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SignificanceSemiconductor interfaces are among the most important in use in modern technology. The properties they exhibit can either enable or disable the characteristics of the materials they connect for functional performance. While much is known about important junctions involving conventional semiconductors such as Si and GaAs, there are several unsolved mysteries surrounding interfaces between oxide semiconductors. Here we resolve a long-standing issue concerning the measurement of anomalously low dielectric constants in SrTiO

Significance Semiconductor interfaces are among the most important in use in modern technology. The properties they exhibit can either enable or disable the characteristics of the materials they connect for functional performance. While much is known about important junctions involving conventional semiconductors such as Si and GaAs, there are several unsolved mysteries surrounding interfaces between oxide semiconductors. Here we resolve a long-standing issue concerning the measurement of anomalously low dielectric constants in SrTiO 3 films with record high electron mobilities. We show that the junction between doped and undoped SrTiO 3 required to make dielectric constant measurements masks the dielectric properties of the undoped film. Through modeling, we extract the latter and show that it is much higher than previously measured.

The study of subtle effects on transport in semiconductors requires high-quality epitaxial structures with low defect density. Using hybrid molecular beam epitaxy (MBE), SrTiO 3 films with a low-temperature mobility exceeding 42,000 cm 2 V −1 s −1 at a low carrier density of 3 × 10 17 cm −3 were achieved. A sudden and sharp decrease in residual resistivity accompanied by an enhancement in the superconducting transition temperature were observed across the second Lifshitz transition where the third band becomes occupied, revealing dominant intraband scattering. These films further revealed an anomalous behavior in the Hall carrier density as a consequence of the antiferrodistortive (AFD) transition and the temperature dependence of the Hall scattering factor. Using hybrid MBE growth, phenomenological modeling, temperature-dependent transport measurements, and scanning superconducting quantum interference device imaging, we provide critical insights into the important role of inter- versus intraband scattering and of AFD domain walls on normal-state and superconducting properties of SrTiO 3 .

Studies on the hydrogen incorporated M1 phase of VO2 film have been widely reported. However, there are few works on an M2 phase of VO2. Recently, the M2 phase in VO2 has received considerable attention due to the possibility of realizing a Mott transition field-effect transistor. By varying the postannealing environment, systematic variations of the M2 phase in (020)-oriented VO2 films grown on Al2O3(0001) were observed. The M2 phase converted to the metallic M1 phase at first and then to the metallic rutile phase after hydrogen annealing (i.e., for H2/N2 mixture and H2 environments). From the diffraction and spectroscopy measurements, the transition is attributed to suppressed electron interactions, not structural modification caused by hydrogen incorporation. Our results suggest the understanding of the phase transition process of the M2 phase by hydrogen incorporation and the possibility of realization of the M2 phased-based Mott transition field-effect transistor.

We examined the magnetic, structural, and chemical properties of CoFe2O4 films deposited on Al2O3 substrates using RF-magnetron sputtering under Ar and Ar + O2 environments. Perpendicular magnetic anisotropy with large magnetization was observed in the case of the film deposited under the Ar + O2 environment. However, no significant anisotropy appeared in the film deposited under the Ar environment. Interestingly, depth-dependent X-ray photoelectron spectra showed nearly identical stoichiometric ratios of Co/Fe and oxygen vacancies regardless of the deposition environment. Structurally, high-quality crystallinity with in-plane compressive strain, and lower surface and interfacial roughness were induced in the film deposited under the Ar + O2 environment. Cross-sectional transmission electron microscopy and X-ray reflectivity measurements showed that oxygen not only suppressed the deposition rate but also increased the electron density of the film, resulting in better crystallinity. Hence, the presence of oxygen during deposition should be considered as one of the essential parameters for improving the structural and magnetic properties of ferrite films.

A clear experimental explanation of the contribution of Mott and Peierls transitions to the insulator–metal transition (IMT) characteristics in vanadium dioxide (VO2) is still lacking. Examining th...

Theoretically, the edges of a MoS2 flake and S-vacancy within the lattice have nearly zero Gibbs free energy for hydrogen adsorption, which is essentially correlated to the exchange currents in hydrogen evolution reaction (HER). However, MoS2 possesses insufficient active sites (edges and S-vacancies) in pristine form. Interestingly, active sites can be effectively engineered within the continuous MoS2 sheets by treating it with plasma in a controlled manner. Here, we employed N2 plasma on a large-area continuous-monolayer MoS2 synthesized via metal–organic chemical vapor deposition to acquire maximum active sites that are indeed required for an efficient HER performance. The MoS2 samples with maximum active sites were acquired by optimizing the plasma exposure time. The newly induced edges and S-vacancies were directly verified by high-resolution transmission electron microscopy. The 20 min treated MoS2 sample showed maximum active sites and thereby maximum HER activity, onset overpotential of ∼−210 mV vs reversible hydrogen electrode (RHE), and Tafel slope of ∼89 mV/dec. Clearly, the above results show that this approach can be employed for improving the HER efficiency of large-scale MoS2-based electrocatalysts.

Studies on the hydrogen incorporated M1 phase of VO

We investigate the surface electronic structure of SrTiO3 (STO) films grown by a hybrid molecular beam epitaxy that are both stoichiometric and nonstoichiometric by means of x-ray photoelectron spectroscopy and electron energy loss spectroscopy. Increasing the fraction of the surface that is terminated with an SrO layer is correlated with a decrease in the chemical potential whereby the valence band maximum moves closer to the Fermi level, but without a significant change in the bandgap. Inasmuch as SrO-terminated STO (001) has previously been shown to act as an electron scavenger in which carriers from the bulk are trapped, we argue that the high fraction of SrO in the terminal layer is what lowers the chemical potential in Sr-rich STO. Our experimental results provide important insights into various physical phenomena that can occur on STO (001) surfaces and their effect on bulk electronic properties.

The asymmetric response of the cation distribution on [0 0 1] oriented CoFe2O4 films were examined. Depending on the deposition temperature and substrates (MgO(0 0 1) or SrTiO3(0 0 1)), the films exhibited a range of compressive or tensile strain states along the out-of-plane. With increases in deposition temperature, the initial compressive (tensile) strain for the films grown on MgO(0 0 1) (SrTiO3(0 0 1)) was reduced. Furthermore, the out-of-plane (in-plane) magnetic anisotropy of the film grown on MgO(0 0 1) (SrTiO3(0 0 1)) decreased with increasing deposition temperature. The uniaxial magnetic anisotropy energy calculation based on the magnetic anisotropy (crystalline and shape) and strain of films confirmed that the strain reduction with increasing deposition temperature is the main source of the uniaxial magnetic anisotropy reduction, regardless of the sign of the strain. X-ray photoelectron spectroscopy explained the saturation magnetization variation by the cation distribution depending on the strain state. Furthermore, the calculated magnetic moment based on the cation distribution were well matched for the high crystalline films.

In this study, the surface-chemistry-dependent hydrophobicity and antibacterial properties of ITO films were examined. After annealing of the films, their surface root-mean-square roughness increased and remained the same regardless of the annealing environment. However, depending on the annealing environment (O 2 , N 2 , Ar, or vacuum), the contact angle increased with increasing Sn 2+ contents in the films. Furthermore, the antibacterial effect of the annealed films decreased regardless of the annealing environment. The depth-dependent chemical state determined by X-ray photoelectron spectroscopy showed an increase in the Sn content at the surfaces of all films. In particular, the as-grown film exhibited the highest Sn content at the surface and also the strongest antibacterial effect.

A seemingly simple oxide with a rutile structure, RuO2, has been shown to possess several intriguing properties ranging from strain-stabilized superconductivity to a strong catalytic activity. Much interest has arisen surrounding the controlled synthesis of RuO2 films, but unfortunately, utilizing atomically controlled deposition techniques, such as molecular beam epitaxy (MBE), has been difficult due to the ultra-low vapor pressure and low oxidation potential of Ru. Here, we demonstrate the growth of epitaxial, single crystalline RuO2 films on different substrate orientations using the novel solid-source metal–organic (MO) MBE. This approach circumvents these issues by supplying Ru using a “pre-oxidized” solid MO precursor containing Ru. High-quality epitaxial RuO2 films with a bulk-like room-temperature resistivity of 55 μΩ cm were obtained at a substrate temperature as low as 300 °C. By combining x-ray diffraction, transmission electron microscopy, and electrical measurements, we discuss the effect of substrate temperature, orientation, film thickness, and strain on the structure and electrical properties of these films. Our results illustrating the use of a novel solid-source metal–organic MBE approach pave the way to the atomic-layer controlled synthesis of complex oxides of “stubborn” metals, which are not only difficult to evaporate but also hard to oxidize.

Pd takes various chemical forms according to the types of catalysts and even its pristine chemical state is often subject to change in operation. To understand the mechanism how Pd-based catalysts activate reactions, it is important to have a capability to identify the surface chemical state of Pd-based catalysts in contact with reactants. In this study, we conducted in situ near ambient pressure-x-ray photoelectron spectroscopy (NAP-XPS) in gas conditions with water and oxygen for hydrous Pd oxide, which is also known as palladium hydroxide (Pd(OH)2) and it is the key component of the Pearlman’s catalyst widely used for organic reactions. We found that hydrous Pd oxide showed different spectral features to that of anhydrous Pd oxide under gaseous water environments. We believe that this NAP-XPS study provides new information for a better identification of hydrous Pd oxide under reaction conditions, which is not available from conventional XPS in ultrahigh vacuum.

Na-doping concentration dependent electrical and structural properties of NiO ceramics were investigated. Various samples of NaxNi1−xO (x = 0.00, 0.01, 0.03, 0.05, 0.07, 0.09, 0.11) were prepared by using conventional solid state reaction. Most of the samples showed the decreased electrical resistivity with the increase of the Na-doping concentration while maintaining phase-pure face-centered cubic (fcc) structure, except in the case of the sample containing 11% of Na. For the sample containing 11% of Na, the electrical resistivity slightly increased due to the presence of secondary phase. The possible physical origin of this reduced electrical resistivity is related to the increased number of oxygen vacancies, with the increase of the Na-doping concentration for phase-pure NaxNi1−xO ceramics.

Amorphous silicon-zinc-tin-oxide (a-SZTO) has been fabricated by radio frequency (RF) magnetron sputtering at room temperature. Direct investigation on the change of the band gap of a-SZTO depending on Si contents was performed simply by combining the results of x-ray photoelectron spectroscopy (XPS), Kelvin probe microscopy, and UV-VIS optical spectrometer. The chemical properties, such as metal cation binding energy and oxygen vacancies related O 1s peak were measured by using XPS. We report on the electrical performance and the thermal stability of SZTO TFTs related with energy band gap analysis depending on Si contents. The activation energy and the density of states were calculated by applying temperature stress (TS). As Si contents increased, we have observed E C -E F was increased. The relative position of the E C shifts far from the E F because the shallow and/or deep states, such as ionized oxygen vacancies were decreased. It was confirmed that sample 3 with high Si content had the best stability (ΔV TH of 0.21 V) on thermal stress. This investigation on the variation of energy band gap and the stability of a-SZTO has represented possible step towards not only the control of band gap but also controllable achievement of electrical property and stability using optimized Si contents.

Physical property variation in dopant-free ZnO films was investigated. Film annealing under various environments (O 2 , in-Air, N 2 and vacuum) resulted in better crystallinity than in the as-grown film. In particular, the film annealed under the N 2 environment showed better crystallinity and electrical properties than films annealed in other environments. Based on spectroscopic analysis, we found a correlation between physical (structural, electrical) and chemical properties: The crystallinity of ZnO films is closely related to Zn O bonding, whereas carrier concentration is associated with V O (oxygen vacancy).

The structural, electrical, and chemical properties of Sn-doped Fe2O3 powders were investigated. Various quantities of Sn-doped Fe2O3 powders were synthesized using solid-state reactions. Rietveld analysis for the powders that were doped below 2% revealed a phase-pure Sn-doped Fe2O3 structure (i.e., identical to Fe2O3 structure). Alternatively, the analysis for the powders that were doped more than 3% exhibited secondary phase. The unit cell volume and electrical conductivity of the phase-pure samples increased with an increase in the doping concentration. X-ray photoelectron spectroscopy measurements showed an increased Fe2+ state with the increase in Sn doping concentration. Therefore, the improved electrical conductivity and unit cell volume with the increase in doping concentration of the phase-pure powders might be related to the increased Fe2+ state.

This study examined the microscopic aspects of macroscopic physical property variations of hydrogen annealed VO2 films, deposited on Al2O3(0001) substrates by RF magnetron sputtering. The temperature-dependent electrical resistivity showed that the as-grown film exhibited a metal-insulator-transition (MIT) at 55.20 °C and 49.26 °C during heating and cooling, respectively. On the other hand, no MIT was observed for the film annealed under a hydrogen environment. Spectroscopic measurements during the in-situ hydrogenation process showed that hydrogen annealing (∼0.3 mbar, up to 300 °C) promoted the V3+ state above 100 °C. Raman spectroscopy and X-ray diffraction confirmed that the as-grown film changed from a monoclinic to rutile structure during hydrogen annealing. In addition, the shift of the (020) diffraction peak position of the hydrogen-annealed film to a lower angle compare to that of the known rutile VO2 film was attributed to the expansion of the unit cell. In addition, local structure analysis showed that an increase in octahedral symmetry after hydrogen annealing is one of the main explanations for the metallic characteristics of the hydrogen-annealed film.

Amorphous CoFe 2 O 4 films with different thicknesses were deposited on MgO(001) substrates to examine the correlation between the oxygen vacancy and magnetic properties of the films. As the film thickness increased, the films became packed and smooth with well-defined interfaces. The magnetic hysteresis loop on the out-of-plane and in-plane showed an increased saturation magnetization and decreased coercivity when the film thickness was increased. Furthermore, the number of oxygen vacancies decreased with increasing films thickness. Those observations suggest that the presence of the correlation between the magnetic property variations and number of oxygen vacancies.

A series of transition metal Ni x Co 1−x (N(CN) 2 ) 2 dicyanamide molecular magnet, 0 ≤ x ≤ 1, was chemically synthesized. Structure and magnetic properties were investigated by IR spectroscopy, neutron powder diffraction and temperature-dependent magnetization measurement. The neutron powder diffraction measurement established that all samples showed the rutile structure with Pnnm space group. The detailed Rietveld refinement results revealed a systematic decrease of unit-cell volume with increasing Ni content. We also observed a concurrent increase of the Curie temperature with Ni content due to the increase of superexchange interaction.

A polymer electrolyte membrane water electrolysis system can produce high-purity hydrogen gases in a highly efficient manner. However, the level of hydrogen gas production is still small. In addition, noble-metal catalysts for the reaction in acidic environments, as well as an additional drying step to remove water contained in the hydrogen, are required. Therefore, water electrolysis system with high efficiency and lower cost, an alkaline anion-exchange membrane system that can produce high-purity hydrogen without a noble-metal catalyst, is needed. Nano-size NiCo2O4 powders were prepared by using a sol-gel method to achieve an efficient and economical water electrolysis system. When the powder was calcined at 450 °C, the crystallinity and the cyclic voltammogram measurement showed the best values. In addition, the 15-wt.% polytetrafluoroethylene mixed NiCo2O4 powders exhibited the largest cyclic voltammetry active area and the highest oxygen evolution reaction activity with the appropriate stability.

We herein investigated the magnetic moment of thickness-dependent SnO2 films at room temperature. The saturation magnetization of SnO2 films grown on the r-plane sapphire substrate was decreased with increasing the film thickness. An areal saturation magnetization, a magnetic moment in the unit surface area, did not vary with increasing film thickness. Hall effect measurement indicated that the charge carrier from oxygen vacancy was presented in the epitaxial SnO2 films. Furthermore, X-ray photoelectron spectroscopy analysis revealed that the oxygen vacancy profiles were consistent with that of magnetization. These results suggest that oxygen vacancies at the surface region are the source of the magnetic moment in SnO2 films.

The effects of annealing environments and dopants on the structural, chemical, optical, and electrical properties of NiO are investigated. Pristine NiO exhibits electrically insulating behavior regardless of the annealing environments (e.g., vacuum and O2 environments). On the other hand, Li-doped NiO exhibits electrical resistivities of ∼350 Ω cm and ∼9 Ω cm for the ceramics annealed under vacuum and O2, respectively with reduced optical band-gap. The structural analysis revealed that all the samples exhibit a cubic structure (Fm-3m, SG 225) without any measurable secondary phases. However, Li-doped NiO samples showed a reduction in the unit cell volume as compared to the undoped NiO samples. X-ray photoelectron spectroscopy revealed different residual Li content depending on the annealing environments, and the presence of induced Ni3+ state in the Li-doped NiO. These induced Ni3+ states and doped Li are responsible for the observed change in the structural and electrical properties of the Li-doped NiO samples.

Over the last decades, the spin-coating (SC) technique has been widely used to prepare thin films of various materials in the liquid phase on arbitrary substrates. The technique simply relies on the centrifugal force to spread a coating solution radially outward over the substrate. This mechanism works fairly well for solutions with low surface tension to form thin films of reasonable junctions on smooth substrates. Here, we present a modified SC technique, namely, ultrasonic-assisted spin-coating (UASC), to form thin films of coating solution having high surface tension on rough substrates with excellent junctions. The UASC technique couples SC with an external ultrasonic wave generator to provide external perturbation to locally break down big drops of the coating material into smaller droplets via Rayleigh instability. Because of their lower mass, these tiny droplets gain low momenta and move slowly both in radial and azimuthal directions, giving them an enough time to effectively permeate within pores, thereby yielding excellent junctions. Furthermore, we also investigated the effect of junction improvement on conventional and inverted bulk heterojunction organic solar cells. Intriguingly, the organic solar cells fabricated by the UASC method showed an improved efficiency compared to typical SC owing to efficient charge transfer across the junction. These results clearly imply that UASC is a simple and powerful technique which can significantly enhance the device performance by improving the junction. Moreover, we believe that UASC can be more effective for the preparation of devices composed of multilayers of different materials having complicated nanostructures.

We report the physical characteristics of InGaZnO4 films deposited at various temperatures. The films were deposited on Al2O3(0001) substrates using pulsed laser deposition technique. Based on X-ray diffraction and field emission scanning electron microscopy measurements, the crystal structure changed from amorphous to polycrystalline as deposition temperature increased to 550 °C. Furthermore, UV–vis measurements revealed a decrease in tail state i.e. improvement of local ordering, resulting in an increase of optical band-gap energy as deposition temperature increased. The core-level X-ray photoelectron spectra also showed an increase (decrease) in metal-oxide (oxygen deficiency) bond as the deposition temperature increased. The carrier concentration, Hall mobility and conductivity variation with deposition temperature are related to the competition between oxygen deficiency and grain boundary formation.

Preparation of Cu-Se compounds, chalcogenide-based materials, is studied in this paper. The Cu-Se compounds were synthesized by using a hydrothermal method. This method possesses the advantages of simple preparation, facile manipulation, and the use of nontoxic supercritical water as a solvent. The chemical composition of the Cu-Se compounds was controlled by varying the copper source, reaction time, temperature, and surfactant. When CuCl2 was used as a copper source, the Cu-Se compound tended to form a CuSe2 phase. This phase was also formed at short reaction times and low reaction temperatures. Additionally, we investigated the change in the chemical composition of various Cu-Se compounds by using an acid treatment. The CuSe phase easily decomposed in acidic media while the CuSe2 phase showed a high resistance to acidic media. This synthesized CuSe2 can be used as a Cu(In,Ga)Se2 (CIGS) precursor in a nonselenization method. This preparation method is expected to have potential applications in energy-related devices.

A simulation of the helicopter/ship dynamic interface has been developed and applied to simulate a UH-60A operating from an LHA class ship. Time accurate CFD solutions of the LHA airwake are interfaced with a flight dynamics simulation based on the GENHEL model. The flight dynamics model was updated to include improved inflow modeling and gust penetration effects of the ship airwake. A maneuver controller was used to simulate pilot control inputs for specified approach and departure trajectories. The CFD solutions show significant time varying flow effects in the airwake. Time histories of the aircraft angular rate and pilot control activity indicate that the time varying nature of the airwake has significant effect on aircraft response and pilot workload.

Our ultimate goal of this study is to form a monolayer of plasmonic anisotropic gold nanoparticles on the surface of disposable plastics via chemical processes to use LSPR sensor. The process creates hydroxyl group by method of plasma cleaning or piranha treatment, subsequently substitute hydroxyl groups for Amine functions by using amino silane solution. Then, the surface is coated with the gold nanoparticles. For the plasmonic gold nanoparticles, gold bipyramids (Au BPs) and gold nanorods (Au NRs) with a surfactant, cetyltrimethylammonium bromide (CTAB) bilayer were employed. Then, their surfaces were modified with methoxy polyethylene glycol thiol (mPEG-SH). Based on these plasmonic gold nanoparticle substrates, we have investigated adsorption sensitivity and kinetics of self-assembly of thiolated organic molecules.

A new passivity condition for designing a 1-DOF haptic system is derived with consideration of human arm impedance. The method includes a human arm impedance model of a newly defined ideal mass-spring system. The developed passivity condition guarantees that the virtual wall requires more work than the ideal mass-spring system to compress. The condition guarantees that extractable energy from the virtual wall is less than the ideal system during the release phase. Eventually, the total energy of the haptic system is guaranteed to be less than or equal to that of an ideal mass-spring system whose passivity is guaranteed for all the possible user movements. The developed condition is also shown to be necessary and sufficient for the passivity of the haptic system.