Negatively Charged Nanosheets Significantly Enhance the Energy-Storage Capability of Polymer-Based Nanocomposites.

Polymer-based dielectric materials play a key role in advanced electronic devices and electric power systems. Although extensive research has been devoted to improve their energy-storage performances, it is a great challenge to increase the breakdown strength of polymer nanocomposites in terms of achieving high energy density and good reliability under high voltages. Here, a general strategy is proposed to significantly improve their breakdown strength and energy storage by adding negatively charged Ca2 Nb3 O10 nanosheets. A dramatically enhanced breakdown strength (792 MV m-1 ) and the highest energy density (36.2 J cm-3 ) among all flexible polymer-based dielectrics are observed in poly(vinylidene fluoride)-based nanocomposite capacitors. The strategy generalizability is verified by the similar substantial enhancements of breakdown strength and energy density in polystyrene-based nanocomposites. Phase-field simulations demonstrate that the further enhanced breakdown strength is ascribed to the local electric field, produced by the negatively charged Ca2 Nb3 O10 nanosheets sandwiched with the positively charged polyethyleneimine, which suppresses the secondary impact-ionized electrons and blocks the breakdown path in nanocomposites. The results demonstrate a new horizon of high-energy-density flexible capacitors.

Electric-Field-Controlled Nonvolatile Magnetization Rotation and Magnetoresistance Effect in Co/Cu/Ni Spin Valves on Piezoelectric Substrates.

Electric-field control of magnetism is a key issue for the future development of low-power spintronic devices. By utilizing the opposite strain responses of the magnetic anisotropies in Co and Ni films, a Co/Cu/Ni/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (PMN-PT) spin-valve/piezoelectric heterostructure with ∼7 nm Cu spacer layer was properly designed and fabricated. The purely electric-field-controlled nonvolatile and reversible magnetization rotations in the Co free layer were achieved, whereas the magnetization of the Ni fixed layer was almost unchanged. Accordingly, not only the electroresistance but also the electric-field-tuned magnetoresistance effects were obtained, and more importantly at least six nonvolatile magnetoresistance states in the strain-tuned spin valve were achieved by setting the PMN-PT into different nonvolatile piezo-strain states. These findings highlight potential strategies for designing electric-field-driven multistate spintronic devices.

Solid-State Synapse Based on Magnetoelectrically Coupled Memristor.

Brain-inspired computing architectures attempt to emulate the computations performed in the neurons and the synapses in the human brain. Memristors with continuously tunable resistances are ideal building blocks for artificial synapses. Through investigating the memristor behaviors in a La0.7Sr0.3MnO3/BaTiO3/La0.7Sr0.3MnO3 multiferroic tunnel junction, it was found that the ferroelectric domain dynamics characteristics are influenced by the relative magnetization alignment of the electrodes, and the interfacial spin polarization is manipulated continuously by ferroelectric domain reversal, enriching our understanding of the magnetoelectric coupling fundamentally. This creates a functionality that not only the resistance of the memristor but also the synaptic plasticity form can be further manipulated, as demonstrated by the spike-timing-dependent plasticity investigations. Density functional theory calculations are carried out to describe the obtained magnetoelectric coupling, which is probably related to the Mn-Ti intermixing at the interfaces. The multiple and controllable plasticity characteristic in a single artificial synapse, to resemble the synaptic morphological alteration property in a biological synapse, will be conducive to the development of artificial intelligence.

Ultrahigh Energy Density in SrTiO3 Film Capacitors.

Solid-state dielectric film capacitors with high-energy-storage density will further promote advanced electronic devices and electrical power systems toward miniaturization, lightweight, and integration. In this study, the influence of interface and thickness on energy storage properties of SrTiO3 (STO) films grown on La0.67Sr0.33MnO3 (LSMO) electrode are systematically studied. The cross-sectional high resolution transmission electron microscopy reveals an ion interdiffusion layer and oxygen vacancies at the STO/LSMO interface. The capacitors show good frequency stability and increased dielectric constant with increasing STO thickness (410-710 nm). The breakdown strength (Eb) increases with decreasing STO thickness and reaches 6.8 MV/cm. Interestingly, the Eb under positive field is enhanced significantly and an ultrahigh energy density up to 307 J/cm3 with a high efficiency of 89% is realized. The enhanced Eb may be related to the modulation of local electric field and redistribution of oxygen vacancies at the STO/LSMO interface. Our results should be helpful for potential strategies to design devices with ultrahigh energy density.