Qianru Lin

Compositional dependence of electrocaloric effect in lead-free (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 ceramics

We studied the electrocaloric effect (ECE) in (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT–xBCT) ceramics over a wide composition range (x = 0.3–0.7) using an indirect method based on the thermodynamics Maxwell relations. The maximum adiabatic temperature change ΔT was found to shift from the proximity of Curie temperature to higher temperatures, owing to a synergistic effect of field induced change in the phase transition temperature and alignment and growth of polar nano-regions. Coexistence of positive and negative ECEs were confirmed in BCT-rich compositions (x ≥ 0.5). The abnormal negative ECE is presumably attributed to the rhombohedral to tetragonal transition occurred at relatively low temperatures. The most promising ECE was found in BZT–0.4BCT ceramics, which exhibited a uniform ECE in the temperature range of interest, with a promising ΔT of 0.58 K at ∼125 °C under a moderate electric field of 28 kV cm−1.

Engineering Silver Nanowire Networks: From Transparent Electrodes to Resistive Switching Devices

Metal nanowires (NWs) networks with high conductance have shown potential applications in modern electronic components, especially the transparent electrodes over the past decade. In metal NW networks, the electrical connectivity of nanoscale NW junction can be modulated for various applications. In this work, silver nanowire (Ag NW) networks were selected to achieve the desired functions. The Ag NWs were first synthesized by a classic polyol process, and spin-coated on glass to fabricate transparent electrodes. The as-fabricated electrode showed a sheet resistance of 7.158 Ω □-1 with an optical transmittance of 79.19% at 550 nm, indicating a comparable figure of merit (FOM, or ΦTC) (13.55 × 10-3 Ω-1). Then, two different post-treatments were designed to tune the Ag NWs for not only transparent electrode but also for threshold resistive switching (RS) application. On the one hand, the Ag NW film was mechanically pressed to significantly improve the conductance by reducing the junction resistance. On the other hand, an Ag@AgOx core-shell structure was deliberately designed by partial oxidation of Ag NWs through simple ultraviolet (UV)-ozone treatment. The Ag core can act as metallic interconnect and the insulating AgOx shell acts as a switching medium to provide a conductive pathway for Ag filament migration. By fabricating Ag/Ag@AgOx/Ag planar structure, a volatile threshold switching characteristic was observed and an on/off ratio of ∼100 was achieved. This work showed that through different post-treatments, Ag NW network can be engineered for diverse functions, transforming from transparent electrodes to RS devices.

Morphology control and large piezoresponse of hydrothermally synthesized lead-free piezoelectric (Bi0.5Na0.5)TiO3 nanofibres

Lead-free piezoelectric bismuth sodium titanate (BNT) nanostructures were synthesised using a low-temperature hydrothermal technique. It is found that the phase and morphology of the products are strongly dependent on the composition and concentration of the precursors, as well as the processing conditions. Through optimising the synthesis parameters, well-crystallized BNT nanofibers with 150–200 nm diameters and ∼5 μm length were obtained. The BNT fibres show a pure perovskite phase with (011) orientation along the length direction. A piezoelectric constant of d33 = ∼15 pm V−1 in the diameter direction was observed for these BNT nanofibers.

Tunable resistance switching in solution processed chromium-doped strontium titanate nanoparticles films

In this work, resistance switching behaviours in solution processed chromium (Cr)-doped strontium titanate (SrTiO3) films have been investigated. Undoped SrTiO3 film shows I-V characteristics of typical nonlinear resistor and no resistance hysteresis loops are observed. On the contrary, Cr-doped SrTiO3 films show stable and reversible hysteresis loops, which can be controlled by applying different voltage bias. Based on a series of characterization results, including X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS), we infer that Ti4+ is substituted by Cr3+, giving rise to increased concentration of oxygen vacancies. Therefore, the observed resistance switching phenomenon is attributed to voltage driven oxygen vacancy migration. Furthermore, gradually decreased overall resistance is also realized under repeated sweeping cycles.

Periodicity dependence of the built-in electric field in (Ba0.7Ca0.3)TiO3/Ba(Zr0.2Ti0.8)O3 ferroelectric superlattices

Symmetric ferroelectric superlattices consisting of (Ba0.7Ca0.3)TiO3 (BCT) and Ba(Zr0.2Ti0.8)O3 (BZT) layers were successfully grown on La0.7Sr0.3MnO3 electroded (001)-oriented SrTiO3 substrates by laser molecular beam epitaxy. With the monitor of reflective high-energy electron diffraction, the growth mode and rate were precisely controlled to realize the desired superlattice periodicity as confirmed by both X-ray diffraction and transmission electron microscopy results. The microscopic piezoelectric response and macroscopic ferroelectric properties were investigated as a function of periodicity of the BCTm/BZTm (m = 3, 5, 10, and 15 unit cells) superlattices. The existence of a built-in electric field was confirmed in all the superlattices and its strength was highly dependent on the periodicity. The excellent tunability of built-in electric field opens a path for designing microelectronic devices with various functionalities based on BCTm/BZTm superlattices.

Built-in electric field in compositionally graded (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 thin films

A built-in electric field was generated in compositionally graded (1 − x)Ba(Zr0.2Ti0.8)O3–x(Ba0.7Ca0.3)TiO3 (BZT–xBCT) thin films deposited on SrRuO3-electroded (001) SrTiO3 single crystal substrates through pulsed laser deposition. By tailoring the gradient direction with compositionally upgraded (UG) and downgraded (DG) heterostructures, the origin of the internal bias was investigated and the structure–property relationship was also revealed. It was confirmed that the magnitude of the built-in electric field was dominated by the flexoelectric effect in the BZT–xBCT compositionally graded structure. Compared to UG multilayer, a higher remanent polarization of Pr ∼ 17 μC cm−2 in the DG multilayer was observed. This enhancement stemmed from the retained in-plane compressive strain as confirmed by XRD results. An approximate 65% enhancement in piezoelectricity was also achieved in the DG heterostructure, which can be attributed to the larger built-in electric field.

Digital to analog resistive switching transition induced by graphene buffer layer in strontium titanate based devices

Resistive switching behaviour can be classified into digital and analog switching based on its abrupt and gradual resistance change characteristics. Realizing the transition from digital to analog switching in the same device is essential for understanding and controlling the performance of the devices with various switching mechanisms. Here, we investigate the resistive switching in a device made with strontium titanate (SrTiO3) nanoparticles using X-ray diffractometry, scanning electron microscopy, Raman spectroscopy, and direct electrical measurements. It is found that the well-known rupture/formation of Ag filaments is responsible for the digital switching in the device with Ag as the top electrode. To modulate the switching performance, we insert a reduced graphene oxide layer between SrTiO3 and the bottom FTO electrode owing to its good barrier property for the diffusion of Ag ions and high out-of-plane resistance. In this case, resistive switching is changed from digital to analog as determined by the modulation of interfacial resistance under applied voltage. Based on that controllable resistance, potentiation and depression behaviours are implemented as well. This study opens up new ways for the design of multifunctional devices which are promising for memory and neuromorphic computing applications.