High-Performance Artificial Synapse Based on CVD-Grown WSe2 Flakes with Intrinsic Defects.

High-performance artificial synaptic devices with rich functions are highly desired for the development of an advanced brain-like neuromorphic system. Here, we prepare synaptic devices based on a CVD-grown WSe2 flake, which has an unusual morphology of nested triangles. The WSe2 transistor exhibits robust synaptic behaviors such as excitatory postsynaptic current, paired-pulse facilitation, short-time plasticity, and long-time plasticity. Furthermore, due to its high sensitivity to light illumination, the WSe2 transistor exhibits excellent light-dosage-dependent and light wavelength-dependent plasticity, which endow the synaptic device with more intelligent learning and memory functions. In addition, WSe2 optoelectronic synapses can mimic "learning experience" behavior and associative learning behavior like the brain. An artificial neural network is simulated for pattern recognition of hand-written digital images in the MNIST data set and the best recognition accuracy could reach 92.9% based on weight updating training of our WSe2 device. Detailed surface potential analysis and PL characterization reveal that the intrinsic defects generated in growth are dominantly responsible for the controllable synaptic plasticity. Our work suggests that the CVD-grown WSe2 flake with intrinsic defects capable of robust trapping/de-trapping charges holds great application prospects in future high-performance neuromorphic computation.

Template-Assisted Synthesis of a Large-Area Ordered Perovskite Nanowire Array for a High-Performance Photodetector.

One-dimensional (1D) organic-inorganic hybrid perovskite nanowires (NWs) with well-defined structures possess superior optical and electrical properties for optoelectronic applications. However, most of the perovskite NWs are synthesized in air, which makes the NWs susceptible to water vapor, resulting in large amounts of grain boundaries or surface defects. Here, a template-assisted antisolvent crystallization (TAAC) method is designed to fabricate CH3NH3PbBr3 NWs and arrays. It is found that the as-synthesized NW array has designable shapes, low crystal defects, and ordered alignment, which is attributed to the sequestration of water and oxygen in air by the introduction of acetonitrile vapor. The photodetector based on the NWs exhibits an excellent response to light illumination. Under the illumination of a 532 nm laser with 0.1 µW and a bias of -1 V, the responsivity and detectivity of the device reach 1.55 A/W and 1.21 × 1012 Jones, respectively. The transient absorption spectrum (TAS) shows a distinct ground state bleaching signal only at 527 nm, which corresponds to the absorption peak induced by the interband transition of CH3NH3PbBr3. Narrow absorption peaks (a few nanometers) indicate that the energy-level structures of CH3NH3PbBr3 NWs only have a few impurity-level-induced transitions leading to additional optical loss. This work provides an effective and simple strategy to achieve high-quality CH3NH3PbBr3 NWs, which exhibit potential application in photodetection.

Braiding Lateral Morphotropic Grain Boundaries in Homogenetic Oxides.

Interfaces formed by correlated oxides offer a critical avenue for discovering emergent phenomena and quantum states. However, the fabrication of oxide interfaces with variable crystallographic orientations and strain states integrated along a film plane is extremely challenging by conventional layer-by-layer stacking or self-assembling. Here, the creation of morphotropic grain boundaries (GBs) in laterally interconnected cobaltite homostructures is reported. Single-crystalline substrates and suspended ultrathin freestanding membranes provide independent templates for coherent epitaxy and constraint on the growth orientation, resulting in seamless and atomically sharp GBs. Electronic states and magnetic behavior in hybrid structures are laterally modulated and isolated by GBs, enabling artificially engineered functionalities in the planar matrix. This work offers a simple and scalable method for fabricating unprecedented innovative interfaces through controlled synthesis routes as well as providing a platform for exploring potential applications in neuromorphics, solid-state batteries, and catalysis.

Exfoliation of 2D van der Waals crystals in ultrahigh vacuum for interface engineering.

Two-dimensional (2D) materials and their heterostructures have been intensively studied in recent years due to their potential applications in electronic, optoelectronic, and spintronic devices. Nonetheless, the realization of 2D heterostructures with atomically flat and clean interfaces remains challenging, especially for air-sensitive materials, which hinders the in-depth investigation of interface-induced phenomena and the fabrication of high-quality devices. Here, we circumvented this challenge by exfoliating 2D materials in an ultrahigh vacuum. Remarkably, ultraflat and clean substrate surfaces can assist the exfoliation of 2D materials, regardless of the substrate and 2D material, thus providing a universal method for the preparation of heterostructures with ideal interfaces. In addition, we studied the properties of two prototypical systems that cannot be achieved previously, including the electronic structure of monolayer phospherene and optical responses of transition metal dichalcogenides on different metal substrates. Our work paves the way to engineer rich interface-induced phenomena, such as proximity effects and moiré superlattices.

High-Performance Phototransistors Based on MnPSe3 and Its Hybrid Structures with Au Nanoparticles.

Layered metal thiophosphates with a general formula MPX3 (M is a group VIIB or VIII element and X is a chalcogen) have emerged as a novel member in a two-dimensional (2D) family with fascinating physical and chemical properties. Herein, the photoelectric performance of the few-layer MnPSe3 was studied for the first time. The multilayer MnPSe3 shows p-type conductivity and its field-effect transistor delivers an ultralow dark current of about 0.1 pA. The photoswitching ratio reaches ∼103 at a wavelength of 375 nm, superior to that of other thiophosphates. A responsivity and detectivity of 392.78 mA/W and 2.19 × 109 Jones, respectively, have been demonstrated under irradiation of 375 nm laser with a power intensity of 0.1 mW/cm2. In particular, the photocurrent can be remarkably increased up to 30 times by integrating a layer of Au nanoparticle array at the bottom of the MnPSe3 layer. The metal-semiconductor interfacial electric field and the strain-induced flexoelectric polarization field caused by the underlying nanorugged Au nanoparticles are proposed to contribute together to the significant current improvement.