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Although tin monoxide (SnO) is an interesting compound due to its p-type conductivity, a widespread application of SnO has been limited by its narrow band gap of 0.7 eV. In this work, we theoretically investigate the structural and electronic properties of several SnO phases under high pressures through employing van der Waals (vdW) functionals. Our calculations reveal that a metastable SnO (β-SnO), which possesses space group P21/c and a wide band gap of 1.9 eV, is more stable than α-SnO at pressures higher than 80 GPa. Moreover, a stable (space group P2/c) and a metastable (space group Pnma) phases of SnO appear at pressures higher than 120 GPa. Energy and topological analyses show that P2/c-SnO has a high possibility to directly transform to β-SnO at around 120 GPa. Our work also reveals that β-SnO is a necessary intermediate state between high-pressure phase Pnma-SnO and low-pressure phase α-SnO for the phase transition path Pnma-SnO →β-SnO → α-SnO. Two phase transition analyses indicate that there is a high possibility to synthesize β-SnO under high-pressure conditions and have it remain stable under normal pressure. Finally, our study reveals that the conductive property of β-SnO can be engineered in a low-pressure range (0–9 GPa) through a semiconductor-to-metal transition, while maintaining transparency in the visible light range.

Iron(III) titanates are composed of earth-abundant elements and are attracting rapidly growing interest as highly promising candidates for solar-energy as well as optoelectronics applications. In this article, we report on the successful synthesis of pseudobrookite Fe2TiO5 thin films with RF sputtering followed by post-deposition annealing in air. The chemical composition, crystal structure, surface morphology, and optical properties of the films were characterized both experimentally and theoretically. The film was confirmed to be single phase and exhibited a highly crystalline orthorhombic structure with a preferential crystal orientation of (131) with excellent adhesion on both glass and silicon substrates, indicating that an epitaxial substrate is not required. Dielectric functions from spectroscopic ellipsometry and density functional theory (DFT) also provide a good understanding of the optical characteristics of the films.

Although tin monoxide (SnO) is an interesting compound due to its p-type conductivity, a widespread application of SnO has been limited by its narrow band gap of 0.7 eV. In this work, we theoretically investigate the structural and electronic properties of several SnO phases under high pressure through employing van der Waals (vdW) functionals. Our calculations reveal that a metastable SnO (A-SnO), which possesses space group P21/c and a wide band gap of 1.9 eV, is more stable than l-SnO at pressures higher than 80 GPa. Moreover, a stable (space group P2/c) and a metastable (space group Pnma) phases of SnO appear at pressures higher than 120 GPa. Energy and topological analyses show that P2/c-SnO has a high possibility to directly transform to t-SnO at around 120 GPa. Our work also reveals that h-SnO is a necessary intermediate state between high-pressure phase Pnma-SnO and low-pressure phase o-SnO for the phase transition path Pnma-SnO ®u-SnO ® g-SnO. Two phase transition analyses indicate that there is a high possibility to synthesize h-SnO under high-pressure conditions and have it remain stable under normal pressure. Finally, our study reveals that the conductive property of -SnO can be engineered in a low-pressure range (0-9 GPa) through a semiconductor-to-metal transition, while maintaining transparency in the visible light range.

The existence of a topological Dirac nodal loop is predicted for hydrogenated monolayer boron sheets with a nonsymmorphic symmetry. The three-center two-electron bonds in boron compounds restrict the electronic system to be insulating or semimetallic with a Dirac nodal loop. Two types of electronic structures are distinguished by the Z_2 topological index and confirmed by first-principles total-energy calculations. The topological taxonomy, developed in this research, can be applied to other two-dimensional materials and to seek novel Dirac fermions.

The effects of Sr substitution in BaSi2, a promising photoabsorber for thin-film solar cells, were examined computationally and experimentally. The calculations showed that Ba0.5Sr0.5Si2 with Sr atoms at the A1 site (A1-Sr-BSS) is more stable than that with Sr atoms at the A2 site (A2-Sr-BSS). Both materials are indirect bandgap semiconductors. The indirect and direct bandgaps of A1-Sr-BSS are larger than those of BaSi2, while those of A2-Sr-BSS are smaller. The indirect bandgap of A1-Sr-BSS is 0.01 eV higher than that of BaSi2, while that of A2-Sr-BSS is 0.06 eV lower. This trend is attributable to the difference in the electronic properties around the conduction band minimum, which is determined by the type of atoms (Ba or Sr) that occupy the A1 site. Single-crystal X-ray diffraction and diffuse reflectance measurements of Ba1-xSrxSi2 (0.0 ≤ x ≤ 0.8) revealed that the Ba atoms at the crystallographic site A1 are preferentially substituted by Sr atoms and that the bandgap, Eg, decreases with x (1.24 eV at x = 0 and 1.15 eV at x = 0.65). The preferential occupation of the A1 site by Sr atoms is consistent with the computational result that A1-Sr-BSS is more stable than A2-Sr-BSS. However, the experimentally observed decrease in Eg with x is closer to the decrease of −0.06 eV in the indirect bandgap of A2-Sr-BSS than to the 0.01 eV increase in the case of A1-Sr-BSS. The reason for this discrepancy is discussed.

Realization of efficient p-type conductivity in wide gap oxides is a challenging task partly due to the localized nature of non-bonding oxygen 2p states of which the valence band maximum consists. In this study, effects of anisotropic strain on an accepter level of Al-doped anatase TiO2 are investigated using LDA+U calculations. Strain engineering effectively increases the cation states in the valence band maximum of TiO2. It is demonstrated that a deep acceptor level induced by substitutional Al for Ti is turned into a delocalized shallow level under a tensile strain of as much as 8%. This effect is confirmed by the analysis of thermodynamic transition level which is largely shifted from 0.8 eV above to below the valence band maximum, being a shallow acceptor, as the tensile strain is increased.

Band-gap engineering of oxide materials is of great interest for optoelectronics, photovoltaics, and photocatalysis applications. In this study, electronic structures of perovskite oxynitrides, LaTiO2 N and SrNbO2 N, and solid solutions, (SrTiO3 )1-x (LaTiO2 N)x and (SrTiO3 )1-x (SrNbO2 N)x , are investigated using hybrid density functional calculations. Band gaps of LaTiO2 N and SrNbO2 N are much smaller than that of SrTiO3 owing to the formation of a N 2p band, which is higher in energy than the O 2p band. The valence- and conduction-band offsets of SrTiO3 /LaTiO2 N and SrTiO3 /SrNbO2 N are computed, and the adequacy for H2 evolution is analyzed by comparing the positions of the band edges with respect to the standard hydrogen electrode (SHE). The band gap of (SrTiO3 )1-x (LaTiO2 N)x and (SrTiO3 )1-x (SrNbO2 N)x solid solutions are also discussed.

We studied the formation of Ba1−xSrxZrO3 (0 ≤ x ≤ 1) nanoparticles under highly super-saturated conditions, using supercritical water. It is known that B-site Zr in the perovskite structure plays a dominant role at the nucleation stage, with high nucleation rates under supercritical conditions; this is due to the significantly lower solubility of Zr, compared with A-site ions (i.e., Zr precipitates faster, and A-site ions are taken up into particle after the Zr nucleation). However, in this study, it was found that A-site Ba and Sr significantly influenced the particle size, the A-site deficiency rate, and the surface-OH density of the nanoparticles. The differences in particle size suggested that the ripening or coalescence that occurred after the nucleation stage was dominant in determining the particle size, even under highly super-saturated conditions such as those realized in the supercritical hydrothermal synthesis. The characteristic nanostructure formed in the supercritical water was analyzed in detail; variables such as the A-site deficiency rate and the surface-OH density were investigated. The existence of vacancies at the A-site was confirmed using X-ray absorption fine structure, and a highly defective structure was obtained, particularly when the Ba content was high. The surface state of the nanoparticles was also studied using X-ray photoelectron spectroscopy and first-principles calculations, with the aim of understanding the differences in particle size, and the effects of the A-site deficiencies; the amount of surface-OH corresponded to the A-site deficiency rate, and had an inverse relationship with the particle size.

The main aim of this study is to scrutinize promising plasmonic materials by understanding their electronic structure and correlating them to the optical properties of selected refractory materials. For this purpose, the electronic and optical properties of the conductive ceramics TiC, ZrC, HfC, TaC, WC, TiN, ZrN, HfN, TaN, and WN are studied systematically by means of first-principles density functional theory. A full ab initio procedure to calculate plasma frequency from the electronic band structure is discussed. The dielectric functions are calculated by including electronic interband and intraband transitions. Our calculations confirm that transition metal nitrides, such as TiN, ZrN, and HfN, are the strongest candidates, exhibiting performance comparable to that of conventional noble metals in the visible to the near-infrared regions. On the other hand, carbides are not suitable for plasmonic applications because they show very large losses in the same regions. From our calculated dielectric functions, the scattering and absorption efficiencies of nanoparticles made of these refractory materials are evaluated. It is revealed that TiN and TaC are the best candidate materials for applications in photothermal energy conversion over a broad spectral region. Furthermore, quality factors for localized surface plasmon resonance and surface plasmon polaritons are calculated to compare quantitative performances, and ZrN and HfN are found to be comparable to conventional plasmonic metals such as silver and gold, Comment: 9 pages, 7 figures and 1 table

A series of Sn(Nb 1− x Ta x ) 2 O 6 solid solutions were successfully synthesized as novel visible-light-active photocatalysts. The characterizations of powder X-ray diffraction and UV–vis absorption spectrum indicate that these solid solutions possess continuous changes in the crystallography features and optical band gaps, and are therefore a group of continuous solid solutions. Their photocatalytic properties were evaluated via methylene blue (MB) photodegradation. The Sn(Nb 0.8 Ta 0.2 ) 2 O 6 sample showed the best photocatalytic performance, full mineralization ability, and an excellent stability. After 120 min of irradiation, the degradation and mineralization percentages of MB over Sn(Nb 0.8 Ta 0.2 ) 2 O 6 reached 99.5% and 98.9%, respectively; in addition, the degradation ratio remained at 97% after five-cyclic test. The further study on the active species and the theoretical calculation based on density functional theory helped to clarify the mechanism of the photodegradation reaction. On this basis, we propose the strategies to design high-performance photocatalysts employed in hole-dominated and electron-dominated photocatalytic applications, respectively.

SrGe 2 and BaGe 2 were characterized for structural, electronic and optical properties by means of diffuse reflectance and first-principles density functional theory. These two germanides crystallize in the BaSi 2 -type structure, in which Ge atoms are arranged in tetrahedral configuration. The calculation indicates a charge transfer from Sr (or Ba) atoms to Ge atoms along with the formation of covalent bonds among Ge atoms in Ge tetrahedral. The computational results confirm that these two germanies are Zintl phase described as Sr 2 Ge 4 (or Ba 2 Ge 4 ), which are characterized by positively charged [Sr 2 (or Ba 2 )] 2.59+ and negatively charged [Ge 4 ] 2.59− units acting as cation and anion, respectively. These compounds are indirect gap semiconductors with band gap estimated to be E g = 1.02 eV for BaGe 2 and E g = 0.89 eV for SrGe 2 which are in good agreement with our experimental measured values ( E g = 0.97 eV for BaGe 2 and E g = 0.82 eV for SrGe 2 ). Our calculations demonstrate that the band gaps are narrowed by application of hydrostatic pressure; the pressure coefficients are estimated to be −10.54 for SrGe 2 and −10.06 meV/GPa for BaGe 2 . Optical properties reveal that these compounds have large absorption coefficient (∼7.5 × 10 4 cm −1 at 1.5 eV) and the estimated high frequency (static) dielectric constant are, e ∞ ( e 0 ) ≈ 12.8(20.97) for BaGe 2 and e ∞ ( e 0 ) ≈ 14.27(22.87) for SrGe 2 .

Air pollution by automobile exhaust is becoming a global challenge; it is not limited to developing countries where traffic volumes are rapidly increasing.[1] Small vehicles, such as motorcycles and scooters with an engine displacement of 100 mL or less,[2] are one of the major causes for such air pollution; only a small percentage of these vehicles is equipped with exhaust purifiers.[3] Current exhaust purifiers comprise catalysts based on platinum-group metals (PGMs), such as Pt, Pd, and Rh.[4–7] The supply of PGMs may not be reliable for meeting the increasing demands for exhaust purification since deposits of PGMs in the Earth's crust are limited and localized.[8] Furthermore, PGM-based catalysts are not suited to the low-temperature exhaust of small vehicles (

A visible-light-sensitive tin sulfide photocatalyst was designed based on a ubiquitous element strategy and density functional theory (DFT) calculations. Computational analysis suggested that tin monosulfide (SnS) would be more efficient than SnS2 as a photocathode for hydrogen production because of the low ionization potential and weak ionic character of SnS. To test this experimentally, nanoparticles of SnS were loaded onto a mesoporous electrode using a wet chemical method, and the bandgap of the synthesized SnS quantum dots was found to be tunable by adjusting the number of successive ionic layer adsorption and reaction (SILAR) cycles, which controls the magnitude of the quantum confinement effect. Efficient hydrogen production was achieved when the bandgap of SnS was wider than that of the bulk form.

Colorful rutile TiO2 was prepared by heating Ti2O3 at 550–900 °C to develop novel visible-light-sensitive and eco-friendly photocatalysts for environmental remediation under visible-light irradiation. The colors of the prepared samples, which ranged from grayish green to yellowish off-white via yellow differed from the reported colors of reduced TiO2, such as blue and black. The TiO2 prepared in this study was characterized by X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and UV–Visible spectroscopy. These measurements showed that the TiO2 contained Ti3+-interstitial sites. The TiO2 was sensitive to visible light, and calculation of the band diagram demonstrated that this visible-light absorption is caused mainly by formation of Ti3+-interstitial sites in rutile TiO2. Among the prepared samples, the TiO2 prepared by heating Ti2O3 at 700 °C shows the highest photocatalytic activity under visible-light irradiation. In addition, the sample was further and mildly ground using a bead-milling machine. The ground sample possessed higher surface area and better photocatalytic activity.

Density functional calculations are performed for lanthanum-oxide clusters in order to study the local dielectric properties of such clusters using the dielectric constant defined at local points. An increase in coordination number brings about an increase in electron population on the central lanthanum atom, leading to an increase in the local dielectric constant., Please also see, errata, Ph.D. thesis, Pawel Szarek 京都大学学術情報リポジトリ(京都大学図書館機構) [Szarek, Pawel, Theoretical Study of Electronic States of Chemical Bonds.(化学結合の電子状態に関する理論的研究.) http://hdl.handle.net/2433/66212 ]

The oxygen evolution reaction that occurs during water oxidation is of considerable importance as an essential energy conversion reaction for rechargeable metal–air batteries and direct solar water splitting. Cost-efficient ABO3 perovskites have been studied extensively because of their high activity for the oxygen evolution reaction; however, they lack stability, and an effective solution to this problem has not yet been demonstrated. Here we report that the Fe4+-based quadruple perovskite CaCu3Fe4O12 has high activity, which is comparable to or exceeding those of state-of-the-art catalysts such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ and the gold standard RuO2. The covalent bonding network incorporating multiple Cu2+ and Fe4+ transition metal ions significantly enhances the structural stability of CaCu3Fe4O12, which is key to achieving highly active long-life catalysts., Perovskites are cost-effective and efficient catalysts for electrocatalytic oxygen evolution, however often lack stability. Here, the authors report a tetravalent iron based quadruple perovskite which is both active and stable, due to the covalent bonding network incorporating many of its iron and copper ions.

Perovskite strontium stannate SrSnO3is a promising photocatalyst. However, its band gap is too large for efficient solar energy conversion. In order to sensitize SrSnO3toward visible-light activities, the effects of doping with various selected cations and anions are investigated by using hybrid density functional calculations. Results show that doping can result in dopant level to conduction band transitions which lie lower in energy compared to the original band gap transition. Therefore, it is expected that doping SrSnO3can induce visible-light absorption.

First-principles calculations were used to compute the dielectric permittivities of hypothetical [La,Sc]2O3 solid solutions in the cubic (bixbyite) and hexagonal La2O3 phases. Dielectric enhancement is predicted at small Sc concentrations due to the rattling ion effect. Similar calculations for a model amorphous La2O3 structure show little change in permittivity when a small amount of Sc is substituted for La. In this case, the local environment around the Sc changes in a way that compensates for the rattling ion effect., 4 pages, 3 figures

Thin films of SrTiO3 (STO) and Rh-doped SrTiO3 (Rh-STO) were synthesized by sol-gel method and loaded with Ag nanoparticles. Pristine STO films exhibited anodic photocurrent while Rh-STO exhibited cathodic photocurrent. An enhancement in the overall cathodic photocurrent is observed with Ag nanoparticle loading and an additional enhancement in the visible light range is seen from the incident photon-to-current efficiency spectrum due to synergetic effect of Rh doping and Ag loading in STO.

A series of magnesium vanadates (MgV 2O6, Mg2V 2O7, and Mg3V 2O8) were synthesized to investigate the effect of cation concentration on photocatalytic performance. The samples were characterized by X-ray diffraction, field emission-scanning electron microscopy, UV-visible diffuse reflectance spectroscopy, and fluorescence spectroscopy. The photocatalytic O2 evolution experiments under visible light irradiation showed Mg2V 2O7 exhibits the best performance, while Mg3V 2O8 has the lowest activity. The density functional theory calculations indicated that the lowest unoccupied states of Mg3V 2O8 are the mostly localized by the cation layers. The fluorescence spectra and fluorescence decay curves gave evident performances of excited states of magnesium vanadates and pointed out MgV 2O6 has a very short excited electron lift-time. Mg2V 2O7 performs high photocatalytic activity because of its high electron mobility and long electron life-time.

We have achieved a remarkably wide range (~1.2 V) of differences in flatband voltage (Vfb) by using a combination of metal alloy (Pt-W)/La2O3 gate stacks. The controllable range is about 3 times greater than that of Hf-based gate stacks. The wide range of Vfb can be maintained even after annealing in forming gas and oxidizing gas ambients. We consider that this is attributed to charge transfer at the Pt- La2O3 interfaces inducing increase in the effective work functions, especially at high Pt composition ratios. We believe that La2O3 is one of the most promising high-k dielectric materials for future CMOS devices in 32nm-node and beyond.

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SrTiO3 is a promising photocatalyst for the production of hydrogen from water splitting under solar light. Cr doping is an effective treatment for adjusting its absorption edge to the visible-light range, although the performance of Cr-doped SrTiO3 is strongly affected by the oxidation number of the Cr ions. In this study, we theoretically predict that elevating the Fermi level, i.e., n-type carrier doping in SrTiO3, can stabilize the desirable oxidation number of chromium (Cr3+), contributing to a higher activity for H2 evolution. Our computational results, based on hybrid density-functional calculations, reveal that such an n-type condition is realized by substituting group-V metals (Ta, Sb, and Nb), group-III metals (La and Y), and fluorine atoms for the Ti, Sr, and O sites in SrTiO3, respectively. From our systematic study of the capability of each dopant, we conclude that La is the most effective donor for stabilizing Cr3+. This prediction is successfully evidenced by experiments showing that the La and Cr codoped SrTiO3 dramatically increases the amount of H2 gas evolved from water under visible-light irradiation, which demonstrates that our guiding principle based on Fermi level tuning by the codoping scheme is valid for the design of advanced photocatalysts.

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Realization of an abrupt HfO2/Si interface without unintentional oxidation of the silicon substrate is a crucial task for the development of modern field-effect transistors. Here, we present a theoretical model which suggests that deposition of a dopant material on the HfO2 layer turns it into an oxygen absorber, suppressing the formation of SiOx at the interface. Tantalum is predicted as an effective dopant in HfO2 for this purpose.

The solubility of silicon in high-permittivity (high-κ) oxides significantly affects the performance of field-effect transistors. Our comparative study of silicon impurities in La2O3 and HfO2 reveals that the stability of silicon at substitutional sites strongly depends on its coordination number. When substituted for lanthanum, a silicon atom fits comfortably in La2O3, thanks to the formation of a SiO4 tetrahedral structure. In addition, the substitutional silicon acts as a donor impurity in La2O3, increasing oxygen content in the oxide. This contributes to absorbing silicon and oxygen from the interface region, leading to the formation of lanthanum silicate at the La2O3/silicon interface.

application/pdf, Charged defects in a gate insulating oxide significantly degrade electric properties of the field-effect transistors. We report on our analysis of the effects of Mg incorporation into HfO2 upon reduction in the positive charges associated with oxygen vacancies VO+2. Our comprehensive study using first-principles calculations revealed that a Mg atom substituted for Hf is stable in charge negative MgHf-2 and strongly binds with VO+2, neutralizing the defect. This contributes to the suppressing of the electron traps at the defect site, improving the reliability of Hf-based gate oxides.

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