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[en] We report on strategies for characterizing hexagonal coincidence phases by analyzing the involved spatial moiré beating frequencies of the pattern. We derive general properties of the moiré regarding its symmetry and construct the spatial beating frequency as the difference between two reciprocal lattice vectors of the two coinciding lattices. Considering reciprocal lattice vectors , with lengths of up to n times the respective (1, 0) beams of the two lattices, readily increases the number of beating frequencies of the nth-order moiré pattern. We predict how many beating frequencies occur in nth-order moirés and show that for one hexagonal lattice rotating above another the involved beating frequencies follow circular trajectories in reciprocal-space. The radius and lateral displacement of such circles are defined by the order n and the ratio x of the two lattice constants. The question of whether the moiré pattern is commensurate or not is addressed by using our derived concept of commensurability plots. When searching potential commensurate phases we introduce a method, which we call cell augmentation, and which avoids the need to consider high-order beating frequencies as discussed using the reported moiré of graphene on SiC(0001). We also show how to apply our model for the characterization of hexagonal moiré phases, found for transition metal-supported graphene and related systems. We explicitly treat surface x-ray diffraction-, scanning tunneling microscopy- and low-energy electron diffraction data to extract the unit cell of commensurate phases or to find evidence for incommensurability. For each data type, analysis strategies are outlined and avoidable pitfalls are discussed. We also point out the close relation of spatial beating frequencies in a moiré and multiple scattering in electron diffraction data and show how this fact can be explicitly used to extract high-precision data. (paper)
[en] Highlights: • Negative capacitance effect is observed in metal–P(VDF-TrFE)–Al2O3–metal stacked structure firstly. • Weiss mean field model is employed to simulate S-shaped negative capacitance behavior. • The switching time of the ferroelectric film increases with increasing dielectric layer thickness. - Abstract: The dielectric layer in the sandwich structural device plays a very important role in determining the electrical properties of the ferroelectric film. In this paper, we investigate the effect of the dielectric layers with different thicknesses on switching performance of ferroelectric P(VDF-TrFE) thin films. The hysteresis loops become slanting with increasing thickness of the dielectric layer. A negative slope of the ‘real’ hysteresis loop is apparently observed which demonstrates negative capacitance effect caused by the dielectric layer. This behavior is simulated qualitatively by the Weiss mean field model considering an interfacial dielectric layer in series with a ferroelectric layer. The agreement between experiments and simulations supports that negative capacitance results from the positive feedback among electric dipoles. Furthermore, the switching time of the ferroelectric film increases with the increase of dielectric layer thickness. This study shows that the ferroelectric sandwich structure provides great potential towards low power negative capacitance devices.
[en] Tungsten oxide (WO3) thin films on fluorine-doped tin oxide (FTO) coated glass substrates are fabricated by pulsed laser deposition for photoelectrochemical water splitting. The effects of oxygen pressure during deposition on the structure, morphology, optical properties and photoelectrochemical performance of the films have been systematically evaluated. Columnar growth characteristic of WO3 thin films is observed at oxygen pressure higher than 8 Pa. The grain size increases with increasing oxygen pressure in the range of 1–13 Pa then decreases with further increasing oxygen pressure from 13 Pa to 30 Pa. Pure monoclinic phase of WO3 has been obtained for samples deposited above 13 Pa. Decreasing oxygen pressure below 8 Pa leads to an oxygen deficient surface with a substoichiometric phase. The film prepared at 13 Pa exhibits the best photoelectrochemical performance with a photocurrent density as high as 1.9 mA/cm2 under a xenon lamp illumination in 0.1 M Na2SO4 electrolyte. Furthermore, hydrogen evolution conducted with a three-electrode configuration and a two-electrode configuration is presented and discussed. - Highlights: • WO3 thin films are prepared by pulsed laser deposition with varying oxygen pressure. • The morphology, structure, optical properties and PFC performance are evaluated. • Hydrogen evolution is conducted with a three-electrode and a two-electrode configuration. • The WO3 thin film deposited at 13 Pa is demonstrated to be an efficient photoanode.