2D materials: Metallic when narrow

Subnanometre metallic wires can be engineered from semiconducting sheets of transition-metal dichalcogenides by means of a focused electron beam.

Reducing the size of a crystal to the nanoscale can lead to a rearrangement of its atomic lattice and the creation of new material properties. Such rearrangements have been observed before in thin films and narrow wires fabricated from bulk materials of a single element1, 2. The use of materials made from more than one element has the potential to offer additional degrees of freedom because the composition of the crystals can also be modified3, and the atomic rearrangement of such structures has already been investigated using top-down methodologies such as electron-beam lithography4. Molybdenum sulphide ribbons with a width of around 0.35 nm have, for example, been fabricated by creating holes in a MoS2 sheet using electron irradiation in a transmission electron microscope (TEM)4. Writing in Nature Nanotechnology, Junhao Lin, Wu Zhou and colleagues now report the fabrication of subnanometre wires in MoS2 and other semiconducting transition-metal dichalcogenide sheets, and show that these nanowires are metallic5.

The researchers — who are based at Vanderbilt University, Oak Ridge National Laboratory, the University of Tsukuba, Fisk University and the University of Tennessee — used mechanically exfoliated MoS2, MoSe2 and WSe2 monolayer flakes and exposed them to an electron beam with relatively low energy in a scanning TEM. With the instrument, they were able to create holes side-by-side at designated sites in the monolayer flakes (Fig. 1a), and could observe in situ the detailed process of nanowire formation. The lighter atoms in the parent sheets can be removed by the energy beam through knock-off effects4 or ionization effects5, or a combination of both. Thus, when two holes are created close to each other, the holes can rapidly increase in size and produce ribbons between them (Fig. 1a). When the ribbon width is narrowed past a critical size, the ribbon can turn into a wire with uniform width, and becomes robust enough to survive under the electron-beam irradiation. The team created, for example, three nearby holes in a MoSe2 flake, resulting in the formation of three wires in a Y-shaped connection (Fig. 1b). The subnanometre wires were found to be flexible and could rotate and bend under the electron irradiation while maintaining their atomic structure.

Figure 1: Fabrication of subnanometre wires using electron-beam irradiation.

a, Holes form and coalesce in a transition-metal dichalcogenide sheet exposed to an electron beam (pink) in a TEM4. b, Y-shaped connection of the subnanometre wires created by introducing and growing three holes adjacently using a scanning TEM5. c, The controlled creation of holes with nanometre spacing using focused electron beams.

The lattice structure of the nanowires was directly imaged from several viewing angles and found to be composed of stacked transition metal (M) and chalcogen (X) triangles in staggered positions. This lattice structure has been observed before in chemically synthesized pseudo-one-dimensional ternary molybdenum chalcogenide crystals6, and nanowires of such molybdenum chalcogenides with a stoichiometry of MX have previously been shown to be metallic7. Lin and colleagues fabricated a two-terminal device with a MoSe wire and proved that the wires are metallic using in situ electrical conductance measurements in the TEM. This further confirms the MX structure of the formed nanowires.

Fabrication of one-dimensional nanostructures — especially graphene nanoribbons — has been extensively studied by different bottom-up and top-down methods8, 9, 10, 11. Narrowing two-dimensional materials into ribbons by electron beam etching or chemical etching cannot produce ribbons with controlled widths and smooth edges8, 9. Chemical bottom-up approaches can create high-quality graphene nanoribbons and more complex structures (like T- and Y-shaped connections)10, but they are unlikely to be used on a large scale11. In contrast, the subnanometre wires produced in transition-metal dichalcogenides by electron-beam irradiation have a definite structure, smooth edges and are reproducible4, 5. Furthermore, Lin and colleagues show that it is possible to detect the variations of conductance during the phase transition from MX2 to MX, and apply rotation or bending load to the nanowire through electron irradiation for in situ mechanical testing. The researchers expect the fabrication method to be scalable, because all nanowires eventually reach a stable structure, regardless of the initial shape of the parent layer.

This top-down approach is also versatile, and it could serve as a general energy-beam-based top-down lithography technology for nanoscale devices that consist of subnanometre elements from binary or more complex two-dimensional materials (Fig. 1c). Using the technique, Lin and colleagues created holes at controlled spacings, with the smallest separation between two nanowires being less than 100 nm. However, reducing this spacing further could be challenging. Furthermore, the efficiency of this fabrication method is limited by the low throughput of electron-beam lithography. Nevertheless, the work should inspire researchers to fabricate nanowires from other materials, and ultimately the creation of functional devices.

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