Unleashing the Potential of Phosphorene

After being exfoliated from black phosphorus, phosphorene has dramatically different electronic and mechanical properties from its parent material. The atomically thin, two-dimensional layers are powerful semiconductors with high potential for use in thin, flexible electronics. But phosphorene has one problematic weakness: air.

When exposed to ambient conditions, phosphorene becomes increasingly unstable and rapidly degrades. Mark Hersam, the Bette and Neison Harris Chair in Teaching Excellence at Northwestern University’s McCormick School of Engineering, has developed a new plan to keep phosphorene’s physical nature—and potential—intact.

“Most practical high-performance black phosphorus applications will be used in air—not in a vacuum,” Hersam said. “So we needed to create a strategy to enable next generation applications based on the material.”

Hersam and his team used several types of microscopy to study the process of phosphorene degradation and found that water and oxygen irreversibly reacted with the material to form an oxidized phosphorus species. To counter this effect, Hersam deposited atomic layers of aluminum oxide, or alumina, over the phosphorene. This encapsulation layer suppressed degradation, allowing the phosphorene to maintain its stability when exposed to air for more than two weeks.

“In addition to being unstable in air, black phosphorus degrades at elevated temperatures,” Hersam said. “We used alumina because it can be grown at low temperatures through atomic layer deposition.”

Funded by the National Science Foundation, Office of Naval Research, and the Keck Foundation, the research was described in the paper “Effective passivation of exfoliated black phosphorus transistors against ambient degradation,” which appeared in the November 7 issue of NanoLetters. Joshua Wood and Spencer Wells, a postdoctoral fellow and a graduate student in Hersam’s lab, respectively, were the paper’s first authors. Additional contributors included Tobin Marks, the Vladimir N. Ipatieff Professor of Catalytic Chemistry and professor of materials science and engineering, and Lincoln Lauhon, professor of materials science and engineering.

The team is now using higher resolution methods, such as atomic resolution scanning tunneling microscopy, to further probe the structural and chemical mechanisms involved in phosphorene’s air degradation. They are also extending their encapsulation approach to other reactive nanomaterials.

“Our strategy for enhancing environmental stability opens possibilities for black phosphorus,” Hersam said. “It should accelerate efforts to implement the material in practical electronic and optoelectronic applications.”