Browsing by Author "Terrones, Humberto"

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    CVD-grown monolayered MoS2 as an effective photosensor operating at low-voltage
    (IOP Publishing, 2014) Perea-López, Néstor; Lin, Zhong; Pradhan, Nihar R.; Iñiguez-Rábago, Agustín; Elías, Ana Laura; McCreary, Amber; Lou, Jun; Ajayan, Pulickel M.; Terrones, Humberto; Balicas, Luis; Terrones, Mauricio
    We report the fabrication of a photosensor based on as-grown single crystal monolayers of MoS2synthesized by chemical vapor deposition (CVD). The measurements were performed using Au/Ti leads in a two terminal configuration on CVD-grown MoS2 on a SiO2/Si substrate. The device was operated in air at room temperature at low bias voltages ranging from −2 V to 2 V and its sensing capabilities were tested for two different excitation wavelengths (514.5 nm and 488 nm). The responsivity reached 1.1 mA W−1 when excited with a 514.5 nm laser at a bias of 1.5 V. This responsivity is one order of magnitude larger than that reported from photo devices fabricated using CVD-grown multilayered WS2. A rectifying-effect was observed for the optically excited current, which was four times larger in the direct polarization bias when compared to the reverse bias photocurrent. Such rectifying behavior can be attributed to the asymmetric electrode placement on the triangular MoS2 monocrystal. It is envisioned that these components could eventually be used as efficient and low cost photosensors based on CVD-grown transition metal dichalcogenide monolayers.
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    Sulfur Vacancy Related Optical Transitions in Graded Alloys of MoxW1-xS2 Monolayers
    (Wiley, 2024) Ghafariasl, Mahdi; Zhang, Tianyi; Ward, Zachary D.; Zhou, Da; Sanchez, David; Swaminathan, Venkataraman; Terrones, Humberto; Terrones, Mauricio; Abate, Yohannes
    Engineering electronic bandgaps is crucial for applications in information technology, sensing, and renewable energy. Transition metal dichalcogenides (TMDCs) offer a versatile platform for bandgap modulation through alloying, doping, and heterostructure formation. Here, the synthesis of a 2D MoxW1-xS2 graded alloy is reported, featuring a Mo-rich center that transitions to W-rich edges, achieving a tunable bandgap of 1.85 to 1.95 eV when moving from the center to the edge of the flake. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy showed the presence of sulfur monovacancy, VS, whose concentration varied across the graded MoxW1-xS2 layer as a function of Mo content with the highest value in the Mo-rich center region. Optical spectroscopy measurements supported by ab initio calculations reveal a doublet electronic state of VS, which is split due to the spin-orbit interaction, with energy levels close to the conduction band or deep in the bandgap depending on whether the vacancy is surrounded by W atoms or Mo atoms. This unique electronic configuration of VS in the alloy gave rise to four spin-allowed optical transitions between the VS levels and the valence bands. The study demonstrates the potential of defect and optical engineering in 2D monolayers for advanced device applications.
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    Vertical and in-plane heterostructures from WS2/MoS2 monolayers
    (Nature Publishing Group, 2014) Gong, Yongji; Lin, Junhao; Wang, Xingli; Shi, Gang; Lei, Sidong; Lin, Zhong; Zou, Xiaolong; Ye, Gonglan; Vajtai, Robert; Yakobson, Boris I.; Terrones, Humberto; Terrones, Mauricio; Tay, Beng Kang; Lou, Jun; Pantelides, Sokrates T.; Liu, Zheng; Zhou, Wu; Ajayan, Pulickel M.
    Layer-by-layer stacking or lateral interfacing of atomic monolayers has opened up unprecedented opportunities to engineer two-dimensional heteromaterials. Fabrication of such artificial heterostructures with atomically clean and sharp interfaces, however, is challenging. Here, we report a one-step growth strategy for the creation of high-quality vertically stacked as well as in-plane interconnected heterostructures of WS2/MoS2 via control of the growth temperature. Vertically stacked bilayers with WS2 epitaxially grown on top of the MoS2 monolayer are formed with preferred stacking order at high temperature. A strong interlayer excitonic transition is observed due to the type II band alignment and to the clean interface of these bilayers. Vapour growth at low temperature, on the other hand, leads to lateral epitaxy of WS2 on MoS2 edges, creating seamless and atomically sharp in-plane heterostructures that generate strong localized photoluminescence enhancement and intrinsic p–n junctions. The fabrication of heterostructures from monolayers, using simple and scalable growth, paves the way for the creation of unprecedented two-dimensional materials with exciting properties.