Part II. Effect of Surface Properties on Sensitivity of Gas Sensor Using Carbon Nanotube and Graphene
Chapter 4. Carbon Nanotube-based Gas Sensor with Functionalization
4.2 Sensing Mechanism of Nanotube-Based Gas Sensor
When the analyte is exposed, the mechanism for a response of carbon nanotube-based gas sensor is not clear. It attempted to explain the sensing mechanism using the properties of the non-pair electron of the nanotube for the P orbital conjugated system. Therefore, explaining the sensing mechanism began with an understanding of the band structure rather than the molecular orbital, which is solid-state materials properties. So, investigating I-V characteristics with applying gate voltage is an essential part of investigating the properties of CNT-based sensors. If the electron donor molecule adsorbs on the CNTs surface, the n-doped property in CNTs appears. In case of I-V curve (Figure 4.1), it shifts to negative, and energy of valence band and conduction band is lowered. Understanding of this band structure explains the nanotube sensing mechanism. Based on this phenomenon, studies have been reported to selectively detect specific gases by coating electron donor/acceptor molecules on the surface of nanotubes. Generally, a single nanotube sensor can’t be used as a commercial gas
sensor because it burns up when high current flows. Randomly networked nanotube films are used in sensors and are in contact with both electrodes. In this geometry, there are several studies on where an analyte is exposed to nanotubes. i) exposed vapor are detected in individual CNT (Intra-CNT), ii) are detected in nanotubes and nanotube junctions (CNTs-CNTs junction), and iii) are detected in the interface between nanotubes and electrodes (CNT-electrode junction).
Figure 4.1 Sensing mechanism through changes in charge carrier concentration and mobility in semiconducting CNT. The ideal I-V curve shift to negative direction before exposure (black) and after exposure of electron donor molecule (red). When the electron donor molecule is adsorbed, the energy of the conduction band and valence band decreases [3].
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i) (Intra-CNT) The sensing mechanism of intra-CNT is by the interaction of analyte and individual nanotubes. When the analyte is adsorbed on the surface of the nanotube, it is detected by affecting the charge carrier mobility of the nanotube itself or by the defect caused by the analyte.
Some researchers have studied the phenomenon of direct charge transfer with nanotubes using N-type and P-type analyte. In general, nanotubes are known to have p-doped properties because oxygen molecules are physically adsorbed on the surface of the nanotube in ambient conditions. Exposure of the p-type analyte to nanotubes with such p-doped properties increases the hole conduction and consequently reduces the resistance of the nanotubes [143, 162]. Conversely, exposure of the N-type analyte reduces hole conduction and increases the resistance of the nanotubes [143, 163, 164]. Figure 4.2 shows that when NH3 was exposed using a single carbon nanotube, the current dropped and the NH3 molecule was detected. When the nanotube was completely covered with SiO passivation and the NH3 molecule was exposed, there was no change in current and it wasn’t detected. When SiO
passivation is selectively deposited on the contact area of the nanotube with the electrode, exposure of NH3 sharply drops the current. Based on these studies, the researchers concluded that the nanotubes themselves are the domain sensing part of the analyte when they are used as sensors.
ii) (CNT-CNT junction) In the case of a nanotube sensor in the form of a CNT-CNT junction network, the junction of the nanotube affects the electrical properties of the entire network. Even if a small contact resistance occurs at the junction of a nanotube, the entire network is greatly affected.
There is a study that the charge tunneling effect of nanotubes was affected by the distance between the
[No passivated]
NH3
[SiO contact-passivated]
NH3
SiO2 SiO
Si SiO2
[SiO fully passivated]
NH3
SiO
Figure 4.2 Study for sensing mechanism of CNT using SiO passivation. A passivation layer is deposited at the electrode/CNT junction, but is sensitive to NH3. The part that senses NH3 sensitively is the wall of the nanotube [13]
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nanotubes.[165, 166] When the analyte was adsorbed on the junction of the nanotubes, the conductivity between the nanotubes was affected. This was because when the analyte was adsorbed on the CNT-CNT junction, the junction spacing was farther away and the conductivity was changed sensitively. In Figure 4.3, there are NR-CG600 and NR-CG200 samples composing a composite with natural rubber (NR), natural rubber with CNTs (NR-CNT) and reduced graphene oxide (RGO) with CNTs in natural rubber. The gap distance of CNTs in NR-CG600 and NR-CG200, which have RGO between the nanotubes and the nanotube junction gap, was farther than the NR-CNT. The conductivity of RGO was lower than CNTs. The role of RGO here was to simply lower the junction gap of CNTs.
When the toluene, p-xylene and benzene solutions are dropped on the prepared sample (Figure 4.3), NR-CG600 and NR-CG-200, where the junction gap of CNT was larger between NR-CNT showed a sensitive response. That is, the distance between the junctions of CNTs proves to play an important role in sensing.
Figure 4.3 Study for sensing mechanism by CNT-CNT junction. (a) experimental setup.
Organic solvent was dropped on rubber/nanotube/RGO composite. Response curve of NR-CNT, NR-CG600 and NR-CG200 using toluene (b), p-xylene (c) and benzene (d) [15].
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iii) (CNT-electrode junction) The analyte sensing occurs at the interface between the nanotube and the electrode. A Schottky barrier occurs when nanotube and metal electrode is contact. A number of studies have been reported on how these Schottky barriers affect analyte sensing. A single tube was placed on substrate and contact with both electrodes and a passivation layer was deposited either in the center of the nanotube or on the electrode and nanotube interface to study the response to the analyte. Figure 4.4 shows the response when NH3 is exposed to a single tube. When the gate voltage was positive, the sensitivity increased, and the sensitivity varied depending on the gate voltage. By selectively depositing a passivation layer on CNT and Au electrode using Si3N4, there was no change in current even when NH3 was exposed at the same concentration. Conversely, when the NH3 was exposed to CNT and electrode junctions by depositing the Si3N4 layer at the center of the CNT, the current decreases due to exposure of NH3 vapor. Although Si3N4 is deposited in the center of the nanotubes, baseline shift occurs but the response to NH3 vapor is clearly distinguished. Thus, the study reported that Schottky junction between CNT and electrode junction plays an important role in sensing. These findings are contradictory to the findings of the within-CNT study. So this debate is going to continue. [13, 17]
[No passivated] [Contacts passivated] [Central channel passivated]
Figure 4.4 Study for sensing mechanism by nanotube/electrode contact using single nanotube. (a) It shows response to NH3 and the degree of sensitivity changes depending on the gate voltage. (b) When CNT-electrode contact is passivated, there is no response to NH3. (c) The length of nanotube passivated by Si3N4 except CNT-electrode have sensitivity to NH3 exposure [17].
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