The most common semiconductors, including all group II-VI materials, all group III-V materials and the elements germanium and silicon, are unstable with respect to oxidation in air. Nanostructures these materials must be protected from oxidation, either by a native oxide (e.g., SiO₂ on Si), or by the covalent attachment of a passivation layer (e.g., triphenyl phosphine on CdSe). Molybdenum disulfide (MoS₂) is a layered semiconductor that resists oxidation even in moist air at temperatures up to 85 ^oC. This attribute makes MoS₂ an attractive semiconductor material for nanoscience applications.

We synthesized molybdenum disulfide nano- and micro-ribbons using a two-step, Electrochemical/Chemical synthetic method in which MoO₂ ÒprecursorÓ nanowires were first electrodeposited size-selectively on a highly oriented pyrolytic graphite (HOPG) surface. These precursor wires were then converted to MoS₂ by exposure to H₂S at 800-900 ^oC. The MoS₂ ribbons prepared using this method had the 2H crystal structure of bulk MoS₂, were organized into parallel arrays of hundred of ribbons, and were up to a millimeter in length. The electronic properties of these nanoribbon arrays were probed after transferring them onto an insulator surface. Our method for synthesizing these nanoribbons is shown schematically below:

Scheme 1. Synthesis of MoS₂ nanoribbons involving step-edge selective electrodeposition of MoO₂ followed by conversion to MoS₂ in H₂S at 800 ^oC.

The first step involves step edge-selective electrodeposition of MoO₂, as shown in Figure 1.

Figure 1. (a) Cyclic voltammograms at 20 mV s^-1 for an HOPG working electrode in an aqueous plating solution containing 1 mM Na₂MoO₄, 1.0 M NaCl, 1.0 M NH₄Cl at pH 8.0. The hatched region shows the potential range employed for step-edge selective growth of MoO₂ nanowires. (b) Current versus time for the electrodeposition of MoO₂ nanowires at -0.80 V for durations of 25 s, 50 s, 100 s, 200 s. These curves are distributed along the current axis for clarity; zero current for each is indicated as a horizontal bar. (c) Diameter of MoO₂ nanowires as the measured by SEM plotted as a function of the square root of the deposition time. Error bars indicate +1 s in the diameter distribution.

Scanning electron micrographs (SEMs) of MoO₂ precursor nanowires and the 2H-MoS₂ nanoribbons obtained from these precursors are shown in Figure 2. MoO₂ wires were hemicylindrical in cross-section and narrowly dispersed in diameter. MoO₂ particles were also deposited in parallel with wires at an areal density of 10⁸ to 10⁹ cm^-2. Some of these particles fused with MoO₂ wires during growth, as can be seen in the SEM image of Fig. 2 (leftmost images). Conversion to MoS₂ at 800 ^oC was complete within a few hours, however with continued heating in H₂S grain growth occurred and MoS₂ nanostructures evolved from hemicylindrical wires into ribbons over a period of 3-4 days. For MoS₂ wires heated in H₂S for 24 hrs. at 800 ^oC (Fig. 2, center images), a basal MoS₂ layer formed in contact with the graphite surface. Atop this layer, which was 2 - 30 nm in total thickness as measured by atomic force microscopy (AFM), many "supernatant" crystallites of MoS₂ are observed. With continued heating in H₂S for 84 hrs. (Fig. 2, right images), the lateral dimensions of this layer increased and the number of supernatant MoS₂ crystallites seen atop this layer became smaller. The total height of the MoS₂ ribbons obtained after 84 hrs was close to one tenth of the total width (i.e., about 35 nm for 350 nm diameter ribbons). The SEM images of Fig. 2 show that the morphological transition from a "wire" to a "ribbon" morphology was nearly complete over this 84 hr. period.

Figure 2. (Left) SEM images of the precursor MoO₂ nanowires. (Center) SEM images of MoS₂ nanoribbons annealed at 800^oC for 24hrs. (Right) SEM images of MoS₂ nanoribbons annealed at 800^oC for 84hrs.