A scanning tunneling microscope image of molybdenum sulfide. Image reprinted with permission from ACS Nano 12, 3, 2569–2579. Copyright 2018 American Chemical Society.
A scanning tunneling microscope image of molybdenum sulfide. Image reprinted with permission from ACS Nano 12, 3, 2569–2579. Copyright 2018 American Chemical Society.

Perfection is not everything, according to an international team of researchers whose study into two-dimensional (2D) materials shows that defects can enhance the materials’ physical, electrochemical, magnetic, energy and catalytic properties.

"Electronic devices, like transistors, are usually made from relatively bulky stacked layers of metal, oxides and crystalline semiconductors," said Shengxi Huang, assistant professor of electrical engineering at Penn State. "We would like to make them with two-dimensional materials so that they can be faster, smaller and more flexible."

To do this, the researchers are investigating single atomic layers of molybdenum sulfide. They report the results of their investigation in a paper in ACS Nano.

Molybdenum sulfide is a molecule made up of one molybdenum atom with two attached sulfur atoms. The molecules line up with the molybdenum atom in the middle and the sulfur atoms on the top and bottom to form a 2D, single-layer film. The researchers placed these films on a variety of substrates – gold, single-layer graphene, hexagonal boron nitride and cerium dioxide – and then irradiated them to create defects in the lattice structure.

Creating 2D materials is not a perfect manufacturing process and defects are always present in the lattice. The researchers wanted to determine how such defects changed the physical and electrochemical properties of the molybdenum sulfide. Irradiation causes some of the molybdenum sulfide to lose a sulfur atom from the surface. With these less-than-perfect films, the researchers could see how the materials changed using a variety of microscopic and spectroscopic techniques.

Simulations of lattice defects allowed the researchers to manipulate the materials and produce structures that matched the experimentally defective films. They found that the material properties predicted by their simulations matched their experimental results.

"We found that the sulfur defects improved the physical characteristics of the material," said Huang. "By choosing the locations and number of defects, we should be able to tune the material's band structure, improving its electronic capabilities."

Experimentally, the researchers found that many more sulfur atoms are lost than molybdenum atoms, because the sulfur is at the surface of the 2D material while the molybdenum is protected in the middle. They also noted that because so many sulfur atoms leave the material, the defects caused by the absence of sulfur overwhelm any effect the absence of a molybdenum in the lattice might have.

Investigating how different substrates enhanced or did not enhance the properties of the 2D material, the researchers found that "the substrates can tune the electronic energy levels in molybdenum sulfide due to charge transfer at the interface". The material properties of the substrate also change the properties of the 2D single layer. For example, cerium dioxide, because it is an oxide, altered the electrical properties of the material differently than the other substrates.

Smaller, faster and more flexible electronics are not the only possible outcome of tuning these 2D materials. "If we have the right amount of sulfur vacancies, we can enhance chemical processes like hydrogen evolution from water," said Huang.

Materials like molybdenum sulfide are used as catalysts in chemical reactions. Huang refers to the splitting of water, a process used to create gaseous hydrogen and oxygen from liquid water, where properly defective molybdenum sulfide could enhance the process, reducing energy use and costs while increasing the amount of hydrogen produced.

Molybdenum is a transition metal and other members of this atomic group, including tungsten, niobium, zirconium, titanium and tantalum, can form molecules called dichalcogenides when combined with sulfur and other chalcogenides such as selenium and tellurium. Dichalcogenides can be made into 2D materials that could also be tuned to enhance their properties.

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.