Investigations of the sorption behaviour of amorphous nitrogen-rich carbon nitride films as sensitive layers for cantilever-based chemical sensors

We present investigations of the sorption behaviour of amorphous nitrogen-rich carbon nitride films (CNx) towards water vapour and volatile organic compounds, for example methanol, ethanol, i-propanol and acetone, in order to evaluate their potential as sensitive layers for cantilever-based chemical sensor applications. The CNx films were deposited by inductively coupled plasma chemical vapour deposition (ICP-CVD) utilizing transport reactions from a solid carbon source. In order to study the influence of the thickness of the sensitive layer on its sensitivity and selectivity, two series of cantilevers coated with 120 nm and 240 nm CNx films were prepared. We found that the variation of the film thickness affected the sorption process of the CNx film quantitatively as well as qualitatively. For thin films (120 nm), the sensor dynamic responses (frequency shift) increased with increasing molecular weight of the analytes. The largest responses were observed towards acetone and i-propanol; here, the cantilever acted as a resonant microbalance. When the film thickness was increased from 120 to 240 run, the analytes with higher dipole moments caused stronger response signals. In this case we observed, for example, an increase of the sensitivity towards methanol by a factor of more than three. The performance of the cantilever-based sensors functionalised with CNx coatings was compared to that of organic polymers, and the observed peculiarities were explained by the chemical nature of the sensitive materials.