An international team of researchers, led by Penn State, has developed ultrasensitive gas sensors based on the infusion of boron atoms into the tightly bound matrix of carbon atoms known as graphene. The group is composed of researchers from six countries and includes the 2010 Noble laureate and graphene pioneer Konstantin Novoselov, and Morinobu Endo, the discoverer of carbon nanotubes.

Graphene is well known for its remarkable strength and ability to transport electrons at high speed, but it is also a highly sensitive gas sensor. By adding boron atoms, the boron graphene (BG) sensors were able to detect noxious gas molecules at extremely low concentrations, parts per billion in the case of nitrogen oxides and parts per million for ammonia, the two gases tested to date. This translates to a 27 times greater sensitivity to NOx and a 105 times greater sensitivity to ammonia compared to pristine graphene. The researchers believe these results will open a path to high-performance sensors that can detect trace amounts of many other molecules.

"This is a project that we have been pursuing for the past four years. We were previously able to dope graphene with atoms of nitrogen, but boron proved to be much more difficult," said Mauricio Terrones, professor of physics, chemistry and materials science at Penn State. "Once we were able to synthesize what we believed to be boron graphene, we collaborated with experts in the United States and around the world to confirm our research and test the properties of our material."

Both boron and nitrogen lie next to carbon on the periodic table, making their substitution feasible. But boron compounds are very air sensitive and decompose rapidly when exposed to the atmosphere. One-centimeter-square sheets were synthesized at Penn State in a one-of-a-kind bubbler-assisted chemical vapor deposition system. The result was large-area, high-quality B- doped graphene sheets.

Once fabricated, the BG samples were sent to the Honda Research Institute USA Inc., located in Columbus OH, who tested the samples against their own highly sensitive gas sensors. The Novoselov lab at the University of Manchester, UK, studied the transport mechanism of the sensors. Theory collaborators in the US and Belgium matched the STM images to experimental images, confirmed the presence of the boron atoms in the graphene lattice and their effect when interacting with ammonia or nitrogen oxide molecules. Collaborators in Japan and China also contributed to the research.

"This multidisciplinary research paves a new avenue for further exploration of ultrasensitive gas sensors," said Dr. Avetik Harutyunyan, Chief Scientist and project leader of Honda Research Institute USA Inc. "Our approach combines novel nanomaterials with continuous UV light radiation in the sensor designs that have been developed in our laboratory by lead researcher Dr. Gugang Chen in the last 5 years. We believe that further development of this technology may break the parts per quadrillion (PPQ) level of detection limit, which is up to six orders of magnitude better sensitivity than current state-of-the-art sensors."

These sensors can be used for labs and industries that use ammonia, a highly corrosive health hazard, or to detect NOx, a dangerous atmospheric pollutant emitted from automobile tailpipes. In addition to detecting toxic or flammable gases, theoretical work indicates that boron-doped graphene could lead to improved lithium ion batteries and field effect transistors.

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