6 research outputs found

    Hydrogen Evolution Catalyzed by a Molybdenum Sulfide Two-Dimensional Structure with Active Basal Planes

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    Molybdenum disulfide has been demonstrated as a promising catalyst for hydrogen evolution reaction (HER). However, its performance is limited by fractional active edge sites and the strong dependence on hydrogen coverage. In this study, we find an enhanced HER performance in a two-dimensional substoichiometric molybdenum sulfide. Both first-principles calculations and experimental results demonstrate that the basal plane is catalytically active toward HER, as evidenced by an optimum Gibbs free energy and a low reaction overpotential. More interestingly, the HER performance is insensitive to hydrogen coverage and can be improved under compressive in-plane biaxial strains. Our results suggest not only an improved HER performance of substoichiometric molybdenum sulfide due to its chemical reactive basal plane but also a way to tune the performance

    Orientation and Electronic Structures of Multilayered Graphene Nanoribbons Produced by Two-Zone Chemical Vapor Deposition

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    The orientation and electronic structure of multilayered graphene nanoribbons with an armchair-edge (AGNRs) were determined by low-temperature scanning tunneling microscopy in this study. The orientation of AGNRs was found to be an edge-on structure when positioned as a top layer, while previous reports showed a face-on structure for monolayered AGNRs on Au(111). According to density functional theory calculations, AGNRs in a top layer preferentially form as edge-on structures rather than face-on structures due to the balance of CH−π and π–π interactions between AGNRs. Scanning tunneling spectroscopy and density functional theory calculations revealed that the electronic structures of multilayered AGNRs are similar to those in a gas-phase due to the lack of interaction between AGNRs and the Au(111) substrate. The observation of AGNRs in mutilayers might suggest the conformation-assisted mechanism of dehydrogenation when there is no contact with the Au(111) substrate

    Substoichiometric Molybdenum Sulfide Phases with Catalytically Active Basal Planes

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    Molybdenum sulfide (MoS<sub>2</sub>) is widely recognized for its catalytic activities where the edges of the crystals turn over reactions. Generating sulfur defects on the basal plane of MoS<sub>2</sub> can improve its catalytic activity, but generally, there is a lack of model systems for understanding metal-centered catalysis on the basal planes. Here, we synthesized a new phase of substoichiometric molybdenum sulfide (s-MoS<sub><i>x</i></sub>) on a sulfur-enriched copper substrate. The basal plane of s-MoS<sub><i>x</i></sub> contains chemically reactive Mo-rich sites that can undergo dynamic dissociative adsorption/desorption processes with molecular hydrogen, thus demonstrating its usefulness for hydrogen-transfer catalysis. In addition, scanning tunneling microscopy was used to monitor surface-directed Ullmann coupling of 2,8-dibromo-dibenzothiophene molecules on s-MoS<sub><i>x</i></sub> nanosheets, where the 4-fold symmetric surface sites on s-MoS<sub><i>x</i></sub> direct C–C coupling to form cyclic tetramers with high selectivity

    Gate-Tunable Giant Stark Effect in Few-Layer Black Phosphorus

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    Two-dimensional black phosphorus (BP) has sparked enormous research interest due to its high carrier mobility, layer-dependent direct bandgap and outstanding in-plane anisotropic properties. BP is one of the few two-dimensional materials where it is possible to tune the bandgap over a wide energy range from the visible up to the infrared. In this article, we report the observation of a giant Stark effect in electrostatically gated few-layer BP. Using low-temperature scanning tunnelling microscopy, we observed that in few-layer BP, when electrons are injected, a monotonic reduction of the bandgap occurs. The injected electrons compensate the existing defect-induced holes and achieve up to 35.5% bandgap modulation in the light-doping regime. When probed by tunnelling spectroscopy, the local density of states in few-layer BP shows characteristic resonance features arising from layer-dependent sub-band structures due to quantum confinement effects. The demonstration of an electrical gate-controlled giant Stark effect in BP paves the way to designing electro-optic modulators and photodetector devices that can be operated in a wide electromagnetic spectral range

    Two-Dimensional Polymer Synthesized <i>via</i> Solid-State Polymerization for High-Performance Supercapacitors

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    Two-dimensional (2-D) polymer has properties that are attractive for energy storage applications because of its combination of heteroatoms, porosities and layered structure, which provides redox chemistry and ion diffusion routes through the 2-D planes and 1-D channels. Here, conjugated aromatic polymers (CAPs) were synthesized in quantitative yield <i>via</i> solid-state polymerization of phenazine-based precursor crystals. By choosing flat molecules (2-TBTBP and 3-TBQP) with different positions of bromine substituents on a phenazine-derived scaffold, C–C cross coupling was induced following thermal debromination. CAP-2 is polymerized from monomers that have been prepacked into layered structure (3-TBQP). It can be mechanically exfoliated into micrometer-sized ultrathin sheets that show sharp Raman peaks which reflect conformational ordering. CAP-2 has a dominant pore size of ∼0.8 nm; when applied as an asymmetric supercapacitor, it delivers a specific capacitance of 233 F g<sup>–1</sup> at a current density of 1.0 A g<sup>–1</sup>, and shows outstanding cycle performance

    Surface Functionalization of Black Phosphorus via Potassium toward High-Performance Complementary Devices

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    Two-dimensional black phosphorus configured field-effect transistor devices generally show a hole-dominated ambipolar transport characteristic, thereby limiting its applications in complementary electronics. Herein, we demonstrate an effective surface functionalization scheme on few-layer black phosphorus, through in situ surface modification with potassium, with a view toward high performance complementary device applications. Potassium induces a giant electron doping effect on black phosphorus along with a clear bandgap reduction, which is further corroborated by in situ photoelectron spectroscopy characterizations. The electron mobility of black phosphorus is significantly enhanced to 262 (377) cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> by over 1 order of magnitude after potassium modification for two-terminal (four-terminal) measurements. Using lithography technique, a spatially controlled potassium doping technique is developed to establish high-performance complementary devices on a single black phosphorus nanosheet, for example, the p–n homojunction-based diode achieves a near-unity ideality factor of 1.007 with an on/off ratio of ∼10<sup>4</sup>. Our findings coupled with the tunable nature of in situ modification scheme enable black phosphorus as a promising candidate for further complementary electronics
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