2D materials like molybdenite (MoS2) and tungstenite (WS2) are creating a flurry of interest in labs around the world because they have dramatically different electronic and mechanical properties from their 3D counterparts. This means that they could find use in novel device applications, such as low-power electronic circuits, low-cost or flexible displays, sensors and even flexible electronics that can be coated onto a wide variety of surfaces.

The most well known 2D materials are graphene (which is a sheet of carbon just one atom thick) and the transition metal dichalcogenides (TMDs). These so-called van der Waals (vdW) structures have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is a chalcogen (such as S, Se and Te). TMDs have an added advantage in that they go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers. These monolayers efficiently absorb and emit light and so could be ideal for making a variety of optoelectronics devices such as photodetectors and light-emitting diodes.

Creating well defined heterostructures

“To explore the full potential of these layered semiconductors, however, will require precisely modulating their chemical, structural and electronic properties to create well-defined heterostructures,” explains team leader Xiangfeng Duan. “These structures will be much like traditional semiconductor heterostructures, which make up all modern electronics and optoelectronics devices.”

The researchers say that they have now succeeded in laterally growing 2D-layered heterostructures consisting of tungstenite-tungsten selenide (WS2-WSe2) and molybdenite-molybdenum diselenide (MoS2-MoSe2). They have also proved that these structures can be used to create a series of functional devices, including p-n diodes, photodiodes and complementary inverters.

Switching reactants around

The team designed a thermal chemical vapour deposition (CVD) process in which the vapour reactants are generated by evaporating the selected solid source material. The reactants can be switched around (in the processing furnace itself) by moving the solid source components in and out of the hot zone of the furnace.

“For example, to produce a WS2-WSe2 heterostructure, we first grow the WS2 domains using a CVD process by thermally evaporating a solid WS2 source in the central hot zone of a one-inch tube furnace in an argon atmosphere,” Duan tells nanotechweb.org. “The peripheral edges of the triangular domains produced feature unsaturated dangling bonds that function as the active growth front as we continue adding and incorporating precursor atoms to extend the 2D crystals in the horizontal direction.”

By moving the WS2 solid source out of the hot zone and the WSe2 solid source into the hot zone, without exposing it to ambient conditions, the chemical vapour source switches from WS2 to WSe2 in the middle of the process to allow for lateral hetero-epitaxial growth at the peripheral active growth front and produce WS2-WSe2 heterostructures, he adds. “Such in situ chemical source switching is crucial for retaining the active edge growth front for sequential epitaxial growth of the 2D films.”

The team, reporting its work in Nature Nanotechnology, says that essentially any type of 2D heterostructure can be grown using a similar strategy. Indeed, the researchers are now busy further extending their approach to grow more complex multi-heterostructures or superlattices either laterally or in the vertical direction.