Linxing Zhang

Schottky junction solar cells based on graphene with different numbers of layers

We have investigated the performance of Schottky junction solar cells based on silicon and graphene with 1–6 layers. The open-circuit voltage of solar cells shows an increase when increasing the number of graphene layers. However, the power conversion efficiency and short-circuit current density increase monotonically when the number of graphene layers is less than 4 and reduces as the number of graphene layers further increases. Our results demonstrate that the number of layers related to the work function and transmission of graphene plays a critical role in determining the performance of solar cells.

Effects of oxygen vacancy on the electronic structure and multiferroics in sol–gel derived Pb0.8Co0.2TiO3 thin films

The single phase Pb(0.8)Co(0.2)TiO3 thin films were synthesized on a Pt/Ti/SiO2/Si substrate by the sol-gel route. The present films exhibited homogeneous microstructure with low porosity. O 1s X-ray photoelectron spectroscopy (XPS) was used to detect the amount of oxygen vacancies. The ferroelectric measurements showed that the ferroelectricity deteriorates with the increase in the number of oxygen vacancies. X-ray absorption spectroscopy (XAS) and XPS were used to study the electronic structure. The results indicated that the decreased ferroelectricity might be ascribed to the weakened hybridization between O 2p and Pb 6s and Ti 3d orbitals. The ferromagnetic behaviors were also observed in the thin films and saturated magnetization raised monotonously with the oxygen vacancy rising due to the enhanced F-center exchange interaction. Magnetoelectric coupling of the films weakened with oxygen vacancy increase.

Large resistive switching and switchable photovoltaic response in ferroelectric doped BiFeO3-based thin films by chemical solution deposition

The destructive readout signal distinguishing the ferroelectric polarization state in a memory device has long been seen as the bottleneck for its commercial application. Both ferroelectric resistive switching and the switchable photovoltaic effect provide multiple choices for non-destructive readout. The polycrystalline ferroelectric BiFeO3-based thin films fabricated by cost-effective chemical solution growth exhibit large resistive switching (with ON/OFF ratios ∼104), which is comparable to switchable diodes and tunnel junctions. Furthermore, switchable photovoltaic response shows stable switching and good retention properties. The standard hysteretic loops of resistive switching current, short circuit current and open circuit voltage versus poling voltage directly indicate hysteretic modulation by ferroelectric polarization, which is the first evidence in polycrystalline BiFeO3 based films. The present films would be potential candidates for non-destructive ferroelectric memory devices with multiple selections.

Large remanent polarization in multiferroic NdFeO3-PbTiO3 thin film

The single phase 0.1NdFeO3-0.9PbTiO3 thin film was fabricated on Pt(111)/Ti/SiO2/Si substrate by a sol-gel route. High energy synchrotron radiation glancing incidence X-ray diffraction and conventional X-ray diffraction were employed to determine the phase structure and crystal orientation. Large remanent polarization (2Pr ≈ 85 μC cm−2) was obtained by ferroelectric hysteresis loop and positive up negative down measurements. The oxidation state of Fe element in the film was investigated by X-ray photoelectron spectroscopy and X-ray absorption spectra methods. The results showed the coexistence of Fe2+ and Fe3+ ions with existence of oxygen vacancies. Weak magnetism (∼11 emu/cc) and obvious magnetoelectric coupling were observed in this multiferroic film.

A kinetic model for Ca treatment of Al-killed steels using FactSage macro processing

A kinetic model was developed to investigate the evolution of inclusions during the Ca treatment of Al-killed steels. Evolutions of steel chemistry and inclusion composition in Al-killed Ca-treated steels from previous experimental works were accurately predicted using the current kinetic model. The model was widely used to predict the evolution of inclusions during the Ca treatment of Al-killed steels with varying concentrations of Al, S, O and Ca. During the Ca injection of Al-killed steels, a large number of CaS and CaO inclusions are transiently generated in the Ca-rich zone due to the excessive Ca. Al2O3 inclusions are gradually modified to calcium aluminates in the bulk steel with the addition of Ca. The superfluous Ca or S may lead to the formation of solid CaS in the liquid steel.

TiO2/CdS porous hollow microspheres rapidly synthesized by salt-assistant aerosol decomposition method for excellent photocatalytic hydrogen evolution performance

TiO2/CdS porous hollow microspheres have been one-pot rapidly synthesized by a salt-assisted aerosol decomposition method, and exhibit an excellent photocatalytic activity of 996 μmol h(-1) (50 mg photocatalysts with loading Ru co-catalyst) for hydrogen evolution from aqueous solutions containing sacrificial reagents (SO3(2-) and S(2-)) under visible light (λ ≥ 420 nm). Its high photocatalytic performance is attributed to the surface morphology, crystallinity and heterostructures. The present facile method can be extended to fabricate other heterostructures consisting of oxides or sulfides.

High efficient GaN-based laser diodes with tunnel junction

High-efficient GaN-based laser diodes (LDs) with tunnel junction are designed by replacing conventional p-type AlGaN cladding layers and p-type GaN contact with lower-resistant n-type AlGaN cladding layers and n-type GaN contact. In addition, the characteristics of the LDs with tunnel junction are numerically investigated by using the commercial software lastip. It is found that the performance of these LDs is greatly improved. As a comparison, the absorption loss and non-radiative recombination are greatly reduced. The threshold current and series resistance are decreased by 12% and 59%, respectively, and the slope efficiency is raised up by 22.3%. At an injection current of 120 mA, the output power and wall-plug-efficiency are increased by 34% and 79%, respectively.

Controllable negative thermal expansion, ferroelectric and semiconducting properties in PbTiO3–Bi(Co2/3Nb1/3)O3 solid solutions

Semiconductor functional materials have been widely applied in electronic devices. However, the reliability and lifetime of the devices suffer from the undesirable mismatch of the coefficients of thermal expansion. It is important to develop semiconductor materials with controllable thermal expansion. Here, we develop a remarkable group of ferroelectric semiconductor materials (1 − x)PbTiO3–xBi(Co2/3Nb1/3)O3 (PT–100xBCN) that have not only tunable electronic and optical properties, but also controllable negative, zero, and positive thermal expansion. Through the BCN chemical substitution, the bandgap (Eg) is reduced from 2.60 eV to 2.26 eV to achieve semiconducting properties in PbTiO3-based ferroelectrics. Meanwhile, we have achieved a gradual transition from negative, to zero, and then to positive thermal expansion (αV: −12.3 × 10−6 to 7.4 × 10−6 K−1). Besides, PT–BCN exhibits ferroelectric and piezoelectric properties. The relatively high piezoelectric coefficient d33 ∼ 200 pC N−1 was achieved in the composition of PT–42BCN near the morphotropic phase boundary. PT–BCN shows good negative temperature coefficient (NTC) thermistor characteristics. The coexistence of light absorption, thermal expansion, and electronic properties in PT–BCN makes it a promising material for future applications. The present study would offer an approach to developing and exploring new multifunctional semiconducting materials with controllable thermal expansion.

Temperature-independent ferroelectric property and characterization of high-TC 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films

Ferroelectric property stability against elevated temperature is significant for ferroelectric film applications, such as non-volatile ferroelectric random access memories. The high-T C 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films show the temperature-independent ferroelectric properties, which were fabricated on Pt(111)/Ti/SiO2/Si substrates via sol-gel method. The present thin films were well crystallized in a phase-pure perovskite structure with a high (100) orientation and uniform texture. A remanent polarization (2P r) of 77 μC cm−2 and a local effective piezoelectric coefficient d 33 * of 60 pm/V were observed in the 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films. It is interesting to observe a behavior of temperature-independent ferroelectric property in the temperature range of room temperature to 125 °C. The remanent polarization, coercive field, and polarization at the maximum field are almost constant in the investigated temperature range. Furthermore, the dielectric loss and fatigue properties of 0.2Bi(Mg1/2Ti1/2)O3-0.8PbTiO3 thin films have been effectively improved by the Mn-doping.

Characteristics of complex oxides in Co based ODS alloys

Abstract Co based oxide dispersion strengthening (ODS) alloys strengthened by oxide nanoparticles and γ′ precipitates are promising high temperature structural materials. Phase evolution, thermal stability and interfacial structure of the oxides were characterised, and the formation mechanism of the complex oxides was clarified. Co based ODS alloys exhibit inhomogeneous dispersion of the oxides due to the presence of relatively large Y–Al–O and fine Y–Hf–O complex oxides. Oxide particle size was controlled by the relative amount of Y–Al–O and Y–Hf–O oxides. The addition of Hf inhibits the formation of Y–Al–O oxides, resulting in the refinement of the oxides. The coherency of the matrix/oxide interface is size dependent. Large Y–Al–O complex oxides are incoherent with the matrix, while small Y–Hf–O nanoparticles tend to be partially coherent with the matrix. The extremely fine Y2Hf2O7 complex oxides demonstrate excellent thermal stability during heat treatment. Based on the observations of partially crystallised nanoparticles and the oxides with core/shell structure, a three-stage mechanism was used to elucidate the formation mechanism of the complex oxides.