Selectivity and Cross-Sensitivity of Transfer-Free Fabricated Nanocrystalline Graphene Field-Effect Gas Sensors
Authors: Noll Dennis , Schwalke Udo 
Published: February 2020

In the course of an increasing awareness for the protection of public health a higher demand for environmental gas sensors, especially for the detection of volatile organic compounds and toxic gases, has been noted. Reasonably-priced devices to cover this demand can be solid-state gas sensors that can be easily fabricated and scaled using planar processing. Furthermore, an appropriate choice of materials may enable the possibility of monolithic integration within silicon CMOS circuitry thus providing the chance of co-integrating read-out and signal processing electronics on the same chip. As a two-dimensional carbon nanomaterial graphene shows a high surface-to-volume ratio and promising electronic properties for the detection of gaseous species. Detection capabilities down to a single molecule have been demonstrated by Schedin et al. [1], using a pristine graphene flake made from mechanical cleavage of highly ordered pyrolytic graphite.
In this contribution we port on a novel method for graphene-based sensor fabrication. By means of a CMOS compatible method we have fabricated hundreds of transfer-free nanocrystalline graphene field-effect transistors (ncGFETs) on silicon dioxide [2], which can be used for detection of gaseous species. Under application of a HP4156A semiconductor parameter analyzer the gaseous sensitivity of our ncGFETs is electrically characterized in terms of backgate FET input characteristics as well as in a resistive configuration. For the resistive configuration the backgate bias has been fixed near the charge neutrality point of our nanocrystalline graphene devices at VBG = 0V. All measurements were done using our self-constructed vacuum probing station that can be systematically flooded with calibrated gas mixtures. The gas concentration is then derived from the vacuum pressure using the ideal gas law.
Sensitivities of our ncGFETs to different volume concentrations of various toxic gases including ammonia (NH3), nitrogen dioxide (NO2), and carbon monoxide (CO) will be presented. E.g. for devices heated to 425K a sensitivity S of roughly 80% is
S = (G-G0)/G0

achieved for 4 parts-per-million-volume (ppmv) of ammonia whereat NO2 already shows a sensitivity of 100% for a concentration of 4 parts-per-trillion-volume (pptv = 10-6 ppmv). The higher sensitivity of our ncGFETs compared to other graphene devices is attributed to modifications of grain boundary potential barriers, which is the dominating mechanism in e.g. SnO2-based Taguchi-type sensors [3]. Furthermore, the influence of water vapor (H2O (g)) on our devices is discussed.
Nevertheless, the cross-sensitivity of Taguchi-type sensors, like our ncGFETs, remains a problem. Speciation of gas molecules is feasible by the use of sensor arrays consisting of devices with different sensitivities and electronic characteristics towards different gas species. Therefore, in order to modify the sensitivities towards the different gas species of our ncGFETs towards the different gas species we have characterized the sensitivity and responsivity at various temperatures. Furthermore, during exposure to the different gases the analysis of the backgate ncGFET input characteristics shows shifts of the charge neutrality point, as well as changes in the hysteresis of our devices. A novel method to selectively discriminate between various gaseous species, which is based on the analysis of the hysteresis in the ncGFETs will be presented and discussed.
[1]F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, Nat Mater, 6, 652 (2007).
[2] D. Noll and U. Schwalke, 2017 12th Ieee International Conference on Design & Technology of Integrated Systems in Nanoscale Era (Dtis 2017) (2017).
[3] R. K. Srivastava, P. Lal, R. Dwivedi and S. K. Srivastava, Sensor Actuat B-Chem, 21, 213 (1994).

Figure 1