconstruct in monolayer P to leave a single dangling bond, giving a negative U defect with a +/− transition level at 0.24 eV above the valence band edge. The V− state forms an unusual 4-fold coordinated site. In few-layer and bulk black P, the defect becomes a positive U site. The divacancy is much more stable than the monovacancy, and it reconstructs to give no deep gap states. Substitutional dopants such as C, Si, O or S do not give rise to shallow donor or acceptor states but instead reconstruct to form non-doping sites analogous to DX or AX centers in GaAs. Impurities on black P adopt the 8-N rule of bonding, as in amorphous semiconductors, rather than simple substitutional geometries seen in tetrahedral semiconductors.

The interaction between oxygen vacancies and doping atoms in ZnO

Abstract Three different kinds of typical p-type impurities, (Ag, N and K) ZnO crystals, with and without oxygen vacancy are studied, respectively. The lattice structure, band structure, density of state and formation energy of oxygen vacancies were calculated on intrinsic crystal and defect crystal with oxygen vacancies. The calculation results reveal that lattice constant and unit cell volume increased in accepter doped cells. The formation energy of oxygen vacancies is lowest in K-doped cells, and the Ag-doped one has the highest hole conductivity. It is easier for K-doped to form oxygen vacancies. Finally, effects of oxygen vacancies to the conductivity of three doped systems were studied.

Three-electron collective Auger decay in CH3F

SynopsisWe report on experimental and theoretical study of collective Auger decay of double inner-valence vacancy in fluoromethane, in which two electrons simultaneously fill the vacancies and a third electron is ejected to continuum. Calculations predict the respective lifetime of the double vacancy state to be in the femtosecond range, typical for two-electron Auger decay. Measured electron spectra indirectly support this rather surprising result.

New insight into the enhanced photocatalytic activity of N-, C- and S-doped ZnO photocatalysts

In general, N-, C- and S-doped ZnO exhibit much higher phototcatalytic activity than the pure ZnO. However, the essential factors and underlying mechanism regarding the enhancement of photocatalytic activity are still unclear. In this work, the electronic structures, optical properties and effective masses of charge carriers are investigated by first-principle density functional theory calculation. Due to the nature of p-type doping, N and C doping can generate vacant states above the Fermi level and shift the conduction band into lower energy region, resulting in narrowing of band gap. Thus, N- and C-doped ZnO demonstrate much stronger light absorption in both visible and ultraviolet region. In contrast, because of the absence of vacant states, only limited enhancement of light absorption is observed for S-doped ZnO whose improved photocatalytic performance can only be attributed to the direct reduction of band gap. The calculation of the effective masses show that ZnO typically possess light electrons and heavy holes, confirming its intrinsic character of n-type semiconductor, while N, C and S doping can generally render electrons lighter and holes heavier, resulting in slower recombination rate of photogenerated electron–hole pairs. Noticeably, C doping can discourage such recombination to the greatest extent and separate electron–hole pairs most efficiently compared with N and S doping, serving as a potentially promising pathway to increase the quantum efficiency of ZnO-based photocatalysts. This work will provide some new insights into the understanding of doping effect over the enhancement of photocatalytic activity of N-, C- and S-doped ZnO.

Effect of electron-phonon interaction on the formation of one-dimensional electronic states in coupled Cl vacancies

The formation of extended electron states in one-dimensional nanostructures is of key importance for the function of molecular electronics devices. Here we study the effects of strong electron-phonon interaction on the formation of extended electronic states in intentionally created Cl vacancy pairs and chains in a NaCl bilayer on Cu(111). The interaction between the vacancies was tailored by fabricating vacancy pairs and chains of different orientation and separation with atomic precision using vertical manipulation. Small separation of divacancies led to the formation of symmetric and antisymmetric vacancy states and localized interface-states. By scanning tunneling spectroscopy (STS) we measured their energy splitting and broadening as a function of the inter-vacancy separation. Unexpectedly, the energy splitting between the vacancy states is enlarged by level repulsion resulting from phonon dressing of the electronic states, as evidenced by theory. Already for a few coupled vacancies we observe an emerging band structure of the defect band.