RSC Advances REVIEW View Article Online View Journal | View Issue A review of molybdenum disulfide (MoS2) based photodetectors: from ultra-broadband, self- Cite this: RSC Adv., 2020, 10,30529 powered to flexible devices Hari Singh Nalwa 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 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 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 WÀ1 up to 1010 AWÀ1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10À9 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 This article is licensed under a MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of Received 9th April 2020 a very wide range of MoS -based photodetectors have been analyzed in terms of their Accepted 17th July 2020 2 photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement DOI: 10.1039/d0ra03183f Open Access Article. Published on 19 August 2020. Downloaded 9/25/2021 4:52:00 AM. wavelengths and incident laser power densities. Conclusions and the future direction are also outlined rsc.li/rsc-advances on the development of MoS2 and other 2D TMD-based photodetectors. Advanced Technology Research, 26650 The Old Road, Valencia, California 91381, USA. E-mail: [email protected] Dr Hari Singh Nalwa is a distinguished independent scientist and scholar working in the elds of nanotechnology and materials science. Dr Nalwa has authored more than 170 scientic articles, 26 book chapters, and 125 volumes of scientic books, as well as 18 patents in cross-disciplinary research areas of nanotechnology, materials science and polymer science. Dr Nalwa's research interests include ferroelectric polymers, conducting polymers, organic nonlinear optical materials for integrated optics, low- and high-dielectric constant materials for microelectronics packaging, 3D printing, two-dimensional (2D) nanomaterial-based bulk heterojunction and dye-sensitized solar cells, and multifunctional sensors for wearable technology. He received the “Award of Excellence” from the Association of American Publishers for the “Handbook of Nanostructured Materials and Nanotechnology,” a 5-volume set (Academic Press, 2000), and “Best Reference Work Award” from the American Society for Engineering Education for “The Encyclopedia of Nanoscience and Nanotechnology,” a 10-volume set (American Scientic Publishers, 2004). He is Founder, President, and Chief Executive Officer (CEO) of American Scientic Publishers (http://www.aspbs.com), which he established in 2000. This journal is © The Royal Society of Chemistry 2020 RSC Adv.,2020,10,30529–30602 | 30529 View Article Online RSC Advances Review 1. Introduction Many research articles have been published on atomically thin layered MoS2 based photodetectors, however, a comprehen- Advances in the elds of electronics, optoelectronics and sive review summarizing the recent developments in MoS2 photonics have created a great demand for new functional mate- photodetectors is completely lacking in the scienti c literature. rials that possess ease of synthesis, processing and fabrication and This review brie y introduces TMDs, including the applications enable desired tailoring of the physical and chemical properties by of MoS2 atomic layers in developing photodetectors. The tuning chemical functionalization and/or formation of hybrid structures of optoelectronic properties by boosting the carrier mobility of for potential applications in electronic and optoelectronic and incident light absorption by MoS2 atomic layers through the – devices.1,2 In this context, earth-abundant two-dimensional (2D) use of plasmonic and halide perovskite MoS2 hybrid hetero- – – – nanomaterials, including graphene3 12 and transition metal structures, 2D 0D and 2D 2D MoS2 heterostructures, interface – ff dichalcogenides (TMDs),13 20 have emerged as novel functional coupling e ect (ICE), or chemical doping of MoS2 lms is dis- materials of choice due to their low-cost production, easy pro- cussed in order to evaluate the performance of MoS2 photode- cessing and easy deposition on different substrates with precise tectors from the perspective of their based phototransistors, thickness control of the atomic layers via mechanical and chem- photoconductors and photodiode components. Particular ical exfoliation, chemical vapor deposition (CVD) and atomic layer emphasis is placed on atomic layered MoS2-based ultra- etching (ALE) methods. Flexible graphene nanosheets have been broadband photodetectors, from their fundamental develop- explored for developing wearable and portable devices, including ment to self-powered to exible photodetectors for wearable – – eld-effect transistors (FETs),21,22 sensors,23 25 supercapacitors,26 28 optoelectronics. The performance of pristine MoS2 atomic layers lithium-ion batteries,29 triboelectric nanogenerators,30 solar and MoS2 hybrid heterostructures with graphene, CNTs, TMDs, – cells31 33 and photodetectors.34,35 Following the research progress of ZnO and surface functionalized MoS2 atomic layers for devel- graphene, a similar wide range of applications for exible atomic oping exible photodetectors is discussed in terms of their layered TMDs have been anticipated and are now slowly emerging, broadband photoresponsivity, detectivity, NEP, photogain, EQE, 36–40 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. from wearable electronics to optoelectronics. Flexible photo- photoresponse speed, mechanical exibility and environmental detectors are becoming a key component of optoelectronic tech- stability. Strain-induced and self-powered MoS2 based hybrid nology for a wide range of applications in the elds of surveillance, photodetectors has also been summarized. Finally, the chal- so robotics, sensors for wearable and portable healthcare and lenges in developing exible photodetectors from TMDs are sports, light-emitting diodes (LEDs), high-speed optical commu- analyzed. This review should be a useful source for and inspire nication, and biomedical imaging.34,35,41 Flexible photodetectors a wide range of audience, including researchers working in the are gaining much attention for use in wearable optoelectronics, for elds of optoelectronics, sensors, materials science, nanotech- which many different types of nanostructures, such as atomic nology, physics, electrical engineering, and communications. layered nanosheets, nanowires (NWs), bers, quantum dots and This article is licensed under a 3D networks of inorganic and organic materials and their nano- fi hybrids, have been studied. These nanostructures have been used 2. Molybdenum disul de (MoS2) for as phototransistors, photoconductors and photodiodes for devel- photodetectors Open Access Article. Published on 19 August 2020. Downloaded 9/25/2021 4:52:00 AM. oping photodetectors from a wide variety of nanomaterials, including silicon (Si) and germanium (Ge),42,43 selenium (Se),44 GaP Fig.1showsthewiderangeoftheelectromagneticspectrum and InP,45,46 CdS,47,48 ZnSe,49 ZnO and its hybrids with PbS, ZnS, covered by 2D nanomaterials, from the near-infrared (NIR) 50–55 56 and mid-IR (MIR) to the far-IR (FIR), the related applications CdO, gold (Au) and polymers, ZnGa2O4, Zn2GeO4 and 57 58,59 60 61 62 63 64 of these nanomaterials in electronics, optoelectronics and In2Ge2O7, CuInSe2, In2S3, In2Se3, Sb2S3, Sb2Se3, Bi2S3, 65 66 67 68 69 70 71 72 73 photonics,andtheatomicandbandstructuresof2Dmate- SnS, SnS2, ZrS3, Zn3P2, PbI2, MoO3, GaS, SnO2, ZnTe, GaTe,74 perovskites,75,76 their hybrid composites with ZnO, gold, rials, including hexagonal boron nitride (h-BN), molybdenum 90 poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) conjugated disul de (MoS2), black phosphorus (BP) and graphene. h-BN 91–95 polymer, and graphene,77–80 polythiophene,81 carbon nanotubes is an insulator with a large bandgap of 6.0 eV, whereas 96–98 99–101 (CNTs),82 graphene nanocomposites with CNTs83 and with ZnO MoS2 and BP are semiconductors with sizeable 84 bandgaps that vary with the number of atomic layers. Gra- quantum dots, transition-metal trichalcogenides (MX3,whereM 102,103 representsatransitionmetal,Ti,Zr,Hf,Nb,orTa,andXis