NEWS | Feb. 27, 2015

Dr. Paul Sheehan Named a Fellow of the American Vacuum Society

By Donna McKinney

Dr. Paul E. Sheehan, a research chemist in the Surface Chemistry Branch of the Chemistry Division at the U.S. Naval Research Laboratory (NRL), was inducted as a Fellow of the American Vacuum Society (AVS) by Dr. Steven George, 2014 AVS President, and Dr. Ellen Fisher, 2014 AVS Awards Committee Chair, at the awards ceremony during the 61st Annual AVS International Symposium and Exhibition. Sheehan was named an AVS Fellow in recognition of his exceptional contributions to the detailed understanding of sp2 carbon nanostructure properties, methods for nanoscale patterning of materials, and the basis of biological and chemical sensor performance.

Sheehan has studied nanoscale phenomena and surface reactions for over two decades. He was a University Fellow at the University of North Carolina where he received a bachelor's degree in Chemistry-based Materials Science in 1993 while doing undergraduate research in the group of Prof. Royce Murray. He then studied nanomechanics at Harvard University where he received his master's degree (1995) and his doctorate (1998) in Chemical Physics under the direction of Prof. Charles Lieber. He then received a National Research Council Fellowship to pursue biosensing using magnetoelectronics at NRL under the direction of Dr. Richard Colton.

In 2001, NRL hired Sheehan to pursue research focused on the use of scanning probe microscopy for the fabrication and characterization of nanostructures. In 2008, Sheehan became Head of the NRL Surface Nanoscience and Sensor Technology Section. The Section is a highly interdisciplinary team comprising about fourteen biochemists, chemists, engineers, and physicists who study nanometer scale phenomena at surfaces as well as bioelectronics for sensing and biotic/abiotic interfaces. Sheehan's current research focuses on the chemical functionalization of graphene to enhance its performance in biosensing and electronics as well as the generation of nanostructures for interfacing with biology. His research has been funded by the Navy, Air Force Office of Scientific Research, Defense Advanced Research Projects Agency, and Defense Threat Reduction Agency.

Manipulating sp2 carbon

The detailed exploration of the structures formed from sp2 carbon—fullerenes, carbon nanotubes, and graphene—has been a major focus of the physical sciences over the past three decades. The generation of these structures, the testing of their theoretically predicted properties, and their application have all met with substantial success. Some of Sheehan's earliest work explored the mechanical properties of carbon nanotubes and SiC nanorods, showing that the elastic modulus of the nanotubes matched the predicted (and superlative) value of ~1 TPa. This was achieved by using a scanning probe to push on nanotubes whose ends were pinned. ISI named the publication one of the top 10 papers in materials science for that decade.

With the advent of graphene research, Sheehan explored the chemistry and functionalization of this fascinating material, helping to understand how new chemistries such as fluorination impacted its electronic, mechanical, and magnetic properties. Graphene's properties are quite subtle. Sheehan and co-workers recently showed that, unlike bulk graphite fluoride, fluorination of graphene was metastable and depends highly on the underlying substrate. He had previously explored this theme of graphene's interaction with its substrate in showing that electronic conduction in graphene on SiC is in fact anisotropic due to charge scattering by the underlying step edges. They subsequently published a series of papers exploring the changes in conduction in graphene due to functionalization, most recently showing that electronic conductivity in graphene can be completely eliminated by hydrogenation and then completely restored to its pristine state by simple heating.

Development of new methods for patterning materials at the nanoscale

The manipulation of matter at the nanoscale has been a dominant theme in Sheehan's career. He has written several reviews on nanolithography as well as developed several advances in scanning probe techniques to modify locally both soft and hard materials. He made significant contributions to the understanding of the mode of patterning of Dip Pen Nanolithography (DPN), where material deposits from an AFM tip onto a substrate. He showed that a water meniscus was not needed to transfer molecules from the tip to the surface as previously thought, and offered a detailed model of the mass transfer processes occurring in the system. Based on the insights gained from that effort, he went on to develop a variant of DPN called thermal DPN where a heated scanning probe controlled the flow of a molecule by varying its viscosity.

More recently, he has focused on using the heat scanning probes to pattern graphene into functional devices. This could mean either using thermal DPN to write thin polymer masks for subsequent processing or by creating molecular templates. A more fundamental insight was that the heatable scanning probes could control local temperature with nanometer resolution and so induce reactions at that length scale. This led to the highly local removal of oxygen from graphene oxide to form thin nanoribbons of conducting graphene.

Understanding the performance of biological and chemical sensors

Beyond the manipulation of matter at the nanoscale, Sheehan has had an ongoing interest in the physical phenomena undergirding sensor performance. Upon arriving at NRL, he pursued a novel approach to biodetection where a magnetic bead would be bound to a giant magnetoresistive (GMR) sensor if a target biomolecule such as DNA was present. The benefit of this approach is that magnetic interference is relatively rare in biological systems and the giant response by the GMR sensor to the presence of the magnetic bead makes this a highly sensitive and selective approach. Indeed, it remains one of the most effective means of directly detecting low concentrations of biomolecules.

Work on the microfabricated sensors stimulated his interest in the effect of scaling sensor size. In 2005, he published a simple paper on the scaling of biosensors to the nanoscale, a popular undertaking at the time. The upshot was that, for many applications, mass transport made this an unwise choice. Others used the results to point out that many reported results were in fact impossible. His interest in sensing extended to biological approaches to sensing where he modeled how magnetotactic bacteria know how to swim north. With his developing interest in graphene nanostructures, he set out to understand how to use this new material to build inexpensive yet sensitive detectors for both chemical and biological agents. The fundamental insight was that graphene oxide, a very inexpensive derivative of graphene, could be readily and cheaply formed into sensors that had lower electronic noise and greater amenability to chemical functionalization than the carbon nanotube networks used to date. Efforts to fully utilize and understand these new materials continue to this day.

Other recognition

His research has garnered other recognition and accolades too. His nanofabrication work has been widely reported in the general science and popular press, including in the New York Times, CBS's SmartPlanet, C&EN, and TV Globo Brazil. His nanofabrication work was selected as a Department of Defense R&D accomplishment in Defense Nanotechnology Research and Development Programs. One of his biosensor papers was cited as the Most Outstanding Contribution out of >1000 submissions to the Biosensors 2008 conference, the leading conference in the biosensors community. It too was highlighted in the popular press in the Economist and on National Public Radio (NPR). In 2009, Sheehan and his co-inventors received the NRL Edison Patent award for their patent on Thermal Dip Pen Nanolithography. Sheehan also has received three Alan Berman Research Publication Awards in his 13 years as a federal employee.