Oxygen Point Defects Research Articles

Tuning Memristivity by Varying the Oxygen Content in a Mixed Ionic–Electronic Conductor

AbstractThe rising interest shown for adaptable electronics and brain‐inspired neuromorphic hardware increases the need for new device architectures and functional materials to build such devices. The rational design of these memory components also benefits the comprehension and thus the control over the microscopic mechanisms at the origin of memristivity. In oxide‐based valence‐change memories, the control of the oxygen drift and diffusion kinetics is a key aspect in obtaining the gradual analog‐type change in resistance required for artificial synapse applications. However, only a few devices are designed with this in mind, as they are commonly built around ionic insulating active materials. This shortcoming is addressed by using a mixed ionic–electronic conductor as functional memristive material. This work demonstrates how the oxygen content in La2NiO4+δ (L2NO4), tuned through post‐annealing treatments, has a critical influence on the memory characteristics of L2NO4‐based memristive devices. The presence of interstitial oxygen point defects in L2NO4 affects both its structure and electrical properties. High oxygen stoichiometry in the pristine state leads to an increased electrical conductivity, ultimately resulting in an improved memory window with highly multilevel, analog‐type memory programing capabilities, desirable for analog computing and synaptic applications in particular.

A new far-red emission from Zn2SnO4 powder synthesized by modified solid state reaction method

In this work, far-red-emitting Zn2SnO4 phosphor has been successfully synthesized by a facile modified solid-state reaction method from ZnO and SnO2 powders. FESEM images reveal that as-milled particles are nearly spherical, with size in the range of 10–30 nm. The particle size increases with increasing annealing temperature and it reaches the value of about 4 μm at 1200 °C. XRD patterns indicate that Zn2SnO4 phase starts occurring at ~700 °C with its single phase at 900 °C and the best crystalline quality of Zn2SnO4 phase is obtained at 1000 °C. UV–Vis spectra analysis reveals that ZnO phase reappears at high annealing temperature (≥1100 °C), reflecting the deterioration of the single phase of Zn2SnO4. A broad visible band centered at 684 nm, which has never been explicitly reported, is clearly observed in the PL spectrum of the single Zn2SnO4 crystals. The intensity of this new band is influenced by annealing temperatures. It reaches the maximum value at 1000 °C, corresponding to the highest quality of the Zn2SnO4 phase. By using data fitting, the PL spectrum is deconvoluted into four Gaussian peaks at ~2.20 eV (563.5 nm), ~1.98 eV (626.1 nm), ~1.80 eV (688.8 nm), and ~1.68 eV (738.0 nm). The peak at ~2.20 eV (563.5 nm) is ascribed to oxygen vacancy (Vo) while the other emission peaks are possibly associated with a significant number of trapped states due to the high concentration of deficiencies such as oxygen vacant defects, zinc interstitials, zinc/tin vacancies (VZn/VSn), Zn/Sn stoichiometry or point defects.

Modifications of ion beam sputtered tantala thin films by secondary argon and oxygen bombardment.

Amorphous tantala (Ta2O5) thin films were deposited by reactive ion beam sputtering with simultaneous low energy assist Ar+ or Ar+/O2+ bombardment. Under the conditions of the experiment, the as-deposited thin films are amorphous and stoichiometric. The refractive index and optical band gap of thin films remain unchanged by ion bombardment. Around 20% improvement in room temperature mechanical loss and 60% decrease in absorption loss are found in samples bombarded with 100-eV Ar+. A detrimental influence from low energy O2+ bombardment on absorption loss and mechanical loss is observed. Low energy Ar+ bombardment removes excess oxygen point defects, while O2+ bombardment introduces defects into the tantala films.

Theoretical Investigation of the Interaction of the CuInSe2 Absorber Material with Oxygen, Hydrogen, and Water

We performed ab initio calculations to study oxygen and hydrogen point defects in the CuInSe2 (CISe) solar-cell material. We found that H interstitial defects (when one H atom is surrounded by four...

Screened Coulomb hybrid density functional investigation of oxygen point defects on ZnO nanowires

In this study, oxygen vacancies and adatoms have been considered on the surface of both hexagonal anssd triangular ZnO nanowires. Their effect on the electronic structure and optical spectra of the nanowires have been investigated using the exact exchange hybrid density functional theory calculations. A surface oxygen vacancy gives rise to appearance of a band gap state at almost 0.7 eV above the valence band of the both types of the nanowires while an oxygen adatom show bulk-like electronic properties. A shape dependence is also indicated by the calculated physical quantities of oxygen related point defects on ZnO nanowires.

Modified Oxygen Defect Chemistry at Transition Metal Oxide Heterostructures Probed by Hard X-ray Photoelectron Spectroscopy and X-ray Diffraction

Transition metal oxide heterostructures are interesting due to the distinctly different properties that can arise from their interfaces, such as superconductivity, high catalytic activity, and magnetism. Oxygen point defects can play an important role at these interfaces in inducing potentially novel properties. The design of oxide heterostructures in which the oxygen defects are manipulated to attain specific functionalities requires the ability to resolve the state and concentration of local oxygen defects across buried interfaces. In this work, we utilized a novel combination of hard X-ray photoelectron spectroscopy (HAXPES) and high resolution X-ray diffraction (HRXRD) to probe the local oxygen defect distribution across the buried interfaces of oxide heterolayers. This approach provides a nondestructive way to qualitatively probe locally the oxygen defects in transition metal oxide heterostructures. We studied two trilayer structures as model systems: the La0.8Sr0.2CoO3−δ/(La0.5Sr0.5)2CoO4−δ/La0.8Sr0...

Study of phase transformation induced by electronic excitation in pure and yttrium doped ZrO2 thin films

The study of polymorphic ZrO2 and its structural stability of high temperature phases, such as tetragonal (t-ZrO2) and cubic (c-ZrO2) phases has been a pivotal issue for technological applications. In this work, nanocrystalline ZrO2 thin film was fabricated using the sol-gel route and comparative investigations were carried out for structural phases, surface morphology and optical band-gap for pure and yttrium doping. The changes in structural and optical studies were studied before and after irradiation with Ag9+, 120 MeV ion beam. The structural findings have shown complete transformation of phase with doping and irradiation. Two direct bands electronic structures have also evolved for doped samples under irradiation. These multiple band transitions in optical measurement could be attributed to the presence of oxygen point defects state in forbidden band gap. Presence of point defects is believed to be responsible for phase transformation/stabilization in pure zirconium oxide under irradiation.

Interactions of superhydrophobic carbon soot coatings with short alkyl chain alcohols and fluorocarbon solutions

Carbon soot is a nanomaterial, whose extreme water repellency determines its potential industrial applicability in anti-fouling coatings, anti-icing systems, chemical sensors and drag reducing surfaces. The wettability and mechanical durability of the soot can be adjusted through an appropriate chemical treatment, but the underlying mechanism is yet unknown. In this paper, we provide new insights regarding the missing link between the fundamentals of soot-alcohol-fluorocarbon interactions and the derivative physicochemical features/properties of the coating. This is achieved by immersion of three types of superhydrophobic soot coatings in aqueous solutions of methanol, ethanol and isopropanol with variable concentration, and subsequent functionalization with fluorocarbon emulsion. The detailed structural and chemical analysis using scanning electron microscopy, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, reveals that the thickness, porosity and ratio of chemical bonds in the soot alter proportionally to the quantity of oxygen functional groups and point defects in the coating, as well as the alkyl chain length of alcohols. Moreover, the chemical treatment with fluorocarbon causes the formation of a thin fluoropolymer film on top of the soot that reinstates its superhydrophobicity and imparts oleophobicity of the surface. These findings are fundamental for further synthesis of soot coatings with controllable surface properties.

Thermal defect annealing of swift heavy ion irradiated ThO2

Isochronal annealing, neutron total scattering, and Raman spectroscopy were used to characterize the structural recovery of polycrystalline ThO2 irradiated with 2-GeV Au ions to a fluence of 1×1013ions/cm2. Neutron diffraction patterns show that the Bragg signal-to-noise ratio increases and the unit cell parameter decreases as a function of isochronal annealing temperature, with the latter reaching its pre-irradiation value by 750°C. Diffuse neutron scattering and Raman spectroscopy measurements indicate that an isochronal annealing event occurs between 275–425°C. This feature is attributed to the annihilation of oxygen point defects and small oxygen defect clusters.

Role of O and Se defects in the thermoelectric properties of bismuth oxide selenide

Bismuth oxygen selenide, Bi2O2Se, is a promising thermoelectric material because of its reduced thermal conductivity. In this study, we perform the first-principles calculation and utilize the solution of Boltzmann transport equation in a constant relaxation-time approximation to compute the electronic and thermoelectric properties of Bi2O2Se with O and Se defects. Oxygen vacancies trap bands located inside the band gap of Bi2O2Se, and the compound becomes a conductor. These bands lead to drastic reduction in the Seebeck coefficient. When vacancies are filled by selenide atoms (selenide point defect), the materials return to be a semiconductor and the Seebeck coefficient increases. The increase of S is also found in the system with defects formed by the substitution of oxygen atoms into selenide sites (oxygen point defect) in the pristine compound. The power factor significantly increases during p-type doping compared with that during n-type doping for the selenide point defect. However, differences in the two doping cases are less distinguished for the oxygen point defect. Hence, the selenide point defect, Bi2O2−δSe1+δ with p-type doping, is an effective way to increase the power factor and eventually the thermoelectric efficiency of Bi2O2Se.

Optical, spectral and thermal properties of natural pumice glass

Pumice is a natural Si-rich material displaying a complex cathodo- (CL) and thermoluminescence (TL) glow curves. The UV-IR CL emission consists of (i) a UV waveband in the range of 340–420nm,(ii) blue band at 450–480nm and (iii) a broad emission in the green-red region (at 550–650nm) that could be respectively linked to Non Bridging Oxygen Hole Centers (Si–O•), self-trapped excitons and point defects (Mn2+ −0.03%- and Fe −1.15%-). Thermal treatments performed on the TL glow curves allowed us to determine that the trap system could be associated with a continuum in the trap distribution, since successive thermal pretreatments in the range of 200–310°C induce an emission that shifts linearly to higher temperatures when the thermal pretreatment (Tstop) is increased, while the intensity of the maxima decreases similarly to the peak area. The evaluation of the Ea values, s value and the trap system calculated by VHR, IR and Glow curve fitting methods considering three possible distribution function for n(E): gaussian, exponential and uniform, has given matching values for the 280°C TL peak.

Oxygen Point Defect Chemistry in Ruddlesden-Popper Oxides (La1-xSrx)2MO4±δ (M = Co, Ni, Cu).

Stability of oxygen point defects in Ruddlesden-Popper oxides (La1-xSrx)2MO4±δ (M = Co, Ni, Cu) is studied with density functional theory calculations to determine their stable sites, charge states, and energetics as functions of Sr content (x), transition metal (M), and defect concentration (δ). We demonstrate that the dominant O point defects can change between oxide interstitials, peroxide interstitials, and vacancies. In general, increasing x and atomic number of M stabilizes peroxide over oxide interstitials as well as vacancies over both peroxide and oxide interstitials; increasing δ destabilizes both oxide interstitials and vacancies but barely affects peroxide interstitials. We also demonstrate that the O 2p-band center is a powerful descriptor for these materials and correlates linearly with the formation energy of all defects. The trends of formation energy versus x, M, and δ and the correlation with O 2p-band center are explained in terms of oxidation chemistry and electronic structure.

First principles study of band line up at defective metal-oxide interface: oxygen point defects at Al/SiO2 interface

The dielectric breakdown at metal-oxide interfaces is a critical electronic device failure mechanism. Electronic tunneling through dielectric layers is a well-accepted explanation for this phenomenon. Theoretical band alignment studies, providing information about tunneling, have already been conducted in the literature for metal-oxide interfaces. However, most of the time materials were assumed defect free. Oxygen vacancies being very common in oxides, their effect on band lineup is of prime importance in understanding electron tunneling in realistic materials and devices. This work explores the effect of oxygen vacancy and oxygen di-vacancy at the Al/SiO2 interface on the band line up within Density Functional Theory using PBE0 hybrid exchange and correlation functional. It is found that the presence of defects at the interface, and their charge state, strongly alters the band line up.

Low temperature solution process-based defect-induced orange-red light emitting diode.

We report low-temperature solution-processed p-CuO nanorods (NRs)/n-ZnO NRs heterojunction light emitting diode (LED), exploiting the native point defects of ZnO NRs. ZnO NRs were synthesized at 90 °C by using hydrothermal method while CuO NRs were synthesized at 100 °C by using microwave reaction system. The electrical properties of newly synthesized CuO NRs revealed a promising p-type nature with a hole concentration of 9.64 × 1018 cm−3. The current-voltage characteristic of the heterojunction showed a significantly high rectification ratio of 105 at 4 V with a stable current flow. A broad orange-red emission was obtained from the forward biased LED with a major peak at 610 nm which was attributed to the electron transition from interstitial zinc to interstitial oxygen point defects in ZnO. A minor shoulder peak was also observed at 710 nm, corresponding to red emission which was ascribed to the transition from conduction band of ZnO to oxygen vacancies in ZnO lattice. This study demonstrates a significant progress toward oxide materials based, defect-induced light emitting device with low-cost, low-temperature methods.

Towards Identification of Oxygen Point Defects by Means of Position Averaged CBED

Towards Identification of Oxygen Point Defects by Means of Position Averaged CBED

Formation and Movement of Cationic Defects During Forming and Resistive Switching in SrTiO3 Thin Film Devices

The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen‐deficient, highly electron‐doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal–insulator–metal structures fabricated from a thin Fe‐doped SrTiO3 (STO) film on n‐conducting Nb‐doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X‐ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect‐rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor‐doped substrate. In particular, the apparent dichotomy between the observation of acceptor‐type defects and increased electronic conductivity in STO is addressed.

Optical properties of ideal γ-Al2O3 and with oxygen point defects: an ab initio study

The optical properties of pure γ-Al2O3 and in the presence of oxygen point defects are investigated by the density functional theory approach using the PBE-GGA and TB-mBJ-GGA schemes.

Investigation of oxygen point defects in cubic ZrO2 by density functional theory

The energetics of formation and migration of the oxygen vacancy and interstitial in cubic ZrO2 are investigated by density functional theory calculations. In an O-rich environment, the negatively charged oxygen interstitial is the most dominant defect whereas, the positively charged oxygen vacancy is the most dominant defect under O-poor conditions. Oxygen interstitial migration occurs by the interstitialcy and the direct interstitial mechanisms, with calculated migration energy barriers of 2.94eV and 2.15eV, respectively. For the oxygen vacancy, diffusion is preferred along the 〈100〉 direction, and the calculated energy barriers are 0.26eV for VO2+, 0.27eV for VO1+ and 0.54eV for VO0. These results indicate that oxygen diffusivity is higher through the vacancy-migration mechanism.

Atmosphere controlled conductivity and Maxwell-Wagner relaxation in Bi0.5K0.5TiO3—BiFeO3 ceramics

Here, we report on a giant dielectric relaxation in (1 − x)Bi0.5K0.5TiO3—xBiFeO3 ceramics below ∼300 °C, which becomes more pronounced with increasing BiFeO3 content. The relaxation was shown to be of Maxwell-Wagner type and associated with charge depletion at the electroded interfaces. It was also shown that the relaxation could be controlled or, eventually, removed by heat treatment in controlled partial pressure of oxygen. This was rationalized by the relationship between the electrical conductivity and variation in the oxidation state of Fe, which is strongly coupled to the partial pressure of oxygen. The results are discussed with emphasis on oxygen diffusion and point defect equilibria involving oxygen vacancies and iron in divalent and tetravalent state. Finally, the barrier-free dielectric properties of the (1 − x)Bi0.5K0.5TiO3—xBiFeO3 ceramics are reported.

2.6 MeV proton irradiation effects on the surface integrity of depleted UO2

The effect of low temperature proton irradiation in depleted uranium dioxide was examined as a function of fluence. With 2.6MeV protons, the fluence limit for preserving a good surface quality was found to be relatively low, about 1.4 and 7.0×1017protons/cm2 for single and poly crystalline samples, respectively. Upon increasing the fluence above this threshold, severe surface flaking and disintegration of samples was observed. Based on scanning electron microscopy (SEM) and X-ray diffraction (XRD) observations the causes of surface failure were associated to high H atomic percent at the peak damage region due to low solubility of H in UO2. The resulting lattice stress is believed to exceed the fracture stress of the crystal at the observed fluencies. The oxygen point defects from the displacement damage may hinder the H diffusion and further increase the lattice stress, especially at the peak damage region.