A polar-switchable and controllable negative phototransistor for information encryption.

Anomalous negative phototransistors have emerged as a distinct research area, characterized by a decrease in channel current under light illumination. Recently, their potential applications have been expanded beyond photodetection. Despite the considerable attention given to negative phototransistors, negative photoconductance (NPC) in particular remains relatively unexplored, with limited research advancements as compared to well-established positive phototransistors. In this study, we designed ferroelectric field-effect transistors (FeFETs) based on the WSe2/CIPS van der Waals (vdW) vertical heterostructures with a buried-gated architecture. The transistor exhibits NPC and positive photoconductance (PPC), demonstrating the significant role of ferroelectric polarization in the distinctive photoresponse. The observed inverse photoconductance can be attributed to the dynamic switching of ferroelectric polarization and interfacial charge transfer processes, which have been investigated experimentally and theoretically using Density Functional Theory (DFT). The unique phenomena enable the coexistence of controllable and polarity-switchable PPC and NPC. The novel feature holds tremendous potential for applications in optical encryption, where the specific gate voltages and light can serve as universal keys to achieve modulation of conductivity. The ability to manipulate conductivity in response to optical stimuli opens up new avenues for developing secure communication systems and data storage technologies. Harnessing this feature enables the design of advanced encryption schemes that rely on the unique properties of our material system. The study not only advances the development of NPC but also paves the way for more robust and efficient methods of optical encryption, ensuring the confidentiality and integrity of critical information in various domains, including data transmission, and information security.

Anisotropic Phonon Behavior and Phase Transition in Monolayer ReSe2 Discovered by High Pressure Raman Scattering.

Re-based transition metal dichalcogenides have attracted extensive attention owing to their anisotropic structure and excellent properties in applications such as optoelectronic devices and electrocatalysis. The present study methodically investigated the evolution of specific Raman phonon mode behaviors and phase transitions in monolayer and bulk ReSe2 under high pressure. Considering the distinctive anisotropic characteristics and the vibration vectors of Re and Se atoms exhibited by monolayer ReSe2, we perform phonon dispersion calculations and propose a methodology utilizing pressure-dependent polarized Raman measurements to explore the precise structural evolution of monolayer ReSe2 under the stress fields. Varied behaviors of the Eg-like and Ag-like modes, along with their specific vector transformations, have been identified in the pressure range 0-14.59 GPa. The present study aims to offer original perspectives on the physical evolution of Re-based transition metal dichalcogenides, elucidating their fundamental anisotropic properties and exploring potential applicability in diverse devices.

Stabilizing High-Frequency Magnetic Properties of Stretchable CoFeB Films by Ribbon-Patterned Periodic Wrinkles.

The intrinsic nonstretchable feature of magnetic films has significantly limited its applications on wearable high-frequency devices. Recent studies have proved that the wrinkling surface structure based on the growth on polydimethylsiloxane (PDMS) is an effective route to obtain stretchable magnetic films. However, it is still a great challenge to simultaneously achieve a desired stretchability and stretching-insensitive high-frequency properties of magnetic films. Herein, we reported a convenient method to stabilize the high-frequency properties of stretchable magnetic films by depositing magnetic ribbon-patterned films on prestrain PDMS membranes. The ribbon-patterned wrinkling CoFeB films have far fewer cracks than the continuous film, which indicates a nice strain-relief effect and thus confers the stability of high-frequency properties for the films under stretching. However, the wrinkle bifurcation and the uneven thickness at the ribbon edge could adversely affect the stability of its high-frequency properties. The 200 µm wide ribbon-patterned film shows the best stretching-insensitive behaviors and maintains a constant resonance frequency of 3.17 GHz at strain from 10% to 25%. Moreover, a good repeatability has been demonstrated by performing thousands of stretch-release cycles, which did not significantly deteriorate its performances. The ribbon-patterned wrinkling CoFeB films with excellent stretching-insensitive high-frequency properties are promising for application in flexible microwave devices.

Light control of droplets on photo-induced charged surfaces.

The manipulation of droplets plays a vital role in fundamental research and practical applications, from chemical reactions to bioanalysis. As an intriguing and active format, light control of droplets, typically induced by photochemistry, photomechanics, light-induced Marangoni effects or light-induced electric fields, enables remote and contactless control with remarkable spatial and temporal accuracy. However, current light control of droplets suffers from poor performance and limited reliability. Here we develop a new superamphiphobic material that integrates the dual merits of light and electric field by rationally preparing liquid metal particles/poly(vinylidene fluoride-trifluoroethylene) polymer composites with photo-induced charge generation capability in real time, enabling light control of droplets on the basis of photo-induced dielectrophoretic force. We demonstrate that this photo-induced charged surface (PICS) imparts a new paradigm for controllable droplet motion, including high average velocity (∼35.9 mm s-1), unlimited distance, multimode motions (e.g. forward, backward and rotation) and single-to-multiple droplet manipulation, which are otherwise unachievable in conventional strategies. We further extend light control of droplets to robotic and bio-applications, including transporting a solid cargo in a closed tube, crossing a tiny tunnel, avoiding obstacles, sensing the changing environment via naked-eye color shift, preparing hydrogel beads, transporting living cells and reliable biosensing. Our PICS not only provides insight into the development of new smart interface materials and microfluidics, but also brings new possibilities for chemical and biomedical applications.

Directly measuring flexoelectric coefficients µ11 of the van der Waals materials.

Flexoelectricity originates from the electromechanical coupling interaction between strain gradient and polarization, broadly applied in developing electromechanical and energy devices. However, the study of quantifying the longitudinal flexoelectric coefficient (µ11) which is important for the application of atomic-scale two-dimensional (2D) materials is still in a slow-moving stage, owing to the technical challenges. Based on the free-standing suspension structure, this paper proposes a widely applicable method and a mensurable formula for determining the µ11 constant of layer-dependent 2D materials with high precision. A combination of in situ micro-Raman spectroscopy and piezoresponse force microscopy (PFM) imaging was used to quantify the strain distribution and effective out-of-plane electromechanical coupling, respectively, for µ11 constant calculation. The µ11 constants and their physical correlation with the variable mechanical conditions of naturally bent structures have been obtained extensively for the representative mono-to-few layered MX2 family (M = W and Mo; X = S and Se), and the result is perfectly consistent with the estimated order-of-magnitude of the µ11 value (about 0.065) of monolayer MoS2. The quantification of the flexoelectric constant in this work not only promotes the understanding of mechanical and electromechanical properties in van der Waals materials, but also paves the way for developing novel 2D nano-energy devices and mechanical transducers based on flexoelectric effects.

Tunable lattice dynamics and dielectric functions of two-dimensional Bi2O2Se: striking layer and temperature dependent effects.

Two-dimensional (2D) Bi2O2Se semiconductors with a narrow band gap and ultrahigh mobility have been regarded as an emerging candidate for optoelectronic devices, whereas the ambiguous phonon characteristics and optical properties still limit their future applications. Herein, high-quality centimeter-scale 2D Bi2O2Se films are successfully synthesized to disclose the lattice dynamics and dielectric functions under the control of thickness and temperature. It has been demonstrated that the stronger electrostatic Bi-Se interactions result in a stiffened phonon vibration of thicker Bi2O2Se layers. Three excitons (Ea, Eb, and Ec) exhibit significant red shifts with layer stacking. Interestingly, the dielectric properties in the visible-near infrared region (Ea and Eb) are dominated by the combined effect of the joint density of states and mass density, whereas the dielectric properties in the ultraviolet region (Ec) are dominated by the exciton effect. Furthermore, the temperature-sensitivity of the phonon frequency and exciton transition energies is revealed to be layer-dependent. In particular, the optical response of Eb excitons exhibits a prominent dependence on temperature, which indicates a promising optical modulation by temperature in the visible spectrum. This study enriches the knowledge about phonon dynamics and dielectric properties for 2D Bi2O2Se, which provides an essential reference for high-performance related optoelectronic devices.

Pressure- and Temperature-Induced Structural Phase Diagram of Lead-Free (K0.5Na0.5)NbO3-0.05LiNbO3 Single Crystals: Raman Scattering and Infrared Study.

Ferroelectric lead-free KxNa1-xNbO3 (KNN) perovskite, whose piezoelectric properties can be comparable to those of traditional Pb-based systems, has aroused wide concern in recent years. However, the specific influences of the stress field on KNN's structure and piezoelectric properties have not been well clarified and there are few descriptions about the temperature-pressure phase diagram. Here, we analyzed the phonon mode behavior and structural evolution of K0.5Na0.5NbO3-0.05LiNbO3 (KNN-LN) and MnO2-doped single crystals with pressure- and temperature-dependent phase structure variations by theoretical calculation, polarized Raman scattering, and infrared reflectance spectra. The different phase structures can be predicted at high pressure using the CALYPSO method with its same-name code. The rhombohedral → orthorhombic → tetragonal → cubic phase transition process can be discovered in detail by Raman spectra under different temperatures and pressures. The phase coexistence on the thermal phase boundary was confirmed by basic anastomosis. Meanwhile, it was found that the substitution of Mn in the NbO6 octahedron aggravates the deformation of high pressure on KNN-LN and the substitution of Mn at the B-site intensifies the structural evolution more severely than at the A-site. The present study aims at exploring octahedra tilt, phonon vibrations, and the internal structure on the general critical phase boundary in KNN-LN crystals. It provides effective help for the study of lead-free perovskite phase transformation and the improvement in piezoelectric properties under a high-pressure field.

Ultra-Stable, Endurable, and Flexible Sb2TexSe3-x Phase Change Devices for Memory Application and Wearable Electronics.

Flexible memory and wearable electronics represent an emerging technology, thanks to their reliability, compatibility, and superior performance. Here, an Sb2TexSe3-x (STSe) phase change material was grown on flexible mica, which not only exhibited superior nature in thermal stability for phase change memory application but also revealed novel function performance in wearable electronics, thanks to its excellent mechanical reliability and endurance. The thermal stability of Sb2Te3 was improved obviously with the crystallization temperature elevated 60 K after Se doping, for the enhanced charge localization and stronger bonding energy, which was validated by the Vienna ab initio simulation package calculations. Based on the ultra-stability of STSe, the STSe-based phase change memory shows 65 000 reversible phase change ability. Moreover, the assembled flexible device can show real-time monitoring and recoverability response in sensing human activities in different parts of the body, which proves its effective reusability and potential as wearable electronics. Most importantly, the STSe device presents remarkable working reliability, reflected by excellent endurance over 100 s and long retention over 100 h. These results paved a novel way to utilize STSe phase change materials for flexible memory and wearable electronics with extreme thermal and mechanical stability and brilliant performance.

2D Transition Metal Dichalcogenide with Increased Entropy for Piezoelectric Electronics.

Piezoelectricity in 2D transition metal dichalcogenides (TMDs) has attracted considerable interest because of their excellent flexibility and high piezoelectric coefficient compared to conventional piezoelectric bulk materials. However, the ability to regulate the piezoelectric properties is limited because the entropy is constant for certain binary TMDs other than multielement ones. Herein, in order to increase the entropy, a ternary TMDs alloy, Mo1- x Wx S2 , with different W concentrations, is synthesized. The W concentration in the Mo1- x Wx S2 alloy can be controlled precisely in the low-supersaturation synthesis and the entropy can be tuned accordingly. The Mo0.46 W0.54 S2 alloy (x = 0.54) has the highest configurational entropy and best piezoelectric properties, such as a piezoelectric coefficient of 4.22 pm V-1 and a piezoelectric output current of 150 pA at 0.24% strain. More importantly, it can be combined into a larger package to increase the output current to 600 pA to cater to self-powered applications. Combining with excellent mechanical durability, a mechanical sensor based on the Mo0.46 W0.54 S2 alloy is demonstrated for real-time health monitoring.

Designing Monoclinic Heterophase Coexistence for the Enhanced Piezoelectric Performance in Ternary Lead-Based Relaxor Ferroelectrics.

Enhanced piezoelectric, dielectric properties and thermal stability in ternary relaxor-PbTiO3 based ferroelectric crystals are expected to develop the next-generation of electromechanical devices. However, due to their increased disorder compared to other ferroelectrics, designing a controllable phase boundary structure and engineered domain remains a challenging task. Here, we construct a monoclinic heterophase coexisting in a ternary Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystal with optimized composition and an ultrahigh piezoelectric coefficient of 1400 pC N-1, to quantify the correlation between spontaneous nanopolarity and phase heterogeneity, in an attempt to understand the origin of the exceptional functionalities. By designing an in situ high-resolution spectroscopic-microscopic technique, we have observed Ma and Mc heterophase mixtures spatially separated by the monoclinic heterophase boundary (MHB), which are responsible for the ferroelectric-dominated and relaxor-ferroelectric-dominated nanodomain structure, respectively. Internal energy mapping from optical soft mode dynamics reveals the inhomogeneous polarization and local symmetry on both sides of the MHB. Various molecular polarizabilities and localized octahedral distortions correlate directly with monoclinic regions and electromechanical contribution. This work clarifies the heterogeneity between structure, energy, and polar order and provides a new design freedom for advanced relaxor ferroelectrics.

Feasible Way to Achieve Multifunctional (K, Na)NbO3-Based Ceramics: Controlling Long-Range Ferroelectric Ordering.

It is challenging to achieve highly tunable multifunctional properties in one piezoelectric ceramic system through a simple method due to the complicated relationship between the microscopic structure and macroscopic property. Here, multifunctional potassium sodium niobate [(K, Na)NbO3 (KNN)]-based lead-free piezoceramics with tunable piezoelectric and electrostrictive properties are achieved by controlling the long-range ferroelectric ordering (LRFO) through antimony (Sb) doping. At a low Sb doping, the slightly distorted NbO6 octahedron and the softened B-O repulsion well maintain the LRFO and induce plenty of nanoscale domains coexisting with a few polar nanoregions (PNRs). Thereby, the diffused rhombohedral-orthorhombic-tetragonal (R-O-T) multiphase coexistence with distinct dielectric jumping is constructed near room temperature, by which the nearly 2-fold increase in the piezoelectric coefficient (d33 ∼ 539 pC/N) and the temperature-insensitive strain (the unipolar strain varies less than 8% at 27-120 °C) are obtained. At a high Sb doping, the LRFO is significantly destroyed, leading to predominant PNRs. Thus, a typical relaxor is obtained at the ferroelectric-paraelectric phase transition near room temperature, in which a large electrostrictive coefficient (Q33 = 0.035 m4/C2), independent of the electric field and temperature, is obtained and comparable to that of lead-based materials. Therefore, our results prove that controlling the LRFO is a feasible way to achieve high-performance multifunctional KNN-based ceramics and is beneficial to the future composition design for KNN-based ceramics.

Flexo-photoelectronic effect in n-type/p-type two-dimensional semiconductors and a deriving light-stimulated artificial synapse.

Flexoelectricity and photoelectricity with their coupled effect (the so-called flexo-photoelectronic effect), are of increasing interest in the study of electronics and optoelectronics in van der Waals layered semiconductors. However, the related device design is severely restricted owing to the ambiguous underlying physical nature of flexo-photoelectronic effects originating from the co-manipulation of light and strain-gradients. Here, flexoelectric polarization and the flexo-photoelectronic effect of few-layered semiconductors have been multi-dimensionally investigated from high-resolution microscopic characterization on the nanoscale, physics analysis, and deriving a device design. We found that two back-to-back built-in electric fields form in bent InSe and WSe2, and greatly modulate the transport behaviors of photogenerated carriers, further facilitating the separation of photogenerated electron-hole pairs and trapping the holes/electrons in InSe or WSe2 channels, recorded in realtime by a home-made technique of lighting Kelvin probe force microscopy (KPFM). The slow release of trapped carriers contributes to the photoconductance relaxation after illumination. Utilizing the photoconductance relaxation, a light-stimulated artificial synapse based on the flexo-photoelectronic effect of bent InSe has been achieved. Significantly, all the pair-pulse facilitation (PPF) behavior, spike frequency-dependent excitatory post-synaptic current (EPSC) and the transition from short-term memory (STM) to long-term memory (LTM) have been successfully realized in this artificial synapse. This work adds to the investigation of flexo-photoelectronic effects on 2D optoelectronics, and moves towards the development of 2D neuromorphic electronics.

Optically Modulated HfS2-Based Synapses for Artificial Vision Systems.

The simulation of human brain neurons by synaptic devices could be an effective strategy to break through the notorious "von Neumann Bottleneck" and "Memory Wall". Herein, opto-electronic synapses based on layered hafnium disulfide (HfS2) transistors have been investigated. The basic functions of biological synapses are realized and optimized by modifying pulsed light conditions. Furthermore, 2 × 2 pixel imaging chips have also been developed. Two-pixel visual information is illuminated on diagonal pixels of the imaging array by applying light pulses (λ = 405 nm) with different pulse frequencies, mimicking short-term memory and long-term memory characteristics of the human vision system. In addition, an optically/electrically driven neuromorphic computation is demonstrated by machine learning to classify hand-written numbers with an accuracy of about 88.5%. This work will be an important step toward an artificial neural network comprising neuromorphic vision sensing and training functions.

Effects of SF6 decomposition components and concentrations on the discharge faults and insulation defects in GIS equipment.

Gas-insulated switchgear (GIS) is widely used across multiple electric stages and different power grid levels. However, the threat from several inevitable faults in the GIS system surrounds us for the safety of electricity use. In order to improve the evaluation ability of GIS system safety, we propose an efficient strategy by using machine learning to conduct SF6 decomposed components analysis (DCA) for further diagnosing discharge fault types in GIS. Note that the empirical probability function of different faults fitted by the Arrhenius chemical reaction model has been investigated into the robust feature engineering for machine learning based GIS diagnosing model. Six machine learning algorithms were used to establish models for the severity of discharge fault and main insulation defects, where identification algorithms were trained by learning the collection dataset composing the concentration of the different gas types (SO2, SOF2, SO2F2, CF4, and CO2, etc.) in the system and their ratios. Notably, multiple discharge fault types coexisting in GIS can be effectively identified based on a probability model. This work would provide a great insight into the development of evaluation and optimization on solving discharge fault in GIS.

New Pressure Stabilization Structure in Two-Dimensional PtSe2.

The frequency shifts and lattice dynamics to unveil the vibrational properties of platinum diselenide (PtSe2) are investigated using pressure-dependent polarized Raman scattering at room temperature up to 25 GPa. The two phonon modes Eg and A1g display similar hardening trends; both the Raman peak positions and full widths at half-maximum have distinct mutation phenomena under high pressure. Especially, the split Eg mode at 4.3 GPa confirms the change of the lattice symmetry. With the aid of the first-principles calculations, a new pressure stabilization structure C2/m of PtSe2 has been found to be in good agreement with experiments. The band structures calculations reveal that the new phase is a novel type-I Dirac semimetal. The results demonstrate that the pressure-dependent Raman spectra combined with theoretical predictions may open a new window for searching and controlling the phase structure and Dirac cones of two-dimensional materials.

Lattice vibration characteristics in layered InSe films and the electronic behavior of field-effect transistors.

Understanding how temperature affects the structural and electronic properties for two-dimensional (2D) semiconductors could promote the application and development of nanoelectronic devices. Here, the temperature dependence of lattice structure for indium selenide (InSe) nanosheets and the corresponding electronic properties of 3 nm indium-deposited InSe field-effect transistors (FETs) are systematically demonstrated. Analyses of Raman spectra suggest that the difference of phonon frequency (Δω) for the A[Formula: see text] mode is found to be 3.14 cm-1, which is larger than that of the E[Formula: see text] mode due to the stronger electron-phonon coupling for the A[Formula: see text] mode. The device performance based on indium-deposited InSe is systematically explained using Kelvin probe force microscopy (KPFM) and the predicted energy band structure. Furthermore, FETs based on temperature and variable thickness InSe flakes are designed as applicable devices. Our findings are of fundamental importance to explain the underlying physics in intrinsic InSe transistors and improve further applications.

Mixed-Dimensional Van der Waals Heterostructure Photodetector.

Van der Waals (vdW) heterostructures, integrated two-dimensional (2D) materials with various functional materials, provide a distinctive platform for next-generation optoelectronics with unique flexibility and high performance. However, exploring the vdW heterostructures combined with strongly correlated electronic materials is hitherto rare. Herein, a novel temperature-sensitive photodetector based on the GaSe/VO2 mixed-dimensional vdW heterostructure is discovered. Compared with previous devices, our photodetector exhibits excellent enhanced performance, with an external quantum efficiency of up to 109.6% and the highest responsivity (358.1 mA·W-1) under a 405 nm laser. Interestingly, we show that the heterostructure overcomes the limitation of a single material under the interaction between VO2 and GaSe, where the photoresponse is highly sensitive to temperature and can be further vanished at the critical value. The metal-insulator transition of VO2, which controls the peculiar band-structure evolution across the heterointerface, is demonstrated to manipulate the photoresponse variation. This study enables us to elucidate the method of manipulating 2D materials by strongly correlated electronic materials, paving the way for developing high-performance and special optoelectronic applications.

Constructing polymers towards ultrathin nanosheets with dual mesopores and intrinsic photoactivity.

We developed ultrathin dual-mesoporous polymer nanosheets by combining co-assembly of different templates with in situ synthesis of functional polymers, which featured inherent smaller and template-directed larger mesopores (2.6 nm and 15 nm, respectively), ultrathin nanolayers (20 nm), high surface area (268 m2 g-1), intrinsic fluorescent properties and effective detectability for organophosphates.

High Responsivity and External Quantum Efficiency Photodetectors Based on Solution-Processed Ni-Doped CuO Films.

Photodetectors based on p-type metal oxides are still a challenge for optoelectronic device applications. Many effects have been paid to improve their performance and expand their detection range. Here, high-quality Cu1-xNixO (x = 0, 0.2, and 0.4) film photodetectors were prepared by a solution process. The crystal quality, morphology, and grain size of Cu1-xNixO films can be modulated by Ni doping. Among the photodetectors, the Cu0.8Ni0.2O photodetector shows the maximum photocurrent value (6 × 10-7 A) under a 635 nm laser illumination. High responsivity (26.46 A/W) and external quantum efficiency (5176%) are also achieved for the Cu0.8Ni0.2O photodetector. This is because the Cu0.8Ni0.2O photosensitive layer exhibits high photoconductivity, low surface states, and high crystallization after 20% Ni doping. Compared to the other photodetectors, the Cu0.8Ni0.2O photodetector exhibits the optimal response in the near-infrared region, owing to the high absorption coefficient. These findings provide a route to fabricate high-performance and wide-detection range p-type metal oxide photodetectors.

Probing Effective Out-of-Plane Piezoelectricity in van der Waals Layered Materials Induced by Flexoelectricity.

Many van der Waals layered 2D materials, such as h-BN, transition metal dichalcogenides (TMDs), and group-III monochalcogenides, have been predicted to possess piezoelectric and mechanically flexible natures, which greatly motivates potential applications in piezotronic devices and nanogenerators. However, only intrinsic in-plane piezoelectricity exists in these 2D materials and the piezoelectric effect is confined in odd-layers of TMDs. The present work is intent on combining the free-standing design and piezoresponse force microscopy techniques to obtain and directly quantify the effective out-of-plane electromechanical coupling induced by strain gradient on atomically thin MoS2 and InSe flakes. Conspicuous piezoresponse and the measured piezoelectric coefficient with respect to the number of layers or thickness are systematically illustrated for both MoS2 and InSe flakes. Note that the promising effective piezoelectric coefficient (deff 33 ) of about 21.9 pm V-1 is observed on few-layered InSe. The out-of-plane piezoresponse arises from the net dipole moment along the normal direction of the curvature membrane induced by strain gradient. This work not only provides a feasible and flexible method to acquire and quantify the out-of-plane electromechanical coupling on van der Waals layered materials, but also paves the way to understand and tune the flexoelectric effect of 2D systems.