Oxygen-poor Condition Research Articles

Oxygen-close-packed (310)-plane substrates of β-Ga2O3 grown by the casting method

The highly anisotropic crystal structure of β-Ga2O3 gives rise to a variety of crystal planes, among which the (310) plane is a potentially stable close-packed plane for the O sublattice. In this paper, we report the β-Ga2O3 single crystal and substrates with a (310) major plane grown by the spontaneous nucleation technique in the casting method. High-quality crystal growth and substrate processing were confirmed by the 25.67 arc sec full width at half maximum and the 0.25 nm surface roughness. The nanoindentation experiments revealed the (310) substrate's better elastic recovery than that of (100) substrate. The Young's modulus and hardness of (310) substrates were 200 and 7.6 GPa, respectively. The surface barrier height and the Schottky barrier height were 1.25 and 0.92 eV, respectively. First principles calculations identified the (310)-Ga-I plane as the most stable surface configuration of the (310) plane under oxygen-poor condition, with a surface energy density of 1.48 J/m2. The (310) twin boundary formation around the O sublattice has a high energy density of 0.55 J/m2, suggesting its unlikelihood of spontaneous formation. These properties of (310) plane facilitate a high-quality crystal processing and epitaxial growth, thus endowing potential applications in high-quality power devices. Furthermore, the growth and fabrication of the (310) plane provide a route toward understanding the properties of β-Ga2O3 and advancing the growth techniques of oxide crystals.

Geochemical Characteristics of the Upper Permian Longtan Formation from Northeastern Sichuan Basin: Implications for the Depositional Environment and Organic Matter Enrichment

AbstractA set of organic‐rich shales of the upper Permian Longtan Formation, which is widely developed in the northeastern part of the Sichuan Basin, is a key formation for the next step of exploration and development. At present, most studies on this set of formations have focused on the reservoir characteristics and reservoir formation mechanism of the shales, and basic studies on the palaeoenvironment and organic matter (OM) enrichment mechanism have not been fully carried out. In this paper, we recovered the sedimentary palaeoenvironment by mineralogical, elemental geochemical and organic geochemical analyses, and explored the enrichment mechanism of OM under the constraints of palaeoenvironmental evolution. The shales can be divided into two stages of sedimentary evolution: compared with the shales of the Lower Longtan Formation, the shales of the Upper Longtan Formation are relatively rich in quartz, poor in clay and carbonate minerals, and the OM type changes from type III to type II2. The depositional environment has undergone a change from sea level rise, from warm and wet climate to dry and cold climate, and from oxygen‐poor condition restricted to open reduction environment; the land source input has decreased, the siliceous mineral content has increased, the biological productivity has improved, and the deposition rate has changed from high to low. A depositional model was established for the shales of the Longtan Formation, reflecting the differential reservoir formation pattern of organic matter. For the Lower Longtan Formation shales, the most important factors controlling OM content are terrestrial source input and deposition rate, followed by paleoclimate and paleo‐oxygen conditions. For the Upper Longtan Formation shales, the most important controlling factor is paleoproductivity, followed by sedimentation rate. The depositional model constructed for the Upper and Lower Longtan Formation shales can reproduce the enrichment of organic matter and provide a basis for later exploration and development.

Reconstruction and stability of Fe3O4 (001) surface: An investigation based on particle swarm optimization and machine learning

Magnetite nanoparticles show promising applications in drug delivery, catalysis, and spintronics. The surface of magnetite plays an important role in these applications. Therefore, it is critical to understand the surface structure of Fe3O4 at atomic scale. Here, using a combination of first-principles calculations, particle swarm optimization (PSO) method and machine learning, we investigate the possible reconstruction and stability of Fe3O4(001) surface. The results show that besides the subsurface cation vacancy (SCV) reconstruction, an A layer with Fe vacancy (A-layer-VFe) reconstruction of the (001) surface also shows very low surface energy especially at oxygen poor condition. Molecular dynamics simulation based on the iron–oxygen interaction potential function fitted by machine learning further confirms the thermodynamic stability of the A-layer-VFe reconstruction. Our results are also instructive for the study of surface reconstruction of other metal oxides.

Oxygen mediated defect evolution in RF sputtered Ga2O3 thin films on p-Si substrate

Impact of oxygen concentration in governing the structural and optical properties of RF sputter Ga 2 O 3 thin film deposited on p-Si at 500 o C by controlling the oxygen flow rate (OFR) has been demonstrated in this work. To maintain the oxygen rich and oxygen poor condition, the argon (Ar) to oxygen (O 2 ) ratio systematically varied from 1:1, 1:2, to 1:0, 2:1 and, 3:1 respectively. Such OFR reliant change in crystalline quality of the grown Ga 2 O 3 thin film is investigated by X-ray Diffraction (XRD) study. The atomic force microscopy results further illustrate an increase in film roughness owing to the inevitable introducing of defects/ oxygen vacancy ( V O ) during deposition with higher OFR. The modification in material stoichiometry via generation of V O and defects in surplus of oxygen is further supported with XPS analysis. Moreover, a rising trend of optical bandgap (Eg) due to transformation from oxygen rich to oxygen poor condition was revealed by spectroscopic ellipsometery study and further correlated with the improved crystallinity of the film. A maximum Eg of 4.44 eV was obtained for the Ga 2 O 3 thin film deposited with 3:1 Ar to O 2 ratio. Finally, the photoluminescence (PL) investigation confirms a strong correlation between defects/ V O generation with OFR variation. Where, the decrease in blue band contribution in lower oxygen concentration was correlated with reduced Vo concentration via its active participation in gallium vacancy V Ga and Vo pair (V Ga +Vo) formation. Finally, the impact of OFR on the Ga 2 O 3 /Si heterojunction device property is also investigated by utilizing the current–voltage (I-V) measurements, which shows a superior device performance for film grown in oxygen poor condition owing to the presence of lower defect concentration, surface roughness, and interface states. • Oxygen mediated structural, chemical and optical transformation of Ga 2 O 3 thin film deposited on p-Si substrate at 500 o C temperature by RF sputtering. • Impact of oxygen extremes on the defect evolution of Ga 2 O 3 thin film by photoluminescence study. • The heterojunction properties of β-Ga 2 O 3 /Si are studied electrically and such heterojunctions exhibit relatively superior rectification properties. • 3:1 is the optimum Ar to O 2 ratio to achieve high quality, defect-free Ga 2 O 3 film on Si.

Liquid metal embrittlement susceptibility of an oxide-dispersion strengthened FeCrAl alloy in contact with liquid lead-bismuth eutectic at 350 °C

Liquid metal embrittlement (LME) susceptibility of an oxide-dispersion strengthened (ODS) Fe14Cr4.5Al-0.5Y2O3 (wt%) alloy in lead-bismuth eutectic (LBE) at 350 °C has been studied using slow strain rate tensile tests. The results showed that LME occurred in this alloy regardless of the oxygen concentration dissolved in LBE, but an oxygen-rich condition led to a much smaller area of cleavage on fracture surfaces as compared to an oxygen-poor condition. Most interestingly, deep secondary cracks were observed in all specimens tested in either oxygen-rich or -poor LBE, which result from LME-induced cracking of the (prior) elongated grain boundaries formed during hot rotary swaging.

BiOBr/ Bi4O5Br2/PDI constructed for visible-light degradation of endocrine disrupting chemicals: Synergistic effects of bi-heterojunction and oxygen evolution

• Bi- heterojunction photocatalyst BiOBr/Bi 4 O 5 Br 2 /PDI was constructed for EDCs removal. • PDI in BiOBr/Bi 4 O 5 Br 2 enhance photocatalytic activity and broaden light-harvesting range. • Bi- heterojunction in composite facilitate H 2 O conversion to O 2 for •O 2 – generation. • BiOBr/Bi 4 O 5 Br 2 /PDI remain high removal rate of BPA under oxygen-poor condition. • Degradation pathway and influence of coexistent substance on BPA removal are revealed. To remove endocrine disrupting chemicals (EDCs), visible-light response photocatalyst BiOBr/Bi 4 O 5 Br 2 /perylene diimide (PDI) with bi-heterojunction was constructed. Under visible-light irradiation, BiOBr/Bi 4 O 5 Br 2 /PDI could degrade 90% Bisphenol A (BPA) within 75 min, while degrade 100% 17α-ethynyl estradiol (EE2) and 17β-estradiol (E2) within 15 min. Radicals quenching experiment and EPR indicated both •O 2 – and holes were the main substances for EDCs degradation, and the possible degradation pathway of EDCs are proposed based on the LC-MS analysis results. In the composite of BiOBr/Bi 4 O 5 Br 2 /PDI, the matching energy band structure between Bi 4 O 5 Br 2 and BiOBr facilitated the formation of heterojunction for strengthening the space charge separation. Meanwhile, PDI with strong photosensitivity combined with BiOBr/Bi 4 O 5 Br 2 not only enhanced visible-light photocatalytic activity but also broadened the light-harvesting range. Owning to the unique one-dimensional conjugated structure and internal electric field effect, PDI could also promote the photo-carriers transfer and separation. With the bi-heterojunction effect, photo-generated electrons were transferred to BiOBr conduction band while holes were accumulated on PDI valence band. Simultaneously, holes could oxidize water with the production of oxygen following being reduced to •O 2 – by photo-generated electrons. Even under oxygen-poor conditions, the production of •O 2 – can reach 32.7 × 10 -5 mol•g −1 •h −1 , resulting in more than 85% BPA degradation within 75 min.

Mechanism of Organic Matter Accumulation in Black Shales of the Yuertusi Formation in the Tarim Basin: Insights From Paleoenvironmental Variation During the Early Cambrian

The paleoenvironment during the Early Cambrian is closely related to the accumulation mechanism of organic matter (OM) from the Lower Cambrian black shales. However, paleoenvironment remains a controversial issue. Here, we reported a lot of detailed data of sedimentary stratigraphy and geochemistry of the Lower Cambrian Yuertusi Formation in the Aksu area, Tarim Basin. The Yuertusi Formation from the Yutixi outcrop consists mainly of silicalite at the base, two sets of black shales, and crystalline dolostone. Based on the redox conditions traced by U/Th, V/Cr, Ni/Co, and V/Sc, the hydrothermal activity traced by Ce/Ce*, Cr/Zr, U/Th, Fe/Ti, and (Fe + Mn)/Ti ratios, as well as paleo-productivity traced by Ba, Cu, Rb/Sr, and other parameters, variations were observed in the depositional environments of the Yuertusi Formation: 1) the silicalite at the base was deposited under an euxinic condition and intense hydrothermal activity. Mo-U co-variation analysis revealed that the north margin of Tarim Basin belonged to the unrestricted marine during the Early Cambrian, 2) the lower black shales were deposited under an oxygen-poor condition and weak hydrothermal activity, and 3) the upper black shales were deposited under oxygen-poor, sub-oxic conditions and almost no hydrothermal activity. Although the hydrothermal activity improved paleo-productivity, the TOC values were low on the whole, which may be due to the intense hydrothermal activity that damaged the formation of source rocks. Comprehensive studies showed a gradually oxidizing environment and weakening paleo-productivity during the Yuertusi Formation deposited. The anoxic conditions were conducive to the preservation of OM, and the high-quality source rocks represented by the black shales of the Yuertusi Formation were formed, especially the first set of black shales. However, the enrichment of OM may be affected by the intense hydrothermal activity.

Impact of carbon–carbon defects at the SiO2/4H-SiC (0001) interface: a first-principles calculation

Poor quality of the SiO2/4H-SiC (0001) interface is a long-standing issue limiting the performance of silicon carbide (SiC)-MOSFETs. However, the origin of the interface defects is still not fully understood. In this article, five types of carbon–carbon defects introduced to the SiO2/4H-SiC (0001) interface are systematically investigated by first-principle calculations. The thermal oxidation process of 4H-SiC is analyzed through the variation of the chemical potentials based on the chemical reaction equation. The trend of the formation energy with different oxidation conditions show that oxygen-poor condition is effective to suppress the formation of carbon–carbon defects. We obtain the accurate electronic structure with hybrid density functional and the distribution of electron states for the carbon–carbon defects. The interfacial carbon–carbon defects in SiC tend to cause gap states near the conduction band minimum (CBM). Furthermore, the calculated charge transition levels (CTLs) also show that the 0/− CTLs of the carbon–carbon defects are close to the CBM acting as acceptors. All the theoretical calculation results provide insight into understanding the atomic structures and electronic properties for the carbon–carbon defects at the SiO2/4H-SiC (0001) interface, which are fundamental to improve the performance of n-channel SiC-MOSFETs.

Benthic Foraminiferal Response to the Millennial-Scale Variations in Monsoon-Driven Productivity and Deep-Water Oxygenation in the Western Bay of Bengal During the Last 45 ka

In this study, we presented a high-resolution benthic foraminiferal assemblage record from the western Bay of Bengal (BoB) (off Krishna–Godavari Basin) showing millennial-scale variations during the last 45 ka. We studied temporal variations in benthic foraminiferal assemblages (relative abundances of ecologically sensitive groups/species, microhabitat categories, and morphogroups) to infer past changes in sea bottom environment and to understand how monsoon induced primary productivity-driven organic matter export flux and externally sourced deep-water masses impacted the deep-sea environment at the core site. Our records reveal a strong coupling between surface productivity and benthic environment on glacial/interglacial and millennial scale in concert with Northern Hemisphere climate events. Faunal data suggest a relatively oxic environment when the organic matter flux to the sea floor was low due to low primary production during intensified summer monsoon attributing surface water stratification and less nutrient availability in the mixed layer. Furthermore, records of oxygen-sensitive benthic taxa (low-oxygen vs. high-oxygen benthics) indicate that changes in deep-water circulation combined with the primary productivity-driven organic matter flux modulated the sea bottom oxygen condition over the last 45 ka. We suggest that the bottom water at the core site was well-ventilated during the Holocene (except for the period since 3 ka) compared with the late glacial period. At the millennial timescale, our faunal proxy records suggest relatively oxygen-poor condition at the sea floor during the intervals corresponding to the cold stadials and North Atlantic Heinrich events (H1, H2, H3, and H4) compared with the Dansgaard/Oeschger (D-O) warm interstadials. The study further reveals oxygen-poor bottom waters during the last glacial maximum (LGM, 19–22 ka) which is more pronounced during 21–22 ka. A major shift in sea bottom condition from an oxygenated bottom water during the warm Bølling–Allerød (B/A) (between 13 and 15 ka) to the oxygen-depleted condition during the cold Younger Dryas (YD) period (between 10.5 and 13 ka) is noticed. It is likely that the enhanced inflow of North Atlantic Deep Water (NADW) to BoB would have ventilated bottom waters at the core site during the Holocene, B/A event, and probably during the D-O interstadials of marine isotope stage (MIS) 3.

Efficiency and mechanism for the control of phosphorus release from sediment by the combined use of hydrous ferric oxide, calcite and zeolite as a geo-engineering tool

In this research, the efficiency and mechanism for the suppression of phosphorus (P) liberation from sediment by the combined use of hydrous ferric oxide (HFO), calcite and zeolite as a geo-engineering tool were investigated. The results showed that the coexisting calcite enhanced the adsorption of phosphate onto HFO via the release of Ca2+ from calcite and the formation of phosphate-bridged Fe-P-Ca ternary complex on the surface of HFO. HFO/calcite/zeolite mixture exhibited good phosphate sorption performance, and the maximum phosphate sorption capacity was 35.9 mg P/g. The sorption of phosphate on HCZ could be primarily due to the surface adsorption of phosphate onto calcite and zeolite, the surface adsorption of phosphate onto HFO via the mechanisms of inner-sphere Fe-phosphate complex and phosphate-bridged Fe-P-Ca complex formation, and the precipitation of calcium phosphate. HFO/calcite/zeolite combined capping and amendment both can greatly cut down on the risk of the release of sedimentary P to the overlying water under the oxygen-poor condition, and the generation of a static layer with low concentrations of soluble reactive P and diffusive gradients in thin film-labile P in sediment was vital to the control of phosphorus liberation from sediment into the overlying water by these methods. Most (approximately 70%) of the total P bound by the HFO/calcite/zeolite combined capping layer existed in the form of moderately or highly stable P, which is not easy to be re-released under the circumstances of normal pH and reducing environment. Results of this work demonstrate that the combined used of HFO, calcite and zeolite as a geo-engineering tool is a sediment remediation strategy with a great application prospect for managing the internal loading of P in surface waterbodies.

Effects of intrinsic defects and doping on SrFe12O19: A first-principles exploration of the structural, electronic and magnetic properties

Effects of intrinsic defects and doping on the M−type strontium hexagonal ferrites (SrFe12O19) has been systematically studied using density functional theory combined with the generalized gradient approximation (GGA) + U method. The energies of possible intrinsic point defects and doping in SrFe12O19 have been determined, and effects of those on the electronic and magnetic properties have been analyzed. The results show that, under an oxygen poor condition, the defects of iron vacancy are difficult to form, while the defects of oxygen vacancy are easy to form. In the case of oxygen rich condition, the defects of iron vacancy are difficult to form when Fermi energy level is close to VBM, however, when Fermi energy level approaches CBM, the defects of iron vacancy in some sublattice sites are tend to form, especially in the 2a sublattice. Meanwhile, under the oxygen rich condition, the defect formation energy of Co substituting for Fe is very low, which indicates that Fe is easy to be substituted by Co, preferentially in 4f2 sublattice, followed by 2a sublattice. The lattice constant of La-Co co-doped is smaller than that of perfect SrFe12O19, meanwhile, the total magnetic moment changes little. The density of states shows that La-Co co-doped SrFe12O19 exhibits n-type semiconductor characteristics.

Interplay between invasive single atom Pt and native oxygen vacancy in anatase TiO2(1 0 1) surface: A theoretical study

The effects of invasive single atom Pt on native oxygen vacancy migration in anatase TiO2(1 0 1) surface, as well as their coupling effect on water dissociation are investigated by means of the first-principles calculations based on density functional theory (DFT). Our results show that introducing Pt atom as Pt-ads and Pt5c modes could accelerate the Ov migration effectively. However, the roles of Pt2c and Pt6c are to decelerate the Ov migration. Oxygen-poor condition is beneficial for forming Pt-ads/TiO2-x1(1 0 1) and Pt2c/TiO2-x2(1 0 1), whereas oxygen-rich condition is beneficial for forming Pt5c/Ti1-yO2-x1(1 0 1) and Pt6c/Ti1-yO2-x1(1 0 1). Under their optimal conditions, Pt5c and Pt6c substitutions could make oxygen Ov formation easier, while Pt-adsorption and Pt2c substitution show no effect on Ov formation. The effect of imported Pt atoms mainly applies to the paths where they are involved directly. When Pt substituted either Ti5c or Ti6c, Ov tends to distribute around it and decreases its coordination number to 4 for maintaining a planar coordinated configuration of Pt-O. The adjacent bimetal Pt4c-Ti4c site in Pt6c/TiO2-x(1 0 1) can serve as the active center for water dissociation reaction. These findings can provide a “quasi-dynamic” perspective to understand the catalytic process of single atom supported on metal oxide surfaces.

Structure and energetics of carbon defects in SiC (0001)/SiO2 systems at realistic temperatures: Defects in SiC, SiO2, and at their interface

We report systematic first-principles calculations that reveal the atomic configurations, stability, and energy levels of carbon defects in SiC (0001)/SiO2 systems. We clarify the stable position (i.e., in SiC, SiO2, or at SiC/SiO2 interfaces) of defects depending on the oxidation environment (an oxygen-rich or -poor condition). At finite temperatures, the chemical potential of atomic species was corrected referring to thermochemical tables in order to obtain the temperature-dependent defect formation energies. Under an oxygen-rich condition, we found that the dicarbon antisite [(C2)Si] in SiC is one of the favorable defects at a typical oxidation temperature of 1600 K and it creates a localized level near the conduction band edge of SiC, being a critical defect for n-channel metal-oxide-semiconductor field-effect transistors (MOSFETs). A variety of carbon-dimer defects at a SiC/SiO2 interface, such as Si—CO—CO2, Si—CO—CO—Si, and Si—(CO)—CO2, are stable under the oxygen-rich condition at 1600 K, and they create localized levels relatively close to the valence band edge of SiC, thus being critical defects for p-channel MOSFETs. In the viewpoint of static energetics, our results suggest that the oxidation of SiC under a high-temperature oxygen-poor condition is effective in suppressing the generation of carbon defects.

Wastewater treatment and microbial community dynamics in a sequencing batch reactor operating under photosynthetic aeration

A sequencing batch bioreactor (SBR) treating municipal wastewater was photosynthetically aerated using microalgae cultivated in a photobioreactor (PBR). Symbiotic interactions and CO2/O2 exchange were established between activated sludge in the SBR and microalgae in the PBR through hydrophobic hollow fiber membranes. Photosynthetic aeration enhanced COD removal in the SBR from 52.2% (without external aeration) to 90.3%, whereas N-NH4+ and P-PO43- removal increased by 63.5% and 90.4%, respectively. The SBR performance under photosynthetic aeration was comparable to that under mechanical aeration. However, no nitrification was observed in the SBR, indicating oxygen limitation and poor growth condition for nitrifiers. In the PBR, there was a rapid increase in biomass concentration and it stabilized at 3.0 g/L after 22 days of operation. High nitrogen demand in the PBR indicated the steady flow of inorganic carbon from the SBR through the membranes. Prolonged oxygen limitation and massive sludge attachment on the membranes resulted in low suspended sludge concentration in the SBR. Microbial community analysis indicated gradual enrichment of facultative and strictly anaerobic microorganisms in the SBR. These results highlight the potential of microalgae in lowering the cost of wastewater aeration and underline the challenges in sustaining symbiotic gas exchange during long-term.

Tunability of p- and n-channel TiOx thin film transistors

To acquire device-quality TiOx films usually needs high-temperature growth or additional post-thermal treatment. However, both processes make it very difficult to form the p-type TiOx even under oxygen-poor growth condition. With the aid of high energy generated by high power impulse magnetron sputtering (HIPIMS), a highly stable p-type TiOx film with good quality can be achieved. In this research, by varying the oxygen flow rate, p-type γ-TiO and n-type TiO2 films were both prepared by HIPIMS. Furthermore, p- and n-type thin film transistors employing γ-TiO and TiO2 as channel layers possess the field-effect carrier mobilities of 0.2 and 0.7 cm2/Vs, while their on/off current ratios are 1.7 × 104 and 2.5 × 105, respectively. The first presented p-type γ-TiO TFT is a major breakthrough for fabricating the TiOx-based p-n combinational devices. Additionally, our work also confirms HIPIMS offers the possibility of growing both p- and n-type conductive oxides, significantly expanding the practical usage of this technique.

Oxygen adsorption on Pt-Pd nanoclusters by DFT and ab initio atomistic thermodynamics

Monometallic Pt and Pd, and bimetallic Pt-core/Pd12-shell and Pd-core/Pt12-shell were built and geometrically optimized by density functional theory calculations in consideration of oxygen adsorption. The different oxygen adsorption sites on the cluster were considered: top site on one shell atom, bridge site between two shell atoms and hollow site among three shell atoms. For each adsorption site, a few coverages were considered, both oxygen-rich condition and oxygen-poor condition. The pressure-temperature phase diagrams were determined for 13-atom Pt-Pd clusters with adsorbed oxygens in terms of ab initio atomistic thermodynamics method. The most stable phase structure and adsorption of oxygen at certain temperature and pressure were illustrated. Additionally the results showed that the cluster structures were most likely to be changed under oxygen-poor condition. This study is helpful to understand their catalyst processes in a few oxygen-involved reactions such as formic acid electro-oxidation, direct methanol fuel cell, oxygen reduction reaction in fuel cells, oxidation of sulfur dioxide and diesel oxidation and so on.

Thermal stability of 2DEG at amorphous LaAlO3/crystalline SrTiO3 heterointerfaces.

At present, the generation of heterostructures with two dimensional electron gas (2DEG) in amorphous LaAlO3 (a-LAO)/SrTiO3 (STO) has been achieved. Herein, we analysed thermal stability of 2DEG at a-LAO/STO interfaces in comparison with 2DEG at crystalline LaAlO3 (c-LAO)/STO interfaces. To create 2DEG at LAO/STO interface, regardless of growing temperature from 25 to 700 °C, we found that environment with oxygen deficient during the deposition of LAO overlayer is essentially required. That indicates that the oxygen-poor condition in the system is more essential than the crystalline nature of LAO layer. 2DEG at a-LAO/STO interface is depleted upon ex situ annealing at 300 °C under 300 Torr of oxygen pressure, while that in c-LAO/STO interface is still maintained. Our result suggests that the LAO overlayer crystallinity critically affects the thermal-annealing-induced depletion of 2DEG at a-LAO/STO interface rather than the generation of 2DEG. We clearly provide that amorphous TiOx can efficiently prevent the thermal degradation of 2DEG at the a-LAO/STO interface, which gives a cornerstone for achieving thermal-stable 2DEG at a-LAO/STO interface.

Comprehensive study on critical role of surface oxygen vacancies for 2DEG formation and annihilation in LaAlO3/SrTiO3 heterointerfaces

Here we report comprehensive study of 2DEG at a-LAO/STO interfaces in comparison with 2DEG at crystalline LaAlO3 (c-LAO)/STO interfaces. We observe that the oxygen deficient environment during the deposition of LAO overlayer is essentially required to create 2DEG at LAO/STO interface regardless of growth temperature from 25°C to 700°C, indicating that the oxygen-poor condition in the system is more important than the crystallinity of LAO layer. The critical thickness (2.6 nm) of 2DEG formation at a-LAO/STO heterostructure is thicker than (1.6 nm) that at c-LAO/STO. Upon ex-situ annealing at 300°C under 300 mTorr of oxygen pressure, 2DEG at a-LAO/STO interface is annihilated, while that in c-LAO/STO interface is still maintained. With combing these findings and scanning transmission electron microscope (STEM) analysis, we suggest that oxygen vacancies at the LAO surface is attributed to the origin of 2DEG formation at the LAO/STO and the crystallinity of the LAO overlayer plays a critical role in the annihilation of 2DEG at a-LAO/STO interface rather than in the formation of 2DEG. This work provides a framework to understand the importance of prohibiting the LAO surface from being oxidized for achieving thermally stable 2DEG at a-LAO/STO interface.

Synergistic Resistive Switching Mechanism of Oxygen Vacancies and Metal Interstitials in Ta2O5

Ta2O5 is extensively studied as a data-storage material for resistance random access memory (RRAM). The resistive switching (RS) in Ta2O5-based RRAM is generally believed to be due to the diffusion of oxygen vacancy (Vo) inside the oxide, while the role of metal interstitials is paid less attention. Here, on the basis of first-principles calculations, we show that the role of interstitial Ta (Tai) is competitive under the oxygen-poor condition and should also contribute to RS in Ta2O5. This is obvious by our calculated comparable energy barriers for the diffusion of Vo and Tai, which are 3.5 and 3.7 eV, respectively. Furthermore, the presence of electric field in working devices will enhance the migration of Tai due to its higher charge states compared to Vo. Meanwhile, Tai will introduce more defect states closer to the conduction bands and, thus, is more effective on tuning the electronic structure of Ta2O5. The present work unravels the contribution of Ta cations in RS of tantalum oxide-based RRAM, pre...

Insights Into Oxygen Adsorption on Ag-Cu Nanocluster as an Oxidation Catalyst by Ab Initio Atomistic Thermodynamics.

To understand the catalytic activity of Ag-Cu cluster as catalyst of ethylene oxidation, bare Ag and Cu and Ag core/Cu12 shell and Cu core/Ag12 shell are built and optimized by density functional theory calculations incorporating the effect of the atmospheric environment. Consequently different oxygen adsorption sites on the cluster are considered: top site of the shell atoms, bridge site of the shell atoms and hollow site of the shell atoms. For each adsorption site, a few coverages are considered, both oxygen-rich condition and oxygen-poor condition. The phase diagrams of oxygen and 13-atom Ag-Cu clusters are finally determined. The pressure-temperature phase diagrams are determined for the oxygen and 13-atom Ag-Cu clusters, which reveal important insights into this system and its catalytic properties. The phase diagrams of oxygen and 13-atom Ag-Cu clusters are finally determined. The results predict the most stable phase: structure and adsorption of oxygen at certain temperature and certain pressure. Under oxygen-poor condition, the cluster structures are most likely to be changed. This information must be helpful to understand the catalytic process of Ag-Cu cluster as catalyst of ethylene oxidation.