Charge State Of Vacancy Research Articles

First-principles DFT+Uinvestigation of charged states of defects and fission gas atoms inCeO2

Cerium dioxide $({\mathrm{CeO}}_{2})$ is considered as a model material for the experimental study of radiation damage in the standard nuclear fuel uranium dioxide $({\mathrm{UO}}_{2})$. In this paper, we present a first-principles study in the framework of the DFT+$U$ approach to investigate the charged point defects and the incorporation of the fission gases Xe and Kr in ${\mathrm{CeO}}_{2}$ and compare it with published data in ${\mathrm{UO}}_{2}$. All intrinsic charge states are considered for point defects in contrast to previous published studies. Our calculations prove that ${\mathrm{CeO}}_{2}$ shows similar behavior to ${\mathrm{UO}}_{2}$ in the formation of point defects with the same charge states under stoichiometric and nonstoichiometric conditions. The charge states of vacancies have an important effect on the incorporation of fission gas atoms in ${\mathrm{CeO}}_{2}$. The bound Schottky defect with the two oxygen vacancies along the (100) direction is found to be energetically preferable to trap Xe and Kr atoms both in ${\mathrm{CeO}}_{2}$ and ${\mathrm{UO}}_{2}$. Xe and Kr atoms in the cation vacancy sites under nonformal charge states (different from $4\ensuremath{-}$) in ${\mathrm{CeO}}_{2}$, unlike in ${\mathrm{UO}}_{2}$, lose electrons to their neighboring atoms, which is traced back to the absence of the +5 valence state for Ce in contrast to its existence for U.

(Invited) Environmental Resistance of Resistive Random Access Memory

Environmental resistance of memory devices must be clarified for putting them into practical use. This should be emphasized in particular for resistance random access memory (ReRAM), which switching mechanism is still unclear, because responses to environmental stresses are closely related to the mechanism. As for the influence of radiation exposure, with increasing density of memory devices, the issue of the generation of soft errors by the radiation, ex. cosmic rays, is becoming more and more serious. So far, soft errors are concerned about only in some semiconductor memories such as SRAM and DRAM, in which stored data are easily inverted by weak current that is generated by incident secondary cosmic rays. On the other hand, there is very little report on reliabilities of ReRAM against exposure to radiation, which might be due to a common belief that ReRAM, which resistive switching is believed to be caused by ion migration, is free from this kind of error. However, if resistive switching of ReRAM is caused by change in the charge state of oxygen vacancies (VO) as proposed by Kamiya et al. [1], soft errors of data inversion might be occurred by charge excitation such as due to ultraviolet irradiation (UV) irradiation. In this paper, we fabricated Pt/NiO/indium tin oxide (ITO) structure, which has transparent ITO electrode, and therefore the irradiation of light to the NiO memory layer became possible. By using this structure, we investigated data retention characteristics against UV irradiation to ReRAM [2]. As a result, soft errors were confirmed to be caused by UV irradiation in both low- and high-resistance states. An analysis of the wavelength dependence of light irradiation on data retention characteristics suggested that electronic excitation from the valence to the conduction band and to the energy level generated due to the introduction of VO caused the errors. Based on a statistically estimated soft error rates, the errors were suggested to be caused by the cohesion and dispersion of VO owing to the generation of electron-hole pairs and valence changes by the ultraviolet irradiation. Therefore, it was suggested that establishing a method to improve resistance to UV irradiation in consideration of the band gap, in-gap states, and the charge state of VO in metal oxides may be a key factor for practical use of ReRAM. The effect of moisture and atmosphere gas on the reliability of ReRAM will also be discussed. [1] K. Kamiya et al., APL 100, 073502 (2012). [2] K. Kimura et al., APL 108, 123501 (2016).

Impact of nitrogen and carbon on defect equilibrium in ZrO2

We investigate the electronic properties of nitrogen and carbon impurities in ZrO2 using density functional theory with a hybrid functional. It is commonly accepted that N substitutes on the O site and is stable in the −1 charge state (NO−). The NO− acceptors are then compensated by donor defects such as O vacancies in the +2 charge state. We test the validity of this assumption by determining the formation energies of all relevant charge states of nitrogen and oxygen vacancies as a function of Fermi level. We also examine the effects of carbon, a common unintentional impurity in ZrO2. We find that carbon impurities have relatively low formation energies and would indeed incorporate easily during growth of ZrO2, but they do not affect the equilibrium between nitrogen and oxygen vacancies. Our results show that the Kröger-Vink picture is valid for common growth conditions and the dominant defects are NO− and VO+2.

Multiferroic nature of intrinsic point defects inBiFeO3: A hybrid Hartree-Fock density functional study

To achieve a fundamental understanding of the multiferroic behavior and electronic properties of intrinsic vacancies in $\mathrm{BiFe}{\mathrm{O}}_{3}$, here we performed first-principles calculations based on hybrid Hartree-Fock density functional theories, which can accurately describe defect electronic structures. Oxygen vacancies, which behave as deep donors with high concentrations under oxygen-poor conditions, reduce the magnetic moments at neighboring Fe ions in the neutral state, while charged oxygen vacancies induce additional ferroelectric polarizations. Cation vacancies, on the other hand, are likely to form under oxygen-rich conditions and result in multiferroic properties distinct from those induced by oxygen vacancies. Bi vacancies act as triple-shallow acceptors and strongly suppress spontaneous polarization regardless of charge states, while Fe vacancies locally interfere with both electric and spin polarization and are thus regarded as multiferroic singular points in $\mathrm{BiFe}{\mathrm{O}}_{3}$. A rich variety of the multiferroic behavior of vacancies can be systematically understood from the localized/delocalized features of defect states, and the different formation conditions for vacancies provide a strategy to tailor the multiferroic properties of $\mathrm{BiFe}{\mathrm{O}}_{3}$ through control of the concentration and charge states of vacancies.

Strain Manipulated Magnetic Properties in ZnO and GaN Induced by Cation Vacancy

The effects of isotropic strains on the magnetic properties in ZnO and GaN induced by cation vacancies are comparatively investigated by density functional theory calculations. The magnetic moments and the couplings between vacancies in different charged states are calculated as a function of strains. The modulation of strain on the magnetic properties relies on the materials and the charge states of cation vacancies in them. As the occurrence of charge transfer in ZnO:V Zn under compression, the coupling between $$V_{\rm{Zn}}^{0} $$ is antiferromagnetic (AFM) and it could be stabilized by strains. Tensions can strengthen the ferromagnetic (FM) coupling between $$V_{\rm{Zn}}^{0} $$ but weaken that of $$V_{\rm{Ga}}^{ - } $$ . The neutral V Ga are always AFM coupling under strains from −6 to +6% and could be stabilized by compressions. The interactions between $$V_{\rm{Ga}}^{ - } $$ are always FM with ignorable variations under strains; however, the FM couplings between $$V_{\rm{Ga}}^{2 - } $$ could be strengthened by compressions. These varying trends of magnetic coupling under strains are interpreted by the band coupling models. Therefore, strain-engineering provides a route to manipulate and design high Curie temperature ferromagnetism derived and mediated by intrinsic defect for spintronic applications.

Oxygen vacancy diffusion in bulk SrTiO3 from density functional theory calculations

Point defects and their diffusion contribute significantly to the properties of perovskite materials. However, even for the prototypical case of oxygen vacancies in SrTiO3 (STO), the predictions of oxygen vacancy activity vary widely. Here we present a comprehensive and systematic study of the diffusion barriers in bulk STO. Using density functional theory (DFT), we assess the role of different supercell sizes, density functionals, and charge states. Our results show that vacancy-induced octahedral rotations, which are limited by the boundary conditions of the supercell, can significantly affect the computed oxygen vacancy diffusion energy barrier. In addition, we find that the diffusion energy barrier of a charged oxygen vacancy is lower than that of a neutral one. This difference is magnified in small supercells. We demonstrate that with increasing supercell size, the effects of the oxygen vacancy charge state and the type of DFT exchange and correlation functional diminish, and all DFT predicted migration energy barriers asymptote to a range of 0.39–0.49eV, which is smaller than the reported experimental values. This work provides important insights and guidance that should be considered for investigations of point defect diffusion in perovskite materials and in oxide superlattices.

Modulation of nitrogen vacancy charge state and fluorescence in nanodiamonds using electrochemical potential

The negatively charged nitrogen vacancy (NV(-)) center in diamond has attracted strong interest for a wide range of sensing and quantum information processing applications. To this end, recent work has focused on controlling the NV charge state, whose stability strongly depends on its electrostatic environment. Here, we demonstrate that the charge state and fluorescence dynamics of single NV centers in nanodiamonds with different surface terminations can be controlled by an externally applied potential difference in an electrochemical cell. The voltage dependence of the NV charge state can be used to stabilize the NV(-) state for spin-based sensing protocols and provides a method of charge state-dependent fluorescence sensing of electrochemical potentials. We detect clear NV fluorescence modulation for voltage changes down to 100 mV, with a single NV and down to 20 mV with multiple NV centers in a wide-field imaging mode. These results suggest that NV centers in nanodiamonds could enable parallel optical detection of biologically relevant electrochemical potentials.

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.

Determining charge state of graphene vacancy by noncontact atomic force microscopy and first-principles calculations

Graphene vacancies are engineered for novel functionalities, however, the charge state of these defects, the key parameter that is vital to charge transfer during chemical reactions and carrier scattering, is generally unknown. Here, we carried out atomic resolution imaging of graphene vacancy defects created by Ar plasma using noncontact atomic force microscopy, and made the first determination of their charge state by local contact potential difference measurements. Combined with density functional theory calculations, we show that graphene vacancies are typically positively charged, with size-dependent charge states that are not necessarily integer-valued. These findings provide new insights into carrier scattering by vacancy defects in graphene, as well as its functionalization for chemical sensing and catalysis, and underline the tunability of these functions by controlling the size of vacancy defect.

Optically modulated charge transfer in TiO2-Au nano-complexes

Highly ordered TiO2 nanotube array (TNA) films were fabricated by anodic oxidation of Ti foil. Au nanoparticles (NPs) are decorated onto the top of TNA films with the aid of ion-sputtering and thermal annealing. The steady-state photoluminescence (PL) spectra, as well as x-ray photoelectron spectroscopy (XPS) and Raman analysis, confirm the presence of Ti3+ valence states in the prepared TNA films. The UV–vis absorption spectra show that the photo-response of as-prepared samples is extended from UV to the visible light region. From the nanosecond time-resolved transient photoluminescence (NTRT-PL) spectra, Ti3+-related PL intensity is observed to vary distinctly with the deposition time of Au NPs. Such a phenomenon could be explained by considering the modulation of oxygen vacancy densities and charge states in TNA films by surface loading Au NPs. The enhanced visible-light photocatalytic activities of the Au-TNA composite were evaluated through the photodegradation of methyl orange (MO) in aqueous solution by UV–vis absorption spectrometry. The decoration of Au NPs plays an essential role in enhancing visible-light photocatalytic activity, because energetic photoelectrons are able to inject to the conduction band of TiO2 owing to the surface plasmon resonance (SPR) effect. It is hoped that our current work will provide a simple strategy to synthesize defect-related composite for photocatalytic applications.

The Interplay between Electronic and Ionic Transport in the Resistive Switching Process of Random Access Memory Devices

In resistive random access memory devices (ReRAM), in order to achieve high endurance and good retention characteristics repetitive control over the resistance change between low resistance (LRS) and high resistance (HRS) states is desired. The binary transition metal oxides TiOx, NiOx, HfOx, AlOx, TaOx have been specifically promising to achieve this goal and are the preferential candidates for the next generation of resistance change based devices. To address the critical issues of ReRAM operation, however, atomistic modeling approaches based on quantum mechanical principles are needed to achieve the optimization of material properties and structures of ReRAM cells, as well as of selection devices and memory array configurations.To explain the process of resistance change observed experimentally, various switching mechanisms were proposed recently and discussed in the technical literature. Generally, in transition metal oxide system such as TiO2, HfO2, NiO, Al2O3, Ta2O5, the diffusion of oxygen vacancies to cluster and create filaments [1-8] and the diffusion of oxygen atoms away from the oxide region to form a thin interfacial reduced oxide, are considered the key ionic processes taking place during forming and switching. Filamentary models for transition metal oxides had been proposed theoretically and the formation energy implications of a conductive filament channel formation corresponding to the “ON” state or LRS [4-9] was investigated for TiO2, HfO2 and Al2O3 (Fig. 1). It was found that during the clustering process of oxygen vacancies, conductive channels are formed by electron delocalization trends induced by the vacancies. The accompanying electronic effects of charge trapping under applied electrical field during the switching process were also recently addressed [7-9]. Hole injection into a reduced transition metal oxide containing a formed filament were found to favor the dissolution, since isolated vacancies prefer the 2+ charge state configuration. On the other hand electron injection induces filament formation and a transition of vacancy charge state to 1+or neutral configuration. The energy barriers of oxygen vacancy out-diffusion from a filament in various charge states are shown in Fig. 2 as a function of chemical potential and Fermi level control. In addition, preferential impurity doping [10] in these types of systems can favorably affect the forming of the conductive filaments and the transition process between the “ON” and “OFF” states (Fig. 3). In conclusion, several competing processes may undergo simultaneously at the microscopic level during the switching of ReRAM devices. Atomistic simulations based on density functional theory can provide an accurate description of the role of each component and describe the mechanism of ReRAM switching.[1] B. Magyari-Köpe, S.G. Park, H.-D. Lee and Y. Nishi, J. Mater. Sci. 47, pp 7498, 2012. [2] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B, 82, 115109, 2010. [3] H.-D. Lee, B. Magyari-Köpe, and Y. Nishi, Phys. Rev. B 81, 193202, 2010. [4] S.G. Park, B. Magyari-Köpe, and Y. Nishi, Tech. Digest VLSI Tech. Symp., 2011. [5] B. Magyari-Köpe, M. Tendulkar, S.G. Park, H.-D. Lee, and Y. Nishi, Nanotechnology 22, 254029, 2011. [6] S.G. Park, B. Magyari-Köpe, and Y. Nishi, IEEE Electron Device Letters, 32, 197, 2011. [7] K. Kamiya, M.Y. Yang, S.G. Park, B. Magyari-Köpe, Y. Nishi, M. Niwa, and K. Shiraishi, Appl. Phys. Lett.,100, 073502, 2012. [8] K. Kamiya, M.Y. Yang, B. Magyari-Köpe, M. Niwa, Y. Nishi, and K. Shiraishi, IEEE Trans. Electron Devices, vol. 60, pp. 3400, 2013. [9] K. Kamiya, M.Y. Yang, B. Magyari-Köpe, M. Niwa, Y. Nishi, and K. Shiraishi, Proc. IEEE IEDM, pp. 478, 2012. [10] L. Zhao, S.G. Park, B. Magyari-Köpe and Y. Nishi, Tech. Digest VLSI Tech. Symp. , 2013.

Effect of vacancies on structural, electronic and optical properties of monolayer MoS2: A first-principles study

Effects of vacancies on structural, electronic and optical properties of monolayer MoS2 were investigated using first-principles plane-wave pseudopotential method based on density functional theory. Results show that the band structure and band gap of perfect monolayer MoS2 are in good agreement with the available experimental and theoretical data. Structural analysis indicates that ions surrounding Mo vacancies show an outward relaxation, while that ions surrounding S vacancies exhibit slightly inward relaxation. Electronic analysis implies that the band gaps of defective monolayer MoS2 are smaller than that of perfect one. After introduction of neutral S-vacancy, monolayer MoS2 has changed from direct to indirect band gap. Mo vacancies bring about acceptor-like levels and p-type conductivities, whereas S vacancies lead to donor-like levels and n-type conductivities. With the increasing charge states of vacancies, the band gaps get smaller and the defect energy levels become deeper. Moreover, as the charge states of vacancies increase, the static dielectric constants of monolayer MoS2 with Mo vacancies decrease, whereas the static dielectric constants of monolayer MoS2 with S vacancies increase.

Lithium and oxygen vacancies and their role in Li2O2charge transport in Li–O2batteries

It is now well accepted that charge transport through Li2O2 is a serious limitation in Li–O2 batteries. We report formation energies for the different charge states of Li, O and O2 vacancies in Li2O2 that could have important implications for charge transport through Li2O2. Charge transition levels are given as a function of the location of the Fermi level in Li2O2 relative to the valence band maximum (VBM). We argue that in Li–O2 discharge/charge electrochemistry, the Fermi level is pinned by LiO2 interface states at ∼0.35 eV above the VBM and this causes the vacancies to be in positively charged states (weakly violating local charge neutrality). We show by non-equilibrium Greens function calculations of charge transport through a Au|Li2O2 + VLiq |Au metal–insulator–metal structure (with VLiq a Li vacancy of charge state q), that the +1 and 0 charge state induces considerable scattering for tunneling holes. This tunneling is the previously proposed dominant mechanism of charge transport in Li–O2 batteries at practical current densities at room temperature, although we also proposed a contribution from hole polaron migration at very low currents and higher temperatures. We suggest that scattering of the tunneling holes by the positively charged vacancies (and possibly hole polarons) is the origin of the resistive loss observed in Li–O2 discharges in bulk electrolysis cells (where other forms of resistance are negligible). Thus, we argue that charged vacancies hinder charge transport through Li2O2 in Li–O2 electrochemical discharges.

Effect of vacancy charge state on positron annihilation in silicon

The charge-state-dependent lattice relaxation of mono-vacancy in silicon is studied using the first-principles pseudopotential plane-wave method. We observe that the structural relaxation for the first-neighbor atoms of the mono-vacancy is strongly dependent on its charge state. The difference in total electron density between with and without charge states in mono-vacancy and its relevant change due to the localized positron are also examined by means of first-principles simulation, demonstrating the strong interplay between positron and electron. Our calculations reveal that the positron lifetime decreases with absolute charge value increasing.

Doping dependence of self-diffusion in germanium and the charge states of vacancies

Self-diffusion in boron-doped germanium has been studied at temperatures between 526 and 749 °C with secondary ion mass spectrometry. Self-diffusion under acceptor doping is retarded compared to intrinsic conditions. This demonstrates the contribution of charged vacancies in self-diffusion. Taking into account the dominance of doubly negatively charged vacancies under donor doping, the doping dependence of self-diffusion is best described with an inverse level ordering for singly and doubly negatively charged vacancies for all doping conditions. The level ordering explains the dominance of doubly charged vacancies under donor doping and their decreasing contribution with increasing acceptor doping until neutral vacancies mediate self-diffusion.

The influence of neighboring vacancies and their charge state on the atomic migration of LaAlO3

We investigated the effects of neighboring vacancies and their charge state on ion migration in rhombohedral-LaAlO3. Our results show that the oxygen migration energy is greatly influenced by the charge state of the oxygen vacancies, which is attributed to the neutral oxygen vacancy generating a localized electronic state at the vacancy site, while the charged oxygen vacancy has delocalized electronic state. In contrast, the charge state of the vacancy does not influence the migration of cations. Furthermore, we found that cation vacancies around the migrating ion enhance the cation migration, whereas they inhibit oxygen migration.

Effects of Oxide Roughness at Metal Oxide Interface: MgO on Ag(001)

Defects in thin oxide films on metal substrates affect metal work function and determine the chemical and physical properties of an oxide. However, accurately predicting properties of these heterogeneous systems is still challenging. Here we use a new approach to treat a mixed metal/metal oxide system within density functional theory, which is based on the application of the auxiliary density matrix method (J. Chem. Theory Comput.2010, 6, 2348) to calculate the exchange interaction at a sharp interface between the two materials, as implemented in the CP2K code. This method is used to calculate the shift of the Ag work function in the MgO/Ag(001) system as a function of the MgO film morphology as well as charge state, position, and density of oxygen vacancies. An accurate band alignment between metal and oxide allows us to predict the relative stabilities of different charge states of oxygen vacancies in MgO as a function of their position with respect to the interface with Ag. Our results confirm that F+ ...

First principles modeling of charged oxygen vacancy filaments in reduced TiO2–implications to the operation of non-volatile memory devices

First principles density functional theory calculations were employed to study the band structure of reduced rutile TiO2 and the implications of oxygen vacancy charge states on the switching mechanisms observed in resistance change random access memory devices. The formation of conductive paths in TiO2 composed of oxygen vacancy filaments were investigated using the LDA+Ud+Up method to calculate the band-structure, electron localization functions, partial charge densities of defect states and the Bader charge decompositions. The thermodynamic stability of charged oxygen vacancies on resistive switching are discussed based on the defect formation energies and a resistive switching mechanism is proposed based on models of filament formation and rupture.

Ab initiostudy of intrinsic defects in zirconolite

Zirconolite, CaZrTi${}_{2}$O${}_{7}$, is a proposed ceramic for the use in disposal of highly active nuclear waste. Density functional theory (DFT) has been used in conjunction with a random search technique to identify the stable interstitial sites for the intrinsic defects in zirconolite. There is a significant dependence on charge states for the vacancy defect structures, with the formation of an O${}_{2}$ molecule in certain charge states of Ti and Zr vacancies. The low coordination of the oxygen ions provided sites on which DFT localized holes associated with the cation vacancies. The Frenkel defect energies calculated from the combination of the vacancy and interstitial formation energies show that the oxygen defects have a strong dependence on the chemical environment with a range of 1.5 eV between the oxygen Frenkel pair energies.

Role of oxygen vacancies in the origin of ferromagnetism in Mn‐doped ZnO

AbstractThe relationship between the oxygen vacancy and ferromagnetism in Mn‐doped ZnO has been studied based on the first‐principles calculations. Three possible charge states of oxygen vacancies, i.e., neutral (VO0), 1+ (VO1+) and 2+ (VO2+) are considered. Results show that the lattice relaxations around oxygen vacancies are large difference under different charge states. It is found that VO1+ and VO2+ oxygen vacancies induce ferromagnetism. However, Mn‐doped ZnO system shows ferromagnetism with VO0 oxygen vacancies in hydrogenated environment, the ferromagnetism is attributed to the interstitial H, which forms a bridge bond and mediates d ‐d coupling and stabilizes the ferromagnetic state. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)