Effects of post-anneal conditions on the dielectric properties of CaCu3Ti4O12 thin films prepared on Pt/Ti/SiO2/Si substrates

High-dielectric-constant CaCu3Ti4O12 (CCTO) thin films were prepared on Pt/Ti/SiO2/Si(100) substrates by pulsed-laser deposition. The 480 nm thick polycrystalline films have preferred orientation and show obvious crystallization on the surface. The temperature-dependence of dielectric constant and loss of the Pt/CCTO/Pt capacitors is comparable with that of epitaxial CCTO films grown on oxides substrates. We found that the dielectric properties are very sensitive to the post-annealing atmosphere and temperature. Post-annealing in nitrogen atmosphere produces larger low-frequency dielectric relaxation as the annealing temperature increases, while annealing in oxygen atmosphere at high temperature suppresses the relaxation but lowers the dielectric constant. Such results are attributed to the presence of insulating grain boundary barrier layers.Comment: 10 pages, 3 figure

Carbon quantum dots coated BiVO4 inverse opals for enhanced photoelectrochemical hydrogen generation

Carbon quantum dots (CQDs) coated BiVO4 inverse opal (io-BiVO4) structure that shows dramatic improvement of photoelectrochemical hydrogen generation has been fabricated using electrodeposition with a template. The io-BiVO4 maximizes photon trapping through slow light effect, while maintaining adequate surface area for effective redox reactions. CQDs are then incorporated to the io-BiVO4 to further improve the photoconversion efficiency. Due to the strong visible light absorption property of CQDs and enhanced separation of the photoexcited electrons, the CQDs coated io-BiVO4 exhibit a maximum photo-to-hydrogen conversion efficiency of 0.35%, which is 6 times higher than that of the pure BiVO4 thin films. This work is a good example of designing composite photoelectrode by combining quantum dots and photonic crystal.Published versio

Effect of charge compensation on the photoelectrochemical properties of Ho-doped SrTiO3 films

When Ho3+ ions are substituted at Sr2+ sites in SrTiO3 (STO), the excess positive charges are compensated via three complementary routes: (1) strontium vacancies, (2) titanium vacancies, and (3) conduction electrons. In this study, we show that the photoelectrochemical properties of Ho-doped STO films are dependent on the charge compensation mechanisms. The compensation mechanism via the titanium vacancies exhibits the highest photocurrent density, which is 1.7 times higher than that of the pure STO sample. Based on the measured dielectric properties and electrochemical impedance spectroscopy data, we propose that the enhanced dielectric constant of the films can enlarge the width of the space charge region at the film/liquid interface, which eventually leads to the increase of the photocurrent density. Further enhancement of photocurrent density is obtained in the samples decorated with appropriate amounts of Pt nanoparticles, showing the advantage of composites for achieving the efficient photoelectrochemical property.Published versio

Enhanced ferroelectric photoelectrochemical properties of polycrystalline BiFeO3 film by decorating with Ag nanoparticles

Polycrystalline BiFeO3 (BFO) films are fabricated on Pt/Ti/SiO2/Si(100) substrate as photoelectrode using sol-gel method. The microstructure, optical, and photoelectrochemical (PEC) properties of the films are characterized and optimized by controlling the film thickness. Moreover, the PEC properties of the BFO films are dependent on ferroelectricpolarization, which is mainly ascribed to the modulation of band structure at the BFO/electrolyte interface by the polarization. Further enhancement of PEC properties is obtained by decorating the samples with appropriate amounts of Ag nanoparticles, which is attributed to the reduced electron-hole recombination, and localized surface plasmon resonance effect of Ag nanoparticles.Published versio

Achromatic THz absorption of conductive nanofilms

According to the theory, an ultrathin conductive film can achromatically dissipate electromagnetic waves with frequency ranging from radio to terahertz. A moderate absorption effect, which gives rise to a maximal absorbance of 50%, can be found if an impedance matching condition is satisfied. We have experimentally demonstrated the frequency-irrelevant, maximal absorption by employing a conductive nanofilm and launching terahertz waves at Brewster angle when the sheet (square) resistance of the film meets the impedance matching condition. In the entire terahertz spectral range covered by our experiments, the frequency-independent optical properties were consistent with the theoretical calculations

High-Efficiency Ferroelectric-Film Solar Cells with an n-type Cu₂O Cathode Buffer Layer

Becasue of the existence of interface Schottky barriers and depolarization electric field, ferroelectric films sandwiched between top and bottom electrodes are strongly expected to be used as a new kind of solar cells. However, the photocurrent with a typical order of μA/cm² is too low to be practical. Here we demonstrate that the insertion of an n-type cuprous oxide (Cu₂O) layer between the Pb­(Zr,Ti)­O₃ (PZT) film and the cathode Pt contact in a ITO/PZT/Pt cell leads to the short-circuit photocurrent increasing 120-fold to 4.80 mA/cm² and power conversion efficiency increasing of 72-fold to 0.57% under AM1.5G (100 mW/cm²) illumination. Ultraviolet photoemission spectroscopy and dark <i>J</i>–<i>V</i> characteristic show an ohmic contact on Pt/Cu₂O, an n⁺–n heterojunction on Cu₂O/PZT and a Schottky barrier on PZT/ITO, which provide a favorable energy level alignment for efficient electron-extraction on the cathode. Our work opens up a promising new method that has the potential for fulfilling cost-effective ferroelectric-film photovoltaic

A single-molecule magnet assembly exhibiting a dielectric transition at 470 K

10.1039/c2sc21023aCHEMICAL SCIENCE3123366-337

A single-molecule magnet assembly exhibiting a dielectric transition at 470 K

International audienceA triangular Dy(III) single-molecule magnet (SMM) exhibiting ferroelectric bistability is assembled in an acentric space group Pna2(1). Hysteresis loops associated with its SMM behavior together with a two-step slow relaxation of the magnetization are observed below 30 K. A transition with dielectric anomalies between a paraelectric and a ferroelectric phase occurs at 470 K