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      Van der Waals epitaxial growth and optoelectronics of large-scale WSe 2/SnS 2 vertical bilayer p–n junctions

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          Abstract

          High-quality two-dimensional atomic layered p–n heterostructures are essential for high-performance integrated optoelectronics. The studies to date have been largely limited to exfoliated and restacked flakes, and the controlled growth of such heterostructures remains a significant challenge. Here we report the direct van der Waals epitaxial growth of large-scale WSe 2/SnS 2 vertical bilayer p–n junctions on SiO 2/Si substrates, with the lateral sizes reaching up to millimeter scale. Multi-electrode field-effect transistors have been integrated on a single heterostructure bilayer. Electrical transport measurements indicate that the field-effect transistors of the junction show an ultra-low off-state leakage current of 10 −14 A and a highest on–off ratio of up to 10 7. Optoelectronic characterizations show prominent photoresponse, with a fast response time of 500 μs, faster than all the directly grown vertical 2D heterostructures. The direct growth of high-quality van der Waals junctions marks an important step toward high-performance integrated optoelectronic devices and systems.

          Abstract

          Growth of large area and defect-free two-dimensional semiconductor layers for high-performance p–n junction applications has been a great challenge. Yang et al. prepare millimeter-scaled WSe 2/SnS 2 vertical heterojunctions by two-step van der Waals epitaxy, which show excellent optoelectronic properties.

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          Emerging photoluminescence in monolayer MoS2.

          Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS(2), a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS(2) crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS(2) provides new opportunities for engineering the electronic structure of matter at the nanoscale.
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            Experimental Observation of Quantum Hall Effect and Berry's Phase in Graphene

            When electrons are confined in two-dimensional (2D) materials, quantum mechanically enhanced transport phenomena, as exemplified by the quantum Hall effects (QHE), can be observed. Graphene, an isolated single atomic layer of graphite, is an ideal realization of such a 2D system. Here, we report an experimental investigation of magneto transport in a high mobility single layer of graphene. Adjusting the chemical potential using the electric field effect, we observe an unusual half integer QHE for both electron and hole carriers in graphene. Vanishing effective carrier masses is observed at Dirac point in the temperature dependent Shubnikov de Haas oscillations, which probe the 'relativistic' Dirac particle-like dispersion. The relevance of Berry's phase to these experiments is confirmed by the phase shift of magneto-oscillations, related to the exceptional topology of the graphene band structure.
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              Atomically thin MoS2: A new direct-gap semiconductor

              The electronic properties of ultrathin crystals of molybdenum disulfide consisting of N = 1, 2, ... 6 S-Mo-S monolayers have been investigated by optical spectroscopy. Through characterization by absorption, photoluminescence, and photoconductivity spectroscopy, we trace the effect of quantum confinement on the material's electronic structure. With decreasing thickness, the indirect band gap, which lies below the direct gap in the bulk material, shifts upwards in energy by more than 0.6 eV. This leads to a crossover to a direct-gap material in the limit of the single monolayer. Unlike the bulk material, the MoS2 monolayer emits light strongly. The freestanding monolayer exhibits an increase in luminescence quantum efficiency by more than a factor of 1000 compared with the bulk material.
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                Author and article information

                Contributors
                wdhu@mail.sitp.ac.cn
                anlian.pan@hnu.edu.cn
                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group UK (London )
                2041-1723
                4 December 2017
                4 December 2017
                2017
                : 8
                : 1906
                Affiliations
                [1 ]GRID grid.67293.39, Key Laboratory for Micro-Nano Physics and Technology of Hunan Province, State Key Laboratory of Chemo/Biosensing and Chemometrics, and School of Physics and Electronics, , Hunan University, ; Changsha, 410082 Hunan China
                [2 ]ISNI 0000000119573309, GRID grid.9227.e, State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, , Chinese Academy of Sciences, ; 200083 Shanghai, China
                [3 ]ISNI 0000 0004 1761 0489, GRID grid.263826.b, SEU-FEI Nano-Pico Center, Key Lab of MEMS of Ministry of Education, , Southeast University, ; 210096 Nanjing, China
                [4 ]ISNI 0000 0001 2314 964X, GRID grid.41156.37, National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, , Nanjing University, ; 210093 Nanjing, China
                [5 ]ISNI 0000 0000 9632 6718, GRID grid.19006.3e, Department of Chemistry and Biochemistry and California NanoSystems Institute, , University of California at Los Angeles, ; Los Angeles, CA 90095 USA
                Author information
                http://orcid.org/0000-0001-5436-0077
                http://orcid.org/0000-0002-6656-2669
                http://orcid.org/0000-0002-0962-5424
                http://orcid.org/0000-0002-2750-5004
                Article
                2093
                10.1038/s41467-017-02093-z
                5715014
                29203864
                82716b78-b848-4d80-9142-06aac5017918
                © The Author(s) 2017

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

                History
                : 15 June 2017
                : 3 November 2017
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