The translation of biological synapses onto a hardware platform is an important step toward the realization of brain‐inspired electronics. However, to mimic biological synapses, devices till‑date continue to rely on the need for simultaneously altering the polarity of an applied electric field or the output of these devices is photonic instead of an electrical synapse. As the next big step toward practical realization of optogenetics inspired circuits that exhibit fidelity and flexibility of biological synapses, optically‑stimulated synaptic devices without a need to apply polarity‑altering electric field are needed. Utilizing a unique photoresponse in black phosphorus (BP), here reported is an all‑optical pathway to emulate excitatory and inhibitory action potentials by exploiting oxidation‑related defects. These optical synapses are capable of imitating key neural functions such as psychological learning and forgetting, spatiotemporally correlated dynamic logic and Hebbian spike‑time dependent plasticity. These functionalities are also demonstrated on a flexible platform suitable for wearable electronics. Such low‐power consuming devices are highly attractive for deployment in neuromorphic architectures. The manifestation of cognition and spatiotemporal processing solely through optical stimuli provides an incredibly simple and powerful platform to emulate sophisticated neural functionalities such as associative sensory data processing and decision making.

Imprinting vision as memory is a core attribute of human cognitive learning. Fundamental to artificial intelligence systems are bioinspired neuromorphic vision components for the visible and invisible segments of the electromagnetic spectrum. Realization of a single imaging unit with a combination of in‐built memory and signal processing capability is imperative to deploy efficient brain‐like vision systems. However, the lack of a platform that can be fully controlled by light without the need to apply alternating polarity electric signals has hampered this technological advance. Here, a neuromorphic imaging element based on a fully light‐modulated 2D semiconductor in a simple reconfigurable phototransistor structure is presented. This standalone device exhibits inherent characteristics that enable neuromorphic image pre‐processing and recognition. Fundamentally, the unique photoresponse induced by oxidation‐related defects in 2D black phosphorus (BP) is exploited to achieve visual memory, wavelength‐selective multibit programming, and erasing functions, which allow in‐pixel image pre‐processing. Furthermore, all‐optically driven neuromorphic computation is demonstrated by machine learning to classify numbers and recognize images with an accuracy of over 90%. The devices provide a promising approach toward neurorobotics, human–machine interaction technologies, and scalable bionic systems with visual data storage/buffering and processing.

Layered black phosphorus (BP), a promising 2D material, tends to oxidize under ambient conditions. While such defective BP is typically considered undesirable, defect engineering has in fact been exploited in contemporary materials to create new behaviors and functionalities. In this spirit, new opportunities arising from intrinsic defect states in BP, particularly through harnessing unique photoresponse characteristics, and demonstrating three distinct optoelectronic applications are demonstrated. First, the ability to distinguish between UV-A and UV-B radiations using a single material that has tremendous implications for skin health management is shown. Second, the same device is utilized to show an optically stimulated mimicry of synaptic behavior opening new possibilities in neuromorphic computing. Third, it is shown that serially connected devices can be used to perform digital logic operations using light. The underpinning photoresponse is further translated on flexible substrates, highlighting the viability of the technology for mechanically conformable and wearable systems. This demonstration paves the way toward utilizing the unexplored potential offered by defect engineering of 2D materials for applications spanning across a broad range of disciplines.

Elemental two-dimensional black phosphorus (BP) is a highly anisotropic versatile material capable of exhibiting wide ranging electronic characteristics ranging from semi-metallic to semiconducting. Its thickness dependent tunable energy gap makes it an exciting prospect for deployment in a variety of applications. The main hurdle limiting diverse applications incorporating BP is its ambient instability. BP degrades rapidly under room conditions, affecting its structure and properties. In this report, we cover the recent progress that has occurred towards protecting BP from ambient degradation. We review the major developments in effectively countering the problem and compare their relative degrees of success. This is provided in the context of the mechanisms governing the atmospheric instability of this material. A targeted focus is kept on the various causes of degradation of BP in atmospheric conditions and the protection strategies that have been implemented so far. IntroductionTwo-dimensional (2D) materials have attracted unprecedented attention over the past decade for their striking properties that are not normally present in their bulk form. Recently, 2D

COMMUNICATION (1 of 8)deployed in the form of van der Waals (vdW) heterostructures that enable fascinating coupled properties from stacked individual layers of 2D sheets which can be exploited in several applications, [2,6,7] including tunneling transistors, [8] quantum hall systems, [9] electrochemical hydrogen evolution reaction, [10] optoelectronics, [4,11] and electronics. [3,12] The p-n junctions are the building blocks of the semiconductor industry, in which the p-n junction heterostructures made from ultrathin materials are of great interest in specialized electronics, optoelectronics, and photonics due to their intriguing coupled properties of the different crystals. [5][6][7]13] Several methods for exfoliation and/or deposition exist such as chemical vapor deposition (CVD), [7] pulsed laser deposition (PLD), [14] molecular beam epitaxy (MBE), [15] pick-and-lift vdW technique, [16] and mechanical exfoliation. [17] These conventional approaches are time consuming and require complicated fabrication processes, [18] yet resulting in devices with small effective areas. [19] Liquid metals are emerging materials which can be used in microfluidics components, [20] sensors, [21] electrodes, [22] phototransistors, [23] flexible and stretchable devices, [24] disease treatment, [22] biomedical field, [22] and in synthesis of low-dimensional materials. [25] Liquid metals have been shown to form a naturally occurring atomically thin layer of oxide at their interface with air, [26][27][28][29] and using liquid metal as a reaction solvent can give access to a sizable portion of oxide elements including oxides which are intrinsically nonlayered crystals. [26] The exfoliated oxides can be converted to sulfides and phosphates. [30] Combination of these atomically thin layers should provide a vast number of vdW heterostructures that are yet to be explored.In this work, atomically thin oxide skin of low melting point liquid metals of tin and indium including p-type tin oxide (SnO) [27] and n-type indium oxide (In 2 O 3 ) [31] are stacked to produce large-area heterostructures with a high degree of homogeneity. Indeed, the p-n vdW heterojunctions feature current rectification properties with exceptionally fast photoresponse times and high sensitivity for UV light. The demonstrated liquid metal synthesis framework offers the possibility of synthesizing and exploring a range of tailored heterostructures for applications in next-generation optoelectronic and photodetection devices.

Intriguing physical and chemical properties of atomically thin semiconductors provide avenues for the development of the next-generation electronics, optoelectronics, and sensing applications. However, many materials are intrinsically nonlayered and therefore difficult to obtain in two dimensions (2D) due to the presence of strong in-plane bonds. Here, we adopted liquid metal synthetic strategies to produce 2D gallium sulfide (Ga2S3), which is an intrinsically nonlayered material. The obtained monoclinic α-Ga2S3 has a relatively high field-effect mobility of 3.5 cm2 V–1 s–1 and features a p-type material with a bandgap of 2.1 eV. Photodetectors that are made based on these synthesized 2D Ga2S3 exhibit relatively strong photodetectivity of 1010 jones and photoresponsivity of 240 A W–1 in visible wavelengths. The 2D Ga2S3 is also found to be suitable for sensing of nitrogen dioxide (NO2) gas at low evaluated temperatures. Excellent electronic, optical, and gas sensing performance demonstrated in this work offers great promises for synthesizing high quality 2D materials based on the liquid metal framework.

A major health concern of the 21st century is the rise of multi-drug resistant pathogenic microbial species. Recent technological advancements have led to considerable opportunities for low-dimensional materials (LDMs) as potential next-generation antimicrobials. LDMs have demonstrated antimicrobial behaviour towards a variety of pathogenic bacterial and fungal cells, due to their unique physicochemical properties. This review provides a critical assessment of current LDMs that have exhibited antimicrobial behaviour and their mechanism of action. Future design considerations and constraints in deploying LDMs for antimicrobial applications are discussed. It is envisioned that this review will guide future design parameters for LDM-based antimicrobial applications.

The unique and long-range ordered-vacancy structure in wafer-scale grown single-unit-cell-thick In2S3 facilitates excellent electronic performance.