It is well known that the electrical performance of carbon nanotube (CNT) devices is extremely sensitive to chemical environment. Studying the interaction between CNTs and chemical gases could, from fundamental point of view, significantly deepen our understanding on nanoscale device physics. Meanwhile, as large scale integration of CNT devices is still challenging at the current stage, individual electronic devices containing CNTs as their key elements for sensing purpose would be more feasible for practical applications of CNTs. In addition to a small diameter, an extremely large surface to volume ratio makes CNT a suitable material for nanoscale chemical sensing. Since the first CNT gas sensor reported in 2000 (Kong, et al., 2000), many types of CNT-based chemical sensors have been demonstrated. CNT networks (Li, et al., 2003), functionalized CNTs (Qi, et al., 2003), CNT and polymer composites (Wei, et al., 2006), etc, are used as sensing elements. Although tremendous progresses have been achieved in this area, the underlying sensing mechanism still remains unclear. NH3 gas detection represents the most typical argument in CNT based gas sensing area. Previously proposed mechanisms include the indirect interaction through the hydroxyl group on SiO2 substrates (Kong, et al., 2000) or pre-adsorbed water layer (Bradley, et al., 2003), adsorption of gas molecules at the interstitial sites in the CNT bundle (Zhao, et al., 2002), direct charge transfer from the adsorbed gas molecules to the CNT channel (Chang, et al., 2001), and modulation of the SB at CNT/metal contacts (Yamada, 2006), etc. Till now, there is no well recognized sensing mechanisms. Hence in this chapter, we will first review the development of CNT based NH3 gas sensors. After that, we will present a systematic study on the sensing mechanisms through selective Si3N4 passivation, which enables us to truly distinguish the sensing signal from the CNT channel and CNT/metal contacts. By comparing the strikingly distinct sensing performance at various testing conditions, we clearly show that the Schottky barrier modulation at the CNT/metal contacts dominates the sensing performance at room temperature. At higher temperatures, say 150oC or above, NH3 molecules start to adsorb on the CNT wall and the charge transfer from the adsorbed NH3 molecules to the CNTs contributes to the sensing signal. Next, we will demonstrate tunable real-time NH3 sensors with three-terminal CNT field-effect transistors. The room-temperature sensitivity and reversibility of such sensors can be greatly enchaned by proper gate voltages. Finally, we will conclude the signicance achieved so far and give an outlook of the future development of CNT based gas sensors.
2. Literature reviews: Contact Vs Channel
It is well know that most of CNTFETs exhibited p-type characteristics in air, which is commonly attributed the contact SB modulation by environmental oxygen (Derycke, et al., 2002, Heinze, et al., 2002). Will this argument still holds for NH3 gas? On one hand, theoretical calculations predicted limited interaction between NH3 gas and CNT at room temperature (Kong, et al., 2000, Bauschlicher & Ricca, 2004, Peng & Cho, 1999). On the other hand, CNT based sensors showed high sensitivity to chemical gas. It is natural for researchers to consider the role of metal-CNT contacts. In order to differentiate, or separate the effect from CNT channel and metal-CNT contacts, the design of experiments is to protect or cover either of them and then compare the sensing performance against NH3 gas. The first effort belongs to Bradley et al. He and his coworkers used thermally evaporated SiO to passivate a short-channel CNTFETs, as shown Fig. 1. They found a good sensitivity to NH3 after contact passivation. (Bradley, et al., 2003)
Zhang et al. argued that when the passivation length was comparable to the depletion length in the CNT, the contacts could be indirectly affected, which could be the case in the work above. In their work, polymethylmethacrylate (PMMA) was applied to protect the CNT/metal contacts from NO2 exposure and their devices became insensitive after contact passivation, as shown in Fig. 2. (Zhang, et al., 2006)
Interestingly, Liu et al. also employed PMMA as a passivation layer. They observed changes in the transfer characteristics upon exposure to NH3 and NO2 for both contact-passivated device and channel passivated devices, suggesting that both the CNT channel and the CNT/metal contacts play a role in the detection process (Liu, et al., 2005). The obvious ambiguity in those reports could arise from the permeable passivation materials used. Moreover, as the experiments were carried out at room temperature and air ambient only, exclusive identification of the sensing mechanisms is not possible. Next, we will introduce a selective Si3N4 passivation technique, with which the sensing signals from the CNT channel and CNT/metal contacts are truly distinguished. Strikingly distinct sensing performance at various testing conditions is observed.
3. Experimental details
CNTs were aligned between Ti/Au source and drain electrodes predefined on a p-type silicon wafer using an ac DEP technique (Li, et al., 2005, Peng, et al., 2006), which is simple and cost effective, suitable for CNT sensor fabrications. The CNT suspension is introduced onto the electrodes with an AC voltage across with a frequency of 2-10MHz. The peak-to-peak voltage ranged from 6V to 16V, depending on the electrodes separation. Due to the induced DEP force, the CNTs in the suspension align with the electric field direction and move towards the electrodes surface. The AC voltage was turned off immediately once SWNTs bridged the electrodes, confirmed by monitoring the resistance across electrodes. The number of the SWNTs between the electrodes can be controlled by adjusting the SWNT
concentration and the manipulation time. A heavily doped Si with a 200 nm thick thermally grown SiO2 top layer was used as the backgate. Note that the CNTs in this work are on top of Au electrodes and the contact regions are fully accessible to the ambient. These devices are typically the SB-CNTFETs. As illustrated in Fig. 3, three device structures were employed in our experiments: (1) an as-prepared CNTFET with the exposed CNT channel and CNT/Au contacts; (2) only the contacts passivated with 500nm Si3N4 layer and (3) only the channel passivated with 500nm Si3N4 layer. Dry air was used as the background gas with a flow rate of 500sccm in the following experiments unless otherwise stated. NH3 gas was selected as the detecting species to study the sensing mechanisms of the CNT sensors.
4. Results and Discussion
4.1. NH3 sensing at room-temperature
An as-prepared CNTFET (Device 1) showed a sensitive response to small concentrations of NH3 at room temperature (see Fig.4 (a)). It is seen that, under a positive gate voltage, both the sensitivity and reversibility were much higher than those under a negative one.
In order to experimentally differentiate whether the sensing responses are from the CNT channel and/or the CNT/Au contacts, we passivated the CNT/Au contacts of Device 1 with a Si3N4 thin film, leaving the CNT channel open. After the passivation, we found that the device (Device 1A) did not respond to NH3 at room temperature, even at a concentration up to 500ppm, as shown in Fig. 4 (b). For comparison, we only passivated the CNT channel with Si3N4 thin film in another CNTFET (Device 2), but uncovering the CNT/Au contacts. Interestingly, Device 2 showed a high sensitivity at room temperature (see Fig. 5). Therefore, we can unambiguously conclude that NH3 gas induced SB modulation is a dominant mechanism for our CNT gas sensors at room temperature.
Actually, PMMA was widely employed as a passivation material to protect the CNT/metal contact regions for gas (Zhang, et al., 2006, Liu, et al., 2005) and protein sensing (Heller, et al., 2008). However, two major problems exist due to the polymer nature of PMMA. Firstly, PMMA is not dense enough to fully passivate the contacts. For example, NO2 was found to penetrate the 2.2μm thick SU-8/PMMA layer in 30mins (Zhang, et al., 2006). Thus, the CNT/metal contacts are inevitably affected by the gradual diffusion of the detecting species through the PMMA layer, so that the role of the contact in the detection could not be eliminated. Secondly, PMMA is thermally unstable above 100oC. This is a critical limitation as the adsorptions of some biomolecules and gas molecules on CNTs are enhanced at high temperatures. In contrast, Si3N4 is much denser and it can completely insulate the contacts from chemical environment (Kaminishi, et al., 2005). Meanwhile, its thermal stability allows for high temperature sensing experiments, as shown later.
4.2. NH3 sensing at elevated temperature
The transfer curves of Device 1A before and after exposure to 500ppm NH3 for 1000s are monitored at T=25oC, T=50oC and T=100oC in Fig. 6 (a), (b) and (c), respectively. No significant change in VTH curves was observed. We suggest that, the adsorption of NH3 on the CNT is not favored at this temperature range.
The transfer curve started to shift towards negative gate voltage after NH3 exposure at 150oC and above, see Fig. 7 (a). Since the contacts were fully isolated from NH3, this parallel shift in the transfer curve suggests that NH3 could adsorb on the CNT wall and donate electrons to the CNT. Consequently, the Fermi level of the CNT moves towards the conduction band edge so that the threshold voltage V TH becomes more negative. When the testing temperature reached 200oC, this phenomenon became more prominent. Progressive shift of the transfer curve in accordance with NH3 concentrations is shown in Fig. 7 (b). Under a first order estimation, the total charge transferred , where the gate capacitance , the SiO2 dielectric constant ε=3.9 and thickness h=200nm. For a SWNT bundle with a length L~5μm and a radius r~5nm, C G ≈0.25 fF. Thus, is approximately 0.625fC or about 4000 electrons, at 200oC with for 500ppm NH3. If the cross-sectional area of NH3 A~0.13nm2, the length of the exposed CNT channel L’~3μm, the coverage ~0.07 (interpolated from the Langmuir plot for 500ppm NH3 on CNT(Bradley, et al., 2003)), the charge transfer rate is about 0.02 electron. This value is reasonably consistent with typical theoretical predictions.
The extracted sensitivity of Device 1 and Device 1A under three gate voltages at T=25oC are shown in Fig. 8 (a). For Device 1, a very high sensitivity and significant gate modulation were observed. When the gate voltage is varied from negative to positive, the dominant carrier injection process changes from tunneling to thermionic emission, and the source-drain current becomes very sensitive to the SB height. A small change in the contact SB height due to NH3 adsorption will be prominently reflected in the source-drain current. Our device structure with a CNT on top of metal electrodes could also enhance the SB modulation effect. Once the contacts passivation is carried out in Device 1A, it essentially does not respond to NH3, implying that the CNT channel is not active to NH3 at room temperature. Fig. 8 (b) compares the sensitivities for Device 1A and Device 2 at T=150oC. At small NH3 concentrations, a low coverage of NH3 on the CNT channel and poor charge transfer efficiency result in a small sensitivity in Device 1A. When the NH3 concentration is increased, the sensitivity for Device 2 becomes saturated, probably due to limited interaction area in the CNT/Au contacts.
4.3. Effect of oxygen on NH3 sensing
Theoretical studies suggest that, NH3 interacts weakly with pristine CNTs with little charge transfer (Kong, et al., 2000, Bauschlicher & Ricca, 2004, Peng & Cho, 1999). Existence of a large activation barrier prevents adsorption of NH3 on perfect CNTs even at high temperatures. However, the adsorption of the gas molecules on defective CNTs could be much easier (Feng, et al., 2005, Robinson, et al., 2007). In addition, the adsorption barrier of NH3 on a defective CNT can be further lowered by pre-dissociated oxygen atoms, leading to an enchanted charge transfer rate, as pointed out by Andzelm et al (Andzelm, et al., 2006). In order to study the influences of oxygen on NH3 adsorption onto the CNT wall, we changed the background gas from dry air to N2. Device 1A was first annealed in N2 environment at 350oC for 2 hrs to degas the adsorbed oxygen. Note that during the high-temperature annealing, remaining oxygen molecules at the contacts can be further desorbed and the device became more n-type. The transfer curves before and after exposure to 500ppm NH3 for 1000s from T=25oC to T=150oC were shown in Fig. 9, and real-time sensing results at T=200oC are shown in Fig. 10. No detectable changes due to NH3 exposure were observed. Comparing with the sensing response observed in dry air environment, we can confirm that the adsorption of NH3 is facilitated by environmental oxygen. It was also found that the sensitivity was restored after the background was changed to dry air again. Our results are consistent with Andzelm et al’s predictions. NH3 could preferentially attach to the defect sites on CNT with pre-dissociated oxygen, as illustrated in Fig. 11.
4.4. Comparisons of the sensing mechanisms in CNT based gas sensors
From our results, we are able to rule out several possibilities of indirect interactions between NH3 and CNT. Firstly, as the testing environment was totally dry, NH3 adsorption through water layer is not applicable here. Secondly, If NH3 could interact through the SiO2 substrate or adsorb inside the CNT bundles, a reduced sensitivity should have been observed after contacts passivation. However, our observation that Device 1A is totally insensitive to NH3 at room temperature does not support this hypothesis. In fact, charge transfer and the SB modulation are the two mechanisms in our CNT sensors.
At room temperature, the weak adsorption of NH3 on the CNT wall does not induce any measurable effect on the source-drain current. The sensing signal mainly arises from the contacts. When NH3 molecules are adsorbed on the CNT/Au interface, the work function of the Au electrode is reduced (Bilic, et al., 2002) and/or the electrostatic charge balance between the CNT and Au is disturbed by the dipoles of NH3 molecules,(Yamada, 2006) leading to an increased SB for hole injection. The sensitivity reflected in the source-drain current are, however, gate voltage dependant. As illustrated in Figure 12 (b), when a negative gate voltage bends the energy band of the CNT upwards, the SB width becomes very narrow and holes could tunnel through the barrier, even when the SB height is increased by NH3. In contrast, at a positive gate voltage, the SB width is too thick for tunneling process. Thus, hole injection is only through thermionic emission over the SB height. The source-drain current is then expressed as:(Appenzeller, et al., 2004) , where is the Schottky barrier height, K B is the Boltzmann’s constant and T is the temperature in Kelvin. In this case, the sensitivity for the SB modulation is . Owing to the exponential dependence on the SB height change , extremely high sensitivity can be achieved. Our results are consistent with Yamada’s theoretical prediction that the SB modulation is most significant when CNT is operating in the depletion mode.(Yamada, 2006)
One disadvantage of the SB dominated sensors is a typical long recovery process at room temperature. Once the operating temperature is increased, the sensitivity degrades shapely due to its exponential dependence with 1/T. The nanoscale CNT/metal contacts could also be prone to subtle environmental noises, causing fluctuations during detection. In contrast, using the CNT channel as a sensing element can avoid the problems. In addition, the large surface to volume ratio of CNT offers another merit. At T>150oC, when the adsorption barrier is further lowered by oxygen, NH3 adsorption on the defect sites of the CNT becomes favored. Upon the consequent charge transfer, the Fermi level of the exposed central CNT channel moves upwards and the energy band shifts downwards with respect to those passivated parts, a potential barrier is therefore created and impedes the current flow, see Fig. 12 (d). Here, we would like to emphasize that, the charge transfer effect can be studied only when the contacts are fully protected. Otherwise, as temperature increases, the sensitivity enhancement from the charge transfer effect and the degradation due to SB modulation counteract each other.
Table 1 compares the SB modulation and charge transfer mechanisms. The sensors with the SB modulation usually demonstrate a high sensitivity at room temperature. Their sensing performance can be adjusted through the gate voltages. In contrast, the sensors under the charge transfer mechanism require high working temperatures, showing a good reversibility.
5. Conclusions and Future Work
In this chapter, we discussed in details the sensing mechanisms of CNT based NH3 detection. By selective Si3N4 passivation, We clearly show that the SB modulation at the CNT/metal contacts dominates the sensing performance at room temperature, and the sensor exhibits high sensitivity and good tunability under appropriate gate voltages. At higher temperatures, say 150oC or above, NH3 molecules start to adsorb on the CNT wall and the charge transfer process from the adsorbed NH3 molecules to the CNTs contributes to the sensing signal. As the mechanisms are identified, the next step is how to improve the sensing performance, and more chanllegingly, how to differentiate between different gas species. One promising way could be functionalizaton method (Feng, et al., 2005, Qi, et al., 2003). For example, using metal catalysts in the form of nanoparticles to decorate CNT, which promotes the interaction with speicific gas speicies. With a combination of several metal nanoparticle, the sensing performance to various gas molecules could be compared and contrasted.