Control of Hole Density in Russellite Bi2WO6 via Intentional Chemical Doping.

Based on the fundamental design concept of modulating the valence band maximum of oxides and subsequent predictions through computational approaches, several lone-pair ns2-based p-type oxide semiconductors, such as Sn2+- or Bi3+-based complex oxides, have been developed. Thus far, the bandgap can be modified via tuning of the chemical composition, whereas the hole density cannot be intentionally controlled because of the poor chemical stability of Sn2+ and/or the formation of oxygen vacancies. The inability to control hole density prohibits the design and realization of emergent electronic devices based on p- and n-type oxide semiconductors. Herein, we report the control of hole density via intentional chemical doping in polycrystalline Bi2WO6. While the holes of polycrystalline Nb- or Ta-doped Bi2WO6 are strongly trapped by grain boundaries, the hole density obtained at high temperatures monotonically increases with the increase in the doping concentration. This study provides important insights into the development of practical p-type oxide semiconductors.

Pt/WO3 Nanoparticle-Dispersed Polydimethylsiloxane Membranes for Transparent and Flexible Hydrogen Gas Leakage Sensors.

Hydrogen gas is a promising, clean, and highly efficient energy source. However, to use combustible H2 gas safety, high-performance and safe gas leakage sensors are required. In this study, transparent and flexible platinum-catalyst-loaded tungsten trioxide (Pt/WO3) nanoparticle-dispersed membranes were prepared as H2 gas leakage sensors. The nanoparticle-dispersed membrane with a Pt:W compositional ratio of 1:13 was transparent and exhibited a sufficient color change in response to H2 gas. The membrane containing 0.75 wt.% of Pt/WO3 nanoparticles exhibited high transparency over a wide wavelength range and the largest transmittance change in response to H2 gas among the others. The heat treatment of the particles at 573 K provided sufficient crystallinity and an accessible area for a gasochromic reaction, resulting in a rapid and sensitive response to the presence of H2 gas. The lower limit of detection of the optimized Pt/WO3 nanoparticle-dispersed membrane by naked eye was 0.4%, which was one-tenth of the minimum explosive concentration. This novel membrane was transparent as well as flexible and exhibited a clear and rapid color response to H2. Therefore, it is an ideal candidate sensor for the safe and easy detection of H2 gas leakage.

Continuous Variation of Secondary Structural Contents of Interfacial Peptides Induced by Hydrogel Fusion.

Three-dimensional assemblies formed by multi-biopolymers perform important biological functions by maintaining the vital activities of living organisms through biochemical reactions that occur at the interfaces of these structures. In this study, we investigated the mechanism of the continuous variation of the secondary structural contents of interfacial peptides induced by the fusion of hydrogels with different charges. The hydrogel fusion induced continuous pH changes at the interface through ionic diffusion from the hydrogel matrices, and the pH value increased rapidly during the early stage (0-200 min) of the fusion process. In addition, the secondary structural content of the interfacial peptides changed continuously between the ß-sheet and random coil conformations during the early stage of the fusion process. The continuous variation in the secondary structural contents of the interfacial peptides was caused by (1) the protonation of peptide molecule amino acid side-chains in the region of pH change and (2) charge shielding due to the electrostatic interactions between the intramolecular peptides, intermolecular peptides, and intramolecular and intermolecular peptides.

Bipolar Semiconducting Properties in α-SnWO4 Based on the Characteristic Defect Structure.

Diodes, memories, logic circuits, and most other current information technologies rely on the combined use of p- and n-type semiconductors. Although oxide semiconductors have many technologically attractive functionalities, such as transparency and high dopability to enable their use as conducting films, they typically lack bipolar conductivity. In particular, the absence of p-type semiconducting properties owing to the innate electronic structures of oxides represents a bottleneck for the development of practical devices. Here, bipolar semiconducting properties are demonstrated in α-SnWO4 within a 100 °C temperature window after appropriate thermal treatment. Comprehensive spectroscopic observations reveal that Sn4+ is present in p-type α-SnWO4 in a notably greater quantity than in n-type. This result strongly suggests that the Sn4+ substitutional defects on the W6+ sites contribute to hole-carrier generation in α-SnWO4. We also find that oxygen vacancies are initially formed in Sn-O-W bonds and migrate to W-O-W bonds with changes in semiconducting properties from p-type to n-type. These findings suggest useful strategies for exploring p-type oxide semiconductors and controlling their carrier type by utilizing the octahedral structure.

Substitutional and interstitial impurity p-type doping of thermoelectric Mg2Si: a theoretical study.

The narrow-gap magnesium silicide semiconductor Mg2Si is a promising mid-temperature (600-900 K) thermoelectric material. It intrinsically possesses n-type conductivity, and n-type dopants are generally used for improving its thermoelectric performance; however, the synthesis of p-type Mg2Si is relatively difficult. In this work, the hole doping of Mg2Si with various impurity atoms is investigated by performing first principles calculations. It is found that the Ag-doped systems exhibit comparable formation energies ΔE calculated for different impurity sites (Mg, Si, and interstitial 4b ones), which may explain the experimental instability of their p-type conductivity. A similar phenomenon is observed for the systems incorporating alkali metals (Li, Na, and K) since their ΔE values determined for Mg (p-type) and 4b (n-type) sites are very close. Among boron group elements (Ga and B), Ga is found to be favorable for hole doping because it exhibits relatively small ΔE values for Si (p-type) sites. Furthermore, the interstitial insertion of Cl and F atoms into the crystal lattice leads to hole doping because of their high electronegativity.

Low-Temperature Spark Plasma Sintering of ZrW2-xMoxO8 Exhibiting Controllable Negative Thermal Expansion.

Molybdenum-doped zirconium tungstate (ZrW2-xMoxO8) has been widely studied because of its large isotropic coefficient of negative thermal expansion (NTE). However, low density and poor sinterability limit its production and application. In this study, relative density greater than 90% single-phase ZrW2-xMoxO8 (0.0 ≤ x ≤ 1.0) sintered bodies were fabricated by spark plasma sintering (500⁻600 °C for 10 min) using ZrW2-xMoxO7(OH)2·2H2O precursor powders as the starting material. High-temperature X-ray diffraction and thermomechanical analysis were used to investigate the change in the order⁻disorder phase transition temperature of the sintered materials; it gradually dropped from 170 °C at x = 0.0 to 78 °C at x = 0.5, and then to below room temperature at x ≥ 0.7. In addition, all sintered bodies exhibited NTE behavior. The NTE coefficient was controllable by changing the x value as follows: from -7.85 × 10-6 °C-1 (x = 0) to -9.01 × 10-6 °C-1 (x = 0.6) and from -3.22 × 10-6 °C-1 (x = 0) to -2.50 × 10-6 °C-1 (x = 1.0) before and after the phase transition, respectively. Rietveld structure refinement results indicate that the change in the NTE coefficient can be straightforwardly traced to the thermodynamic instability of the terminal oxygen atoms, which only have one coordination.