Imperfect Graphene Makes Better Chemical Sensors
A new study shows that chemical sensors made with less perfect graphene may have better sensitivity. Researchers produced graphene chemical sensors with either near-perfect structures or deliberatively defective structures and found that the graphene sensors with edges and line defects were more sensitive in detecting gas analytes.
The research report was published last week in the professional journal Advanced Materials. The research group included scientists from the University of Illinois at Urbana Champaign and Dioxide Materials, a company based in Champaign, IL.
Schematic representation of polycrystalline graphene ribbon chemiresistors. The 3-dimensional color scheme represents the enhanced electric fields at linear defects and the floating/attached molecules represent the electron donor Toluene. Such defective devices show enhanced chemical sensitivity over pristine graphene and carbon nanotube chemiresistors. Image by Alex Jerez, University of Illinois at Urbana-Champaign.
Carbon nanotubes have been known to form superior chemiresistors and the authors have previously shown that point defects in carbon nanotubes contribute to their higher sensitivity in chemical sensing. Based upon this finding, they sought to understand what limits the sensitivity of simple, two-terminal graphene chemiresistors, and study this in the context of inexpensive devices easily manufactured by chemical vapor deposition.
Graphene is a one-atom-thick two-dimensional structure of carbon atoms that are densely packed in a honeycomb crystal lattice. Graphene has excellent electrical, thermal and mechanical properties. Though graphene shares several similarities with carbon nanotubes, graphene is a two-dimensional conductor while carbon nanotubes are essentially one-dimensional conductors.
In their study the authors found that the response of graphene chemiresistors depends on the types and geometry of their defects. The defect in the graphene may be point, line or wrinkle-like and they found that the lines are most likely where the sensing happens.
The researchers found that adsorption at point defects only has a small effect on the overall resistance of the device. Therefore, more perfect graphene chemiresistors are less sensitive to analyte molecules. “On the other hand, micrometer-sized line defects or continuous lines of point defects are different because no easy conduction paths exist around such defects, so the resistance change after adsorption is significant,” they report. The authors speculate that “future work to engineer line defects and edges could further enhance the graphene chemiresistor sensitivity.”
Smaller, faster and more sensitive gas sensors are becoming increasingly important due to their emerging needs in environmental and biomedical applications. This new finding may be another milestone in the development of less expensive and better gas sensors.
Polycrystalline Graphene Ribbons as Chemiresistors. Amin Salehi-Khojin, David Estrada, Kevin Y. Lin, Myung-Ho Bae, Feng Xiong, Eric Pop and Richard I Masel. Advanced Materials. Published online November 23, 2011. DOI.10.1002/adma.201102663.
Dr. Amin Salehi-Khojin and David Estrada