This image illustrates rhenium doping of monolayer molybdenum disulfide. Image: Donna Deng/Penn State.
This image illustrates rhenium doping of monolayer molybdenum disulfide. Image: Donna Deng/Penn State.

A team led by researchers at Penn State has reached a new understanding of why the performance of synthetic two-dimensional (2D) materials is often orders of magnitude worse than predicted. Using this understanding, they also searched for ways to improve these materials' performance in future electronics, photonics and memory storage applications.

Two-dimensional materials have a thickness of, at most, a few atoms. Researchers can make 2D materials by the exfoliation method – peeling a slice of material off a larger bulk material – or by condensing a gas precursor onto a substrate, known as chemical vapor deposition (CVD). The former method produces higher quality materials but is not useful for making devices. The second method is well established in industrial applications but yields low performance 2D films.

The team of researchers has now demonstrated, for the first time, why 2D materials grown by the CVD method have poor performance compared to theoretical predictions. They reported their results in a recent paper in Scientific Reports.

"We grew molybdenum disulfide, a very promising 2D material, on a sapphire substrate," explained Kehao Zhang, a doctoral candidate of Joshua Robinson, associate professor of materials science and engineering at Penn State. "Sapphire itself is aluminum oxide. When the aluminum is the top layer of the substrate, it likes to give up its electrons to the film. This heavy negative doping – electrons have negative charge – limits both the intensity and carrier lifetime for photoluminescence, two important properties for all optoelectronic applications, such as photovoltaics and photosensors."

After determining that the aluminum was giving up electrons to the 2D film, the researchers used a sapphire substrate that was cut in such a way as to expose the oxygen rather than the aluminum on the surface. This enhanced the photoluminescence intensity and the carrier lifetime of 2D molybdenum disulfide grown on the sapphire by 100 times.

In related work, a second team of researchers led by the same Penn State group used doping, which substitutes foreign atoms into the crystal lattice of the film, to improve the properties of the 2D material. They report their work in a paper in Advanced Functional Materials.

"People have tried substitution doping before, but because the interaction of the sapphire substrate screened the effects of the doping, they couldn't deconvolute the impact of the doping," said Zhang, who was also the lead author on the second paper. Using the oxygen-terminated substrate surface from the first paper, the team removed the screening effect from the substrate and were able to dope the 2D molybdenum disulfide film with rhenium atoms.

"We deconvoluted the rhenium doping effects on the material," said Zhang. "With this substrate we can go as high as 1 atomic percent, the highest doping concentration ever reported. An unexpected benefit is that doping the rhenium into the lattice passivates 25% of the sulfur vacancies, and sulfur vacancies are a long-standing problem with 2D materials."

The doping solves two problems: it makes the material more conductive for applications like transistors and sensors, and at the same time improves the quality of the materials by passivating the sulfur vacancies. The team predicts that higher rhenium doping could completely eliminate the effects of sulfur vacancies.

"The goal of my entire work is to push this material to technologically relevant levels, which means making it industrially applicable," Zhang said.

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.