Programmable Optoelectronic Synaptic Transistors Based on MoS2/Ta2NiS5 Heterojunction for Energy-Efficient Neuromorphic Optical Operation.

The optoelectronic synaptic transistors with various functions, broad spectral perception, and low power consumption are an urgent need for the development of advanced optical neural network systems. However, it remains a great challenge to realize the functional diversification of the systems on a single device. 2D van der Waals (vdW) materials can combine unique properties by stacking with each other to form heterojunctions, which may provide a strategy for solving this problem. Herein, an all-2D vdW heterojunction-based programmable optoelectronic synaptic transistor based on MoS2/Ta2NiS5 heterojunctions is demonstrated. The device implements reconfigurable, multilevel non-volatile memory (NVM) states through sequential modulation of multiple optical and electrical stimuli to achieve broadband (532-808 nm), energy-efficient (17.2 fJ), hetero-synaptic functionality in a bionic manner. The intrinsic working mechanisms of the photogating effect caused by band alignment and the interfacial trapping defect modulation induced by gate voltage are revealed by Kelvin-probe force microscopy (KPFM) measurements and carrier transport analysis. Overall, the (opto)electronic synaptic weight controllability for combined in-sensor and in-memory logic processors is realized by the heterojunction properties. The proposed findings facilitate the technical realization of generic all 2D hetero-synapses for future artificial vision systems, opto-logical systems, and Internet of Things (IoT) entities.

In Situ Growth of Wafer-Scale Patterned Graphene and Fabrication of Optoelectronic Artificial Synaptic Device Array Based on Graphene/n-AlGaN Heterojunction for Visual Learning.

The unique optical and electrical properties of graphene-based heterojunctions make them significant for artificial synaptic devices, promoting the advancement of biomimetic vision systems. However, mass production and integration of device arrays are necessary for visual imaging, which is still challenging due to the difficulty in direct growth of wafer-scale graphene patterns. Here, a novel strategy is proposed using photosensitive polymer as a solid carbon source for in situ growth of patterned graphene on diverse substrates. The growth mechanism during high-temperature annealing is elucidated, leading to wafer-scale graphene patterns with exceptional uniformity, ideal crystalline quality, and precise control over layer number by eliminating the release of volatile from oxygen-containing resin. The growth strategy enables the fabrication of two-inch optoelectronic artificial synaptic device array based on graphene/n-AlGaN heterojunction, which emulates key functionalities of biological synapses, including short-term plasticity, long-term plasticity, and spike-rate-dependent plasticity. Moreover, the mimicry of visual learning in the human brain is attributed to the regulation of excitatory and inhibitory post-synapse currents, following a learning rule that prioritizes initial recognition before memory formation. The duration of long-term memory reaches 10 min. The in situ growth strategy for patterned graphene represents the novelty for fabricating fundamental hardware of an artificial neuromorphic system.

In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal.

Polaritons-hybrid light-matter excitations-enable nanoscale control of light. Particularly large polariton field confinement and long lifetimes can be found in graphene and materials consisting of two-dimensional layers bound by weak van der Waals forces1,2 (vdW materials). These polaritons can be tuned by electric fields3,4 or by material thickness5, leading to applications including nanolasers6, tunable infrared and terahertz detectors7, and molecular sensors8. Polaritons with anisotropic propagation along the surface of vdW materials have been predicted, caused by in-plane anisotropic structural and electronic properties9. In such materials, elliptic and hyperbolic in-plane polariton dispersion can be expected (for example, plasmon polaritons in black phosphorus9), the latter leading to an enhanced density of optical states and ray-like directional propagation along the surface. However, observation of anisotropic polariton propagation in natural materials has so far remained elusive. Here we report anisotropic polariton propagation along the surface of α-MoO3, a natural vdW material. By infrared nano-imaging and nano-spectroscopy of semiconducting α-MoO3 flakes and disks, we visualize and verify phonon polaritons with elliptic and hyperbolic in-plane dispersion, and with wavelengths (up to 60 times smaller than the corresponding photon wavelengths) comparable to those of graphene plasmon polaritons and boron nitride phonon polaritons3-5. From signal oscillations in real-space images we measure polariton amplitude lifetimes of 8 picoseconds, which is more than ten times larger than that of graphene plasmon polaritons at room temperature10. They are also a factor of about four larger than the best values so far reported for phonon polaritons in isotopically engineered boron nitride11 and for graphene plasmon polaritons at low temperatures12. In-plane anisotropic and ultra-low-loss polaritons in vdW materials could enable directional and strong light-matter interactions, nanoscale directional energy transfer and integrated flat optics in applications ranging from bio-sensing to quantum nanophotonics.

Plasmonic-enhanced efficiency of AlGaN-based deep ultraviolet LED by graphene/Al nanoparticles/graphene hybrid structure.

The AlGaN-based deep ultraviolet light-emitting diode (DUV LED) has advantages of environmentally friendly materials, tunable emission wavelength, and easy miniaturization. However, the light extraction efficiency (LEE) of an AlGaN-based DUV LED is low, which hinders its applications. Here, we design a graphene/Al nanoparticles/graphene (Gra/Al NPs/Gra) hybrid plasmonic structure, where the strong resonant coupling of local surface plasmons (LSPs) induces a 2.9-times enhancement for the LEE of the DUV LED according to the photoluminescence (PL). The dewetting of Al NPs on a graphene layer by annealing is optimized, resulting in better formation and uniform distribution. The near-field coupling of Gra/Al NPs/Gra is enhanced via charge transfer among graphene and Al NPs. In addition, the skin depth increment results in more excitons being coupled out of multiple quantum wells (MQWs). An enhanced mechanism is proposed, revealing that the Gra/metal NPs/Gra offers a reliable strategy for improving the optoelectronic device performance, which might trigger the advances of LEDs and lasers with high brightness and power density.

Broad spectral tuning of ultra-low-loss polaritons in a van der Waals crystal by intercalation.

Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals are promising candidates for controlling the flow of energy on the nanoscale due to their strong field confinement, anisotropic propagation and ultra-long lifetime in the picosecond range1-5. However, the lack of tunability of their narrow and material-specific spectral range-the Reststrahlen band-severely limits their technological implementation. Here, we demonstrate that intercalation of Na atoms in the van der Waals semiconductor α-V2O5 enables a broad spectral shift of Reststrahlen bands, and that the phonon polaritons excited show ultra-low losses (lifetime of 4 ± 1 ps), similar to phonon polaritons in a non-intercalated crystal (lifetime of 6 ± 1 ps). We expect our intercalation method to be applicable to other van der Waals crystals, opening the door for the use of phonon polaritons in broad spectral bands in the mid-infrared domain.

A graphene-Mo2C heterostructure for a highly responsive broadband photodetector.

Photodetectors based on intrinsic graphene can operate over a broad wavelength range with ultrafast response, but their responsivity is much lower than commercial silicon photodiodes. The combination of graphene with two-dimensional (2D) semiconductors may enhance the light absorption, but there is still a cutoff wavelength originating from the bandgap of semiconductors. Here, we report a highly responsive broadband photodetector based on the heterostructure of graphene and transition metal carbides (TMCs, more specifically Mo2C). The graphene-Mo2C heterostructure enhanced light absorption over a broad wavelength range from ultraviolet to infrared. In addition, there is very small resistance for photoexcited carriers in both graphene and Mo2C. Consequently, photodetectors based on the graphene-Mo2C heterostructure deliver a very high responsivity from visible to infrared telecommunication wavelengths.

Cistrome-GO: a web server for functional enrichment analysis of transcription factor ChIP-seq peaks.

Characterizing the ontologies of genes directly regulated by a transcription factor (TF), can help to elucidate the TF's biological role. Previously, we developed a widely used method, BETA, to integrate TF ChIP-seq peaks with differential gene expression (DGE) data to infer direct target genes. Here, we provide Cistrome-GO, a website implementation of this method with enhanced features to conduct ontology analyses of gene regulation by TFs in human and mouse. Cistrome-GO has two working modes: solo mode for ChIP-seq peak analysis; and ensemble mode, which integrates ChIP-seq peaks with DGE data. Cistrome-GO is freely available at http://go.cistrome.org/.

Bias-switchable negative and positive photoconductivity in 2D FePS3 ultraviolet photodetectors.

Metal-phosphorus-trichalcogenides (MPTs), represented by NiPS3, FePS3, etc, are newly developed 2D wide-bandgap semiconductors and have been proposed as excellent candidates for ultraviolet (UV) optoelectronics. In spite of having superior advantages for solar-blind UV photodetectors, including those free of surface trap states, being highly compatible with versatile integrations as well as having an appropriate band gap, to date relevant study is rare. In this work, the photoresponse characteristic of UV detectors based on few-layer FePS3 has been comprehensively investigated. The responsivity of the photodetector, which is observed to be determined by bias gate voltage, may achieve as high as 171.6 mAW-1 under the illumination of 254 nm weak light, which is comparable to most commercial UV detectors. Notably, both negative and positive photoconductivities exist in the FePS3 photodetectors and can be controllably switched with bias voltage. The eminent and novel photoresponse property paves the way for the further development and practical use of 2D MPTs in high-performance UV photodetections.

Near-Infrared Photodetectors Based on MoTe2 /Graphene Heterostructure with High Responsivity and Flexibility.

2D transition metal dichalcogenides (TMDCs) have attracted considerable attention due to their impressively high performance in optoelectronic devices. However, efficient infrared (IR) photodetection has been significantly hampered because the absorption wavelength range of most TMDCs lies in the visible spectrum. In this regard, semiconducting 2D MoTe2 can be an alternative choice owing to its smaller band gap ≈1 eV from bulk to monolayer and high carrier mobility. Here, a MoTe2 /graphene heterostructure photodetector is demonstrated for efficient near-infrared (NIR) light detection. The devices achieve a high responsivity of ≈970.82 A W-1 (at 1064 nm) and broadband photodetection (visible-1064 nm). Because of the effective photogating effect induced by electrons trapped in the localized states of MoTe2 , the devices demonstrate an extremely high photoconductive gain of 4.69 × 108 and detectivity of 1.55 × 1011 cm Hz1/2 W-1 . Moreover, flexible devices based on the MoTe2 /graphene heterostructure on flexible substrate also retains a good photodetection ability after thousands of times bending test (1.2% tensile strain), with a high responsivity of ≈60 A W-1 at 1064 nm at VDS = 1 V, which provides a promising platform for highly efficient, flexible, and low cost broadband NIR photodetectors.

Flexible Broadband Graphene Photodetectors Enhanced by Plasmonic Cu3-x P Colloidal Nanocrystals.

The integration of graphene with colloidal quantum dots (QDs) that have tunable light absorption affords new opportunities for optoelectronic applications as such a hybrid system solves the problem of both quantity and mobility of photocarriers. In this work, a hybrid system comprising of monolayer graphene and self-doped colloidal copper phosphide (Cu3-x P) QDs is developed for efficient broadband photodetection. Unlike conventional PbS QDs that are toxic, Cu3-x P QDs are environmental friendly and have plasmonic resonant absorption in near-infrared (NIR) wavelength. The half-covered graphene with Cu3-x P nanocrystals (NCs) behaves as a self-driven p-n junction and shows durable photoresponse in NIR range. A comparison experiment reveals that the surface ligand attached to Cu3-x P NCs plays a key role in determining the charge transfer efficiency from Cu3-x P to graphene. The most efficient three-terminal photodetectors based on graphene-Cu3-x P exhibit broadband photoresponse from 400 to 1550 nm with an ultrahigh responsivity (1.59 × 105 A W-1 ) and high photoconductive gain (6.66 × 105 ) at visible wavelength (405 nm), and a good responsivity of 9.34 A W-1 at 1550 nm. The demonstration of flexible graphene-Cu3-x P photodetectors operated at NIR wavelengths may find potential applications in optical sensing, biological imaging, and wearable devices.

Photonics and optoelectronics of two-dimensional materials beyond graphene.

Apart from conventional materials, the study of two-dimensional (2D) materials has emerged as a significant field of study for a variety of applications. Graphene-like 2D materials are important elements of potential optoelectronics applications due to their exceptional electronic and optical properties. The processing of these materials towards the realization of devices has been one of the main motivations for the recent development of photonics and optoelectronics. The recent progress in photonic devices based on graphene-like 2D materials, especially topological insulators (TIs) and transition metal dichalcogenides (TMDs) with the methodology level discussions from the viewpoint of state-of-the-art designs in device geometry and materials are detailed in this review. We have started the article with an overview of the electronic properties and continued by highlighting their linear and nonlinear optical properties. The production of TIs and TMDs by different methods is detailed. The following main applications focused towards device fabrication are elaborated: (1) photodetectors, (2) photovoltaic devices, (3) light-emitting devices, (4) flexible devices and (5) laser applications. The possibility of employing these 2D materials in different fields is also suggested based on their properties in the prospective part. This review will not only greatly complement the detailed knowledge of the device physics of these materials, but also provide contemporary perception for the researchers who wish to consider these materials for various applications by following the path of graphene.

Environmental-Friendly Ag2S QD-Multilayer MoSe2 van Der Waals Heterostructure for High-Performance Broadband Photodetection.

Broadband photodetectors have drawn intensive attention owing to their wide application prospects in optical communication, imaging, astronomy, and so on. Two-dimensional transition-metal dichalcogenides (TMDs) are considered as highly potential candidates for photodetection applications, benefiting from their excellent photoelectric properties. However, most of the photodetectors based on TMDs suffer from low performance in the near-infrared (NIR) region due to the weak optical absorption efficiency near their absorption band edge, which severely constrains their usage for broadband optoelectronics. Here, by taking advantage of the high absorption coefficient and environment-friendly property of Ag2S quantum dots (QDs), the hybrid of multilayer MoSe2/Ag2S QDs is demonstrated with a high-performance broadband photodetection capability (532-1270 nm). The favorable energy band alignment of MoSe2/Ag2S QDs facilitates effective separation and collection of photogenerated carriers, and the heterostructure device exhibits significant enhancement of performance compared to the bare MoSe2 device. High responsivity, detectivity, and external quantum efficiency of 25.5 A/W, 1.45 × 1011 Jones, and 1070% are obtained at a low working voltage of 1 V under 980 nm illumination. The responsivity of the device can reach up to 1.2 A/W at 1270 nm wavelength, which is competitive to the commercial NIR photodetectors. Meanwhile, broadband imaging capability is demonstrated. Our work may open up a facile and eco-friendly approach to construct high-performance broadband photodetectors for next-generation compact optoelectronic applications.

Photogating-assisted tunneling boosts the responsivity and speed of heterogeneous WSe2/Ta2NiSe5 photodetectors.

Photogating effect is the dominant mechanism of most high-responsivity two-dimensional (2D) material photodetectors. However, the ultrahigh responsivities in those devices are intrinsically at the cost of very slow response speed. In this work, we report a WSe2/Ta2NiSe5 heterostructure detector whose photodetection gain and response speed can be enhanced simultaneously, overcoming the trade-off between responsivity and speed. We reveal that photogating-assisted tunneling synergistically allows photocarrier multiplication and carrier acceleration through tunneling under an electrical field. The photogating effect in our device features low-power consumption (in the order of nW) and shows a dependence on the polarization states of incident light, which can be further tuned by source-drain voltages, allowing for wavelength discrimination with just a two-electrode planar structure. Our findings offer more opportunities for the long-sought next-generation photodetectors with high responsivity, fast speed, polarization detection, and multi-color sensing, simultaneously.

Toward Broadband Photodetection: Band Alignment and Interlayer Charge Transfer in 2D Transition Metal Dichalcogenides/3D-Ga2O3 Hybrid-Dimensional Heterostructures.

Recently, various transition metal dichalcogenides (TMDs)/Ga2O3 heterostructures have emerged as excellent candidates for the development of broadband photodetection, exhibiting various merits such as broadband optical absorption, efficient interlayer carrier transfer, a relatively simple fabrication process, and potential for flexibility. In this work, vertically stacked MoSe2/Ga2O3, WS2/Ga2O3, and WSe2/Ga2O3 heterostructures were experimentally synthesized, all exhibiting broadband light absorption, spanning at least from 200 to 800 nm. The absorption coefficients of these TMDs/Ga2O3 heterostructures are significantly improved compared to those of individual Ga2O3 films. The superior performance can be attributed to the type-I band alignment and efficient interlayer carrier transfer, which result from various band offsets along with the different doping conditions of the TMD layers, leading to distinct photoluminescence (PL) emission properties. Through a detailed analysis of the excitation-power-dependent PL spectra, we offer an in-depth discussion of the interlayer carrier transfer mechanism in the TMDs/Ga2O3 heterostructures. Regarding interlayer coupling effects, the shift of the EF of TMD layers plays a crucial role in modulating their trion emission properties. These findings suggest that these three TMDs/Ga2O3 heterostructures have great potential in broadband photodetection, and our in-depth physical mechanism analysis lays a solid foundation for a new device design.

Synergistic-potential engineering enables high-efficiency graphene photodetectors for near- to mid-infrared light.

High quantum efficiency and wide-band detection capability are the major thrusts of infrared sensing technology. However, bulk materials with high efficiency have consistently encountered challenges in integration and operational complexity. Meanwhile, two-dimensional (2D) semimetal materials with unique zero-bandgap structures are constrained by the bottleneck of intrinsic quantum efficiency. Here, we report a near-mid infrared ultra-miniaturized graphene photodetector with configurable 2D potential well. The 2D potential well constructed by dielectric structures can spatially (laterally and vertically) produce a strong trapping force on the photogenerated carriers in graphene and inhibit their recombination, thereby improving the external quantum efficiency (EQE) and photogain of the device with wavelength-immunity, which enable a high responsivity of 0.2 A/W-38 A/W across a broad infrared detection band from 1.55 to 11 µm. Thereafter, a room-temperature detectivity approaching 1 × 109 cm Hz1/2 W-1 is obtained under blackbody radiation. Furthermore, a synergistic effect of electric and light field in the 2D potential well enables high-efficiency polarization-sensitive detection at tunable wavelengths. Our strategy opens up alternative possibilities for easy fabrication, high-performance and multifunctional infrared photodetectors.

High-Speed and Ultrasensitive Solar-Blind Ultraviolet Photodetectors Based on In Situ Grown ß-Ga2O3 Single-Crystal Films.

Deep-level defects in ß-Ga2O3 that worsen the response speed and dark current (Id) of photodetectors (PDs) have been a long-standing issue for its application. Herein, an in situ grown single-crystal Ga2O3 nanoparticle seed layer (NPSL) was used to shorten the response time and reduce the Id of metal-semiconductor-metal (MSM) PDs. With the NPSL, the Id was reduced by 4 magnitudes from 0.389 µA to 81.03 pA, and the decay time (τd1/τd2) decreased from 258/1690 to 62/142 µs at -5 V. In addition, the PDs with the NPSL also exhibit a high responsivity (43.5 A W-1), high specific detectivity (2.81 × 1014 Jones), and large linear dynamic range (61 dB) under 254 nm illumination. The mechanism behind the performance improvement can be attributed to the suppression of the deep-level defects (i.e., self-trapped holes) and increase of the Schottky barrier. The barrier height extracted is increased by 0.18 eV compared with the case without the NPSL. Our work contributes to understanding the relationship between defects and the performance of PDs based on heteroepitaxial ß-Ga2O3 thin films and provides an important reference for the development of high-speed and ultrasensitive deep ultraviolet PDs.

Organic and quantum dot hybrid photodetectors: towards full-band and fast detection.

Photodetectors hold great application potential in many fields such as image sensing, night vision, infrared communication and health monitoring. To date, commercial photodetectors mainly rely on inorganic semiconductors, e.g., monocrystalline silicon, germanium, and indium selenide/gallium with complex and costly fabrication, which are hardly compatible with wearable electronics. In contrast, organic conjugated materials provide great superiority in flexibility and stretchability. In this Highlight, the unique properties of organic and quantum dot photodetectors were firstly discussed to reveal the great complementarity of the two technologies. Subsequently, the recent advance of organic/quantum dot hybrid photodetectors was outlined to highlight their great potential in developing broadband and high-performance photodetectors. Moreover, the multiple functions (e.g., dual-band detection and upconversion detection) of hybrid photodetectors were highlighted for their promising application in image sensing and infrared detection. Lastly, we present a forword-looking discussion on the challenges and our insights for the further advancement of hybrid photodetectors. This work may spark enormous research attention in organic/quantum dot electronics and advance the commercial applications.

Mechanistic understanding of the interfacial properties of metal-PtSe2 contacts.

With the advantages of a moderate band gap, high carrier mobility and good environmental stability, two-dimensional (2D) semiconductors show promising applications in next-generation electronics. However, the accustomed metal-2D semiconductor contact may lead to a strong Fermi level pinning (FLP) effect, which severely limits the practical performance of 2D electronics. Herein, the interfacial properties of the contacts between a promising 2D semiconductor, PtSe2, and a sequence of metal electrodes are systematically investigated. The strong interfacial interactions formed in all metal-PtSe2 contacts lead to chemical bonds and a significant interfacial dipole, resulting in a vertical Schottky barrier for Ag, Au, Pd and Pt-based systems and a lateral Schottky barrier for Al, Cu, Sc and Ti-based systems, with a strong FLP effect. Remarkably, the tunneling probability for most metal-PtSe2 is significantly high and the tunneling-specific resistivity is two orders of magnitude lower than that of the state-of-the-art contacts, demonstrating the high efficiency for electron injection from metals to PtSe2. Moreover, the introduction of h-BN as a buffer layer leads to a weakened FLP effect (S = 0.50) and the transformation into p-type Schottky contact for Pt-PtSe2 contacts. These results reveal the underlying mechanism of the interfacial properties of metal-PtSe2 contacts, which is useful for designing advanced 2D semiconductor-based electronics.

Revealing the Modulation Effects on the Electronic Band Structures and Exciton Properties by Stacking Graphene/h-BN/MoS2 Schottky Heterostructures.

Stacking two dimensional tunneling heterostructures has always been an important strategy to improve the optoelectronic device performance. However, there are still many disputes about the blocking ability of monolayer (1L-) h-BN on the interlayer coupling. Graphene/h-BN/MoS2 optoelectronic devices have been reported for superior device results. In this study, starting with graphene/h-BN/MoS2 heterostructures, we report experimental evidence of 1L-h-BN barrier layer modulation effects about the electronic band structures and exciton properties. We find that 1L-h-BN insertion only partially blocks the interlayer carrier transfer. In the meantime, the 1L-h-BN barrier layer weakens the interlayer coupling effect, by decreasing the efficient dielectric screening and releasing the quantum confinement. Consequently, the optical conductivity and plasmon excitation slightly improve, and the electronic band structures remain unchanged in graphene/h-BN/MoS2, explaining their fascinating optoelectronic responses. Moreover, the excitonic binding energies of graphene/h-BN/MoS2 redshift with respect to the graphene/MoS2 counterparts. Our results, as well as the broadband optical constants, will help better understand the h-BN barrier layers, facilitating the developing progress of h-BN-based tunneling optoelectronic devices.

Wafer-Level, High-Performance, Flexible Sensors Based on Organic Nanoforests for Human-Machine Interactions.

High-performance flexible sensors are essential for real-time information analysis and constructing noncontact communication modules for emerging human-machine interactions. In these applications, batch fabrication of sensors that exhibit high performance at the wafer level is in high demand. Here, we present organic nanoforest-based humidity sensor (NFHS) arrays on a 6 in. flexible substrate prepared via a facile, cost-effective manufacturing approach. Such an NFHS achieves state-of-the-art overall performance: high sensitivity and fast recovery time; the best properties are at a small device footprint. The high sensitivity (8.84 pF/% RH) and fast response time (5 s) of the as-fabricated organic nanoforests are attributed to the abundant hydrophilic groups, the ultra-large surface area with a huge number of nanopores, and the vertically distributed structures beneficial to the transfer of molecules up and down. The NFHS also exhibits excellent long-term stability (90 days), superior mechanical flexibility, and good performance repeatability after bending. With these superiorities, the NFHS is further applied as a smart noncontact switch, and the NFHS array is used as the motion trajectory tracker. The wafer-level batch fabrication capability of our NFHS provides a potential strategy for developing practical applications of such humidity sensors.