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Heterostructures based on diverse two-dimensional (2D) materials are effective for tailoring and further promoting device performance and exhibit considerable potential in photodetection. However, the problem of high-density thermionic carriers can be hardly overcome in most reported heterostructure devices based on type I and type II band alignment, which leads to an unacceptably small Iphoto/Idark and strong temperature dependence that limit the performance of photodetectors. Here, using the MoTe2/h-BN/MoTe2/h-BN heterostructure, we report the hole-dominated Fowler–Nordheim quantum tunneling transport in both on and off states. The state-of-the-art device operating at room temperature shows high detectivity of >108 Jones at a laser power density of

Atomically thin two-dimensional (2D) materials exhibit enormous potential in photodetectors because of novel and extraordinary properties, such as passivated surfaces, tunable bandgaps, and high mobility. High-performance photodetectors based on 2D materials have been fabricated for broadband, position, polarization-sensitive detection, and large-area array imaging. However, the current performance of 2D material photodetectors is not outstanding enough, including response speed, detectivity, and so forth. The way to further promote the development of 2D material photodetectors and their corresponding practical applications is still a tremendous challenge. In this article, these issues of 2D material photodetectors are analyzed and expected to be solved by combining micro-nano characterization technologies. The inherent physical properties of 2D materials and photodetectors can be accurately characterized by Raman spectroscopy, transmission electron microscopy (TEM), and scattering scanning near-field optical microscope (s-SNOM). In particular, the precise probe of lattice defects, doping concentration, and near-field light absorption characteristics can promote the researches of low-noise and high-responsivity photodetectors. Scanning photocurrent microscope (SPCM) can show the overall spatial distribution of photocurrent and analyze the mechanism of photocurrent. Photoluminescence (PL) spectroscopy and Kelvin probe force microscope (KPFM) can characterize the material bandgap, work function distribution and interlayer coupling characteristics, making it possible to design high-performance photodetectors through energy band engineering. These advanced characterization techniques cover the entire process from material growth, to device preparation, and to performance analysis, and systematically reveal the development status of 2D material photodetectors. Finally, the prospects and challenges are discussed to promote the application of 2D material photodetectors.

Uncooled infrared photodetectors have evoked widespread interest in basic research and military manufacturing because of their low-cost, compact detection systems. However, existing uncooled infrared photodetectors utilize the photothermoelectric effect of infrared radiation operating at 8-12 μm, with a slow response time in the millisecond range. Hence, the exploration of new uncooled mid-wavelength infrared (MWIR) heterostructures is conducive to the development of ultrafast and high-performance nano-optoelectronics. This study explores a van der Waals heterojunction on epitaxial HgCdTe (vdWs-on-MCT) as an uncooled MWIR photodetector, which achieves fast response as well as high detectivity for spectral blackbody detection. Specifically, the vdWs-on-MCT photodetector has a fast response time of 13 ns (77 MHz), which is approximately an order of magnitude faster than commercial uncooled MCT photovoltaic photodetectors. Importantly, the device exhibits a photoresponsivity of 2.5 A/W, quantum efficiency as high as 85%, peak detectivity of 2×1010 cm Hz1/2 W-1 under blackbody radiation at room temperature, and peak detectivity of up to 1011 cm Hz1/2 W-1 at 77 K. Thereby, our work facilitates the effective design of high-speed and high-performance heterojunction uncooled MWIR photodetectors. This article is protected by copyright. All rights reserved.

The development of new and multi-functional synaptic transistors has become a research highlight in brain science and brain-like intelligence technologies. Here, we report an optically and electrically modulated carbon nanotube synaptic transistor with a single input terminal and multi-functional output characteristics. The optoelectronic response characteristics of carbon nanotube synaptic transistors can be controlled by the pulsed optical and electrical signals via a lightly doped silicon gate electrode as the input terminal. Some important synaptic behaviors including low-pass filtering and non-volatile memory performance were investigated after training by pulsed light and/or voltage stimulations. High neural activities with multiple input and output performances, such as excitatory, inhibitory and logic behaviour (NOR logic gate), were also observed by simultaneously programming the optical and electrical stimulations. As far as we know, this is the first report of a printed carbon nanotube synaptic transistor with a single gate electrode that has the ability to concurrently receive pulsed electrical and optical inputs as well as to realize multi-functional outputs. The results provide an opportunity for the coupling of multiple inputs through different stimulation methods in emerging brain-like synaptic nanoelectronics.

Low-dimensional Te-based photodetectors exhibit blackbody response and achieve record-set performance metrics., Blackbody-sensitive room-temperature infrared detection is a notable development direction for future low-dimensional infrared photodetectors. However, because of the limitations of responsivity and spectral response range for low-dimensional narrow bandgap semiconductors, few low-dimensional infrared photodetectors exhibit blackbody sensitivity. Here, highly crystalline tellurium (Te) nanowires and two-dimensional nanosheets were synthesized by using chemical vapor deposition. The low-dimensional Te shows high hole mobility and broadband detection. The blackbody-sensitive infrared detection of Te devices was demonstrated. A high responsivity of 6650 A W−1 (at 1550-nm laser) and the blackbody responsivity of 5.19 A W−1 were achieved. High-resolution imaging based on Te photodetectors was successfully obtained. All the results suggest that the chemical vapor deposition–grown low-dimensional Te is one of the competitive candidates for sensitive focal-plane-array infrared photodetectors at room temperature.

2D materials, of which the carrier type and concentration are easily tuned, show tremendous superiority in electronic and optoelectronic applications. However, the achievements are still quite far away from practical applications. Much more effort should be made to further improve their performance. Here, p-type MoSe2 is successfully achieved via substitutional doping of Ta atoms, which is confirmed experimentally and theoretically, and outstanding homojunction photodetectors and inverters are fabricated. MoSe2 p-n homojunction device with a low reverse current (300 pA) exhibits a high rectification ratio (104 ). The analysis of dark current reveals the domination of the Shockley-Read-Hall (SRH) and band-to-band tunneling (BTB) current. The homojunction photodetector exhibits a large open-circuit voltage (0.68 V) and short-circuit currents (1 µA), which is suitable for micro-solar cells. Furthermore, it possesses outstanding responsivity (0.28 A W-1 ), large external quantum efficiency (42%), and a high signal-to-noise ratio (≈107 ). Benefiting from the continuous energy band of homojunction, the response speed reaches up to 20 µs. Besides, the Ta-doped MoSe2 inverter exhibits a high voltage gain (34) and low power consumption (127 nW). This work lays a foundation for the practical application of 2D material devices.

For each generation of semiconductors, the issue of doping techniques is always placed at the top of the priority list since it determines whether a material can be used in the electronic and optoelectronic industry or not. When it comes to 2D materials, significant challenges have been found in controllably doping 2D semiconductors into p- or n-type, let alone developing a continuous control of this process. Here, a unique self-modulated doping characteristic in 2D layered materials such as PtSSe, PtS0.8 Se1.2 , PdSe2 , and WSe2 is reported. The varying number of vertically stacked-monolayers is the critical factor for controllably tuning the same material from p-type to intrinsic, and to n-type doping. Importantly, it is found that the thickness-induced lattice deformation makes defects in PtSSe transit from Pt vacancies to anion vacancies based on dynamic and thermodynamic analyses, which leads to p- and n-type conductance, respectively. By thickness-modulated doping, WSe2 diode exhibits a high rectification ratio of 4400 and a large open-circuit voltage of 0.38 V. Meanwhile, the PtSSe detector overcomes the shortcoming of large dark-current in narrow-bandgap optoelectronic devices. All these findings provide a brand-new perspective for fundamental scientific studies and applications.

Van der Waals heterojunctions based on atomically thin two-dimensional (2D) materials have attracted numerous attention for their special scientific research value and promising applications in photoelectric and micro-nano electronic devices. Especially, the carrier generation, separation, and extraction process in 2D materials can be easily modulated by external field, which may facilitate some multifunctional electronics and optoelectronics. In this paper, we report a unique type-II band alignment ReS2/MoTe2 heterojunction with rectification inversion due to the fact the bottom few-layer MoTe2 can be easily tuned from p-type to n-type state through the applied back-gate voltage. Then we study photodetection properties of ReS2/MoTe2 heterojunction, a relatively fast photoresponse time of 109 μs and a considerable photoresponsivity of 0.34 AW−1 for 520 nm at room temperature show great potential in photodetection. Our studies of ReS2/MoTe2 heterojunction with rectification inversion and high-performance photodetection will facilitate the development of electronics and optoelectronics based on atomically-thin heterojunctions.

2D layered photodetectors have been widely researched for intriguing optoelectronic properties but their application fields are limited by the bandgap. Extending the detection waveband can significantly enrich functionalities and applications of photodetectors. For example, after breaking through bandgap limitation, extrinsic Si photodetectors are used for short-wavelength infrared or even long-wavelength infrared detection. Utilizing extrinsic photoconduction to extend the detection waveband of 2D layered photodetectors is attractive and desirable. However, extrinsic photoconduction has yet not been observed in 2D layered materials. Here, extrinsic photoconduction-induced short-wavelength infrared photodetectors based on Ge-based chalcogenides are reported for the first time and the effectiveness of intrinsic point defects are demonstrated. The detection waveband of room-temperature extrinsic GeSe photodetectors with the assistance of Ge vacancies is broadened to 1.6 µm. Extrinsic GeSe photodetectors have an excellent external quantum efficiency (0.5%) at the communication band of 1.31 µm and polarization-resolved capability to subwaveband radiation. Moreover, room-temperature extrinsic GeS photodetectors with a detection waveband to the communication band of 1.55 µm further verify the versatility of intrinsic point defects. This approach provides design strategies to enrich the functionalities of 2D layered photodetectors.

Low-symmetry 2D materials with unique anisotropic optical and optoelectronic characteristics have attracted a lot of interest in fundamental research and manufacturing of novel optoelectronic devices. Exploring new and low-symmetry narrow-bandgap 2D materials will be rewarding for the development of nanoelectronics and nano-optoelectronics. Herein, sulfide niobium (NbS3 ), a novel transition metal trichalcogenide semiconductor with low-symmetry structure, is introduced into a narrowband 2D material with strong anisotropic physical properties both experimentally and theoretically. The indirect bandgap of NbS3 with highly anisotropic band structures slowly decreases from 0.42 eV (monolayer) to 0.26 eV (bulk). Moreover, NbS3 Schottky photodetectors have excellent photoelectric performance, which enables fast photoresponse (11.6 µs), low specific noise current (4.6 × 10-25 A2 Hz-1 ), photoelectrical dichroic ratio (1.84) and high-quality reflective polarization imaging (637 nm and 830 nm). A room-temperature specific detectivity exceeding 107 Jones can be obtained at the wavelength of 3 µm. These excellent unique characteristics will make low-symmetry narrow-bandgap 2D materials become highly competitive candidates for future anisotropic optical investigations and mid-infrared optoelectronic applications.

Broadband infrared photodetectors based on two-dimensional (2D) materials which are the research focus in the infrared field, have wide applications in remote sensing, thermal imaging, and astronomy observation. In this article, the photodetector based on 2D ferromagnetic material CoSe is studied at room temperature, demonstrating the air-stable, broadband, and up to long wavelength properties. The CoSe material is applied to infrared photodetectors for the first time. The 2D material CoSe is synthesized by using the chemical vapor deposition method. The size of the as-grown CoSe is up to 71.8 μm. The photoresponse of the CoSe photodetector ranges from 450 nm to 10.6 μm. The photoresponsivity of this photodetector is up to 2.58 A W-1 under the 10.6 μm illumination at room temperature. This work provides a new material for broadband photodetector at room temperature and builds a bridge for the magnetoelectronic and broadband photoelectric fields.

Two-dimensional (2D) materials with unique structural and physical properties are urgent to exploit and may be applied in the next-generation electronics and infrared detectors. The homojunction devices are ideal and promising candidates compared to heterojunction devices since there are no interface problems, such as unintentionally induced impurities and charge-trapped sites. In this work, we reported a high-performance MoSe2 homojunction infrared photodetector. MoSe2 homojunction devices directly consist of thick and thin MoSe2. MoSe2 homojunction diodes exhibit different rectified characteristics including forward and reverse rectifications, which are dependent on the back-gate voltage. Remarkably, MoSe2 homojunction photodetectors possess a broadband photoresponse with the wavelength from visible to near-infrared at room temperature. The responsivity of MoSe2 homojunction photodetectors under 940 nm laser illumination is approximately 2.25 A W−1. Additionally, the obtained specific detectivity of MoSe2 homojunction device is over 1010 Jones. Our findings provide an excellent way to fabricate feasible homojunction devices. Meanwhile, homojunction devices also show the wealthy potential in novel electronic and optoelectronic devices.