Giant Superlinear Power Dependence of Photocurrent Based on Layered Ta2 NiS5 Photodetector.

Photodetector based on two-dimensional (2D) materials is an ongoing quest in optoelectronics. 2D photodetectors are generally efficient at low illuminating power but suffer severe recombination processes at high power, which results in the sublinear power-dependent photoresponse and lower optoelectronic efficiency. The desirable superlinear photocurrent is mostly achieved by sophisticated 2D heterostructures or device arrays, while 2D materials rarely show intrinsic superlinear photoresponse. This work reports the giant superlinear power dependence of photocurrent based on multilayer Ta2 NiS5 . While the fabricated photodetector exhibits good sensitivity (3.1 mS W-1 per □) and fast photoresponse (31 µs), the bias-, polarization-, and spatial-resolved measurements point to an intrinsic photoconductive mechanism. By increasing the incident power density from 1.5 to 200 µW µm-2 , the photocurrent power dependence varies from sublinear to superlinear. At higher illuminating conditions, prominent superlinearity is observed with a giant power exponent of γ = 1.5. The unusual photoresponse can be explained by a two-recombination-center model where density of states of the recombination centers (RC) effectively closes all recombination channels. The photodetector is integrated into camera for taking photos with enhanced contrast due to superlinearity. This work provides an effective route to enable higher optoelectronic efficiency at extreme conditions.

Ultrahigh Ferroelectric and Piezoelectric Properties in BiFeO3-BaTiO3 Epitaxial Films Near Morphotropic Phase Boundary.

Ferroelectric solid solutions with composition near the morphotropic phase boundary (MPB) have gained extensive attention recently due to their excellent ferroelectric and piezoelectric properties. Here, we have demonstrated a strategy to realize the controllable preparation of BiFeO3-BaTiO3 (BF-BT) epitaxial films near the MPB. A series of high-quality BF-BT films were fabricated by pulsed laser deposition via adjusting oxygen partial pressure (PO2) using a BF-BT ceramic target. A continuous transition from rhombohedral to tetragonal phase was observed upon increasing PO2. Particularly, the film with a pure tetragonal phase exhibited a large remnant polarization of ∼90.6 µC/cm2, while excellent piezoelectric performance with an ultrahigh strain (∼0.48%) was obtained in the film with coexisting rhombohedral and tetragonal phases. The excellent ferroelectric and piezoelectric properties endow the BF-BT system near the MPB with great application prospects in lead-free electronic devices.

Mechanical Polarization Switching in Hf0.5Zr0.5O2 Thin Film.

HfO2-based films with high compatibility with Si and complementary metal-oxide semiconductors (CMOS) have been widely explored in recent years. In addition to ferroelectricity and antiferroelectricity, flexoelectricity, the coupling between polarization and a strain gradient, is rarely reported in HfO2-based films. Here, we demonstrate that the mechanically written out-of-plane domains are obtained in 10 nm Hf0.5Zr0.5O2 (HZO) ferroelectric film at room temperature by generating the stress gradient via the tip of an atomic force microscope. The results of scanning Kelvin force microscopy (SKPM) exclude the possibility of flexoelectric-like mechanisms and prove that charge injection could be avoided by mechanical writing and thus reveal the true polarization state, promoting wider flexoelectric applications and ultrahigh-density storage of HZO thin films.

Topological Hall effect in SrRuO3thin films and heterostructures.

Transition metal oxides hold a wide spectrum of fascinating properties endowed by the strong electron correlations. In 4dand 5doxides, exotic phases can be realized with the involvement of strong spin-orbit coupling (SOC), such as unconventional magnetism and topological superconductivity. Recently, topological Hall effects (THEs) and magnetic skyrmions have been uncovered in SrRuO3thin films and heterostructures, where the presence of SOC and inversion symmetry breaking at the interface are believed to play a key role. Realization of magnetic skyrmions in oxides not only offers a platform to study topological physics with correlated electrons, but also opens up new possibilities for magnetic oxides using in the low-power spintronic devices. In this review, we discuss recent observations of THE and skyrmions in the SRO film interfaced with various materials, with a focus on the electric tuning of THE. We conclude with a discussion on the directions of future research in this field.

Ferroelectric control of pseudospin texture in CuInP2S6monolayer.

Spin-orbit coupling (SOC) plays an important role in condensed matter physics and has potential applications in spintronics devices. In this paper, we study the electronic properties of ferroelectric CuInP2S6(CIPS) monolayer through first-principles calculations. The result shows that CIPS monolayer is a potential for valleytronics material and we find that the in-plane helical and nonhelical pseudospin texture are induced by the Rashba and Dresselhaus effect, respectively. The chirality of helical pseudospin texture is coupled to the out-of-plane ferroelectric polarization. Furthermore, a large spin splitting due to the SOC effect can be found atKvalley, which can be regarded as the Zeeman effect under a valley-dependent pseudomagnetic field. The CIPS monolayer with Rashbaet aleffects provides a good platform for electrically controlled spin polarization physics.

Strain-engineering on GeSe: Raman spectroscopy study.

Among the IV-VI compounds, GeSe has wide applications in nanoelectronics due to its unique photoelectric properties and adjustable band gap. Even though modulation of its physical characteristics, including the band gap, by an external field will be useful for designing novel devices, experimental work is still rare. Here, we report a detailed anisotropic Raman response of GeSe flakes under uniaxial tension strain. Based on theoretical analysis, the anisotropy of the phonon response is attributed to a change in anisotropic bond length and bond angle under in-plane uniaxial strain. An enhancement in anisotropy and band gap is found due to strain along the ZZ or AC directions. This study shows that strain-engineering is an effective method for controlling the GeSe lattice, and paves the way for modulating the anisotropic electric and optical properties of GeSe.

Ion adsorption-induced reversible polarization switching of a van der Waals layered ferroelectric.

Solid-liquid interface is a key concept of many research fields, enabling numerous physical phenomena and practical applications. For example, electrode-electrolyte interfaces with electric double layers have been widely used in energy storage and regulating physical properties of functional materials. Creating a specific interface allows emergent functionalities and effects. Here, we show the artificial control of ferroelectric-liquid interfacial structures to switch polarization states reversibly in a van der Waals layered ferroelectric CuInP2S6 (CIPS). We discover that upward and downward polarization states can be induced by spontaneous physical adsorption of dodecylbenzenesulphonate anions and N,N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium cations, respectively, at the ferroelectric-liquid interface. This distinctive approach circumvents the structural damage of CIPS caused by Cu-ion conductivity during electrical switching process. Moreover, the polarized state features super-long retention time (>1 year). The interplay between ferroelectric dipoles and adsorbed organic ions has been studied systematically by comparative experiments and first-principles calculations. Such ion adsorption-induced reversible polarization switching in a van der Waals ferroelectric enriches the functionalities of solid-liquid interfaces, offering opportunities for liquid-controlled two-dimensional ferroelectric-based devices.

Ultra-wide temperature electronic synapses based on self-rectifying ferroelectric memristors.

Memristors have been intensively studied in recent years as promising building blocks for next-generation nonvolatile memory, artificial neural networks and brain-inspired computing systems. However, most memristors cannot simultaneously function in extremely low and high temperatures, limiting their use for many harsh environment applications. Here, we demonstrate that the memristors based on high-Curie temperature ferroelectrics can resolve these issues. Excellent synaptic learning and memory functions can be achieved in BiFeO3 (BFO)-based ferroelectric memristors in an ultra-wide temperature range. Correlation between electronic transport and ferroelectric properties is established by the coincidence of resistance and ferroelectricity switch and the direct visualization of local current and domain distributions. The interfacial barrier modification by the reversal of ferroelectric polarization leads to a robust resistance switching behavior. Various synaptic functions including long-term potentiation/depression, consecutive potentiation/depression and spike-timing dependent plasticity have been realized in the BFO ferroelectric memristors over an extremely wide temperature range of -170 °C âˆ¼ 300 °C, which even can be extended to 500 °C due to the robust ferroelectricity of BFO at high temperatures. Our findings illustrate that the BFO ferroelectric memristors are promising candidates for ultra-wide temperature electronic synapse in extreme or harsh environments.

Mediation in the second-order synaptic emulator with conductive atomic force microscopy.

Memristors have been extensively studied for synaptic simulation and neuromorphic computation. Instead of focusing on implementing specific synaptic learning rules by carefully engineering external programming parameters, researchers recently have paid more attention to taking advantage of the second-order memristor that is more analogous to biologic synapses and modulated not only by external inputs but also by internal mechanisms. However, experimental evidence is still scarce. Here, we explore a BiMnO3 memristor by applying simple spike forms. The filament evolution dynamics, including processes of forming and spontaneous decay, were directly observed by the conductive atomic force microscopy (c-AFM) technique. We propose that the unique conductance state of the BMO memristor is regulated by the oxygen vacancy (VO) dynamic process. We believe this primary result is helpful to improve understanding of the internal mechanisms of the second-order oxide memristor, which exhibits promising application in building selectors, memories and neuromorphic-computing systems.

Ultrafast Giant Photostriction of Epitaxial Strontium Iridate Film with Superior Endurance.

Photostriction, optical stimulus driven mechanical deformation in materials, provides a solution toward next-generation technology. Here, the giant photostriction (∼2% change of lattice) of epitaxial strontium iridate (SrIrO3) films under illumination at room temperature is revealed via power-dependent Raman scattering, which is significantly larger as compared to conventional inorganic materials. The time scale and mechanism of this giant photostriction in SrIrO3 are further studied through time-resolved transient reflectivity measurements. The main mechanism is determined to be the electron-phonon coupling. In addition, we find that such an exotic behavior happens within few picoseconds and remains up to 107 cyclic on/off operations. The observation of giant photostriction in SrIrO3 films with superior endurance promises the advance of shape responsive solids that are sensitive to environmental stimuli, which could be widely utilized for multifunctional optoelectronics and optomechanical devices.

Electrolyte-gated SmCoO3 thin-film transistors exhibiting thickness-dependent large switching ratio at room temperature.

A Mott transistor that exhibits a large switching ratio of more than two orders at room temperature is demonstrated by using the electric double layer of an ionic liquid for gating on a strongly correlated electron system SmCoO3. From the thickness dependence of the on-state channel current, we estimate the screening length of the SmCoO3 to be ∼5 nm. The good carrier confinement within the Thomas-Fermi screening length demonstrates that the SmCoO3-channel electric double layer transistor is the first candidate for a two-dimensional Mott transistor.

Strain-mediated phase control and electrolyte-gating of electron-doped manganites.

A prototype Mott transistor, the electric double layer transistor with a strained CaMnO(3) thin film, is fabricated. As predicted by the strain phase diagram of electron-doped manganite films, the device with the compressively strained CaMnO(3) exhibits an immense conductivity modulation upon applying a tiny gate voltage of 2 V.