Elemental Analogues of Graphene: Silicene,Germanene, Stanene,and Phosphorene

The fascinating electronic and optoelectronic properties of free‐standing graphene has led to the exploration of alternative two‐dimensional materials that can be easily integrated with current generation of electronic technologies. In contrast to 2D oxide and dichalcogenides, elemental 2D analogues of graphene, which include monolayer silicon (silicene), are fast emerging as promising alternatives, with predictions of high degree of integration with existing technologies. This article reviews this emerging class of 2D elemental materials – silicene, germanene, stanene, and phosphorene – with emphasis on fundamental properties and synthesis techniques. The need for further investigations to establish controlled synthesis techniques and the viability of such elemental 2D materials is highlighted. Future prospects harnessing the ability to manipulate the electronic structure of these materials for nano‐ and opto‐electronic applications are identified.     

Liquid-Metal Synthesized Ultrathin SnS Layers for High-Performance Broadband Photodetectors

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.     

Long Duration Persistent Photocurrent in 3 nm Thin Doped Indium Oxide for Integrated Light Sensing and In-Sensor Neuromorphic Computation

Miniaturization and energy consumption by computational systems remain major challenges to address. Optoelectronics based synaptic and light sensing provide an exciting platform for neuromorphic processing and vision applications offering several advantages. It is highly desirable to achieve single-element image sensors that allow reception of information and execution of in-memory computing processes while maintaining memory for much longer durations without the need for frequent electrical or optical rehearsals. In this work, ultra-thin (<3 nm) doped indium oxide (In₂O₃) layers are engineered to demonstrate a monolithic two-terminal ultraviolet (UV) sensing and processing system with long optical state retention operating at 50 mV. This endows features of several conductance states within the persistent photocurrent window that are harnessed to show learning capabilities and significantly reduce the number of rehearsals. The atomically thin sheets are implemented as a focal plane array (FPA) for UV spectrum based proof-of-concept vision system capable of pattern recognition and memorization required for imaging and detection applications. This integrated light sensing and memory system is deployed to illustrate capabilities for real-time, in-sensor memorization, and recognition tasks. This study provides an important template to engineer miniaturized and low operating voltage neuromorphic platforms across the light spectrum based on application demand.     

Two‐Dimensional Molybdenum Trioxide and Dichalcogenides

In the quest to discover the properties of planar semiconductors, two‐dimensional molybdenum trioxide and dichalcogenides have recently attracted a large amount of interest. This family, which includes molybdenum trioxide (MoO₃), disulphide (MoS₂), diselenide (MoSe₂) and ditelluride (MoTe₂), possesses many unique properties that make its compounds appealing for a wide range of applications. These properties can be thickness dependent and may be manipulated via a large number of physical and chemical processes. In this Feature Article, a comprehensive review is delivered of the fundamental properties, synthesis techniques and applications of layered and planar MoO₃, MoS₂, MoSe₂, and MoTe₂ along with their future prospects.     

Atomically thin layers of MoS2 via a two step thermal evaporation-exfoliation method

Two dimensional molybdenum disulfide (MoS(2)) has recently become of interest to semiconductor and optic industries. However, the current methods for its synthesis require harsh environments that are not compatible with standard fabrication processes. We report on a facile synthesis method of layered MoS(2) using a thermal evaporation technique, which requires modest conditions. In this process, a mixture of MoS(2) and molybdenum dioxide (MoO(2)) is produced by evaporating sulfur powder and molybdenum trioxide (MoO(3)) nano-particles simultaneously. Further annealing in a sulfur-rich environment transforms majority of the excess MoO(2) into layered MoS(2). The deposited MoS(2) is then mechanically exfoliated into minimum resolvable atomically thin layers, which are characterized using micro-Raman spectroscopy and atomic force microscopy. Furthermore Raman spectroscopy is employed to determine the effect of electrochemical lithium ion exposure on atomically thin layers of MoS(2).     

Ambient Protection of Few‐Layer Black Phosphorus via Sequestration of Reactive Oxygen Species

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.     

CNT/PDMS composite membranes for H2 and CH4 gas separation

Polydimethylsiloxane (PDMS) composites with different weight amounts of multi-walled carbon nanotubes (MWCNT) were synthesised as membranes to evaluate their gas separation properties. The selectivity of the membranes was investigated for the separation of H₂ from CH₄ gas species. Membranes with MWCNT concentrations of 1% increased the selectivity to H₂ gas by 94.8%. Furthermore, CH₄ permeation was almost totally blocked through membranes with MWCNT concentrations greater than 5%. Vibrational spectroscopy and X-ray photoelectron spectroscopy techniques revealed that upon the incorporation of MWCNT a decrease in the number of available Si–CH₃ and Si–O bonds as well as an increase in the formation of Si–C bonds occurred that initiated the reduction in CH₄ permeation. As a result, the developed membranes can be an efficient and low cost solution for separating H₂ from larger gas molecules such as CH₄.     

Nanoscale Resistive Switching in Amorphous Perovskite Oxide (a‐SrTiO3) Memristors

Memristive devices are the precursors to high density nanoscale memories and the building blocks for neuromorphic computing. In this work, a unique room temperature synthesized perovskite oxide (amorphous SrTiO₃: a‐STO) thin film platform with engineered oxygen deficiencies is shown to realize high performance and scalable metal‐oxide‐metal (MIM) memristive arrays demonstrating excellent uniformity of the key resistive switching parameters. a‐STO memristors exhibit nonvolatile bipolar resistive switching with significantly high (10³–10⁴) switching ratios, good endurance (>10⁶I–V sweep cycles), and retention with less than 1% change in resistance over repeated 10⁵ s long READ cycles. Nano‐contact studies utilizing in situ electrical nanoindentation technique reveal nanoionics driven switching processes that rely on isolatedly controllable nano‐switches uniformly distributed over the device area. Furthermore, in situ electrical nanoindentation studies on ultrathin a‐STO/metal stacks highlight the impact of mechanical stress on the modulation of non‐linear ionic transport mechanisms in perovskite oxides while confirming the ultimate scalability of these devices. These results highlight the promise of amorphous perovskite memristors for high performance CMOS/CMOL compatible memristive systems.