As an atom-thick, two-dimensional material with high conductivity, graphene is set to enable a stream of new electronic devices, including particularly sensitive sensors for the detection of various gases, such as those produced by explosives. Now an international team of researchers led by Pennsylvania State University (Penn State) has created a graphene-boron amalgam that can detect particular gases down to mere parts per billion, and may eventually lead to detectors with such sensitivity that they could detect infinitesimally tiny amounts of gas in the order of parts per quadrillion.

By pairing boron atoms with graphene to create what is known as a heteroatom structure (where non-carbon atoms bond with carbon atoms to form part of the molecular ring), the researchers created sensors that are able to detect gas molecules at exceptionally low concentrations; parts per billion for nitrogen oxides (NOx) and parts per million for ammonia. According to the scientists, this equates to a NOx sensitivity 27 times greater and an ammonia detection rate 105 times greater than is possible with untreated graphene.

"This is a project that we have been pursuing for the past four years," says Mauricio Terrones, professor of physics, chemistry and materials science at Penn State. "We were previously able to dope graphene with atoms of nitrogen, but boron proved to be much more difficult. 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."

Graphene is composed of carbon, and boron is an element that sits right beside carbon on the periodic table. This means that they are atomically similar and should combine relatively well. Unfortunately, compounds of boron tend to break down very quickly when exposed to air, so combining the two elements is fraught with problems when it comes to normal graphene production methods.

To overcome this, the researchers used a bespoke bubbler-assisted chemical vapor deposition apparatus to isolate the boron from the atmosphere whilst incorporating the element with the graphene to produce one-square centimeter (0.155-sq in) sheets of boron-doped graphene.

After these sheets were created, they were then transferred to the Honda Research Institute USA in Columbus OH, where they were then compared with known highly-sensitive gas sensors. At the same time, the Novoselov lab at the University of Manchester, UK (where graphene was first synthesized and from where the first commercial graphene light-bulb was produced), examined the electron transport function of the sensors, whilst contributing researchers in the US and Belgium established that boron atoms were melded into the graphene lattice and observed their influence of interaction with ammonia or NOx molecules.

"This multidisciplinary research paves a new avenue for further exploration of ultrasensitive gas sensors," says 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."

Suggested uses for these new types of sensors include safety alerts for workers in laboratories or commercial enterprises that use ammonia, where leaks and spills are not only highly-corrosive but hazardous to health, or the detection of NOx leaks in automotive or industrial areas where exhaust fumes from engines containing the gas can prove lethal in confined spaces.

The scientists also believe that their theoretical research points towards using boron-doped graphene to improve such things as lithium-ion batteries by controlling generated gas levels for optimum efficiency.

The results of this research were recently published in the journal Proceedings of the National Academy of Sciences