Charge-state Transition Research Articles

Preparation and photoluminescence properties of Eu3+ ion-doped Mg2LaTaO6 phosphor

The luminescent properties of Eu3+ ion-doped Mg2LaTaO4 phosphors were synthesized using a vibrating milled solid-state reaction. The XRD results show that the crystal structure of the Mg2LaTaO4:Eu3+ phosphor that is calcined at 1450 °C for 8 h has a cubic structure and the (440) diffraction peak shifts to a higher diffraction angle when the Eu3+ ion concentration increases. The absorption spectra results show that the absorption band d in the range 190–400 nm is attributed to absorption of pure Mg2LaTaO4 by the host lattice. When Mg2LaTaO4 is doped with Eu3+ ions, the broad band that is centered at 295 nm is assigned to the charge transition state (CTS) for the interaction of O2− and Eu3+ with the Mg2LaTaO4 host lattice. The sharp peaks from 310 to 550 nm are attributed to the typical intra-4f transitions of Eu3+ ions. When the phosphor is excited using either 393 or 464 nm there is a red emission at 605 nm that is attributed to the electric dipole transition (5D0 → 7F2) for the Eu3+ ion. The asymmetry ratio is about 2–2.5, which lies well within the range of usual values for oxide glasses, which demonstrates that the Eu3+ ions occupy low-symmetry sites. The maximum intensity of the photoluminescence for the 5D0 → 7F2 transition occurs at an Eu3+ ion concentration of 30 mol%, which demonstrates that there is concentration quenching at higher Eu3+ concentrations. The critical distance is about 7.68 A for the 5D0 (J = 3, 2, 1) transition and the energy transfer is characterized as a wave-function exchange.

Descriptor-Based Approach for the Prediction of Cation Vacancy Formation Energies and Transition Levels

Point defects largely determine the observed optical and electrical properties of a given material, yet the characterization and identification of defects has remained a slow and tedious process, both experimentally and theoretically. We demonstrate a computationally-cheap model that can reliably predict the formation energies of cation vacancies as well as the location of their electronic states in a large set of II-VI and III-V materials using only parameters obtained from the bulk primitive unit cell (2-4 atoms). We apply our model to ordered alloys within the CdZnSeTe, CdZnS, and ZnMgO systems and predict the positions of cation vacancy charge-state transition levels with a mean absolute error of < 0.2 eV compared to the explicitly calculated values, showing useful accuracy without the need for the expensive and large-scale calculations typically required. This suggests the properties of other point defects may also be predicted with useful accuracy from only bulk-derived properties.

Photoinduced small polarons bound to hydrogen defects in rutile TiO2

Photoinduced absorption in rutile ${\mathrm{TiO}}_{2}$ has been studied by means of Fourier transform infrared spectroscopy. It is shown that near-band-gap illumination results in a charge-state transition of a hydrogen defect with an O--H stretch mode at 3500 ${\mathrm{cm}}^{\ensuremath{-}1}$, as evidenced by the appearance of an anticorrelated vibrational mode with a blueshift of $\ensuremath{\sim}1.3\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. The charge-state transition is accompanied by a broad near-infrared absorption band with maximum intensity at $\ensuremath{\sim}7000\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. Our data on both the photoinduced vibrational mode and the near-infrared absorption can be conclusively explained in the framework of a model of small electron polarons bound to a Ti atom adjacent to the O--H group of the hydrogen defect.

First-principles characterization of native-defect-related optical transitions in ZnO

We investigate the electrical and optical properties of oxygen vacancies (VO), zinc vacancies (VZn), hydrogenated VZn, and isolated dangling bonds in ZnO using hybrid functional calculations. While the formation energy of VO is high in n-type ZnO, indicating that this center is unlikely to form, our results for optical absorption signals associated with VO are consistent with those observed in irradiated samples, and give rise to emission with a peak at less than 1 eV. Under realistic growth conditions, we find that VZn is the lowest-energy native defect in n-type ZnO, acting as an acceptor that is likely to compensate donor doping. Turning to optical transitions, we first examine NO as a case study, since N-related transitions have been identified in experiments on ZnO. We also examine how hydrogen, often unintentionally present in ZnO, forms stable complexes with VZn and modifies its optical properties. Compared with isolated VZn, VZn-H complexes have charge-state transition levels lower in the band gap as well as have lower formation energies. These complexes also lead to characteristic vibrational frequencies which compare favorably with experiment. Oxygen dangling bonds show behavior mostly consistent with VZn, while zinc dangling bonds give rise to transition levels near the ZnO conduction-band minimum and emission peaking near 2.4 eV. We discuss our results in view of the available experimental literature.

Rare earth substitutional impurities in germanium: A hybrid density functional theory study

The Heyd, Scuseria, and Ernzerhof (HSE06) hybrid functional by means of density functional theory has been used to model the electronic and structural properties of rare earth (RE) substitutional impurities in germanium (REGe). The formation and charge state transition energies for the REGe (RE = Ce, Pr, Er and Eu) were calculated. The energy of formation for the neutral charge state of the REGe lies between −0.14 and 3.13eV. The formation energy result shows that the Pr dopant in Ge (PrGe) has the lowest formation energy of −0.14eV, and is most energetically favourable under equilibrium conditions. The REGe induced charge state transition levels within the band gap of Ge. Shallow acceptor levels were induced by both the Eu (EuGe) and Pr (PrGe) dopants in Ge. The CeGe and ErGe exhibited properties of negative-U ordering with effective-U values of −0.85 and −1.07eV, respectively.

Rare earth interstitial-complexes in Ge: Hybrid density functional studies

We present results of the structural, energetic and electronic properties of rare earth (RE) interstitial-complexes in Ge (REGeGei; for RE: Ce, Pr, Eu, Er and Tm). We used the Heyd, Scuseria, and Ernzerhof (HSE06) hybrid functional within the framework of density functional theory for all calculations. The energy of formation and charge state transition levels of REGeGei complexes were obtained. For the neutral charge state, the results of the formation energy of the REGeGei, were between 0.21 and 8.14eV. Amongst the REGeGei, while the CeGeGei was energetically the most favourable with a binding energy of 3.90eV, TmGeGei and ErGeGei were not stable with respect to their binding energies. The CeGeGei induced deep donor level with negative-U ordering, the PrGeGei induced shallow levels close to the valence band maximum and the EuGeGei induced a shallow single donor level.

The role of negatively charged oxygen vacancies upon β-MnO2 conductivity

Recent years have witnessed burgeoning development in the field of electrochemical energy storage and conversion. β-MnO2, as one of the most promising electrode materials served in batteries and supercapacitors, has stimulated intensive interest. In the present work, using hybrid functional (HSE06) approaches, we have explored the unexpected role of oxygen vacancies upon β-MnO2 conductivity, by examining the structural properties, vacancy formation energies, and charge-state transition levels. Contrary to most transition metal oxides, we reveal that the oxygen vacancies in β-MnO2 may act as either acceptors or donors, depending on the position of the Fermi level. The negatively charged oxygen vacancies could serve as shallow electron traps, coupled with fairly low formation energies. On one hand, they would possibly contribute to the p-type conductivity of bulk β-MnO2; on the other hand, they would also form in significant concentrations in n-type β-MnO2, thus acting as potential compensating centers and pinning the Fermi energy to the lower part of the band gap. Moreover, it is shown that the positively charged vacancies would act as deep donors in β-MnO2 and are therefore unlikely to induce the n-type conducting behavior. All these findings suggest that the conductivity of β-MnO2 could be effectively controlled by the internal defect engineering, which in turn offers new opportunities for manipulating the performance of β-MnO2.

Electronic structure of substitutionally doped diamond: Spin-polarized, hybrid density functional theory analysis

Individual substitutional defects in diamond have been theoretically investigated using spin-polarized, hybrid density functional theory method. The revised Heyd-Scuseria-Ernzerhof screened hybrid functional (HSE06) was applied for the total energy calculation. We analyzed the equilibrium geometry, formation energy as a function of charge state and defect charge transition levels, in order to predict the relative stability and doping nature of each defect. Calculations revealed that the crystal radius cannot be solely used to anticipate trend in the volume relaxation and subsequently defect formation energy. A thorough analysis of formation energy vs Fermi level diagrams indicate that none of the investigated elements can individually generate a shallow acceptor or donor level, competitive with substitutional B and P.

Zn vacancy as a polaronic hole trap in ZnO

This work explores the Zn vacancy in ZnO using hybrid density functional theory calculations. The Zn vacancy is predicted to be an exceedingly deep polaronic acceptor that can bind a localized hole on each of the four nearest-neighbor O ions. The hole localization is accompanied by a distinct outward relaxation of the O ions, which leads to lower symmetry and reduced formation energy. Notably, we find that initial symmetry-breaking is required to capture this effect, which might explain the absence of polaronic hole localization in some previous hybrid density functional studies. We present a simple model to rationalize our findings with regard to the approximately equidistant thermodynamic charge-state transition levels. Furthermore, by employing a one-dimensional configuration coordinate model with parameters obtained from the hybrid density functional theory calculations, luminescence lineshapes were calculated. The results show that the isolated Zn vacancy is unlikely to be the origin of the commonly observed luminescence in the visible part of the emission spectrum from \textit{n}-type material, but rather the luminescence in the infrared region.

Forces from periodic charging of adsorbed molecules

In a recent publication [Kocić et al., Nano Lett. 15, 4406 (2015)], it was shown that gating of molecular levels in the field of an oscillating tip of an atomic force microscope can enable a periodic charging of individual molecules synchronized to the tip’s oscillatory motion. Here we discuss further implications of such measurements, namely, how the force difference associated with the single-electron charging manifests itself in atomic force microscopy images and how it can be detected as a function of tip-sample distance. Moreover, we discuss how the critical voltage for the charge-state transition depends on distance and how that relates to the local contact potential difference. These measurements allow also for an estimate of the absolute tip-sample distance.

Rare Earth Interstitials in Ge: A Hybrid Density Functional Theory Study

In this work, the results of density functional theory calculations for rare earth (Ce, Pr, Eu, and Er) interstitials in Ge are presented. We employed the hybrid functional of Heyd, Scuseria, and Ernzerhof (HSE06) for all the calculations. We calculated the formation energies and charge state transition levels for the tetrahedral (T) and hexagonal (H) configurations of the Ce, Pr, Eu, and Er interstitials in Ge. While for the T configuration, the charge states of the Ce and Pr did not induce any thermodynamic accessible transition state level within the band gap of Ge, for both the T and H configurations the Eu and Er interstitials in Ge induce deep levels in the band gap. The H configuration of the Ce interstitial in Ge induces a shallow donor level at 0.03 eV below the conduction band. The Eu interstitial exhibits negative-U properties for the (+2/−2) transition level and the Er interstitial displays characteristics of charge state controlled metastability.

A first principle hybrid functional calculation of [formula omitted]-[formula omitted] defect complexes in germanium

By means of density functional theory (DFT), using the screened Heyd, Scuseria, and Ernzerhof (HSE06) hybrid functional we present results of the TmGe3+-VGe defect complexes in germanium (Ge). The formation energies of the first (fnn), second (snn), third (tnn) and fourth (ftnn) nearest neighbour configurations of VGe-VGe were examined. The charge state transition levels for all these configurations were examined as well. The TmGe3+-VGe complexes were found to have a positive binding energies for the neutral charge state in the fnn and ftnn configurations. The thermodynamic transition levels revealed that the TmGe3+-VGe induced shallow levels in the band gap for the fnn, tnn and ftnn configurations and deep level for the tnn configuration. The snn configuration showed no charge state transition level, the − 2 charge state was stable for all Fermi energies in the band gap. The TmGe3+-VGe displayed evidence of a single donor level (+1/0) and an acceptor level (−1/−2) within the band gap. Charge state controlled metastability was exhibited by the TmGe3+-VGe.

Nonequivalent lanthanide defects: Energy level modeling

Empirical charge-state transition level schemes are popular tools to model the properties of lanthanide-doped materials and their construction has become standard practice. Typically, it is implicitly assumed that all lanthanide ions form isostructural defects. However, in practice, multiple nonequivalent defects related to the same lanthanide can occur or different lanthanides can even incorporate in different ways. The consequences of these complications on the impurity energy levels are discussed in this article. It seems that small structural differences around the lanthanide dopant can give rise to important spectral differences in its emission. These are not always clearly reproduced by the charge-state transition level schemes. Improvements to the existing procedure are suggested and applied to the lanthanide ions in the well-studied host crystals SrAl2O4, Sr2Si5N8 and SrGa2S4.

Impact of Point Defects on Proton Conduction in Strontium Cerate

Strontium cerate (SrCeO3) is a well known proton conductor. In applications, it is frequently doped with yttrium to increase stability and promote ionic conductivity; however, the microscopic effects of doping and native defects are not fully understood. We use first-principles hybrid density functional calculations to study the atomic and electronic structure of impurities and defects and their impact on proton conduction. Interactions of hydrogen with cation vacancies are found to negatively impact proton conductivity. We also investigate band alignments as well as the alignment of the thermodynamic charge-state transition levels of hydrogen. Comparison with BaCeO3 allows us to establish trends across the perovskite cerates.

Gallium vacancy complexes as a cause of Shockley-Read-Hall recombination in III-nitride light emitters

We describe a mechanism by which complexes between gallium vacancies and oxygen and/or hydrogen act as efficient channels for nonradiative recombination in InGaN alloys. Our identification is based on first-principles calculations of defect formation energies, charge-state transition levels, and nonradiative capture coefficients for electrons and holes. The dependence of these quantities on alloy composition is analyzed. We find that modest concentrations of the proposed defect complexes (∼1016 cm−3) can give rise to Shockley-Read-Hall coefficients A=(107−109) s−1. The resulting nonradiative recombination would significantly reduce the internal quantum efficiency of optoelectronic devices.

A hybrid functional calculation of Tm3+ defects in germanium (Ge)

In this work, we present ab-initio calculation results for the Tm3+ interstitial (Tmi3+), vacancy–interstitial complex (VGe-Tmi3+) and substitutional (TmGe3+) defects in germanium (Ge) as determined by the density functional theory (DFT) using the Heyd, Scuseria, and Ernzerhof (HSE06) hybrid functional. We calculated the formation energies and the charge state transition levels of different configurations. Our results show that the Tm3+ interstitial exists in the hexagonal configuration with low formation energy. The formation energies for VGe−Tmi3+ and TmGe3+ were as low as 0.84eV. The most energetically favourable defects were the VGe−Tmi3+ in the axial configuration and the TmGe3+. The TmGe3+ and VGe−Tmi3+ introduced a single acceptor ϵ(0/−1) charge state transition level that was positioned deep in the middle of the band gap. The majority of the levels induced by the defects under investigation, were either shallow donor or acceptor level lying close to the band gap edges.

Ab‐initio study of germanium di-interstitial using a hybrid functional (HSE)

In this work, we present ab‐initio calculation results of Ge di-interstitials (I2(Ge)) in the framework of the density functional theory (DFT) using the Heyd, Scuseria, and Ernzerhof (HSE) hybrid functional. The formation energy, transition levels and minimum energy configurations were obtained for I2(Ge) −2, −1, 0, +1 and +2 charge states. The calculated formation energies show that for all charge states of I2(Ge), the double tetrahedral (T) configuration formed the most stable defect with a binding energy of 1.24eV in the neutral state. We found the (+2/+1) charge state transition level for the T lying below the conduction band minimum and (+2/+1) for the split[110]-tetrahedral configuration lying deep at 0.41eV above the valence band maximum. The di-interstitials in Ge exhibited the properties of both shallow and deep donor levels at (+2/+1) within the band gap and depending on the configurations. I2(Ge) gave rise to negative-U, with effective-U values of −0.61 and −1.6eV in different configurations. We have compared our results with calculations of di-interstitials in silicon and available experimental data.

First-principles survey of the structure, formation energies, and transition levels of As-interstitial defects in InGaAs

While point defects in elemental (Si) and compound (GaAs, GaN, AlN) semiconductors have been extensively studied both experimentally and theoretically, only limited theoretical studies of these defects exist for technologically important binary $(\mathrm{S}{\mathrm{i}}_{x}\mathrm{G}{\mathrm{e}}_{1\text{\ensuremath{-}}x})$ and pseudobinary $(\mathrm{I}{\mathrm{n}}_{x}\mathrm{G}{\mathrm{a}}_{1\text{\ensuremath{-}}x}\mathrm{As}$, $\mathrm{I}{\mathrm{n}}_{\mathrm{x}}\mathrm{G}{\mathrm{a}}_{1\text{\ensuremath{-}}x}\mathrm{N}$, $\mathrm{A}{\mathrm{l}}_{x}\mathrm{G}{\mathrm{a}}_{1\text{\ensuremath{-}}x}\mathrm{N})$ semiconductor alloys. Here, we use density-functional theory and a recently developed bounds-analysis approach to survey the atomic structures, formation energies, and charge-state transition levels of the stable and metastable states of As interstitials in the pseudobinary alloy $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$. Our studies consider seven different candidate defect structures for the As interstitial, with calculations performed for selected defect charge states in the range $q=\ensuremath{-}2$ to $+3$. In each case, the mean and standard deviations of the defect-formation energy are determined using statistical sampling methods that place the defect into a wide variety of differing local-alloy environments. When examined from the point of view of the mean formation energy of the defect, the stable configurations of the As interstitial in $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$ are found to resemble previous findings for GaAs, with a ${\mathrm{C}}_{1\mathrm{h}}\text{\ensuremath{-}}\mathrm{p}{001}_{\mathrm{III}}$ interstitial structure in a $q=+1$ charge state favored near midgap and below, and a ${\mathrm{C}}_{2\mathrm{v}}\text{\ensuremath{-}}11{0}_{\mathrm{a}}$ split-interstitial structure in a $q=\ensuremath{-}1$ charge state favored above midgap (the named point-group symmetries refer to the underlying symmetry that the alloy defect would possess if within GaAs). The statistical sampling reveals a strong dependence of the defect-formation energy on the local-alloy environment, with the standard deviation $\ensuremath{\sigma}$ of the formation energy approaching 0.21 eV for the most stable As-interstitial structures. Because the range of ground-state energies encountered by an As-interstitial defect when moving through the alloy is found to be quite large, approaching $\ensuremath{\sim}1.2\phantom{\rule{0.16em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}(\ifmmode\pm\else\textpm\fi{}3\ensuremath{\sigma})$, defect-diffusion pathways in $\mathrm{I}{\mathrm{n}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\mathrm{As}$ will have spatial and temporal complexities not found in GaAs.

Scanning tunneling spectroscopy reveals a silicon dangling bond charge state transition

We report the study of single dangling bonds (DBs) on a hydrogen-terminated silicon (100) surface using a low-temperature scanning tunneling microscope. By investigating samples prepared with different annealing temperatures, we establish the critical role of subsurface arsenic dopants on the DB electronic properties. We show that when the near-surface concentration of dopants is depleted as a result of 1250 °C flash anneals, a single DB exhibits a sharp conduction step in its I(V) spectroscopy that is not due to a density of states effect but rather corresponds to a DB charge state transition. The voltage position of this transition is perfectly correlated with bias-dependent changes in the STM images of the DB at different charge states. Density functional theory calculations further highlight the role of subsurface dopants on DB properties by showing the influence of the DB-dopant distance on the DB state. We discuss possible theoretical models of electronic transport through the DB that could account for our experimental observations.

Periodic Charging of Individual Molecules Coupled to the Motion of an Atomic Force Microscopy Tip.

Individual molecules at the edges of self-assembled islands grown on Ag(111) can be deliberately switched in their charge state with the electric field from a scanning-probe tip. Close to the threshold voltage for a charge state transition, periodic switching of the charge is directly driven by the cantilever motion in frequency-modulated atomic force microscopy (AFM), as can be deduced from the signature in the measured frequency shift. In this regime, the integrated frequency shift yields the tip-sample force that is due to a single additional electron. Further, the signature of the dynamic charging response provides information on the electronic coupling of the molecule to the substrate. In analogy to previous experiments on quantum dots, this may also be used in the future to access excited state properties of single molecules from AFM experiments.