"We took the resulting exfoliated flakes and fabricated transistors out of them. The device metrics were among the best reported for any exfoliated phosphorene, thereby confirming that we had isolated high quality material in a scalable manner without degradation."Mark Hersam, Northwestern University
Over the past two years, phosphorene has attracted increased attention due to its potential for use in thin, flexible electronics. And because it is a natural semiconductor, phosphorene holds promise where miracle material graphene falls short.
"There has been a decade-long attempt to make graphene semiconducting," said Northwestern University's Mark Hersam. "Our group and others have tried to do it with limited success. So why not just use a material that is already a semiconductor?"
In order for phosphorene to reach its full potential, however, it needs to be incredibly thin – preferably at the atomic scale. Until now, researchers have experienced difficulties in exfoliating atomically-thin flakes from the bulk material, called black phosphorous, in a quick and efficient manner. Hersam, however, may have solved this problem. His group recently developed a method that results in a substantially higher exfoliation yield and much thinner flakes than previous efforts.
Supported by the US National Science Foundation and the US Office of Naval Research, the research is described in a paper in the Proceedings of the National Academy of Sciences. Joohoon Kang, a graduate student in Hersam's laboratory, is first author of the paper.
After being exfoliated from black phosphorous, phosphorene has dramatically different electronic and mechanical properties from its parent material. Not only are the atomically thin, two-dimensional layers powerful semiconductors, but they also emit light efficiently, suggesting opportunities in optoelectronics.
"Graphene taught us that the most scalable method was to exfoliate in a solution," said Hersam, professor of materials science and engineering at Northwestern's McCormick School of Engineering. "You begin with a solvent and then add graphite and a surfactant. After introducing energy via sonication, you can exfoliate the graphite down to graphene. It would seem obvious that the same approach would work with phosphorene. The difference, however, is that phosphorene is very reactive chemically, which requires important changes in protocol to achieve exfoliation without degradation."
When exposed to air, the chemical reactivity of phosphorene causes it to deteriorate rapidly, which suggests that certain components of the air, such as water and oxygen, are driving this deterioration and need to be avoided. As a first attempt at doing this, Hersam conducted exfoliation with organic solvents in a closed, air-free and water-free environment.
"The problem with the organic solvent approach is that it is very inefficient," he said. "It results in low exfoliation yield and flakes that are relatively thick."
The breakthrough came when Hersam and his team realized – after a year studying the deterioration process – that phosphorene only deteriorates in the presence of both water and oxygen together. So they tried bubbling an inert gas through water to remove the free oxygen and thereby managed to create an aqueous solvent for exfoliating black phosphorous that avoids deterioration.
By sonicating black phosphorous in a mixture of deoxygenated water and surfactants, Hersam was able to achieve a substantially higher exfoliation yield and produce much thinner flakes that reached the atomically thin limit. In addition to providing superior phosphorene materials, this method uses commonly available, environmentally-benign water as opposed to organic solvents.
"We took the resulting exfoliated flakes and fabricated transistors out of them," Hersam said. "The device metrics were among the best reported for any exfoliated phosphorene, thereby confirming that we had isolated high quality material in a scalable manner without degradation."
This story is adapted from material from Northwestern University's McCormick School of Engineering, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.