Gas sensor devices have traditionally comprised thin films of metal oxides, with tin oxide, zinc oxide and indium oxide being some of the most common materials employed. With the recent discovery of ...novel metal oxide nanostructures, sensors comprising nano-arrays or single nanostructures have shown improved performance over the thin films. The improved response of the nanostructures to different gases has been primarily attributed to the highly single crystalline surfaces as well as large surface area of the nanostructures. In this paper the properties of clean and defected quasi one-dimensional ZnO nanostructures, including hexagonal and triangular nanowires, nanotubes and facetted nanotubes are reviewed. The adsorption of atoms and molecules on the ZnO nanostructures are also reviewed and the findings are compared to studies examining similar reactions on nanostructured metal oxide surfaces for sensing purposes. While both experimental and theoretical approaches have been employed to examine gas sensor reactions, this review focuses on studies that employ electronic structure calculations, which primarily concentrate on using density functional theory. Computational studies have been useful in elucidating the reaction mechanism, binding strength, charge transfer as well as other electronic and structural properties of the nanomaterials and the gas-sensor interaction. Despite these studies there are still significant areas of research that need to be pursued that will assist in the link between theoretical and experimental findings, as well as advancing the current chemical and physical understanding of these novel materials. A summary and outlook for future directions of this exciting area of research is also provided.
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.
An all‐optically tunable neuromorphic imaging element based on black phosphorus (BP) is demonstrated. The unusual wavelength‐dependent photocurrent in BP is harnessed to optically program and erase visual memory elements. Concurrently, the same elements are capable of in‐pixel image pre‐processing in an array and optoelectronic machine learning for image recognition through artificial neural networks.
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.
The potential of self‐propagating defects in black phosphorus (BP) is exploited to create functional optoelectronic capabilities, particularly unique wavelength‐selective photoresponse characteristics. To unveil the potential offered by defect engineering of 2D materials, three distinct optoelectronic applications for UV‐A/B discrimination, light‐stimulated logic operations, and neuromorphic computation are demonstrated in BP devices.
Few‐layer black phosphorous (BP) has emerged as a promising candidate for next‐generation nanophotonic and nanoelectronic devices. However, rapid ambient degradation of mechanically exfoliated BP ...poses challenges in its practical deployment in scalable devices. To date, the strategies employed to protect BP have relied upon preventing its exposure to atmospheric conditions. Here, an approach that allows this sensitive material to remain stable without requiring its isolation from the ambient environment is reported. The method draws inspiration from the unique ability of biological systems to avoid photo‐oxidative damage caused by reactive oxygen species. Since BP undergoes similar photo‐oxidative degradation, imidazolium‐based ionic liquids are employed as quenchers of these damaging species on the BP surface. This chemical sequestration strategy allows BP to remain stable for over 13 weeks, while retaining its key electronic characteristics. This study opens opportunities to practically implement BP and other environmentally sensitive 2D materials for electronic applications.
Few‐layer black phosphorous (BP) has recently emerged as a promising elemental analog to graphene. A chemical sequestration approach is reported that allows BP to remain stable without requiring its isolation from the ambient environment. The strategy allows BP to remain stable for over 13 weeks, while retaining its key electronic characteristics.
Silicene, a two-dimensional honeycomb network of silicon atoms like graphene, holds great potential as a key material in the next generation of electronics; however, its use in more demanding ...applications is prevented because of its instability under ambient conditions. Here we report three types of bilayer silicenes that form after treating calcium-intercalated monolayer silicene (CaSi2) with a BF4(-) -based ionic liquid. The bilayer silicenes that are obtained are sandwiched between planar crystals of CaF2 and/or CaSi2, with one of the bilayer silicenes being a new allotrope of silicon, containing four-, five- and six-membered sp(3) silicon rings. The number of unsaturated silicon bonds in the structure is reduced compared with monolayer silicene. Additionally, the bandgap opens to 1.08 eV and is indirect; this is in contrast to monolayer silicene which is a zero-gap semiconductor.
The electronic properties of thiol‐functionalized 2D MoS2 nanosheets are investigated. Shifts in the valence and conduction bands and Fermi levels are observed while bandgaps remain unaffected. These ...findings allow the tuning of energy barriers between 2D MoS2 and other materials, which can lead to improved control over 2D MoS2‐based electronic and optical devices and catalysts.
We demonstrate, using first-principles molecular-dynamics simulations, that oxidation of silicene can easily take place either at low or high oxygen doses, which importantly helps clarify previous ...inconsistent reports on the oxidation of silicene on the Ag(111) substrate. We show that, while the energy barrier for an O2 molecule reacting with a Si atom strongly depends on the position and orientation of the molecule, the O2 molecule immediately dissociates and forms an Si-O-Si configuration once it finds a barrier-less chemisorption pathway around an outer Si atom of the silicene overlayer. A synergistic effect between the molecular dissociation and subsequent structural rearrangements is found to accelerate the oxidation process at a high oxygen dose. This effect also enhances self-organized formation of sp(3)-like tetrahedral configurations (consisting of Si and O atoms), which results in collapse of the two-dimensional silicene structure and its exfoliation from the substrate. We also find that the electronic properties of the silicene can be significantly altered by oxidation. The present findings suggest that low flux and low temperature of the oxygen gas are key to controlling oxidation of silicene.
Chromism-based optical filters is a niche field of research, due to there being only a handful of electrochromic materials. Typically, electrochromic transition metal oxides such as MoO3 and WO3 are ...utilized in applications such as smart windows and electrochromic devices (ECD). Herein, we report MoO3–x -based electrically activated ultraviolet (UV) filters. The MoO3–x grown on indium tin oxide (ITO) substrate is mechanically assembled onto an electrically activated proton exchange membrane. Reversible H+ injection/extraction in MoO3–x is employed to switch the optical transmittance, enabling an electrically activated optical filter. The devices exhibit broadband transmission modulation (325–800 nm), with a peak of ∼60% in the UV-A range (350–392 nm). Comparable switching times of 8 s and a coloration efficiency of up to 116 cm2 C–1 are achieved.
As lithium-ion (Li-ion) batteries approach their theoretical limits, alternative energy storage systems that can power technology with greater energy demands must be realized. Li-metal batteries, ...particularly Li-air batteries (LABs), are considered a promising energy storage candidate due to their inherent lightweight and energy-dense properties. Unfortunately, LAB practicality remains hindered by inadequate oxygen solubility and diffusion rates within the electrolyte, both which are fundamental for LAB operation. Due to exceptionally high oxygen solubilities, perfluorochemicals (PFCs) have been investigated as a promising solution to this issue. Although PFCs have been reported to enhance LAB performance and longevity when implemented within the cathodic regions of LABs in several studies, the influence of this class of compounds on other components of the battery (including the anode and the electrolyte) is also highly important. This paper reviews the use of PFCs in LABs to date and discusses the performance enhancements resulting from their implementation. We identify and discuss future prospects and emerging research directions for the use of PFCs into LAB design, in the effort toward realization of high-performing LAB technologies.
Atomically thin materials face an ongoing challenge of scalability, hampering practical deployment despite their fascinating properties. Tin monosulfide (SnS), a low‐cost, naturally abundant layered ...material with a tunable bandgap, displays properties of superior carrier mobility and large absorption coefficient at atomic thicknesses, making it attractive for electronics and optoelectronics. However, the lack of successful synthesis techniques to prepare large‐area and stoichiometric atomically thin SnS layers (mainly due to the strong interlayer interactions) has prevented exploration of these properties for versatile applications. Here, SnS layers are printed with thicknesses varying from a single unit cell (0.8 nm) to multiple stacked unit cells (≈1.8 nm) synthesized from metallic liquid tin, with lateral dimensions on the millimeter scale. It is reveal that these large‐area SnS layers exhibit a broadband spectral response ranging from deep‐ultraviolet (UV) to near‐infrared (NIR) wavelengths (i.e., 280–850 nm) with fast photodetection capabilities. For single‐unit‐cell‐thick layered SnS, the photodetectors show upto three orders of magnitude higher responsivity (927 A W−1) than commercial photodetectors at a room‐temperature operating wavelength of 660 nm. This study opens a new pathway to synthesize reproduceable nanosheets of large lateral sizes for broadband, high‐performance photodetectors. It also provides important technological implications for scalable applications in integrated optoelectronic circuits, sensing, and biomedical imaging.
Miniaturized photodetectors are key for the next generation of sensing, communication, and imaging technologies. Single‐atom‐thick SnS layers are printed on a millimeter scale to showcase application in high‐performance photodetectors. These SnS‐based ultrafast photodetectors show a broadband spectral response ranging from deep ultraviolet to near infrared wavelengths (i.e., 280 to 850 nm) with excellent figures of merit.
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