A schematic of plasma-assisted carbon-hydrogen species doping in the tungsten disulfide lattice. Image: Fu Zhang/Penn State.
A schematic of plasma-assisted carbon-hydrogen species doping in the tungsten disulfide lattice. Image: Fu Zhang/Penn State.

Researchers at Penn State have developed a novel doping technique for introducing carbon-hydrogen molecules into a single atomic layer of the semiconducting material tungsten disulfide. According to the researchers, this doping dramatically changes the electronic properties of the material, and thus offers a way to create new types of components for energy-efficient photoelectric devices and electronic circuits.

"We have successfully introduced the carbon species into the monolayer of the semiconducting material," said Fu Zhang, doctoral student in materials science and engineering, and lead author of a paper on the work in Science Advances.

Tungsten disulfide is a member of a class of two-dimensional (2D) materials known as transition metal dichalcogenides (TMDs). Normally, it is an n-type semiconductor, in which negatively charged electrons are the charge carriers; there are also p-type semiconductors, in which positively charged holes are the charge carriers.

In their study, the researchers discovered that substituting some of the sulfur atoms in tungsten disulfide with carbon atoms caused the one-atom-thick material to develop a bipolar effect, turning it into an ambipolar semiconductor that is both n-type and p-type.

"The fact that you can change the properties dramatically by adding as little as two atomic percent was something unexpected," said Mauricio Terrones, senior author of the paper and distinguished professor of physics, chemistry and materials science and engineering.

According to Zhang, once the material is highly doped with carbon, the researchers can produce a degenerate p-type with a very high carrier mobility. "We can build n+/p/n+ and p+/n/p+ junctions with properties that have not been seen with this type of semiconductor," he said.

Semiconductors are commonly used in the transistors found in computers and electronic devices, but this 2D ambipolar semiconductor could find use in various other applications as well. "This type of material might also be good for electrochemical catalysis," Terrones said. "You could improve conductivity of the semiconductor and have catalytic activity at the same time."

Up to now, there have been few papers in the field of doping 2D materials, because it requires multiple processes to take place simultaneously under specific types of conditions. The team's technique utilizes a plasma to lower the temperature at which methane can be cracked to produce carbon-hydrogen molecules down to 752°F. At the same time, the plasma has to be strong enough to knock a sulfur atom out of the atomic layer and substitute a carbon-hydrogen unit.

"It's not easy to dope monolayers, and then to measure carrier transport is not trivial," Terrones says. "There is a sweet spot where we are working. Many other things are required."

Susan Sinnott, professor and head of the Department of Materials Science and Engineering, provided theoretical calculations that guided the experimental work. When Terrones and Zhang observed that doping the 2D material was changing its optical and electronic properties – something they had never seen before – Sinnott's team suggested the best atom to dope with and predicted the subsequent properties, which corresponded with the experiment.

Saptarshi Das, assistant professor of engineering science and mechanics, and his group then measured the carrier transport in various transistors with increasing amounts of carbon substitution. They watched the conductance change radically until they had completely changed the conduction type from negative to positive.

"It was very much a multidisciplinary work," Terrones says.

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.