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      Low-temperature plasma-enhanced atomic layer deposition of 2-D MoS2: large area, thickness control and tuneable morphology

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          Abstract

          A low-temperature plasma enhanced atomic layer deposition process is demonstrated to synthesize high quality 2-D MoS 2 films with tuneable morphology.

          Abstract

          Low-temperature controllable synthesis of monolayer-to-multilayer thick MoS 2 with tuneable morphology is demonstrated by using plasma enhanced atomic layer deposition (PEALD). The characteristic self-limiting ALD growth with a growth-per-cycle of 0.1 nm per cycle and digital thickness control down to a monolayer are observed with excellent wafer scale uniformity. The as-deposited films are found to be polycrystalline in nature showing the signature Raman and photoluminescence signals for the mono-to-few layered regime. Furthermore, a transformation in film morphology from in-plane to out-of-plane orientation of the 2-dimensional layers as a function of growth temperature is observed. An extensive study based on high-resolution transmission electron microscopy is presented to unravel the nucleation mechanism of MoS 2 on SiO 2/Si substrates at 450 °C. In addition, a model elucidating the film morphology transformation (at 450 °C) is hypothesized. Finally, the out-of-plane oriented films are demonstrated to outperform the in-plane oriented films in the hydrogen evolution reaction for water splitting applications.

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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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            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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              Two Dimensional Atomic Crystals

              We report free-standing atomic crystals that are strictly 2D and can be viewed as individual atomic planes pulled out of bulk crystals or as unrolled single-wall nanotubes. By using micromechanical cleavage, we have prepared and studied a variety of 2D crystals, including single layers of boron nitride, graphite, several dichalcogenides and complex oxides. These atomically-thin sheets (essentially gigantic 2D molecules unprotected from the immediate environment) are stable under ambient conditions, exhibit high crystal quality and are continuous on a macroscopic scale.
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                Author and article information

                Journal
                NANOHL
                Nanoscale
                Nanoscale
                Royal Society of Chemistry (RSC)
                2040-3364
                2040-3372
                2018
                2018
                : 10
                : 18
                : 8615-8627
                Affiliations
                [1 ]Department of Applied Physics
                [2 ]Eindhoven University of Technology
                [3 ]5600 MB Eindhoven
                [4 ]The Netherlands
                [5 ]Philips Innovation Services
                [6 ]Laboratory of Inorganic Materials Chemistry
                [7 ]Department Chemical Engineering and Chemistry
                [8 ]Oxford Instruments Plasma Technology
                [9 ]Yatton
                [10 ]UK
                Article
                10.1039/C8NR02339E
                29696289
                56d26a86-116a-4dec-be9a-59ef6fcc7df4
                © 2018

                http://rsc.li/journals-terms-of-use

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