Oxygen Vacancy Levels Research Articles

Oxygen-Deficient Titania with Adjustable Band Positions and Defects; Molecular Layer Deposition of Hybrid Organic–Inorganic Thin Films as Precursors for Enhanced Photocatalysis

Molecular layer deposition (MLD) of TiCl4 and ethylene-glycol (Ti-EG) was recently demonstrated as a vapor phase synthetic route for preparation of photocatalytic thin films via hybrid organic–inorganic thin films. The organic moieties of the hybrid material function as sacrificial components that undergo controlled decomposition during thermal annealing. Anneal temperature was shown to be an important factor determining the overall photocatalytic performances of the treated films with 650 °C optimal for photodegradation of dye molecules and anneal at 520 °C showing optimal performance for the direct photocatalytic production of H2O2. Both systems exhibit activities that are not typically attainable by titania, yet a fundamental understanding of the underlying details leading to these improved reactivities for the specific cases is still lacking. Here we demonstrate that thermal anneal of hybrid organic–inorganic thin films prepared by MLD yield oxygen-deficient titania with controllable levels of oxygen vacancies (OVs) and defect states that are adjusted by the temperature of the anneal process, performed under air. The anneal process result in nonstoichiometric oxide films with unique electronic properties including the tuning of band positions, accessibility to significantly deeper valence band position and controlled formation of electronic defect states that assist in charge separation for Au-titania catalyst. We correlate the oxygen deficiency and electronic structure of the annealed film with the photocatalytic activity for shedding light on the details that lead to the improved reactivity. These results extend the scope of using MLD in the context of photocatalysis with new routes for obtaining nonstoichiometric oxides which are key for enhancing and tailoring the reactivity of metal oxide (photo)catalysis.

Understanding memory effects in Li-ion batteries: evidence of a kinetic origin in TiO2 upon hydrogen annealing.

Memory effects in Li-ion battery materials have been explained on the basis of the thermodynamics of many-particles body, however the role of the (de-)intercalation kinetics is not yet clear. We demonstrate that kinetic aspects, specifically Li-ion mobility, are determining the magnitude of the memory effect in TiO2 by studying samples with different levels of oxygen vacancies.

Impact of cyclic plasma treatment on oxygen vacancy defects in TiN/HfZrO/SiON/Si gate stacks

This work evaluates the defects in HfZrO as a function of Zr addition into HfO2 and when the dielectric was subjected to a slot-plane-antenna (SPA) plasma treatment in a cyclic process to form TiN/HfZrO/SiON/Si gate stacks. The defect energy levels, estimated by temperature-dependent current-voltage measurements, suggest that Zr addition in HfO2 modifies the charge state of the oxygen vacancy formation, V+. The influence of electron affinity variation of Hf and Zr ions on the charged oxygen vacancy levels seems to have contributed to the increase in defect activation energy, Ea, from 0.32 eV to 0.4 eV. The cyclic SPA plasma exposure further reduces the oxygen vacancy formation because of the film densification. When the dielectric was subjected to a constant voltage stress, the charge state oxygen vacancy formation changes to V2+ and improvement was eliminated. The trap assisted tunneling behavior, as observed by the stress induced leakage current characteristics, further supports the oxygen vacancy formation model.

Mechanochemical Preparation and Properties of Nanodimensional Perovskite Materials

The study is focused on the synthesis of LaMO3 (M = Co, Fe, Mn) perovskite materials using combination of precipitation of precursors and mechanical milling at room temperature. Physicochemical properties of products at each step of preparation were studied by powder X-ray di raction, Mossbauer spectroscopy, infrared spectroscopy (in the middle and far regions) and X-ray photoelectron measurements. As-prepared perovskite powders are composed of nanoparticles with very ne crystallite size (about 15 nm) in all cases. The materials have also high dispersion, high extent of microstrains and high level of oxygen vacancies which is very important in relevance to their use as heterogeneous catalysts.

Photoluminescent properties of Ce-doped HfOxNy thin films prepared by magnetron sputtering

The chemical structures and the optical properties of the Ce-doped HfOxNy thin films were prepared by radio frequency (RF) magnetron sputtering and investigated by using X-ray photoelectron spectroscopy (XPS), ultraviolet–visible (UV–vis) absorption spectrophotometer and photoluminescence (PL) spectroscopy. The results showed that the band gap gradually decreased as the nitrogen incorporation content increased from 1.1% to 8.8%. The PL bands located at 380nm and 465nm of the sample as-deposited corresponded to the transition from the oxygen vacancy level to valance band and the 5d-4f transition of the trivalent cerium, respectively. The further study revealed that both two PL bands exhibited red shift phenomenon in the NH3-annealed sample, which is due to the decrease of the band gap.

Control of doping by matrix in few-layer graphene/metal oxide composites with highly enhanced electrical conductivity

Doping of graphene by contacting other materials has significant meaning to the graphene based devices and composites. In this work, highly conducting few-layer graphene (FLG) based composites in which the doping type and level can be manipulated by incorporating FLG with different matrixes are fabricated. Three metal oxides with different level of oxygen vacancies (α-Al2O3, 3%mol yttria stabilized zirconia (3YSZ) and 8%mol yttria stabilized zirconia (8YSZ)) are selected as matrix material. While the electrical conductivity is largely enhanced to 1.4×103–2.1×103Sm−1 in the composites by adding 4.42–5.1vol.% FLG, hole-doping level in composites increases in the sequence of FLG/Al2O3<FLG/3YSZ<FLG/8YSZ from room temperature to moderate temperature, as indicated by thermopower measurement and calculation. It is deduced that the concentration of oxygen vacancy on the surface of oxides plays an important role for tuning the hole-doping level in FLG and the control of doping can be realized accordingly.

Investigation of Photoinduced Electrical Properties in the Heterojunction TiO&lt;sub&gt;2&lt;/sub&gt;/SnO&lt;sub&gt;2&lt;/sub&gt;

TiO2/SnO2 thin films heterostructures are grown by the sol-gel-dip-coating technique. It was found that the crystalline structure of TiO2 depends on the annealing temperature and the substrate type. TiO2 films deposited on glass substrate, submitted to thermal annealing until 550°C, present anatase structure, whereas films deposited on quartz substrate transform to rutile structure when thermally annealed at 1100°C. When structured as rutile, this oxide semiconductor has very close lattice parameters to those of SnO2, making easier the heterostructure assembling. Electrical properties of TiO2/SnO2 heterostructure were evaluated as function of temperature and excitation with different light sources. The temperature dependence of conductivity is dominated by a deep level with energy coincident with the second ionization level of oxygen vacancies in SnO2, suggesting the dominant role of the most external layer material (SnO2) to the electrical transport properties. The fourth harmonic of a Nd:YAG laser line (4.65eV) seems to excite the most external layer whereas a InGaN LED (2.75eV) seems to excite electrons from the ground state of a quantized interfacial channel as well as intrabandgap states of the TiO2 layer.

Light emission from tin-dioxide crystals

The luminescence spectra of SnO2 crystals are studied in the temperature range from 5 to 40 K. Broad luminescence bands in the visible spectral region are attributed to transitions from the donor levels of oxygen vacancies and interstitial tin atoms to the acceptor levels of another type of intrinsic defects that are apparently formed from tin vacancies and interstitial oxygen atoms. It is found that emission related to one of the exciton-defect complexes with the minimum binding energy is enhanced, as the crystal is heated from 5 to 30 K. The distinguishing feature of this complex is its anomalously weak coupling with crystal-lattice vibrations.

Oxygen vacancy defects in Ta2O5 showing long-range atomic re-arrangements

The structure, formation energy, and energy levels of the various oxygen vacancies in Ta2O5 have been calculated using the λ phase model. The intra-layer vacancies give rise to unusual, long-range bonding rearrangements, which are different for each defect charge state. The 2-fold coordinated intra-layer vacancy is the lowest cost vacancy and forms a deep level 1.5 eV below the conduction band edge. The 3-fold intra-layer vacancy and the 2-fold inter-layer vacancy are higher cost defects, and form shallower levels. The unusual bonding rearrangements lead to low oxygen migration barriers, which are useful for resistive random access memory applications.

High performance hydrogenated TiO2 nanorod arrays as a photoelectrochemical sensor for organic compounds under visible light

A photo-electrode of hydrogenated TiO2 nanorod arrays (H-TNRs) was prepared and used as a photoelectrochemical sensor for organic compound detections for the first time. Under visible light, the H-TNR electrode has shown a highly sensitive and steady photocurrent response due to introduction of oxygen vacancy and mid-gap energy levels. The H-TNRs are a promising material for photoelectrochemical detection of organic compounds under visible light.

The quasi-particle electronic structure of charged oxygen vacancies in TiO2

We studied the quasi-particle electronic structure of oxygen vacancies V O in rutile TiO2 using G0W0 approximation on top of GGA + U as a method of choice. In order to determine the degree of electronic localization, we examined the combined G0W0@GGA + U scheme in which both G0W0 and GGA + U methods are individually one step toward a better inclusion of the non-locality of the exchange-correlation potential. Our G0W0@GGA + U results realize the weak nature of electron correlation. Moreover, it was shown that the U-dependency of the energy gap in the perfect bulk is slightly stronger than that of a defected sample while the U-dependence of V O induced levels in different charged states show different behaviors. Eventually, we concluded that applying G0W0@GGA + U, though it does not stabilize oxygen vacancy levels in the gap, however, it does correct the formation energies and transition levels more closely to the experimental measurements.

Multi-band emission in a wide wavelength range from tin oxide/Au nanocomposites grown on porous anodic alumina substrate (AAO)

The photoluminescence (PL) properties of tin oxide nanostructures are investigated. Three samples of different morphology, induced by deposition process and various geometrical features of nanoporous anodic aluminum oxide (AAO) substrate, are analyzed. X-ray photoelectronic spectroscopy (XPS) analysis reveals the presence of two forms of tin oxide on the surface of all studied samples: SnO and SnO2. The former form is typical for reduced surface with bridging oxygen atoms and every other row of in-plane oxygen atoms removed. The oxygen defects give rise to a strong emission in visible region. Two intense PL peaks are observed centered at about 540 (band I) and 620 (band II)nm. The origin of these bands was ascribed to the recombination of electrons from the conduction band (band I) and shallow traps levels (band II) to the surface oxygen vacancy levels. Upon deposition of Au nanoparticles on the top of tin oxide nanostructures the emission at 540 and 620nm disappears and a new band (band III) occurs in the range >760nm. The PL mechanism operating in the studied systems is discussed. The tin oxide/Au nanocomposites can be used as efficient multi-band light emitters in a wide (from visible to near infrared) wavelength range.

Improvement of the fatigue and the ferroelectric properties of PZT films through a LSCO seed layer

The ability to optimizate the preparation of Lead Zirconate Titanate (PZT) films on platinized Si substrate by pulsed laser deposition was demonstrated. The effect of the modification of the interface film/electrode through the use of a (La,Sr)CoO3 (LSCO) seed layer on the remnant polarization, fatigue endurance and stress in PZT films was studied. An improvement on the ferroelectric properties was found with the using of the LSCO layer. A remnant polarization (Pr) of 19.8 μC/cm2 and 4.4 μC/cm2 for films with and without the LSCO layer were found. In the same way the polarization fatigue decreases significantly after deposition of the LSCO layer between the film and substrate. Atomic force microscopy (AFM) images revealed a different growth process in the films. Current–voltage (I–V) measurements showed that the use of LSCO seed layer improves the leakage current and, on the other hand the conduction mechanisms in the film without LSCO, after the fatigue test, was found to be changed from Schottky to Poole–Frenkel. The trap activation energy (about 0.14 eV) determined from Poole–Frenkel mode agrees well with the energy level of oxygen vacancies. The films stresses were estimated by XRD in order to explain the improvement on the structure and consequentially ferroelectric properties of the films. The model proposed by Dawber and Scott was found to be in agreement with our experimental data, which seems to predict that the oxygen vacancies play an important role on fatigue.

Defect induced optical bandgap narrowing in undoped SnO2 nanocrystals

Unusual optical bandgap narrowing is observed in undoped SnO2 nanoparticles synthesized by the solution combustion method. The estimated crystallite size is nearly 7 nm. Though the quantum confinement effect predicts a larger optical bandgap for materials with small crystallite size than the bulk, the optical bandgap in the as synthesized materials is found to be 2.9 eV compared to the reported value of 3.6 eV for bulk SnO2 particles. The yellow-green photoluminescence emissions and the observed narrowing of the bandgap can be attributed to the deep donor levels of oxygen vacancies, owing to the high exothermicity of the combustion reaction and the faster cooling rates involved in the process.

Structures, optical properties, and electrical transport processes of SnO2 films with oxygen deficiencies

The structures, optical and electrical transport properties of SnO2 films, fabricated by rf sputtering method at different oxygen partial pressures, were systematically investigated. It has been found that preferred growth orientation of SnO2 film is strongly related to the oxygen partial pressure during deposition, which provides an effective way to tune the surface texture of SnO2 film. All films reveal relatively high transparency in the visible range, and both the transmittance and optical band gap increase with increasing oxygen partial pressure. The temperature dependence of resisitivities was measured from 380 K down to liquid helium temperatures. At temperature above K, besides the nearest‐neighbor‐hopping process, thermal activation processes related to two donor levels ( and 100 meV below the conduction band minimum) of oxygen vacancies are responsible for the charge transport properties. Below K, Mott variable‐range hopping conduction process governs the charge transport properties at higher temperatures, while Efros–Shklovskii (ES) variable‐range‐hopping conduction process dominates the transport properties at lower temperatures. Distinct crossover from Mott type to ES type variable‐range‐hopping conduction process at several to a few tens kelvin are observed for all SnO2 films.

Defect energy levels in La and Hf germanates on Ge

We have calculated the energy levels of oxygen vacancies in LaGeOx and HfGeOx using a density functional method which does not need a band gap correction. The defect energy levels are aligned to the band gap of Ge by the calculated band offsets. The defect configurations have more complex bonding than those of simple binary oxides. The defect levels of metal germanates lie deeper in the band gap than the equivalent binary metal oxides, so that there should be less charge trapping in the germanates. The defect levels of HfGeOx are found to be deeper and the defect orbitals are more localized than those in HfSiOx.

Oxygen vacancy and dopant concentration dependent magnetic properties of Mn doped TiO2 nanoparticle

Mn doped TiO2 nanoparticles are synthesized by sol–gel method. Incorporation of Mn shifts the diffraction peak of TiO2 to lower angle. The position and width of the Raman peak and photoluminescence intensity of the doped nanoparticles varies with oxygen vacancy and Mn doping level. The electron spin resonance spectra of the Mn doped TiO2 show peaks at g = 1.99 and 4.39, characteristic of Mn2+ state. Reduction in the emission intensity, on Mn doping, is owing to the increase of nonradiative oxygen vacancy centers. Mn doped TiO2, with 2% Mn, shows ferromagnetic ordering at low applied field. Paramagnetic contribution increases as Mn loading increases to 4% and 6%. Temperature dependent magnetic measurement shows a small kink in the ZFC curve at about 40 K, characteristic of Mn3O4. The ferromagnetic ordering is possibly due to the interaction of the neighboring Mn2+ ions via oxygen vacancy (F+ center). Increase in Mn concentration increases the fraction of Mn3O4 phase and thereby increases the paramagnetic ordering.

Formation of an oxygen vacancy-dinitrogen complex in nitrogen-doped hafnium oxide

Nitrogen in a thin HfO2−xNx layer was observed in the form of dinitrogen molecules by atom probe microscope measurements. To evaluate the chemical state, an ab initio calculation based on monoclinic HfO2 was performed with using oxygen vacancy models combined with interstitial or substitutional nitrogen. The formation of a hafnium dinitrogen complex near the oxygen vacancy was found to considerably lower the system energy. This result contrasts the substitutional nitrogen complex passivating the oxygen vacancy level in the cubic structure and suggests that the structural stability enhanced by the oxygen vacancy-dinitrogen complex restrains the formation of grain boundaries in amorphous HfO2, which are responsible for the large leakage current.

Spectroscopic Studies of In&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt; Nanostructures; Photovoltaic Demonstration of In&lt;SUB&gt;2&lt;/SUB&gt;O&lt;SUB&gt;3&lt;/SUB&gt;/p-Si Heterojunction

The thermal oxidation process of the indium nitride (InN) nanorods (NRs) was studied. The SEM studies reveal that the cracked and burst mechanism for the formation of indium oxide (In2O3) nanostructures by oxidizing the InN NRs at higher temperatures. XRD results confirm the bcc crystal structure of the as prepared In2O3 nanostructures. Strong and broad photoluminescence spectrum located at the green to red region with maximum intensity at 566 nm along with a weak ultraviolet emission at 338 nm were observed due to oxygen vacancy levels and free excitonic transitions, respectively. The valence band onset energy of 2.1 eV was observed from the XPS valence band spectrum, clearly justifies the alignment of Fermi level to the donor level created due to the presence of oxygen vacancies which were observed in the PL spectrum. The elemental ratio In:O in as prepared In2O3 was found to be 42:58 which is in close agreement with the stoichiometric value of 40:60. A downward shift was observed in the Raman peak positions due to a possible phonon confinement effect in the nanoparticles formed in bursting mechanism. Such single junction devices exhibit promising photovoltaic performance with fill factor and conversion efficiency of 21% and 0.2%, respectively, under concentrated AM1.5 illumination.

Luminescence study of oxygen vacancies in lanthanum hafnium oxide, La2Hf2O7

Luminescence properties of La2Hf2O7 have been measured for powdered samples prepared by solid state synthesis. These include photoluminescence excitation and emission spectra, x-ray excited spectra, luminescence decay profiles, and reflectivity spectra. The observed luminescence is attributed to self-trapped excitons and transitions involving energy levels of oxygen vacancies. The oxygen-vacancy-related luminescence consists of a broad band in the region 350–750nm with a peak around 460nm. The optical and X-ray excited emission spectra differ due to the more selective nature possible with optical excitation. The experimental results are in general agreement with spectroscopic properties predicted by theoretical electronic structure calculations of oxygen vacancy states by Liu et al. in 2007 [15]. The excitation and emission properties of the oxygen vacancy luminescence are shown to vary with the atmosphere and temperature of the synthesis process.