Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites–MoS2 heterostructures, 2D–0D MoS2/quantum dots (QDs) and 2D–2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W−1 up to 1010 A W−1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10−9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

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A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self-powered to flexible devices

: Uniform growth of pristine two dimensional (2D) materials over large areas at lower temperatures without sacrifice of their unique physical properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into functional devices. This Letter describes a vapor phase growth technique for precisely controlled synthesis of continuous, uniform molecular layers of MoS2 on silicon dioxide and highly oriented pyrolitic graphite substrates of over several square centimeters at 350 deg C. Synthesis of few-layer MoS2 in this ultra-high vacuum physical vapor deposition process yields materials with key optical and electronic properties identical to exfoliated layers. The films are composed of nano-scale domains with strong chemical binding between domain boundaries, allowing lift-off from the substrate and electronic transport measurements from contacts with separation on the order of centimeters.

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Continuous Ultra-Thin MOS2 Films Grown by Low-Temperature Physical Vapor Deposition (Postprint)

Since the first isolation of graphene, new classes of two-dimensional (2D) materials have offered fascinating platforms for fundamental science and technology explorations at the nanometer scale. In particular, 2D transition metal dichalcogenides (TMD) such as MoS2 and WSe2 have been intensely investigated due to their unique electronic and optical properties, including tunable optical bandgaps, direct–indirect bandgap crossover, strong spin–orbit coupling, etc., for next-generation flexible nanoelectronics and nanophotonics applications. On the other hand, organics have always been excellent materials for flexible electronics. A plethora of organic molecules, including donors, acceptors, and photosensitive molecules, can be synthesized using low cost and scalable procedures. Marrying the fields of organics and 2D TMDs will bring benefits that are not present in either material alone, enabling even better, multifunctional flexible devices. Central to the realization of such devices is a fundamental understanding of the organic–2D TMD interface. Here, we review the organic–2D TMD interface from both chemical and physical perspectives. We discuss the current understanding of the interfacial interactions between the organic layers and the TMDs, as well as the energy level alignment at the interface, focusing in particular on surface charge transfer and electronic screening effects. Applications from the literature are discussed, especially in optoelectronics and p–n hetero- and homo-junctions. We conclude with an outlook on future scientific and device developments based on organic–2D TMD heterointerfaces.

The organic-2D transition metal dichalcogenide heterointerface.

The unique properties of hybrid heterostructures have motivated the integration of two or more different types of nanomaterials into a single optoelectronic device structure. Despite the promising features of organic semiconductors, such as their acceptable optoelectronic properties, availability of low-cost processes for their fabrication, and flexibility, further optimization of both material properties and device performances remains to be achieved. With the emergence of atomically thin 2D materials, they have been integrated with conventional organic semiconductors to form multidimensional heterostructures that overcome the present limitations and provide further opportunities in the field of optoelectronics. Herein, a comprehensive review of emerging 2D-organic heterostructures-from their synthesis and fabrication to their state-of-the-art optoelectronic applications-is presented. Future challenges and opportunities associated with these heterostructures are highlighted.

2D-Organic Hybrid Heterostructures for Optoelectronic Applications.

Single-component near-infrared phototransistors based on ambipolar organic semiconductor nanowires have been investigated and compared with their corresponding thin-film counterparts. The nanowire organic phototransistors (NW-OPTs) showed photocurrent/dark-current ratios and photoresponsivities as high as 1.3 × 104 and 440 mA W−1 for the p-type channel, and 3.3 × 104 and 70 mA W−1 for the n-type channel, respectively, upon near-infrared illumination with an intensity of 47.1 mW cm−2. These were much higher values compared to their thin-film counterparts. The enhancement of the near-infrared photoresponse could be attributed to the larger trap density originating from the semiconductor/insulator interface and the semiconductor/air interface. The performance of NW-OPTs was demonstrated to open up new possibilities to improve the near-infrared photoresponse of single-component devices.

Enhanced near-infrared photoresponse of organic phototransistors based on single-component donor-acceptor conjugated polymer nanowires

High performance photodiode based on MoS2/pentacene heterojunction

High performance photoresponsive field-effect transistors based on MoS2/pentacene heterojunction

Performances of photoresponsive organic field-effect transistors (photOFETs) operating in the near infrared (NIR) region utilizing SiO2 as the gate dielectric is generally low due to low carrier mobility of the channel We report on NIR photOFETs based on lead phthalocyanine (PbPc)/C60 heterojunction with ultrahigh photoresponsivity by utilizing poly(vinyl alcohol) (PVA) as the gate dielectric For 808 nm NIR illumination of 169 mW cm−2, an ultrahigh photoresponsivity of 21 A W−1, and an external quantum efficiency of 3230% were obtained at a gate voltage of 30 V and a drain voltage of 80 V, which are 124 times and 126 times as large as the reference device with SiO2 as the gate dielectric, respectively The ultrahigh enhancement of photoresponsivity is resulted from the huge increase of electron mobility of C60 film grown on PVA dielectric AFM investigations revealed that the C60 film grown on PVA is much smooth and uniform and the grain size is much larger than that grown on SiO2 dielectric, which together results in four orders of magnitude increase of the field-effect electron mobility of C60 film

http://iopscience.iop.org/article/10.1088/0957-4484/26/18/185501/pdf

Ultrahigh near infrared photoresponsive organic field-effect transistors with lead phthalocyanine/C60 heterojunction on poly(vinyl alcohol) gate dielectric.

The performance of photoresponsive organic FETs (photOFETs) intimately depends on their optical absorption and charge carrier transport property. We report on red light responsive photOFETs based on various device structures utilizing neodymium phthalocyanine (NdPc2) as the light sensitive material. PhotOFETs based on planar heterojunction, bulk heterojunction (BHJ), and hybrid planar–BHJ (HPBHJ) with NdPc2 and C60 on poly(vinyl alcohol) (PVA) and SiO2 gate dielectric were fabricated and characterized. Among various device structures, HPBHJ-photOFET on PVA dielectric showed the best performance. For the 650-nm-red light illumination, an ultrahigh photoresponsivity of 108 A/W and a maximum photosensitivity of $3.75 \times 10^{4}$ were obtained. The ultrahigh enhancement of the photoresponsivity for HPBHJ-photOFET is resulted from high absorption coefficient of NdPc2 in the red light region, high dissociation efficiency of the photogenerated excitons, and high electron mobility of C60 layer grown on PVA.

Toward Ultrahigh Red Light Responsive Organic FETs Utilizing Neodymium Phthalocyanine as Light Sensitive Material

Hybrid organic-inorganic (HOI) photodiodes have both advantages of organic and inorganic materials, including compatibility of traditional Si-based semiconductor technology, low cost, high photosensitivity and high reliability, showing tremendous value in application. Red light sensitive HOI photodiodes based on the p-Si/copper phthalocyanine (CuPc) hetrojunction were fabricated and characterized. The effects of CuPc layer thickness on the performance were investigated, and an optimal layer thickness of around 30 nm was determined. An analytical expression is derived to describe the measured thickness dependence of the saturation photocurrent. For the device with optimal CuPc layer thickness, a photoresponsivity of 0.35 A/W and external quantum efficiency of 70% were obtained at 9 V reverse voltage bias and 655 nm light illumination of 0.451 mW. Furthermore, optical power dependent performances were investigated.

Qiang Ren

Papers

High performance photodiode based on MoS2/pentacene heterojunction

High performance photoresponsive field-effect transistors based on MoS2/pentacene heterojunction

Ultrahigh near infrared photoresponsive organic field-effect transistors with lead phthalocyanine/C60 heterojunction on poly(vinyl alcohol) gate dielectric.

Toward Ultrahigh Red Light Responsive Organic FETs Utilizing Neodymium Phthalocyanine as Light Sensitive Material

High Performance Photodiode Based on p-Si/Copper Phthalocyanine Heterojunction.