Low Optical Writing Energy Multibit Optoelectronic Memory Based on SnS2 /h-BN/Graphene Heterostructure.

With the rapid development of artificial intelligence and neural network computing, the requirement for information storage in computing is gradually increasing. Floating gate memories based on 2D materials has outstanding characteristics such as non-volatility, optical writing, and optical storage, suitable for application in photonic in-memory computing chips. Notably, the optoelectronic memory requires less optical writing energy, which means lower power consumption and greater storage levels. Here, the authors report an optoelectronic memory based on SnS2 /h-BN/graphene heterostructure with an extremely low photo-generated hole tunneling barrier of 0.23 eV. This non-volatile multibit floating gate memory shows a high switching ratio of 106 and a large memory window range of 64.8 V in the gate range ±40 V. And the memory device can achieve multilevel storage states of 50 under a low power light pulses of 0.32 nW and small light pulse width of 50 ms. Thanks to the Fowler-Nordheim tunneling of the photo-generated holes, the optical writing energy of the optoelectronic memory has been successfully reduced by one to three orders of magnitude compared to existing 2D materials-based systems.

Defective Anatase TiO2-x Mesocrystal Growth In Situ on g-C3 N4 Nanosheets: Construction of 3D/2D Z-Scheme Heterostructures for Highly Efficient Visible-Light Photocatalysis.

Environmental remediation by employing visible-light-active semiconductor heterostructures provides effective solutions for handling emerging contaminants by a much greener and lower cost approach compared with other methods. This report demonstrates that the in situ growth of nanosized single-crystal-like defective anatase TiO2-x mesocrystals (DTMCs) on g-C3 N4 nanosheets (NSs) can produce a 3D/2D DTMC/g-C3 N4 NS heterostructure with the two components held together by chemical bonds to form tight interfaces. This nanostructured heterostructure displayed remarkably improved photocatalytic activity toward the removal of the model pollutants Methyl Orange (MO) and CrVI under visible-light irradiation in comparison with the pristine DTMC and g-C3 N4 NS components, which suggests that both the oxidation and reduction abilities of the DTMC/g-C3 N4 NSs were simultaneously enhanced after fabrication. On the basis of the results of a systematic characterization, a reasonable mechanism for the photocatalytic activity based on a direct Z-scheme heterojunction is proposed and further verified by the measurement of . OH. This novel Z-scheme heterojunction endows the heterostructure with improved photogenerated electron/hole pair separation and a strong redox ability for the efficient degradation of wastewater pollutants. This work will be useful for the design and fabrication of direct Z-scheme heterostructured photocatalysts with novel architectures for applications in energy conversion and environmental remediation.

Anatase TiO2 Mesocrystals: Green Synthesis, In Situ Conversion to Porous Single Crystals, and Self-Doping Ti3+ for Enhanced Visible Light Driven Photocatalytic Removal of NO.

Mesocrystals are of great interest for a wide range of applications owing to their unique structural features and properties. The realization of well-defined metal oxide mesocrystals through a facile and green synthetic approach still remains a great challenge. Here, a novel synthesis strategy is reported for the production of spindle-shaped anatase TiO2 mesocrystals with a single-crystal-like structure, which was simply achieved through the one-step hydrolysis reaction of TiCl3 in the green and recyclable media polyethylene glycol (PEG-400) without any additives. Such anatase mesocrystals were constructed from small nanocrystal subunits (≈1.5-4.5 nm in diameter) and formed through oriented aggregation of the nanocrystals pre-formed in the reaction system. Owing to their novel structural characteristics, the as-synthesized anatase mesocrystals could be easily fused in situ into porous single crystals by annealing in air. More significantly, after being annealed in vacuum, Ti3+ sites could be easily induced in the anatase crystal lattice, resulting in the formation of Ti3+ self-doped anatase mesocrystals. The thus-transformed mesocrystals exhibited enhanced visible light activity towards the photocatalytic oxidation of nitric oxide (NO) to NO3- , which could be largely attributed to their intrinsic Ti3+ self-doped nature, as well as high crystallinity and high porosity of the mesocrystalline architecture.

Growth of wafer-scale graphene-hexagonal boron nitride vertical heterostructures with clear interfaces for obtaining atomically thin electrical analogs.

Two-dimensional (2D) integrated circuits based on graphene (Gr) heterostructures have emerged as next-generation electronic devices. However, it is still challenging to produce high-quality and large-area Gr/hexagonal boron nitride (h-BN) vertical heterostructures with clear interfaces and precise layer control. In this work, a two-step metallic alloy-assisted epitaxial growth approach has been demonstrated for producing wafer-scale vertical hexagonal boron nitride/graphene (h-BN/Gr) heterostructures with clear interfaces. The heterostructures maintain high uniformity while scaling up and thickening. The layer number of both h-BN and graphene can be independently controlled by tuning the growth process. Furthermore, conductance measurements confirm that electrical hysteresis disappears on h-BN/Gr field-effect transistors, which is attributed to the h-BN dielectric surface. Our work blazes a trail toward next-generation graphene-based analog devices.

Engineering the Optoelectronic Properties of 2D Hexagonal Boron Nitride Monolayer Films by Sulfur Substitutional Doping.

Tuning the optical and electrical properties of two-dimensional (2D) hexagonal boron nitride (hBN) is critical for its successful application in optoelectronics. Herein, we report a new methodology to significantly enhance the optoelectronic properties of hBN monolayers by substitutionally doping with sulfur (S) on a molten Au substrate using chemical vapor deposition. The S atoms are more geometrically and energetically favorable to be doped in the N sites than in the B sites of hBN, and the S 3p orbitals hybridize with the B 2p orbitals, forming a new conduction band edge that narrows its band gap. The band edge positions change with the doping concentration of S atoms. The conductivity increases up to 1.5 times and enhances the optoelectronic properties, compared to pristine hBN. A photodetector made of a 2D S-doped hBN film shows an extended wavelength response from 260 to 280 nm and a 50 times increase in its photocurrent and responsivity with light illumination at 280 nm. These enhancements are mainly due to the improved light absorption and increased electrical conductivity through doping with sulfur. This S-doped hBN monolayer film can be used in the next-generation electronics, optoelectronics, and spintronics.

Flexible Optical Synapses Based on In2Se3/MoS2 Heterojunctions for Artificial Vision Systems in the Near-Infrared Range.

Near-infrared (NIR) synaptic devices integrate NIR optical sensitivity and synaptic plasticity, emulating the basic biomimetic function of the human visual system and showing great potential in NIR artificial vision systems. However, the lack of semiconductor materials with appropriate band gaps for NIR photodetection and effective strategies for fabricating devices with synaptic behaviors limit the further development of NIR synaptic devices. Here, a two-terminal NIR synaptic device consisting of the In2Se3/MoS2 heterojunction has been constructed, and it exhibits fundamental synaptic functions. The reduced band gap and potential barrier of In2Se3/MoS2 heterojunctions are essential for NIR synaptic plasticity. In addition, the NIR synaptic properties of In2Se3/MoS2 heterojunctions under strain have been studied systematically. The ΔEPSC of the In2Se3/MoS2 synaptic device can be improved from 38.4% under no strain to 49.0% under a 0.54% strain with a 1060 nm illumination for 1 s at 100 mV. Furthermore, the artificial NIR vision system consisting of a 10 × 10 In2Se3/MoS2 device array has been fabricated, exhibiting image sensing, learning, and storage functions under NIR illumination. This research provides new ideas for the design of flexible NIR synaptic devices based on 2D materials and presents many opportunities in artificial intelligence and NIR vision systems.

Ultralow Power Optical Synapses Based on MoS2 Layers by Indium-Induced Surface Charge Doping for Biomimetic Eyes.

Biomimetic eyes, with their excellent imaging functions such as large fields of view and low aberrations, have shown great potentials in the fields of visual prostheses and robotics. However, high power consumption and difficulties in device integration severely restrict their rapid development. In this study, an artificial synaptic device consisting of a molybdenum disulfide (MoS2 ) film coated with an electron injection enhanced indium (In) layer is proposed to increase the channel conductivity and reduce the power consumption. This artificial synaptic device achieves an ultralow power consumption of 68.9 aJ per spike, which is several hundred times lower than those of the optical artificial synapses reported in literature. Furthermore, the multilayer and polycrystalline MoS2 film shows persistent photoconductivity performance, effectively resulting in short-term plasticity, long-term plasticity, and their transitions between each other. A 5 × 5 In/MoS2 synaptic device array is constructed into a hemispherical electronic retina, demonstrating its impressive image sensing and learning functions. This research provides a new methodology for effective control of artificial synaptic devices, which have great opportunities used in bionic retinas, robots, and visual prostheses.

Atomically Thin Hexagonal Boron Nitride and Its Heterostructures.

Atomically thin hexagonal boron nitride (h-BN) is an emerging star of 2D materials. It is taken as an optimal substrate for other 2D-material-based devices owing to its atomical flatness, absence of dangling bonds, and excellent stability. Specifically, h-BN is found to be a natural hyperbolic material in the mid-infrared range, as well as a piezoelectric material. All the unique properties are beneficial for novel applications in optoelectronics and electronics. Currently, most of these applications are merely based on exfoliated h-BN flakes at their proof-of-concept stages. Chemical vapor deposition (CVD) is considered as the most promising approach for producing large-scale, high-quality, atomically thin h-BN films and heterostructures. Herein, CVD synthesis of atomically thin h-BN is the focus. Also, the growth kinetics are systematically investigated to point out general strategies for controllable and scalable preparation of single-crystal h-BN film. Meanwhile, epitaxial growth of 2D materials onto h-BN and at its edge to construct heterostructures is summarized, emphasizing that the specific orientation of constituent parts in heterostructures can introduce novel properties. Finally, recent applications of atomically thin h-BN and its heterostructures in optoelectronics and electronics are summarized.

Synthesis of High-Quality Multilayer Hexagonal Boron Nitride Films on Au Foils for Ultrahigh Rejection Ratio Solar-Blind Photodetection.

Solar-blind photodetectors have widespread applications due to the unique merit of a "black background" on the earth. However, most solar-blind photodetectors reported previously exhibited quite low rejection ratios (R200nm/R280nm < 103) and were interfered with by light longer than 280 nm. Herein, by an ambient pressure chemical vapor deposition (CVD) method, large-area, clean, and uniform two-dimensional (2D) multilayer h-BN films with different thicknesses have been successfully synthesized on Au foils. The synthesized multilayer h-BN film is transparent to light longer than 280 nm, showing excellent optical and optoelectronic properties to weak solar-blind light (µW/cm2). This sensitive solar-blind h-BN photodetector exhibits ultrahigh rejection ratios (R220nm/R280nm > 103 and R220nm/R290nm > 104), a low dark current (102 fA), and a large detectivity (3.9 × 1010 Jones). It is noteworthy that the rejection ratio (R220nm/R290nm) here is superior to most of those previously reported based on traditional semiconductors. This large-scale, clean, and uniform multilayer h-BN film will contribute to the progress of next-generation optoelectronic devices.

A mixed-dimensional 1D Se-2D InSe van der Waals heterojunction for high responsivity self-powered photodetectors.

Mixed-dimension van der Waals (vdW) p-n heterojunction photodiodes have inspired worldwide efforts to combine the excellent properties of 2D materials and traditional semiconductors without consideration of lattice mismatch. However, owing to the scarcity of intrinsic p-type semiconductors and insufficient optical absorption of the few layer 2D materials, a high performance photovoltaic device based on a vdW heterojunction is still lacking. Here, a novel mixed-dimension vdW heterojunction consisting of 1D p-type Se nanotubes and a 2D flexible n-type InSe nanosheet is proposed by a facile method, and the device shows excellent photovoltaic characteristics. Due to the superior properties of the hybrid p-n junction, the mix-dimensional van der Waals heterojunction exhibited high on/off ratios (103) at a relatively weak light intensity of 3 mW cm-2. And a broadband self-powered photodetector ranging from the UV to visible region is achieved. The highest responsivity of the device could reach up to 110 mA W-1 without an external energy supply. This value is comparable to that of the pristine Se device at 5 V and InSe device at 0.1 V, respectively. Furthermore, the response speed is enhanced by one order of magnitude over the single Se or InSe device even at a bias voltage. This work paves a new way for the further development of high performance, low cost, and energy-efficient photodetectors by using mixed-dimensional vdW heterostructures.