Both graphene and CNTs experience changes in their electrical conductance when exposed to different gases (such as CO2, NO2, and NH3), and they are, therefore, ideal candidates for sensing/measuring applications. In this research, a set of novel gas sensor models employing Field Effect Transistor structure using these materials have been proposed. In the suggested models, different physical properties such as conductance, capacitance, drift velocity, carrier concentration, and the current-voltage (I-V) characteristics of graphene/CNTs have been employed to model the sensing mechanism. An Artificial Neural Network model has also been developed for the special case of a CNT gas sensor exposed to NH3 to provide a platform to check the accuracy of the models. The performance of the models has been compared with published experimental data which shows a satisfactory agreement.
TopCarbon Allotropes
Carbon is tetravalent chemical and non-metallic element which has a high level of stability. Carbon has atomic number six with four electrons in the valence band that tend to form an octet state through covalent bonding. This form of atomic bond is very stable. Carbon plays a crucial role in people’s lives. Because of its stable feature, carbon has occurred in different natural isotopic structures. In addition, carbon comes in different allotropes which are all necessary in different types of applications especially in nanotechnology as is going to be emphasized in this research (Kinoshita, 1988; Diederich & Rubin, 1992). An illustration of carbon allotropes, i.e. graphite, graphene, fullerene and CNT has been provided in Figure 1a to d (Hirsch, Eigler, & Heinemann, 2013). Each allotrope is obtained from a specific carbon structure possessing various electrical properties associated with its structural features (Singh et al., 2011).
One of the aforementioned carbon allotropes is fullerene with 0 dimension (0D) structure as illustrated in Figure 2. An important use of fullerene is various types of sensors (Giannopoulos, 2014; Elhaes & Ibrahim, 2013). Electronic sensors based on fullerene are sensitive to any adsorbed molecule due mainly to the fact that each of the atoms in fullerene is a surface atom. Electrons can move freely on the surface of fullerene (Yang, Zhang, & Xu, 2013; Wu et al., 2013b; Korotcenkov, 2013).
Another carbon allotrope which possesses 1 dimension (1D) structure, namely carbon nanotube, has attracted the attention of all researchers in the sensor development technology over the past few decades (Cole & Zook, 2006; Sayago et al., 2013). In 1991, Sumio Iijima, a Japanese researcher, discovered the multi-walled carbon nanotube (abbreviated as MWCNT) and two years later, he found single-walled carbon nanotubes (abbreviated as SWCNT) (Saito, 1998; Iijima, 1991). Since the flow of electrons through carbon nanotubes depends heavily upon the side wall function, the sensitivity can be manipulated by controlling the defect locations (Bellucci, 2005; Sevik et al., 2011). Research in the area of CNTs has been greatly influenced by the discovery of MWCNTs and SWCNTs (Gulotty et al., 2013). Figures 3 and 4 depict schematics of these two CNT structures, respectively (Baughman, Zakhidov, & de Heer, 2002).
Figure 3.
Schematic of a single-walled carbon nanotube
Figure 4.
Schematic of multi-walled carbon nanotubes