Native Vacancies Research Articles

Elucidating the origin of magnetic ordering in ferroelectric BaTiO3-d thin film via electronic structure modification

With the motive of unraveling the origin of native vacancy induced magnetization in ferroelectric perovskite oxide systems, here we explore the consequences of electronic structure modification in magnetic ordering of oxygen deficient epitaxial BaTiO thin films. Our adapted methodology employs state-of-the-art experimental approaches viz. photo-emission, photo-absorption spectroscopies, magnetometric measurements duly combined with first principles based theoretical methods within the frame work of density functional theory (DFT and DFT+U) calculations. Oxygen vacancy (O) is observed leading partial population of Ti 3d (t2g), which induces defect state in electronic structure near the Fermi level and reduces the band gap. The oxygen deficient BaTiO2.75 film reveals Mott–Hubbard insulator characteristic, in contrast to the band gap insulating nature of the stoichiometric BaTiO3. The observed magnetic ordering is attributed to the asymmetric distribution of spin polarized charge density in the vicinity of O site, which originates unequal magnetic moment values at first and second nearest neighboring Ti sites, respectively. Hereby, we present an exclusive method for maneuvering the band gap and on-site electron correlation energy with consequences on magnetic properties of BaTiO system, which can open a gateway for designing novel single phase multiferroic system.

A mechanism assessment for the anti-corrosion of zirconia coating under the condition of subcritical water corrosion

Natural grown ZrO2 cannot effectively protect Zr alloy due to the poor phase stability. In this work, a pre-defected ZrO2 coating was fabricated on Zr alloy and further bombarded by Ar8+. A spontaneous martensitic transformation in ZrO2 coating was triggered by lattice distortion and gradient mismatch on the interface. Additionally, Ar8+ bombardment induced the formation of interstitial-vacancy pairs in ZrO2. DFT modeling proved that the interstitial-vacancy pairs were superior in stabilizing T-phase ZrO2, compared with native oxygen vacancies. A mechanism for enhanced corrosion resistance was proposed based on the unique interface and defect state.

Electrical Doping Effect of Vacancies on Monolayer MoS2

Doping of transition-metal dichalcogenides (TMDCs) is an effective way to tune the Fermi level to facilitate the band engineering required for different types of devices. For TMDCs, a controversy abounds with regard to the doping role played by vacancy-type defects. Here, we report a detailed study based on first-principles calculations proposing that the native sulfur vacancies (VS) can significantly alter the electrical doping level in MoS2 and tune the material to exhibit conventional n- or p-type semiconductor characteristics. In particular, we reveal that the lower concentration of the single VS (2.8 and 6.3%) yields p-type characteristics, whereas the higher concentration of the single VS or a cluster of VS (12.5, 18.8, and 25.0%) yields n-type characteristics. The trend is consistent with previous X-ray photoelectron spectroscopy and scanning tunneling microscopy results. Employing this method of tuning the electron doping level, we modeled a commonly used metal–semiconductor interface to demonstra...

Toward wafer-scale diamond nano- and quantum technologies

We investigate native nitrogen vacancy (NV) and silicon vacancy (SiV) color centers in a commercially available, heteroepitaxial, wafer-sized, mm thick, single-crystal diamond. We observe single, native NV centers with a density of roughly 1 NV per μm3 and moderate coherence time (T2 = 5 μs) embedded in an ensemble of SiV centers. Using low temperature luminescence of SiV centers as a probe, we prove the high crystalline quality of the diamond especially close to the growth surface, consistent with a reduced dislocation density. Using ion implantation and plasma etching, we verify the possibility to fabricate nanostructures with shallow color centers rendering our material promising for fabrication of nanoscale sensing devices. As this diamond is available in wafer-sizes up to 100 mm, it offers the opportunity to up-scale diamond-based device fabrication.

Physician Workforce Partnerships in Rural American Indian/Alaska Native Communities and the Potential of Post-Graduate Fellowships.

Rural American Indian and Alaska Native (AI/AN) communities face physician vacancy rates over 25%. A variety of programs aim to address those gaps, from early-life STEM initiatives for AI/ANs to physician loan repayment programs. However, unfilled clinical positions and underrepresentation of AI/AN physicians persist. We review existing workforce initiatives, then demonstrate that three recently developed clinical fellowship programs fill an important gap. The fellowships, led by faculty at large academic health centers, place fellows in clinical positions in rural AI/AN communities in partnership with tribal health systems and/or the Indian Health Service. In addition to providing clinical care, the fellowships seek to enhance health systems' capacity development through community-centered initiatives that include training and health promotion. Other academic health centers should consider working together with tribal communities to assess whether replication of the models could reduce local physician staffing gaps and health disparities.

Native Vacancy Enhanced Oxygen Redox Reversibility and Structural Robustness

AbstractCathode materials with high energy density, long cycle life, and low cost are of top priority for energy storage systems. The Li‐rich transition metal (TM) oxides achieve high specific capacities by redox reactions of both the TM and oxygen ions. However, the poor reversible redox reaction of the anions results in severe fading of the cycling performance. Herein, the vacancy‐containing Na4/7[Mn6/7(◻Mn)1/7]O2 (◻Mn for vacancies in the MnO slab) is presented as a novel cathode material for Na‐ion batteries. The presence of native vacancies endows this material with attractive properties including high structural flexibility and stability upon Na‐ion extraction and insertion and high reversibility of oxygen redox reaction. Synchrotron X‐ray absorption near edge structure and X‐ray photoelectron spectroscopy studies demonstrate that the charge compensation is dominated by the oxygen redox reaction and Mn3+/Mn4+ redox reaction separately. In situ synchrotron X‐ray diffraction exhibits its zero‐strain feature during the cycling. Density functional theory calculations further deepen the understanding of the charge compensation by oxygen and manganese redox reactions and the immobility of the Mn ions in the material. These findings provide new ideas on searching for and designing materials with high capacity and high structural stability for novel energy storage systems.

Defect-Mediated Alloying of Monolayer Transition-Metal Dichalcogenides.

Alloying plays a central role in tailoring the material properties of 2D transition-metal dichalcogenides (TMDs). However, despite widespread reports, the details of the alloying mechanism in 2D TMDs have remained largely unknown and are yet to be further explored. Here, we combine a set of systematic experiments with ab initio density functional theory (DFT) calculations to unravel a defect-mediated mechanism for the alloying of monolayer TMD crystals. In our alloying approach, a monolayer MoSe2 film serves as a host crystal in which exchanging selenium (Se) atoms with sulfur (S) atoms yields a MoS2 xSe2(1- x) alloy. Our study reveals that the driving force required for the alloying of CVD-grown films with abundant vacancy-type defects is significantly lower than that required for the alloying of exfoliated films with fewer vacancies. Indeed, we show that pre-existing Se vacancies in the host MoSe2 lattice mediate the replacement of chalcogen atoms and facilitate the synthesis of MoS2 xSe2(1- x) alloys. Our DFT calculations suggest that S atoms can bind to Se vacancies and then diffuse throughout the host MoSe2 lattice via exchanging the position with Se vacancies, further supporting our proposed defect-mediated alloying mechanism. Beside native vacancy defects, we show that the existence of large-scale defects in CVD-grown MoSe2 films causes the fracture of alloys under the alloying-induced strain, while no such effect is observed in exfoliated MoSe2 films. Our study provides a deep insight into the details of the alloying mechanism and enables the synthesis of 2D alloys with tunable properties.

Effects of Native Vacancies on Nb-Doped MgH2 Using Density Functional Theory Calculations

Fil: Gaztanaga, Francisco. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca. Instituto de Fisica del Sur. Universidad Nacional del Sur. Departamento de Fisica. Instituto de Fisica del Sur; Argentina

Ar2+ Beam Irradiation-Induced Multivancancies in MoSe2 Nanosheet for Enhanced Electrochemical Hydrogen Evolution

Poor electronic conductivity and an inert basal plane restrict the further enhancement of hydrogen evolution reaction (HER) in 2H-phase MoSe2. Herein, we synthesized MoSe2 nanosheet arrays on carbon cloth and induced multivacancies in their basal plane via high-energy Ar2+ beam irradiation, which are confirmed by Raman, electron spin resonance, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy analyses. Electrical measurement results indicate that these vacancies in the MoSe2 basal plane can effectively improve their electrocatalytic performance, where the lowest overpotential of −171 mV at the current density of −100 mA/cm2 and Tafel slope of 35 mV/dec were achieved in Ar2+-irradiated (dose of 5 × 1015 ions/cm2) MoSe2 nanosheets. First-principles calculation results reveal that different cases of native vacancies in the MoSe2 basal plane could effectively enhance the conductivity of MoSe2 and produce more catalytic active sites for hydrogen evolution, giving rise to ...

(Invited) Vacancy-Type Defects and Their Carrier Trapping Properties in GaN Studied by Monoenergetic Positron Beams

Positron annihilation is a powerful technique for evaluating point defects in semiconductors. Using this technique, one can detect vacancy-type defects in subsurface regions with high-sensitivity. We have used monoenergetic positron beams to probe native vacancies in GaN grown on Si substrate by metalorganic vapor phase epitaxy. Measurements of Doppler broadening spectra and positron lifetimes indicated that the major defect species in the GaN layers was a Ga vacancy coupled with nitrogen vacancies. The positron trapping rate of the defects decreased with increasing [C], which was attributed to the downward shift of the Fermi level position and the resultant change in the defect charge state. Under illumination, the defect charge state was changed from positive to neutral, which was attributed to the trapping of the excited electrons by the defects. The effect of illumination was suppressed for the GaN layer with [C] = 1018 cm−3, which was ascribed to an excellent electron trapping property of C in GaN.

Electronic and magnetic properties of defect complexes in Eu-doped GaN: First-principles calculations

Various forms of defect complexes are studied in Eu-doped GaN by first-principles calculations with the framework of density functional theory. We consider defect complexes formed by dopant Eu and vacancies or insterstitals with the neutral charge states in GaN, i.e., EuGa-VGa, EuGa-VN, EuGa-Ni, and EuGa-Oi, where EuGa represents the Eu site substituting for Ga site, and VGa, VN, Ni and Oi are Ga vacancies, N vacancies, interstitial N and interstitial O, respectively. The formation energies are calculated for all defect configurations. The formation energies of native Ga vacancies are found to be higher compared to those of N vacancies, while the tetrahedral interstitials are energetically more stable than the octahedral interstitial. The Eu-4f electrons introduce a magnetic moment of about 6.15 μB in the case of single dopant Eu, i.e., EuGa, which is probably suppressed in the presence of octahedral interstitial N around Eu. And Ga vacancies and octahedral interstitial O can contribute the magnetic moments to the systems, while other intrinsic defects do almost not promote the formation of local magnetic moments.

Anomalous Electron Accumulation at Al2O3/in2O3 Interface Via Short-Range Ordered Two-Dimensional Electron Gas

Transparent conducting oxide (TCO) films, which is one of the widely used as window electrodes at the various devices, fulfil high electrical conductivity and optical transparency in the visible region of spectrum. Conventional TCO films are formed by the tuning charge transport of wide band gap oxides like indium gallium zinc oxide (IGZO), zinc oxide (ZnO), tin dioxide (SnO2), and indium oxide (In2O3). Among the various TCO candidates, In2O3 is an attractive TCO candidate since In2O3 can satisfied the visible transparency and high carrier density via doping processes. To achieve the high mobility In2O3 films, doping processes with metal dopants such as Ti, Zr, Mo, and W have been suggested by many previous study. Intrinsic In2O3, however, has n-type semiconductor due to its native oxygen vacancy act as n-type donor and indium interstitial. As another approach to gain highly conductivity TCOs, two-dimensional electron gas (2DEG) channel can be suggested since 2DEG channel can be applied to increase the electrical conductivity of oxide semiconductors up to the metallic conduction level. The tuning of electrical and optical properties in oxides via surface and interfacial 2DEG channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially-confined electrons. Although most previous studies have been carried out in crystalline phase like single crystal SrTiO3 or III-V compound semiconductor, realizing practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge. In this study, we fabricated Al2O3/In2O3 heterojunction system on soda-lime glass consisting of In2O3 deposited by RF reactive sputter with a thickness from 8 to 100 nm and Al2O3 deposited by ALD with a thickness below 100 nm. We suggest that Al2O3/In2O3 heterojunction acts as short-range ordered 2DEG channel with a low temperature (~200 ℃). After deposition of Al2O3 layer, the heterojunction two-dimensional metallic channel was fabricated between Al2O3 and In2O3 interface as showing the metallic conduction with sheet charge density > 10 14/cm2, sheet resistance of 850 Ω/cm2, room temperature Hall mobility of 20.5cm2 V-1 sec-1 depending on the In2O3 thickness. To prove existence of 2DEG channel at the interface, we performed the I-V measurement, time of flight secondary ion mass spectrometry, ultraviolet-visible spectrophotometer, tunneling electron microscope–electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and Hall measurement.

Ferromagnetism in ferroelectric BaTiO3 induced by vacancies: Sensitive dependence on charge state, origin of magnetism, and temperature range of existence

Using density-functional calculations we investigate the possibility and underlying mechanism of generating ferromagnetism (FM) in ferroelectric ${\mathrm{BaTiO}}_{3}$ by native vacancies. For the same vacancy species but different charge states (e.g., ${\mathrm{V}}_{\mathrm{O}}^{0}$ vs ${\mathrm{V}}_{\mathrm{O}}^{2+}$), our paper reveals a marked difference in magnetic behaviors. For instance, while ${\mathrm{V}}_{\mathrm{O}}^{0}$ is ferromagnetic, ${\mathrm{V}}_{\mathrm{O}}^{2+}$ is not. This sensitive dependence, which has often been overlooked, highlights the critical importance of taking into account different charge states. Furthermore, while oxygen vacancies have been often used in experiments to explain the vacancy-induced FM, our calculation demonstrates that Ti vacancies, in particular ${\mathrm{V}}_{\mathrm{Ti}}^{3\ensuremath{-}}$ and ${\mathrm{V}}_{\mathrm{Ti}}^{2\ensuremath{-}}$ with low formation energies, generate even stronger ferromagnetism in ${\mathrm{BaTiO}}_{3}$, with a magnetic moment which is 400% larger than that of ${\mathrm{V}}_{\mathrm{O}}^{0}$. Interestingly, this strong FM of ${\mathrm{V}}_{\mathrm{Ti}}^{}$ can be further enhanced by hole doping. Although both cation vacancies (${\mathrm{V}}_{\mathrm{Ti}}^{\mathrm{q}}$) and anion vacancies (${\mathrm{V}}_{\mathrm{O}}^{0}$) induce FM, their mechanisms differ drastically. FM of anion vacancies originates from the spin-polarized electrons at Ti sites, but FM of cation vacancies stems from the spin-polarized holes at O sites. This paper also sheds light on vacancy-induced FM by discovering that the spin densities of all three considered vacancy species are highly extended in real space, distributed far away from the vacancy. Moreover, we predict that the ferromagnetism caused by ${\mathrm{V}}_{\mathrm{Ti}}^{3\ensuremath{-}}$ is able to survive at high temperatures, which is promising for room-temperature spintronic or multiferroic applications.

Ultrahigh thermoelectric performance in Cu2−ySe0.5S0.5 liquid-like materials

Liquid-like thermoelectric materials have recently received heightened attentions due to their exceptional thermal and electrical transport properties. As a typical example, Cu2−ySe has good electrical transport properties while Cu2−yS has extremely low lattice thermal conductivity. Combining these stirring characters into one material is expected to result in excellent thermoelectric performance. In this study, we found that Cu2−ySe and Cu2−yS can form a solid solution in the composition range down to half Se and half S. XRD, SEM and TEM reveal that Cu2−ySe0.5S0.5 possesses a unique hierarchical microstructure composed of mesoscale polymorphs, nanoscale domains and modulations. Besides, the liquid-like copper ions at high temperature not only strongly scatter lattice phonons but also eliminate some of the transverse phonon vibrations. Combining with the extraordinarily low sound speeds, an overall ultralow thermal conductivity is achieved in Cu2−ySe0.5S0.5 with the values similar to that in Cu2S. Furthermore, the electrical transport performance of Cu2−ySe0.5S0.5 is significantly improved through tuning its native Cu vacancies. High electrical power factors similar to or even superior to Cu2−ySe are observed due to the high weighted mobility. All these favorable factors lead to much enhanced quality factor and thus remarkably high thermoelectric performance in Cu2−ySe0.5S0.5, which reaches a ZT of 2.3 at 1000 K, among the highest values in bulk materials.

Modification of the yellow luminescence in gamma-ray irradiated GaN bulk single crystal

We report the variation of the yellow luminescence (YL) in GaN bulk single crystals by gamma-ray irradiation. The crystals are irradiated at room temperature with gamma-rays of 1.17 and 1.33 MeV from a cobalt-60 source. Gamma-ray dose is 160 kGy. The resistivity varies from 30 Ωcm for an un-irradiated sample to 104 Ωcm for gamma-ray irradiated one. The nitrogen displacement in gamma-ray irradiated samples is observed by Rutherford backscattering channeling experiments using proton beam, suggesting the existence of the deep energy level relating to interstitial nitrogen atoms. The YL from the un-irradiated GaN with a peak at 557 nm (2.22 eV) is observed at around 440 nm to 800 nm, whereas that of the gamma-ray irradiated GaN shows a peak at 532 nm (2.33 eV) although the YL spectrum is almost overlapped with un-irradiated ones. Compton electrons emitted by the gamma-ray irradiation induce the shallow donor located at about 50 meV bellow the conduction band. This energy level is close to that of nitrogen vacancy. The modification of YL is attributed to a transition from the shallow donor induced by the gamma-ray irradiation to the native gallium vacancy.

ZnO and ZnO1−x based thin film memristors: The effects of oxygen deficiency and thickness in resistive switching behavior

In this study, direct-current reactive sputtered ZnO and ZnO1-x based thin film (30 nm and 300 nm in thickness) memristor devices were produced and the effects of oxygen vacancies and thickness on the memristive characteristics were investigated. The oxygen deficiency of the ZnO1-x structure was confirmed by SIMS analyses. The memristive characteristics of both the ZnO and ZnO1-x devices were determined by time dependent current-voltage (I-V-t) measurements. The distinctive pinched hysteresis I-V loops of memristors were observed in all the fabricated devices. The typical homogeneous interface and filamentary types of memristive behaviors were compared. In addition, conduction mechanisms, on/off ratios and the compliance current were analyzed. The 30 nm ZnO based devices with native oxygen vacancies showed the best on/off ratio. All of the devices exhibited dominant Schottky emissions and weaker Poole-Frenkel conduction mechanisms. Results suggested that the oxygen deficiency was responsible for the Schottky emission mechanism. Moreover, the compliance currents of the devices were related to the decreasing power consumption as the oxygen vacancies increased.

Instability of the Sb vacancy in GaSb

We demonstrate that the instability of the Sb vacancy in GaSb leads to a further increase in the acceptor-type defect concentration in proton irradiated undoped, $p$-type GaSb. Using positron annihilation spectroscopy in situ with 10 MeV proton irradiation at 35 K, we find that the irradiation produces both native vacancy defects in GaSb. However, the Sb vacancy is unstable above temperatures of 150 K and undergoes a transition resulting in a Ga vacancy and a Ga antisite. The activation energy of this transition is determined to be $0.6\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\ifmmode\pm\else\textpm\fi{}0.1$ eV. Our results are in line with the established amphoteric defect model and prove that the instability of the Sb vacancy in GaSb has a profound role on the native defect concentration in GaSb.

Cathodo-luminescence of color centers induced in sapphire and yttria-stabilized zirconia by high-energy electrons

We have studied the color-center production in sapphire and yttria-stabilized zirconia (YSZ) single crystals by cathodo-luminescence (CL) spectroscopy for electron energies ranging between 400 and 1250 keV in a high-voltage electron microscope. Emission spectra were collected for in-beam conditions near room temperature and at 200 K. Comparison was made with CL spectra recorded for 3-keV–20-keV electrons in a scanning electron microscope. For high-energy electrons, CL spectra for sapphire revealed broad emission bands centered at photon energies about 3.0 eV and 3.8 eV that were, respectively, assigned to oxygen vacancies (F0 and F+ centers) induced by elastic collisions, on the basis of photoluminescence (PL) data. No such bands were recorded for 3-keV and 5-keV electrons. Two similar bands were also recorded for YSZ (with 9.5 and 18 mol. % yttria) at about 2.8 eV and 4.1 eV that can be, respectively, attributed to the native oxygen vacancies (F2+ centers) and F+ centers. The 4.1-eV band was not seen for 20-keV electrons: it was only produced for high electron energies by elastic collision processes. Instead, the small side band was also found at 2.85 eV for 20-keV electrons. PL excitation contour plots of virgin and irradiated YSZ were also recorded to support our discussion on point-defect identification. CL band intensities show a maximum versus electron energy, whereas point-defect concentrations should increase due to the increase of oxygen atom displacement cross section. The effect of electron energy on the different steps of the CL process is discussed to account for such a behavior.

Vacancy-Induced Semiconductor–Insulator–Metal Transitions in Nonstoichiometric Transition Metal Oxides

Metal–insulator transitions in strongly correlated transition metal oxides (TMOs) induced by electrochemical charging have been attributed to formation of vacancy defects. However, the role of native defects in affecting these transitions is not well understood. This presentation will discuss a new type of phase transition in p-type, nonstoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition along with the complete reversal of conductivity from p- to n-type at room temperature induced by the interaction of native vacancy defects with redox species such as Li+ during electrochemical charging. Direct observation of vacancy-ion interactions using in-situ near-infrared photoluminescence spectroscopy show that this transition is a result of passivation of native nickel (cationic) vacancy defects and subsequent formation of oxygen (anionic) vacancy defects driven by Li+ insertion into the lattice. Changes in the oxidation states of nickel due to defect interactions probed by X-ray photoemission spectroscopy support the above conclusions. In contrast, n-type, nonstoichiometric tungsten oxide shows only insulator-to-metal transition, which is a result of oxygen vacancy formation. The defect-property correlations shown here in these model systems can be extended to other oxides to enable rational engineering of a new robust class of nanoscale, low temperature TMO catalysts and sensors having tailored functionality and selectivity.

Unintentional doping effects in black phosphorus by native vacancies in h-BN supporting layer

Ab initio calculations are used to study the indirect doping effect in black phosphorus (BP) from defects in h-BN support layer. We find defects in substrate can strongly introduce doping to the BP layer by weak van der Waals effect. With considering various native vacancies in h-BN, we find that indirect n-type doping of BP only manifests under case of isolated nitrogen vacancy. P-type doping is presented in most cases including different divacancies, which is in consistent with published experimental reports. Besides, we find presence of BP upon h-BN can also alter the intrinsic magnetic properties of defective h-BN layer.