CVD tungsten monolayers? Just add salt

Researchers from Singapore and Japan have demonstrated the halide-assisted chemical vapor deposition of tungsten selenide and tungsten sulfide monolayers, which can be carried out starting with volatile tungsten oxyhalides from tungsten oxide at atmospheric pressure and a relatively low temperature of 700 to 850 Celsius. The monolayers have high hole and electron mobility. [Appl Mater Today; DOI: 10.1016/j.apmt.2015.09.001]

There has been a lot of interest in atomically thin crystals of transition metal dichalcogenides (TMDs). These two-dimensional  materials consist of a hexagonally packed layer of transition metal atoms sandwiched between two layers of chalcogen atoms. So far, about forty different types of TMDs having wide ranging properties have been made, they can, depending on the precise composition be metallic, semiconducting or semimetallic. It is geometric confinement and their distinct crystal symmetry that give rise to these technologically interesting properties. However, there is a pressing need for controlled methods to generate pure and perfect monolayers for further research and development.

Now, Goki Eda of the National University of Singapore and colleagues there and at the National Institute for Materials Science in Ibaraki, Japan, explain that growing two-dimensional tungsten dichalcogenide crystals using chemical vapor deposition (CVD) needs a steady flow of tungsten in the vapor phase. To achieve such a steady flow, generally requires high temperature and low pressure because of the high sublimation point of the tungsten oxides used as precursor materials. However, Li and colleagues have used an alkali metal halide (MX where M = Na or K and X = Cl, Br or I) as growth promoters to get around these problems.

The team's mechanistic experiments suggest that the growth promoters lead to the formation of volatile tungsten oxyhalide species in the vapor phase, which stabilizes the total concentration of available tungsten above the substrate and so ensures steady and efficient delivery of the precursor to the substrate. Thankfully, the monolayer crystals of tungsten selenide or sulfide do not carry with them any unintentional dopants from the alkali metal and halogen atoms.

Tests on the monolayers show good field-effect transistor (FET) performances with high current on/off ratios (about 107), the researchers report. They observed hole and electron mobilities up to 102 and 26  cm2 V-1 s-1 for the selenide monolayer and an electron mobility of about 14 cm2 V-1 s-1 for a sulfide device.

"The next step in this work is to further reduce the growth temperatures of these materials by identifying suitable growth catalysts," Eda told Materials Today. "This will allow growth of complex heterostructures of delicate materials in a layer-by-layer fashion without compromising their chemical and structural integrity."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".