Nanotechnology at the level of chemical physics doesn’t normally attract popular attention, but a recent paper in Nature Communications is worth highlighting given its implications for the use of two-dimensional materials in electronics.
Graphene is the best known 2d material, and much has been said of its wondrous properties in various applications including electronic components. However, the so-called band-gap problem of graphene is major obstacle to its use in electronic diodes and transistors. Such components act as switches controlling the flow of electrons across them, and the tuning of these switches relies on semiconductor materials that allow the passage of electrons with specific voltages. Graphene in its pure state is not a semiconducting material, and forcing graphene to behave as a semiconductor typically requires it to be doped with atoms of materials other than the hexagonal carbon lattice of which it is made.
Graphene aside, there are hundreds of known 2d materials under active investigation by materials scientists and engineers. One class of materials known as monolayer transition metal dichalcogenides is of particular interest for semiconductor applications. Molybdenum disulphide (MoS2), for example, has considerable potential in electronics.
Like graphene, MoS2 can be produced on an industrial scale by means of chemical vapour decomposition (CVD), and with a heat treatment in a selenium-rich environment converted to molybdenum diselenide (MoSe2), which has a higher electric conductivity. Another related compound, molybdenum ditelluride (MoTe2), is especially interesting, as it is almost transparent semiconductor with a band gap in the infrared region. This makes it particularly useful in electronics, or as an infrared detector.
The problem with MoTe2 is the relatively low temperature required for the conversion from MoS2. CVD is a high-temperature process, so we need another way of converting monolayer MoS2 to telluride compounds. The Korean researchers behind the Nature Communications paper have come up with a sodium scooter process (complete with a fast-food delivery metaphor) which rapidly transports tellurium to MoS2 and tungsten disulphide (WS2) monolayers with the aid of a sodium-based catalyst Na2Te2, where Na2 is a sodium molecule. The reaction temperature is as low as 525°C, which is considerably below the 700°C dissociation temperature of MoTe2, and 1000°C typical of the CVD process.
“We call it scooter, because it delivers quickly,” says Yun Seok Joon, lead author of the Nature Communications study. “MoTe2 molecules decompose to molybdenum and tellurium at high temperatures, but the scooter anchors telluride to MoS2, and acts as a catalyst that lowers the activation temperature of the reaction.”
Yun and his colleagues produced a number of different telluride-based 2d semiconductors, and with these compounds fabricated an electronic diode with MoTe2 on the edge, and MoTe2 on the inner part.
Further reading: Yun et al., Telluriding monolayer MoS2 and WS2 via alkali metal scooter, Nature Communications 8 (2017); doi: 10.1038/s41467-017-02238-0.