Mingrong Shen

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

Because 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/cm(2) is too low to be practical. Here we demonstrate that the insertion of an n-type cuprous oxide (Cu(2)O) layer between the Pb(Zr,Ti)O(3) (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/cm(2) and power conversion efficiency increasing of 72-fold to 0.57% under AM1.5G (100 mW/cm(2)) illumination. Ultraviolet photoemission spectroscopy and dark J-V characteristic show an ohmic contact on Pt/Cu(2)O, an n(+)-n heterojunction on Cu(2)O/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.

Enhancement of magnetization in Eu doped BiFeO3 nanoparticles

Bi1−xEuxFeO3 (BEFOx) nanoparticles have been synthesized by a sol-gel method. The samples maintain the pure rhombohedral structure up to x=0.1, while an impure phase of EuFeO3 is observed at x=0.15. The remenant magnetization of the BEFOx, which is significantly higher than those in nonmagnetically rare-earth doped BiFeO3, is found to increase with the increase of Eu concentration. Ferromagnetic coupling between Eu3+ and Fe3+ ions, evidenced by our first-principles calculations, is found to be the main reason for such a magnetization enhancement. In the mean time, the appearance of EuFeO3 phase is demonstrated to be another factor for the further magnetization enhancement in BEFOx=0.15.

Deposition of diamond‐like carbon films by electrolysis of methanol solution

By electrolysis of the methanol solution, an attempt was made to deposit diamondlike carbon (DLC) films on silicon substrate at temperature of less than 60 °C. Substrates were negatively biased with a dc potential of 0 to −3000 V. IR spectra showed that the O–H, C–H, and C–O vibration bands of electrolyte decreased remarkably after electrolysis and a new peak characterized as the C=C bond appeared. The deposited films were characterized as DLC films by Raman spectroscopy.

Effects of postanneal 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 (PLD). 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 obtained in the epitaxial CCTO films grown on oxides substrates. We found that the dielectric properties are very sensitive to the postannealing atmosphere and temperature. Postannealing 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 and decreases the dielectric constant of the thin films. Such results are attributed to the presence of insulating grain-boundary barrier layers.

Inverse opal structured Ag/TiO2 plasmonic photocatalyst prepared by pulsed current deposition and its enhanced visible light photocatalytic activity

We developed a pulsed current deposition method to fabricate TiO2 inverse opals with highly dispersed Ag nanoparticles (NPs) as a visible light driven plasmonic photocatalyst. It is observed that the incorporation of Ag NPs can significantly improve the photocatalytic activity of TiO2 inverse opals in the visible light region. The sample with a deposition time of 45 s has the highest measured photocatalytic activity, and exceeds that of the Ag/TiO2 inverse opals prepared by a photochemical reduction method. Such an enhancement is ascribed to the optimized localized surface plasmon resonance property of the Ag NPs, and excellent separation of the photoexcited electrons and Ag+ ions, resulting from the uniform Ag NPs produced by pulsed current deposition. The proposed mechanism is further confirmed by hydroxyl radical detection and electrochemical impedance spectroscopy analysis. Our study provides new insight into the design and preparation of advanced visible light photocatalytic materials.

Separation of the Schottky barrier and polarization effects on the photocurrent of Pt sandwiched Pb(Zr0.20Ti0.80)O3 films

Photoelectric behavior of Pt sandwiched Pb(Zr0.20Ti0.80)TiO3 (PZT) films deposited on Pt∕Ti∕SiO2∕Si substrates by a sol-gel method was investigated by testing the short-circuit photocurrent under different film thicknesses. By poling the films step by step with increased magnitude and alternated direction of the dc electric field, interesting photoelectric behavior was found when the PZT films were in virgin or poled up/down state. The photocurrent was also strongly dependent on the film thickness. A simple model was proposed to separate the effects of interface Schottky barriers and bulk ferroelectric polarization of the film on the photocurrent of the Pt/PZT/Pt structure.

Experimental and theoretical evidence of enhanced ferromagnetism in sonochemical synthesized BiFeO3 nanoparticles

BiFeO3 nanoparticles were synthesized by a sonochemical method and their magnetic behavior was investigated both experimentally and theoretically. With an aid of ultrasonic irradiation, the saturated magnetization of BiFeO3 nanoparticles at room temperature was found to be increased effectively, from 0.007 to 0.012 μB/Fe. The postannealing and x-ray photoemission spectroscopy results demonstrate that oxygen vacancies can be generated due to the ultrasonic irradiation and play an important role to increase the ferromagnetism. Our first-principles calculation results are in good agreement with the experimental observations.

Hopping conduction in Mn-doped ZnO

The dc and ac conductivities of Mn-doped ZnO were investigated at temperatures from 10 to 100 K. The temperature dependence of the dc conductivity from 10 to 100 K shows an abrupt change at ∼18 K, manifesting a much lower activation energy for conduction below 18 K. From 10 to 18 K, the ac conductivity, σac(ω), varies as σac(ω)=Aωs in the frequency range from 102 to 106 Hz with s in the range of 0.6–1. The dc and ac conductivity observations suggest that the dominant conduction mechanism at temperatures between 10 to 18 K in these samples is a hopping conduction.

Magnetically separable BiFeO3 nanoparticles with a γ-Fe2O3 parasitic phase: controlled fabrication and enhanced visible-light photocatalytic activity

A facile sol–gel approach with a fixed calcination temperature is developed to prepare BiFeO3 (BFO) nanoparticles, and the gel-drying temperature is adjusted to control the appearance of a γ-Fe2O3 parasitic phase. The room temperature ferromagnetism of the samples is strongly dependent on the gel-drying temperature. When the gel-drying temperature increases from 80 to 140 °C, the saturated magnetization of the corresponding samples jumps from 0.22 emu g−1 to 1.2 emu g−1, allowing the nanoparticles to be magnetically separated in solution. From examination by transmission electron microscopy and X-ray photoelectron spectroscopy, it is confirmed that the γ-Fe2O3 parasitic phase is nucleated during the gel-drying process under high temperatures above 120 °C, and remains in the subsequent annealing process, resulting in the anomalous enhanced magnetization. Comparing with pure BFO nanoparticles prepared under low gel-drying temperature, the BFO/γ-Fe2O3 samples exhibit significantly increased visible-light photocatalytic ability towards rhodamine B. The formation of a heterojunction structure between the BFO and γ-Fe2O3 phases is proposed to be responsible for the enhanced photocatalytic activity. Further enhanced photocatalytic activity is obtained in this study when adding a small amount of H2O2 during photocatalysis, indicating the samples have photo-Fenton-like catalytic activity.

Dielectric enhancement and Maxwell-Wagner effects in polycrystalline ferroelectric multilayered thin films

Dielectric enhancement was observed in polycrystalline BaTiO3/Ba0.6Sr0.4TiO3 multilayered thin films deposited layer-by-layer on Pt/Ti/SiO2/Si substrates via pulsed laser deposition. The dielectric constant of the films was enhanced more than four times with the decrease of the individual layer thickness down to 30 nm, while the dielectric loss was kept at a low level comparable to that of the solid solution Ba0.8Sr0.2TiO3 thin films. The Maxwell-Wagner model is proposed to explain the experimental data, which can predict both the dielectric enhancement and frequency dependence when the individual layer thickness is more than 40 nm.