Infrared-visible fused photodetection presents significant potential for target perception in complex scenarios. However, dual-band imaging inherently generates a considerable amount of redundant data, highlighting a pressing need to perform compressive sensing directly at the pixel level. Here, we report a photodetector composed of a MoS2/h-BN/PdSe2 vertically stacked heterostructure with a common metal electrode interconnecting the bottom PdSe2 channel with the top MoS2 channel. By exploiting the property that infrared light can penetrate deeper than visible light, the bottom PdSe2/Au photovoltaic Schottky junction in this photodetector can detect the infrared light and drive the top MoS2 channel able to detect the visible light. Moreover, by applying voltage at the external drain terminal, the output photocurrent can be further enhanced or suppressed depending on the voltage polarity. The detector receives dual-band optical inputs but outputs only a single electrical signal, allowing for in-pixel dual-band intercorrelated compressive sensing. The physical process of infrared photoresponse that drives the visible photoresponse occurs directly within the detector, enabling the filtration and extraction of targets of interest based on the intensity of infrared irradiation at the pixel level. This work offers a compact and energy-efficient solution for multispectral optical information compressive sensing and processing in complex environments.

Band alignment engineering is crucial for facilitating charge separation and transfer in optoelectronic devices, which ultimately dictates the behavior of Van der Waals heterostructures (vdWH)-based photodetectors and light emitting diode (LEDs). However, the impact of the band offset in vdWHs on important figures of merit in optoelectronic devices has not yet been systematically analyzed. Herein, the regulation of band alignment in WSe2/Bi2Te3- xSex vdWHs (0 ≤ x ≤ 3) is demonstrated through the implementation of chemical vapor deposition (CVD). A combination of experimental and theoretical results proved that the synthesized vdWHs can be gradually tuned from Type I (WSe2/Bi2Te3) to Type III (WSe2/Bi2Se3). As the band alignment changes from Type I to Type III, a remarkable responsivity of 58.12 A W-1 and detectivity of 2.91×1012 Jones (in Type I) decrease in the vdWHs-based photodetector, and the ultrafast photoresponse time is 3.2 µs (in Type III). Additionally, Type III vdWH-based LEDs exhibit the highest luminance and electroluminescence (EL) external quantum efficiencies (EQE) among p-n diodes based on Transition Metal Dichalcogenides (TMDs) at room temperature, which is attributed to band alignment-induced distinct interfacial charge injection. This work serves as a valuable reference for the application and expansion of fundamental band alignment principles in the design and fabrication of future optoelectronic devices.

Optimizing the width of depletion region is a key consideration in designing high performance photovoltaic photodetectors, as the electron-hole pairs generated outside the depletion region cannot be effectively separated, leading to a negligible contribution to the overall photocurrent. However, currently reported photovoltaic mid-infrared photodetectors based on two-dimensional heterostructures usually adopt a single pn junction configuration, where the depletion region width is not maximally optimized. Here, we demonstrate the construction of a high performance broadband mid-infrared photodetector based on a MoS2/b-AsP/MoS2 npn van der Waals heterostructure. The npn heterojunction can be equivalently represented as two parallel-stacked pn junctions, effectively increasing the thickness of the depletion region. Consequently, the npn device shows a high detectivity of 1.3 x 1010 cmHz1/2W-1 at the mid-infrared wavelength, which is significantly improved compared with its single pn junction counterpart. Moreover, it exhibits a fast response speed of 12 mu s, and a broadband detection capability ranging from visible to mid-infrared wavelengths.

Position-sensitive detectors (PSDs) are widely used in optical inspection applications, such as precise measurement of distance and angle, surface profiling, and motion tracking, etc. The lateral photovoltaic effect is the mechanism employed by most PSDs to determine position, but its gain-free nature hinders weak-light detection and applications. Here, a new position recognition mechanism with an intrinsic high gain is proposed and employ it to design a weak-light and large-size photoconductor-type PSD. The position recognition is established by the intrinsic link between the gain and channel length in a photoconductor, and the designed linear correlation between the channel length and position. The photoconductor-type graphene/Si PSD based on the recognition mechanism shows an active length of over 1 mm, a high spatial position resolution at the sub-micrometer level, and a low detection limit power of 100 pW. Compared with Si PSDs, the gain of the photoconductor-type PSD is improved by 3-4 orders of magnitude. In addition, the operational wavelength range of the PSD is expanded to include the near-infrared region, specifically at 1300 and 1550 nm. The new position recognition mechanism paves the way for the development of ultra-sensitive light position detection. A photoconductor-type position recognition mechanism is developed by combining the intrinsic link between gain and channel length in a photoconductor with a predetermined linear correlation between channel length and position. The position-sensitive detector based on the mechanism has a high gain and is ideal for detecting ultra-weak light positions. image

Two-dimensional (2D) van der Waals heterostructures based on various 2D transition metal dichalcogenides are widely used in photodetection applications. However, their response time and photoresponsivity are limited, posing a challenge for their applications in high-sensitivity photodetection. Surface charge transfer doping (SCTD) has emerged as a novel doping approach for low-dimensional materials with high specific surface area and attracted considerable attention, as it is simple and effective, does not damage the lattice, and considers various types of dopants. Herein, we prepare p-i-n junction-based photodetectors via the SCTD of WSe2/ReS2 heterojunctions using p-type dopant F4-TCNQ molecules, where doped WSe2 serves as a p-type semiconductor, undoped WSe2 acts as an intrinsic layer, and ReS2 functions as an n-type semiconductor. The surface-charge-transfer-doped WSe2/ReS2 heterojunction leads to a reduction in the Schottky barrier and an increase in the built-in electric field compared with the as-fabricated heterojunction. In the photovoltaic mode and under 785 nm laser illumination, the photodiode exhibits an increase in responsivity from 0.08 to 0.29 A/W, specific detectivity from 1.89 × 1012 to 8.02 × 1012 Jones, and the external quantum efficiency from 12.67 to 46.29%. Additionally, the p-i-n structure expands the depletion region width, resulting in a photovoltaic response time of 7.56/6.48 μs and a -3 dB cutoff frequency of over 85 kHz, an order of magnitude faster than the pristine response time. Herein, we derive an effective and simple scheme for designing high-performance, low-power optoelectronic devices based on 2D van der Waals heterostructures.

Doping of two-dimensional (2D) semiconductors is necessary to achieve high performance and low power consumption in optoelectronic and logic devices. Herein, we report controllable p- and n-type doping of the transition-metal dichalcogenides (TMDs), tungsten diselenide (WSe2), and molybdenum ditelluride (MoTe2) via argon (Ar) plasma treatment. The doping of TMDs was tuned by controlling Ar plasma treatment conditions. The desired n-type doping in WSe2 and MoTe2 was obtained by applying a short treatment time, resulting in increased electron current and an upshift in binding energy. Owing to prolonged plasma treatment, abnormal p-type doping was achieved in WSe2 and MoTe2. Increased hole current, downshift in binding energy, appearance of oxygen bonds (O–W, Mo, Se, and Te), and redshift in Raman peaks revealed that the abnormal p-type doping behavior for WSe2 and MoTe2 was owing to considerable vacancies and edge defects induced in the top layer of TMDs by Ar plasma treatment. These were immediately occupied by oxygen atoms or oxidized to introduce oxygen doping when TMDs were exposed to air. The formation of a homogeneous oxide film on the top layer of TMDs was further demonstrated by the smooth surface and higher thickness of TMDs with heavy p-type doping compared to those of the pristine sample. A complementary metal-oxide-semiconductor (CMOS) inverter based on p- and n-WSe2 was demonstrated herein. Our study proposes a controllable way to modulate the doping type of TMDs, which has potential applications in the performance modulation of devices, design of novel device structures, and realization of 2D material-based logic circuits.

van der Waals heterostructures (vdWHs) overcoming the lattice and processing limitations of conventional heterostructures provide an opportunity to develop high-performance 2D vdWH solar cells and photodiodes. However, it is challenging to improve the sensitivity and response speed of 2D vdWH photovoltaic devices due to the low light absorption efficiency and electron/hole traps in heterointerfaces. Here, we design a PbS/MoS2/WSe2 heterostructure photodiode in which a light-sensitive PbS quantum dot (QD) layer combined with a MoS2/WSe2 heterostructure significantly enhances the photovoltaic response. The electron current in the heterostructure is increased by the effective collection of photogenerated electrons induced by PbS QDs. The device exhibits a broadband photovoltaic response from 405 to 1064 nm with a maximum responsivity of 0.76 A/W and a specific detectivity of 5.15 × 1011 Jones. In particular, the response speed is not limited by multiple electron traps in the PbS QDs/2D material heterointerface, and a fast rising/decaying time of 43/48 μs and a -3 dB cutoff frequency of over 10 kHz are achieved. The negative differential capacitance and frequency dependence of capacitance demonstrate the presence of interface states in the MoS2/WSe2 heterointerface that hamper the improvement of the response speed. The scheme to enhance photovoltaic performance without sacrificing response speed provides opportunities for the development of high-performance 2D vdWH optoelectronic devices.

Two-dimensional van der Waals heterostructures can combine properties of individual materials to enable high-performance photodetection. Here, a novel ReS2/graphene/WSe2 heterostructure, prepared by dry transfer, demonstrates air-stable, high-performance, polarization-sensitive, and broadband photodetection. Dark current can be strongly suppressed by the built-in electric field of the heterostructure. The specific detectivities are up to 10(10) Jones and 10(9) Jones under zero and reverse bias, respectively. Response time is on the order of a millisecond. The polarization-sensitive photodetection has been observed in the heterostructure due to the low lattice symmetry of ReS2. Broadband photoresponse from visible to infrared range has been demonstrated. A high photoresponsivity of 1.02 A W-1 is achieved for illumination at the wavelength of 785 nm. This work provides a viable approach toward future high-performance, air-stable, and polarization-sensitive broadband photodetectors.

Surface charge transfer doping has been widely utilized to tune the electronic and optical properties of semiconductor photodetectors based on low-dimensional materials. Although many studies have been conducted on the performance (response time, responsivity, etc.) of doped photodetectors and their mechanisms, they merely examined a specific thickness and did not systematically explore the dependence of doping effects on the number of layers. This work performs a series of investigations on ReS2 photodetectors with different numbers of layers and demonstrates that the p-dopant tetrafluorotetracyanoquinodimethane (F4-TCNQ) converts the deep trap states into recombination centers for few-layer ReS2 and induces a vertical p-n junction for thicker ReS2. A response time of 200 ms is observed in the decorated 2-layer ReS2 photodetector, more than two orders of magnitude faster than the response of the pristine photodetector, due to the disappearance of deep trap states. A current rectification ratio of 30 in the F4-TCNQ-decorated sandwiched ReS2 device demonstrates the formation of a vertical p-n junction in a thicker ReS2 device. The responsivity is as high as 2,000 A/W owing to the strong carrier separation of the p-n junction. Different thicknesses of ReS2 enable switching of the prominent operating mechanism between transforming deep trap states into recombination centers and forming a vertical p-n junction. The thickness-dependent doping effect of a two-dimensional material serves as a new mechanism and provides a scheme toward improving the performance of other semiconductor devices, especially optical and electronic devices based on low-dimensional materials.

Surface-enhanced Raman scattering (SERS) has been widely investigated as a powerful trace analysis technique. Two-dimensional (2D) materials are recognized as potential platforms for SERS. Herein, Raman enhancement on a 2D nitrogen-doped graphene (NG) substrate is reported. The 2D NG with different layers is synthesized on a SiO2/Si substrate by a microwave plasma heating (MPH) treatment using silk fibroin (SF) as the precursor. Bonding configurations of nitrogen dopants in graphene are revealed by X-ray photoelectron spectroscopy (XPS). Electrical properties of monolayer, bilayer, and trilayer NG exhibit p-type semiconductor behavior, while four-layer and thicker NG exhibit metallic behavior. The p-type semiconductor behavior can be attributed to O atoms in NG, which is demonstrated by XPS. Then, significant Raman enhancement of Rhodamine 6G (R6G) molecules is achieved by NG as the SERS substrate, which is based on the charge-transfer mechanism, and the Raman enhancement effect is thickness-dependent. For monolayer NG, the detection limit of R6G molecules on NG can be as low as 10(-8) M and the Raman enhancement factor reaches 10(6). Moreover, the detection is stable for days under an ambient environment. This work shows NG to be a potential Raman enhancement platform for sensitive molecular detection.