Four Professors Included in PNAS Special Materials Feature

Northwestern University researchers know the future of novel materials – so much so that the Proceeding of the National Academy of Sciences (PNAS) selected three Northwestern Engineering papers — from four professors — for its Novel Materials Special Feature.

Only eight papers from around the world were included in the feature. The three papers from Northwestern Engineering — by professors Mark C. Hersam, Yonggang Huang, Chad A. Mirkin, and John Rogers – featured research on flexible, biointegrated electronics, atomically thin flakes of phosphorene, and the hierarchical organization of nanomaterials.

Published in PNAS this month, Rogers and Huang co-authored a paper describing a set of materials for flexible electronic systems that can serve as novel classes of biomedical implants with capabilities for long-term, high-performance operation in the body. The key advance is in the development of ultrathin layers of material that perfectly seal the devices from surrounding biofluids, with projected lifetimes that exceed 50 years.

“Thin, flexible sheets of electronics that laminate onto vital organs of the human body offer radically new and powerful engineering design opportunities for future generation pacemakers, deep brain stimulators, and other even more advanced types of implants,” said Rogers, the Louis Simpson and Kimberly Querrey Professor of Materials Science and Engineering, Biomedical Engineering, and Neurological Surgery. “A long-standing challenge is in constructing such flexible systems in ways that prevent penetration of biofluids into the active electrodes, with robust operation throughout the life of the patient.”

As a solution, Rogers’s team developed methods for growing sub-micron thick layers of glass on silicon wafers and for transferring them onto the surfaces of flexible electronic systems. The result is a perfect, biocompatible water barrier that can also allow electrical interactions between the targeted tissue and the underlying electronics.

The paper, “Ultrathin, transferred layers of thermally grown silicon dioxide as biofluid barriers for biointegrated flexible electronic systems,” was supported by the Defense Advanced Research Projects Agency, National Science Foundation, and Army Research Office. Hui Fang, Jianing Zhao, and Enming Song, all members of Rogers’ group, were co-first authors of the paper.

Published in PNAS in April, Hersam’s work demonstrated a new way to exfoliate thinner flakes of phosphorene at higher yield. While phosphorene shows great promise for thin, flexible electronics, it must be incredibly thin to reach its full potential.

“Graphene taught us that the most scalable method was to exfoliate in a solution,” said Hersam, Walter P. Murphy Professor of Materials Science and 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 open air, the chemical reactivity of phosphorene leads to rapid deterioration. The result suggests that components of air, such as water and oxygen, are driving degradation and need to be avoided. Consequently, Hersam initially bypassed this issue by exfoliating with organic solvents in a closed, air-free, and water-free environment.

The breakthrough came when Hersam and his team realized — after a year of studying the degradation process — that phosphorene degrades in the presence of both water and oxygen together. By bubbling an inert gas through water, Hersam deoxygenated it to create an aqueous solvent for exfoliating black phosphorous that avoids degradation. After sonicating black phosphorous in a mixture of deoxygenated water and surfactants, he found substantially higher exfoliation yield and much thinner flakes that reached the atomically thin limit.

The paper, “Stable aqueous dispersions of optically and electronically active phosphorene,” was supported by the National Science Foundation and Office of Naval Research. Joohoon Kang, a graduate student in Hersam’s laboratory, was first author of the paper.

In a PNAS Perspective paper, which was published earlier this month, Mirkin presented a framework that defines how one can understand and control material structure across length scales with inorganic particles. Mirkin is the director of the International Institute for Nanotechnology, George B. Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences and a professor of materials science and engineering, chemical and biological engineering, biomedical engineering, and medicine.

“Uniformity is of both practical and functional importance, necessary to increase structural sophistication and realize the promise of nanostructured materials,” Mirkin and his co-authors wrote. “Thus, we define the nature of nonuniformity at each length scale as a means to guide ongoing research efforts and highlight potential problems in the field.”

The paper “The nature and implications of uniformity in the hierarchical organization of nanomaterials” included material supported by the Air Force Office of Scientific Research and the National Science Foundation.