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Published online 27 May 2017
New synthesis technique enables researchers to create asymmetric molecular monolayers.
An international team of researchers has created a thick material, made of a single molecule, with upper and lower surfaces that have distinct properties — an asymmetry which makes this monolayer a valuable tool for nanotechnology.
The researchers began by using a conventional technique to make a molybdenum disulfide monolayer in which molybdenum atoms are flanked by sulfur atoms above and below. They then exposed the monolayer to hydrogen plasma to strip off the upper sulfur atoms and replaced them with hydrogen1.
“Controlling the stripping process to remove exactly one atomic layer was very challenging,” says Lain-Jong Li of Saudi Arabia’s King Abdullah University of Science and Technology, who led the study. Finally, they heated the monolayer in the presence of powdered selenium, which replaced the hydrogen.
“It’s as though we’re performing surgery on the monolayer. We strip off its skin and replace it with a new selenium skin,” says Li.
The resulting material is a sandwich of selenium, molybdenum, and sulfur, a structure which the team confirmed using electron microscopy and X-ray spectroscopy.
This asymmetry gives the synthetic material properties that are valuable in nanotechnology design and research. For example, since molybdenum's bonds with sulfur and selenium react differently in an electric field, the monolayer deforms in the presence of a vertical voltage, making it potentially useful in nanoelectromechanical devices.
The same technique can be used to make other asymmetric monolayers. “Our dream is to replace the centre atom with another magnetic metal to give the material new functionality,” says Li of the new goal his team is working toward.