Growing two dimensional (2-D) transition metal dichalcogenides

Method brings scalability to this class of semiconductor material


Scanning transmission electron microscopy images of the MoS2–MoSe2 alloy grown directly onto SiO2 membranes. (a) Low-magnification HAADF image, multilayer sample. Inset: nanobeam diffraction pattern. (b) Atomic force microscopy image of monolayer MoS2, with the height profile showing the thickness of the nucleation center. (c) Low-magnification BF image of monolayer MoS2–MoSe2 sheets. Inset: monolayer lattice image (scale bar 1 nm). (d) BF image nucleation center.

Transition metal dichalcogenides (TMDCs) are semiconductors of the type MX2, where M is a transition metal atom (such as molybdenum or tungsten) and X is a chalcogen atom (such as sulphur, selenium, or tellurium). TMDCs provide a promising alternative to graphene as a 2-D, atomic scale material for use in electronics, battery, and semiconductor industries. TMDCs exhibit a unique combination of atomic-scale thickness, direct bandgap, strong spin–orbit coupling, and favorable electronic and mechanical properties, which make them interesting for research and for applications in high-end electronics, spintronics, optoelectronics, energy storage, flexible electronics, DNA sequencing, and personalized medicine.

Even though 2-D TMDCs exhibit a breadth of new properties that are distinct from traditional bulk materials or thin films, developing such materials into large-scale and defect-free atomic layers with controllable thickness on desired substrates is challenging. The state-of-art mechanical exfoliation method produces high quality monolayers of TMDCs, but this technique is not scalable. Likewise, electron beam lithography with etching creates pillars of material that limit its use in device fabrication.

Army researchers have developed a method for controlled and abundant growth of TMDCs by applying a focused ion or electron beam onto an insulator substrate, to produce a charged area on the surface. The charged surface is exposed to water molecules to hydrogenate the charged species, which is then subject to chemical vapor deposition in a tube furnace to grow thin, consistent layers of TMDCs.

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