Graphene (a 2D sheet of carbon atoms arranged in a honeycombed lattice) can be used to detect individual gas molecules adsorbed on its surface thanks, in particular, to its high surface area of around 2600 m2/g. Moreover, the high mobility of charge carriers in graphene, which can vary between around 2500 to 40 000 cm2V–1s–1, means that it can sense a greater number of chemicals and biomarkers compared to other solid materials.

In previous work, the team, led by Eric Pop of the University of Illinois at Urbana-Champaign and Amin Salehi-Khojin at the University of Illinois at Chicago, found that graphene-based sensors could trace gas molecules – but only if the graphene contained internal defects. The fact that electronics devices such as chemical field-effect transistors (chemFETs) based on pristine graphene can also behave as a highly sensitive gas and chemical sensors thus surprised the researchers.

Defects modulate graphene electronic properties

"We have now discovered that the sensitivity of pristine graphene chemFETs is not necessarily intrinsic to graphene but rather that it is facilitated by external defects in an underlying substrate," explains Salehi-Khojin. "These defects modulate the electronic properties of the carbon material."

The team, which includes scientists from the Korea Research Institute of Standards and Science in Daejon and Seoul National University, studied graphene that it had obtained by a technique called mechanical exfoliation. This, now famous, "sticky tape" method was first used to isolate graphene back in 2004 and produces virtually defect-free shavings of the material.

The researchers studied the behaviour of suspended graphene-based sensors as well as graphene sensors placed on defective silica substrates and hydrogen-passivated (or defect-free) silica. They exposed the sensors to a number of known gas molecules via a "pulse-injection" method and then recorded the change in current passing through the samples. The sensitivity of each sensor was defined by the ratio of change in current compared to initial current.

Strong electronic response

"We found that the suspended graphene and graphene on pristine SiO2 samples were insensitive to gas molecules," Salehi-Khojin told nanotechweb.org. "This was not the case for graphene on defective SiO2 samples (with 'dangling' bonds), however, which showed a strong electronic response when exposed to the different gases."

"As mentioned, the defective substrate modulates the electronic properties of graphene and hence pristine graphene chemFET works like a doped graphene chemFET," explained UIC team member Bijandra Kumar.

"We thus conclude that graphene needs defects, either in the form of internal defects on the graphene surface or external defects on the substrate surface, for it to be used for gas sensing," said team member Poya Yasaei, also at UIC.

Being able to control graphene’s electronic properties using an external defective supporting substrate in this way will be very useful for designing and engineering graphene chemFET sensors and other electronic devices in the future, added Salehi-Khojin.

The team is now busy looking at how the density and type of defects in a given substrate modulate the sensing properties of graphene-based sensors.

The present work is detailed in Nano Letters.