Ultrahigh‐Sensitive Broadband Photodetectors Based on Dielectric Shielded MoTe2/Graphene/SnS2 p–g–n Junctions

2D atomic sheets of transition metal dichalcogenides (TMDs) have a tremendous potential for next‐generation optoelectronics since they can be stacked layer‐by‐layer to form van der Waals (vdW) heterostructures. This allows not only bypassing difficulties in heteroepitaxy of lattice‐mismatched semiconductors of desired functionalities but also providing a scheme to design new optoelectronics that can surpass the fundamental limitations on their conventional semiconductor counterparts. Herein, a novel 2D h‐BN/p‐MoTe₂/graphene/n‐SnS₂/h‐BN p–g–n junction, fabricated by a layer‐by‐layer dry transfer, demonstrates high‐sensitivity, broadband photodetection at room temperature. The combination of the MoTe₂ and SnS₂ of complementary bandgaps, and the graphene interlayer provides a unique vdW heterostructure with a vertical built‐in electric field for high‐efficiency broadband light absorption, exciton dissociation, and carrier transfer. The graphene interlayer plays a critical role in enhancing sensitivity and broadening the spectral range. An optimized device containing 5?7‐layer graphene has been achieved and shows an extraordinary responsivity exceeding 2600 A W^?1 with fast photoresponse and specific detectivity up to ≈10¹³ Jones in the ultraviolet–visible–near‐infrared spectrum. This result suggests that the vdW p–g–n junctions containing multiple photoactive TMDs can provide a viable approach toward future ultrahigh‐sensitivity and broadband photonic detectors.     

High‐Responsivity Graphene/InAs Nanowire Heterojunction Near‐Infrared Photodetectors with Distinct Photocurrent On/Off Ratios

Graphene is a promising candidate material for high‐speed and ultra‐broadband photodetectors. However, graphene‐based photodetectors suffer from low photoreponsivity and I_light/I_dark ratios due to their negligible‐gap nature and small optical absorption. Here, a new type of graphene/InAs nanowire (NW) vertically stacked heterojunction infrared photodetector is reported, with a large photoresponsivity of 0.5 AW^?1 and I_light/I_dark ratio of 5 × 10², while the photoresponsivity and I_light/I_dark ratio of graphene infrared photodetectors are 0.1 mAW^?1 and 1, respectively. The Fermi level (E_F ) of graphene can be widely tuned by the gate voltage owing to its 2D nature. As a result, the back‐gated bias can modulate the Schottky barrier (SB) height at the interface between graphene and InAs NWs. Simulations further demonstrate the rectification behavior of graphene/InAs NW heterojunctions and the tunable SB controls charge transport across the vertically stacked heterostructure. The results address key challenges for graphene‐based infrared detectors, and are promising for the development of graphene electronic and optoelectronic applications.     

Van der Waals Epitaxial Growth of Atomic Layered HfS2 Crystals for Ultrasensitive Near‐Infrared Phototransistors

As a member of the group IVB transition metal dichalcogenides (TMDs) family, hafnium disulfide (HfS₂) is recently predicted to exhibit higher carrier mobility and higher tunneling current density than group VIB (Mo and W) TMDs. However, the synthesis of high‐quality HfS₂ crystals, sparsely reported, has greatly hindered the development of this new field. Here, a facile strategy for controlled synthesis of high‐quality atomic layered HfS₂ crystals by van der Waals epitaxy is reported. Density functional theory calculations are applied to elucidate the systematic epitaxial growth process of the S‐edge and Hf‐edge. Impressively, the HfS₂ back‐gate field‐effect transistors display a competitive mobility of 7.6 cm² V^?1 s^?1 and an ultrahigh on/off ratio exceeding 10⁸. Meanwhile, ultrasensitive near‐infrared phototransistors based on the HfS₂ crystals (indirect bandgap ≈1.45 eV) exhibit an ultrahigh responsivity exceeding 3.08 × 10⁵ A W^?1, which is 10⁹‐fold higher than 9 × 10^?5 A W^?1 obtained from the multilayer MoS₂ in near‐infrared photodetection. Moreover, an ultrahigh photogain exceeding 4.72 × 10⁵ and an ultrahigh detectivity exceeding 4.01 × 10¹² Jones, superior to the vast majority of the reported 2D‐materials‐based phototransistors, imply a great promise in TMD‐based 2D electronic and optoelectronic applications.     

Ultraflexible Near‐Infrared Organic Photodetectors for Conformal Photoplethysmogram Sensors

Flexible organic optoelectronic devices simultaneously targeting mechanical conformability and fast responsivity in the near‐infrared (IR) region are a prerequisite to expand the capabilities of practical optical science and engineering for on‐skin optoelectronic applications. Here, an ultraflexible near‐IR responsive skin‐conformal photoplethysmogram sensor based on a bulk heterojunction photovoltaic active layer containing regioregular polyindacenodithiophene‐pyridyl[2,1,3]thiadiazole‐cyclopentadithiophene (PIPCP) is reported. The ultrathin (3 µm thick) photodetector exhibits unprecedented operational stability under severe mechanical deformation at a bending radius of less than 3 µm, even after more than 10³ bending cycles. Deliberate optimization of the physical dimensions of the active layer used in the device enables precise on/off switching and high device yield simultaneously. The response frequency over 1 kHz under mechanically deformed conditions facilitates conformal electronic sensors at the machine/human interface. Finally, a mechanically stretchable, flexible, and skin‐conformal photoplethysmogram (PPG) device with higher sensitivity than those of rigid devices is demonstrated, through conformal adherence to the flexuous surface of a fingerprint.     

High Detectivity and Transparent Few‐Layer MoS2/Glassy‐Graphene Heterostructure Photodetectors

Layered van der Waals heterostructures have attracted considerable attention recently, due to their unique properties both inherited from individual two‐dimensional (2D) components and imparted from their interactions. Here, a novel few‐layer MoS₂/glassy‐graphene heterostructure, synthesized by a layer‐by‐layer transfer technique, and its application as transparent photodetectors are reported for the first time. Instead of a traditional Schottky junction, coherent ohmic contact is formed at the interface between the MoS₂ and the glassy‐graphene nanosheets. The device exhibits pronounced wavelength selectivity as illuminated by monochromatic lights. A responsivity of 12.3 mA W^?1 and detectivity of 1.8 × 10¹⁰ Jones are obtained from the photodetector under 532 nm light illumination. Density functional theory calculations reveal the impact of specific carbon atomic arrangement in the glassy‐graphene on the electronic band structure. It is demonstrated that the band alignment of the layered heterostructures can be manipulated by lattice engineering of 2D nanosheets to enhance optoelectronic performance.     

Perovskite/Organic Bulk‐Heterojunction Integrated Ultrasensitive Broadband Photodetectors with High Near‐Infrared External Quantum Efficiency over 70%

Ultraviolet‐visible‐near infrared (UV‐Vis‐NIR) broadband detection is important for image sensing, communication, and environmental monitoring, yet remains as a challenge in achieving high external quantum efficiency (EQE) in the broad spectrum range. Herein, sensitive broadband integrated photodetectors (PDs) with high EQE levels are reported. The organic bulk‐heterojunction (OBHJ) layer, based on a NIR sensitive organic acceptor, is employed to extend the response spectrum of the perovskite PDs. A key strategy of introducing dual electron transport materials respectively for Vis and NIR regions into the active layer of integrated PDs is applied. Further combined with the proper energy level alignment and reasonable distribution of PC₆₁BM in the active layer, the extraction and transport of photo induced charges in between perovskite and OBHJ is promoted efficiently. The integrated PD with the optimized structure exhibits an EQE mostly beyond 70% in the Vis–NIR region, which is the highest value among the ever reported solution‐processable broadband PDs. The highest responsivity is 0.444 and 0.518 A W^?1 in the Vis and NIR region, respectively. The specific detectivity is beyond 10¹⁰ Jones in the range from 340 to 940 nm, enabling the device to detect weak signals in the UV to NIR broad region.     

Light Confinement Effect Induced Highly Sensitive,Self‐Driven Near‐Infrared Photodetector and Image Sensor Based on Multilayer PdSe2/Pyramid Si Heterojunction

In this study, a highly sensitive and self‐driven near‐infrared (NIR) light photodetector based on PdSe₂/pyramid Si heterojunction arrays, which are fabricated through simple selenization of predeposited Pd nanofilm on black Si, is demonstrated. The as‐fabricated hybrid device exhibits excellent photoresponse performance in terms of a large on/off ratio of 1.6 × 10⁵, a responsivity of 456 mA W^?1, and a high specific detectivity of up to 9.97 × 10¹³ Jones under 980 nm illumination at zero bias. Such a relatively high sensitivity can be ascribed to the light trapping effect of the pyramid microstructure, which is confirmed by numerical modeling based on finite‐difference time domain. On the other hand, thanks to the broad optical absorption properties of PdSe₂, the as‐fabricated device also exhibits obvious sensitivity to other NIR illuminations with wavelengths of 1300, 1550, and 1650 nm, which is beyond the photoresponse range of Si‐based devices. It is also found that the PdSe₂/pyramid Si heterojunction device can also function as an NIR light sensor, which can readily record both “tree” and “house” images produced by 980 and 1300 nm illumination, respectively.     

Near‐Infrared Excitation/Emission and Multiphoton‐Induced Fluorescence of Carbon Dots

Carbon dots (CDs) have significant potential for use in various fields including biomedicine, bioimaging, and optoelectronics. However, inefficient excitation and emission of CDs in both near‐infrared (NIR‐I and NIR‐II) windows remains an issue. Solving this problem would yield significant improvement in the tissue‐penetration depth for in vivo bioimaging with CDs. Here, an NIR absorption band and enhanced NIR fluorescence are both realized through the surface engineering of CDs, exploiting electron‐acceptor groups, namely molecules or polymers rich in sulfoxide/carbonyl groups. These groups, which are bound to the outer layers and the edges of the CDs, influence the optical bandgap and promote electron transitions under NIR excitation. NIR‐imaging information encryption and in vivo NIR fluorescence imaging of the stomach of a living mouse using CDs modified with poly(vinylpyrrolidone) in aqueous solution are demonstrated. In addition, excitation by a 1400 nm femtosecond laser yields simultaneous two‐photon‐induced NIR emission and three‐photon‐induced red emission of CDs in dimethyl sulfoxide. This study represents the realization of both NIR‐I excitation and emission as well as two‐photon‐ and three‐photon‐induced fluorescence of CDs excited in an NIR‐II window, and provides a rational design approach for construction and clinical applications of CD‐based NIR imaging agents.     

Single Pixel Black Phosphorus Photodetector for Near‐Infrared Imaging

Infrared imaging systems have wide range of military or civil applications and 2D nanomaterials have recently emerged as potential sensing materials that may outperform conventional ones such as HgCdTe, InGaAs, and InSb. As an example, 2D black phosphorus (BP) thin film has a thickness‐dependent direct bandgap with low shot noise and noncryogenic operation for visible to mid‐infrared photodetection. In this paper, the use of a single‐pixel photodetector made with few‐layer BP thin film for near‐infrared imaging applications is demonstrated. The imaging is achieved by combining the photodetector with a digital micromirror device to encode and subsequently reconstruct the image based on compressive sensing algorithm. Stationary images of a near‐infrared laser spot (λ = 830 nm) with up to 64 × 64 pixels are captured using this single‐pixel BP camera with 2000 times of measurements, which is only half of the total number of pixels. The imaging platform demonstrated in this work circumvents the grand challenges of scalable BP material growth for photodetector array fabrication and shows the efficacy of utilizing the outstanding performance of BP photodetector for future high‐speed infrared camera applications.     

Near‐Infrared Annihilation of Conductive Filaments in Quasiplane MoSe2/Bi2Se3 Nanosheets for Mimicking Heterosynaptic Plasticity

It is desirable to imitate synaptic functionality to break through the memory wall in traditional von Neumann architecture. Modulating heterosynaptic plasticity between pre‐ and postneurons by another modulatory interneuron ensures the computing system to display more complicated functions. Optoelectronic devices facilitate the inspiration for high‐performance artificial heterosynaptic systems. Nevertheless, the utilization of near‐infrared (NIR) irradiation to act as a modulatory terminal for heterosynaptic plasticity emulation has not yet been realized. Here, an NIR resistive random access memory (RRAM) is reported, based on quasiplane MoSe₂/Bi₂Se₃ heterostructure in which the anomalous NIR threshold switching and NIR reset operation are realized. Furthermore, it is shown that such an NIR irradiation can be employed as a modulatory terminal to emulate heterosynaptic plasticity. The reconfigurable 2D image recognition is also demonstrated by an RRAM crossbar array. NIR annihilation effect in quasiplane MoSe₂/Bi₂Se₃ nanosheets may open a path toward optical‐modulated in‐memory computing and artificial retinal prostheses.