Molybdenum Disulfide Nanowires and Nanoribbons by Electrochemical/Chemical Synthesis

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Abstract
Molybdenum disulfide nanowires and nanoribbons have been synthesized by a two-step, electrochemical/chemical synthetic method. In the first step, MoOx wires (a mixture of MoO2 and MoO3) were electrodeposited size-selectively by electrochemical step-edge decoration on a highly oriented pyrolytic graphite (HOPG) surface. Then, MoOx precursor wires were converted to MoS2 by exposure to H2S either at 500−700°C, producing “low-temperature” or LT MoS2 nanowires that were predominantly 2H phase, or above 800°C producing “high-temperature” or HT MoS2 ribbons that were predominantly 3R phase. The majority of these MoS2 wires and ribbons were more than 50 μm in length and were organized into parallel arrays containing hundreds of wires or ribbons. MoS2 nanostructures were characterized by X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, selected area electron diffraction, X-ray diffraction, UV−visible absorption spectrometry, and Raman spectroscopy. HT and LT MoS2 nanowires were structurally distinct: LT MoS2 wires were hemicylindrical in shape and nearly identical in diameter to the MoOx precursor wires from which they were derived. LT MoS2 wires were polycrystalline, and the internal structure consisted of many interwoven, multilayer strands of MoS2; HT MoS2 ribbons were 50−800 nm in width and 3−100 nm thick, composed of planar crystallites of 3R-MoS2. These layers grew in van der Waals contact with the HOPG surface so that the c-axis of the 3R-MoS2 unit cell was oriented perpendicular to the plane of the graphite surface. Arrays of MoS2 wires and ribbons could be cleanly separated from the HOPG surface and transferred to glass for electrical and optical characterization. Optical absorption measurements of HT MoS2 nanoribbons reveal a direct gap near 1.95 eV and two exciton peaks, A1 and B1, characteristic of 3R-MoS2. These exciton peaks shifted to higher energy by up to 80 meV as the wire thickness was decreased to 7 nm (eleven MoS2 layers). The energy shifts were proportional to 1/ L∥2, and the effective masses were calculated. Current versus voltage curves for both LT and HT MoS2 nanostructures were probed as a function of temperature from −33 °C to 47 °C. Conduction was ohmic and mainly governed by the grain boundaries residing along the wires. The thermal activation barrier was found to be related to the degree of order of the crystallites and can be tuned from 126 meV for LT nanowires to 26 meV for HT nanoribbons.