Schematic showing vapor phase methods used to deposit atomically thin TMDs by direct chalcogenization (via S/Se/Te) of pre-deposited transition metal-containing thin films (metals, metal oxides, thiosalt layers).Transition metal sulfides and selenides known as transition metal dichalcogenides (TMDs), made up of graphene-like sheets of atoms held together by van der Waals forces, hold promise for future optical, electronic, and mechanical devices.
TMDs share similarities with that ubiquitous electronic material silicon, such as a direct band gap in the visible-near IR range, high carrier mobilities and on/off ratios, but can enable nanoelectronics, integration with photonics, and even quantum electronics. Moreover, TMDs can be deposited onto virtually any substrate and can withstand the stresses and strains of flexible supports.
Essential to the exploitation of TMDs in future nanotechnologies is the ability to synthesize high quality bulk and thin film crystals. In recent years, the potential of traditional bulk semiconductor synthesis techniques such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and metal organic chemical vapor deposition (MOCVD) for thin-film TMDs has been rediscovered.
According to Cecilia Mattevi of Imperial College London, there has been tremendous progress in the synthesis of high-quality TDM crystals just a few atoms thick, opening up the way to completely new two-dimensional materials [Reale et al., Applied Materials Today 3 (2016) 11].
For TMDs to take nanoelectronics ‘beyond silicon’, believes Mattevi, synthesis of wafer-sized thin films of these materials is essential. Bulk synthesis methods based on chemical vapor transport (CVT) developed in the 1970s and 1980s are now being extended to produce bulk single crystals of group VI chalcogenides such as MoS2, WS2, WSe2, WTe2, and more recently MoTe2 and MoSe2.
At their heart, these techniques rely on the evaporation of precursor materials in an enclosed tubular furnace. Precursor species evaporated at the hotter ‘source’ end of the furnace are transported to the cooler, ‘sink’ end using inert transport agents (like I2 or Br2). Deposition takes place as precursor molecules mix and coalesce, rather like condensation on glass.
As an alternative to evaporating solid precursors at high temperatures, CVD growth of thin films of TMDs like MoS2 and WS2 has been successfully achieved using highly volatile chemical intermediates at much lower temperatures.
Even thinner layers can be synthesized by turning ultrathin transition metal or metal oxide films into TMDs by heating in a furnace with chalcogen vapors. Single or few atomic layers of MoS2, MoSe2, and WS2 have been produced via PVD, as well as more tricky dichalcogenides such as MoTe2.
Most recently, though, a one-step method has been devised to synthesize atomically thin TMDs by evaporating metal oxide and chalcogen powers simultaneously. This approach produces high-quality TMD monolayers with grain sizes up to the millimeter scale. A similar vapor phase transport technique has also enabled the growth of MoS2 single crystals from evaporated MoS2 powders.