Shaopeng Lin

Highly uniform bipolar resistive switching characteristics in TiO2/BaTiO3/TiO2 multilayer

Nanoscale multilayer structure TiO2/BaTiO3/TiO2 has been fabricated on Pt/Ti/SiO2/Si substrate by chemical solution deposition method. Highly uniform bipolar resistive switching (BRS) characteristics have been observed in Pt/TiO2/BaTiO3/TiO2/Pt cells. Analysis of the current-voltage relationship demonstrates that the space-charge-limited current conduction controlled by the localized oxygen vacancies should be important to the resistive switching behavior. X-ray photoelectron spectroscopy results indicated that oxygen vacancies in TiO2 play a crucial role in the resistive switching phenomenon and the introduced TiO2/BaTiO3 interfaces result in the high uniformity of bipolar resistive switching characteristics.

Impact of applied strain on the electron transport through ferroelectric tunnel junctions

Combining nonequilibrium Green’s functions with density-functional theory, we have investigated the effect of external strain field on the tunneling electroresistance (TER) of ferroelectric material sandwiched between Pt electrodes. The results show that the strain induced para/ferroelectric phase transitions play an important role in the electronic transport properties of the junction. Sizable enhancements in the resistance are found for the strained ferroelectric junctions with a TER ratio of 9000%. Detail analyses show that the Ti–O displacements along the transport direction in ferroelectric barrier change the effective potential profile, resulting in a giant piezoelectric resistance in the ferroelectric tunnel junctions.

Phase diagram of ferroelectric nanowires and its mechanical force controllability

A phenomenological thermodynamic model of ferroelectric nanowires is developed to investigate the size-temperature phase diagrams. Considering existence of the surface tension, size, and external applied stress effects, the approximated expressions of the transition temperatures at which the paraelectric phase loses its stability with respect to the appearance of the spontaneous polarization are derived. At the same time, the size-temperature phase diagrams as functions of the ferroelectric nanowire radius are obtained, and also show its external mechanical stress controllability.

Improvement of pyroelectric figures of merit in zirconia-doped congruent lithium niobate single crystals

Pyroelectric materials are widely investigated for thermal devices. As an efficient standard for evaluation, the pyroelectric figures of merit (FoM) for specific thermal-related applications can clearly represent the performance of voltage responsivity and detectivity. In this paper, six congruent lithium niobate (CLN) single crystals with different ZrO2 doping concentrations were grown by the top-seeded melt growth method including pure CLN. In order to analyze the dielectric and pyroelectric performances, the concentrations, and occupations were extensively studied by ultraviolet and infrared absorption spectra. Our results present an efficient way to improve the FoM of CLN by using zirconia doping. The working temperature of Zr:CLN thermal device can be widened obviously.

Effects of Zr 4+ co-doping on the spectroscopic properties and yellow light emissions of Dy 3+ in LiNbO 3 single crystals

A series of zirconium and dysprosium doped congruent LiNbO3 (Zr:Dy:CLN) single crystals with a fixed concentration of Dy3+(1mol%) and varied concentrations of optical damage resistant ion Zr4+ (0, 1, 2, 3mol%) have been grown successfully by the Czochralski technique. The influence of Zr4+ doping on structure and the spectroscopic characteristics of Dy3+ in Zr:Dy:CLN crystals have been investigated. Detailed Judd-Ofelt theory analysis was carried out to determine the Judd-Ofelt intensity parameters and main spectroscopic characteristics, which coincided well with the measured fluorescence performance. The defects, resulting from Li-deficiency or exceeding Zr4+-doping, showed positive influences on the absorbing hypersensitive transition 6H15/2→6F11/2 + 6F9/2 and the parameter Ω2. The stimulated emission cross sections of transition 4F9/2 → 6H13/2 with different Zr4+ concentration were evaluated. Exceeding Zr4+ doping was demonstrated effective for significantly eliminating the negative impact of threshold concentration on emission cross sections. The results showed a negative correlation between yellow fluorescence branching ratio of transition 4F9/2 → 6H13/2 and the symmetry of crystal field. Besides, we proposed the Zr:Dy:CLN single crystals as a candidate material for yellow laser operation.

Efficient Synthesis of Stoichiometric Lithium Tantalate Powder by a Solid-State Combustion Route

Stoichiometric lithium tantalate (SLT) powder was prepared by a solid-state combustion method from the raw powder of Li2CO3, Ta2O5, and urea, which is characterized by a short reaction time and relatively low temperatures. The urea, added into the mixture (Li2CO3 and Ta2O5), serves as a fuel and an inducer to produce intermediate compounds and cause a solid-state combustion reaction as well. Various temperatures and urea contents were employed to optimize the synthesis of SLT. The chemical process and the quality of LiTaO3 powder were investigated by X-ray diffraction, electron microscope, thermogravimetry, and Raman spectroscopy. The optimized synthesis shows great capability to reduce the volatilization of lithium-ion and presents an efficient way to manufacture high-quality SLT.

Surface and size effects on phase diagrams of ferroelectric nanocylinders

Size-temperature phase diagrams of ferroelectric nanocylinder have been investigated. Taking into account existence of the depolarization field, surface and size effects, an eighth-order polynomial of the modified thermodynamic model has been established. Our results show that the phase diagrams obviously vary with ratio of the length and radius, and can be adjusted by the mechanical loads especially.

Temperature-Dependent and Threshold Behavior of Sm3+ Ions on Fluorescence Properties of Lithium Niobate Single Crystals

Temperature-dependent and threshold behavior of Sm3+ ions on fluorescence properties of lithium niobate (LiNbO3, LN) single crystals were systematically investigated. The test materials, congruent LiNbO3 single crystals (Sm:LN), with various concentrations of doped Sm3+ ions from 0.2 to 2.0 mol.%, were grown using the Czochralski technique. Absorption spectra were obtained at room temperature, and photoluminescence spectra were measured at various temperatures in the range from 73 K to 423 K. Judd–Ofelt theory was applied to calculate the intensity parameters Ωt (t = 2, 4, 6) for 1.0 mol.% Sm3+-doped LiNbO3, as well as the radiative transition rate, Ar, branching ratio, β, and radiative lifetime, τr, of the fluorescent 4G5/2 level. Under 409 nm laser excitation, the photoluminescence spectra of the visible fluorescence of Sm3+ mainly contains 568, 610, and 651 nm emission spectra, corresponding to the energy level transitions of 4G5/2→6H5/2, 4G5/2→6H7/2, and 4G5/2→6H9/2, respectively. The concentration of Sm3+ ions has great impact on the fluorescence intensity. The luminescence intensity of Sm (1.0 mol.%):LN is about ten times as against Sm (0.2 mol.%):LN at 610 nm. The intensity of the fluorescence spectra were found to be highly depend on temperature, as well as the concentration of Sm3+ ions in LiNbO3 single crystals, as predicted; however, the lifetime changed little with the temperature, indicating that the temperature has little effect on it, in Sm:LN single crystals. Sm:LN single crystals, with orange-red emission spectra, can be used as the active material in new light sources, fluorescent display devices, UV-sensors, and visible lasers.

Persistent luminescence found in Mg2+ and Pr3+ co-doped LiNbO3 single crystal

Persistent luminescence (PersL) was found in Mg2+ and Pr3+ co-doped lithium niobate perovskite-like single crystal. Bright red afterglow emission was observed at room temperature for LiNbO3 excited by a wide range of near ultraviolet light, wherein the defect states associated with Mg2+ ions are demonstrated to play an essential role. We propose a charge relaxation mechanism, based on the transition between mid-gap defect states, to describe the PersL phenomenon within the trap–detrap scenario. Given the outstanding ferroelectric, piezoelectric, electro-optical, mechanical, and acoustic-optical properties of the host material, the discovery of PersL in LiNbO3 opens an opportunity to couple various responses to external stimuli, and thus to develop novel applications based on multifunctional persistent luminescent phosphors (MFPLPs).

Bending influence of the electrocaloric effect in a ferroelectric/paraelectric bilayer system

The influence of bending behavior on the electricaloric (EC) effect is investigated by adopting a thermodynamic model of ferroelectric (FE)/paraelectric (PE) bilayer. The polarization transitions driven by the bending effect occur at a certain temperature range, glass-like transitions, which are found to dramatically impact the EC response. A large range of EC response can be obtained from the FE/PE bilayer which is attributed to the varying misfit strain across the film thickness arising from the bending effect. The results suggest that careful choice of thickness ratio between FE and PE layers allows one to harness the intrinsic and extrinsic contributions to obtain the optimized EC response, which presents great potential for solid refrigeration application.