Carbon nanotube gas sensor array for multiplex analyte discrimination

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Carbon nanotube gas sensor array for multiplex analyte discrimination Hoël Guerin a,∗ , Hélène Le Poche b,c , Roland Pohle d , Elizabeth Buitrago a , ˜ Badía a , Jean Dijon b,c , Adrian M. Ionescu a Montserrat Fernández-Bolanos a

École Polytechnique Fédérale de Lausanne (EPFL) – Nanoelectronic Devices Laboratory (NANOLAB), ELB 335, Station 11, 1015 Lausanne, Switzerland Univ. Grenoble Alpes, F-38000 Grenoble, France c CEA, LITEN, DTNM, F-38054 Grenoble, France d Siemens AG Corporate Technology, CT RTC SET CPS-DE, Otto-Hahn-Ring 6, 81739 München, Germany b

a r t i c l e

i n f o

Article history: Received 28 April 2014 Received in revised form 17 September 2014 Accepted 27 October 2014 Available online xxx Keywords: Carbon nanotube Gas sensor Multiplex gas discrimination Metal–nanotube interface

a b s t r a c t The lack of selectivity toward a particular analyte has always been the primary concern regarding CNTbased gas sensors. For that reason, in here we present a gas discrimination strategy that focuses on the electrode–CNT junction. The junction is shown to play a key role in the sensing mechanism. Resistive gas sensors based on horizontal CNT arrays have been fabricated using various designs and different topcontacting metals: Pt, Pd and Au. Arrays of devices have been exposed to a series of gases to monitor their resistive response. It was found for our system that the sensor response does not significantly change as a function of the device design or the available CNT sensing area in between the anchoring electrodes. On the contrary, responses to gases are observed to be specific to each sensor electrode metal. Exposure of locally passivated devices (for which distinct areas have been covered) to NO2 , H2 and NH3 highlights different sensing mechanisms for each gas. Multiplex gas discrimination for room temperature can be achieved by strategically choosing the right metal/CNT combination in a complete sensor system. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Discovered in 1991 by Iijima [1], carbon nanotubes (CNT) have been intensively investigated thanks to their unique electrical and mechanical properties [2,3]. CNTs are both a stiff and light material that exhibits a high Young’s modulus and good electrical conductivity without electron migration issues [4]. As a consequence of these exceptional properties, CNTs have become a promising alternative for a wide range of applications in micro/nano electronics from interconnecting lines [5], field-effect transistor (FET) based logic gates [6], nano-electro-mechanical systems (NEMS) [7] to chemical sensors [8]. Since semiconducting single wall carbon nanotube (SWCNT) FETs were found to exhibit resistance variation upon exposure to NH3 and NO2 in the early 2000s [9], they have been widely investigated for gas sensing applications. Different groups have reported remarkable sensitivities of CNT-based sensors to a wide spectrum of gases such as COx , SOx , NOx , alcohols, organic vapors (important for disease diagnosis), explosive compounds and chemical warfare

∗ Corresponding author. Tel.: +41 216937856. E-mail address: hoel.guerin@epfl.ch (H. Guerin).

agents among others, with detection limits in the order of ppm or ppb [10–12]. Whereas the most common metal oxide semiconductor sensing technology is currently facing power limitations, due to its high operation temperature, and scaling issues [13], CNTs exhibit many properties that are desirable for gas sensing [14]. The nanotubes are almost exclusively composed of surface atoms. The conducting channel of a CNT based sensor is therefore always in direct proximity with the gas analytes. The surface effects of gas molecules generate high responses from the CNTs. The inherent high sensitivity of the CNTs removes the need for supplementary technologies such as gas preconcentration. Thus, CNT sensors can benefit from simple device configurations for low cost, miniaturized sensing applications. Thanks to their p-bonds perpendicular to the CNT surface, CNTs are also electrochemically active at room temperature making them suitable for the development of a low power sensing technology [15]. However, the main drawback to the superior performances of CNT-based gas sensors is the lack of selective detection toward a particular analyte. Consequently, the risk of false alarms is high and many functionalization strategies have been developed in order to obtain a gas specific response. Methods that have yielded good results include the decoration of CNTs with different metal nanoparticles [16,17] or coating with polymers [18,19]. However, such functionalization strategies may

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Fig. 1. Process flow for the fabrication of resistive gas sensors with horizontal CNT arrays directly integrated via a selective and directional catalytic growth process.

affect negatively the overall performance of the device in terms of response time, long-term stability, etc. For instance, polymers may be deteriorated when refreshing of the sensing surface involves the use of UV light or high temperature steps [20–22]. They might also degrade naturally over time. This work presents a gas discrimination strategy using different metal electrodes to contact in situ, CVD grown, horizontal arrays of CNT chemiresistors. Ultimately, the combination of the sensor resistive response with each metal could be used as an electronic signature to identify specific gases as exemplified in this work. The method proposed here is technologically straightforward and does not require any extra processing steps that could harm the sensor’s lifetime or reliability as a trade-off. Finally, the device sensing mechanism is investigated for H2 , NO2 and NH3 , through a localized Al2 O3 passivation layer. 2. Experimental 2.1. CNT gas sensor fabrication The CNT gas sensors have been manufactured on p-doped Si/SiO2 substrates following a process flow that has been extensively described in literatures [23,24]. The main steps of the fabrication process are summarized below and are schematically shown in Fig. 1. The first phase of the process flow consists in fabricating a catalyst support designed to enable the localized growth and integration of horizontal CNT mats through a catalytic chemical vapor deposition (CVD) process [25]. A stack of three layers of TiN, Al2 O3 and TiN are deposited on the substrate by sputtering followed by a final SiO2 layer (Fig. 1a). This oxide is patterned to serve as a selective hard mask (Fig. 1b) in the dry etching process of the upper TiN and Al2 O3 layers to form the iron catalyst support lines (Fig. 1c). Afterwards, the SiO2 hard mask is removed and the lower TiN layer is patterned to form the device electrodes (Fig. 1d). In the second phase of the fabrication process, the actual CNT array in situ growth takes place from one sidewall of the catalyst support to bridge both electrodes. A 1-nm thick layer of iron catalyst is locally deposited by e-beam evaporation with a 45◦ tilt angle on a single sidewall of the support by a lift-off process (Fig. 1e). The growth is performed by a catalytic CVD process at 580 ◦ C with a feedstock gas mixture of acetylene diluted in hydrogen and helium at 0.4 mbar (Fig. 1f). Fe nanoparticles (NP) are formed by heating the thin iron film. The particles serve as nucleation sites for CNT growth. The in situ integration of horizontal CNT arrays is achieved thanks to the selectivity and the directionality of the growth process. It is only activated from the Fe nanoparticle surfaces with aluminum oxide as an underlying material. Due to the diffusion of the iron catalyst inside the TiN layer, no CNTs nucleate on the nitride surfaces as shown in Fig. 2 [26]. Furthermore, the CNT growth is perpendicular to the surface on which the catalyst is deposited leading to the formation of horizontal CNT arrays as illustrated in Fig. 2 [23,24]. The resistive gas sensor fabrication is finalized by dipping the samples

Fig. 2. SEM picture (tilted view) showing the selective growth of CNTs from the Al2 O3 surface and the absence of nucleation on the TiN surfaces. The picture is taken after isopropanol treatment which induces the densification of CNT arrays. The arrays are flattened on the substrate.

in isopropanol. This treatment flattens and densifies the CNT mats onto the oxide and makes contact to the TiN electrode. Then the top-contact metal can be deposited by evaporation/lift-off which easily clamps the CNT arrays onto both electrodes (Fig. 1g). Kong et al. [17] were the first to propose Pd NP-decorated SWCNTs to improve the gas selectivity toward H2 . This approach was further developed by Star et al. [16]. This group fabricated FETs utilizing a SWCNT network as a channel decorated with different metal nanoparticles. They observed that the conductance response to different gases was dependent on the type of metal NPs used to decorate the nanotubes. The whole group of FET array responses could therefore be potentially used as an electronic signature of a given gas species. They showed that the charge transfer between gas molecules and the decorated nanotubes is dependent on a (metal work function dependent) potential barrier formed at the SWNT–NP interface [27,28]. Our method proposes to transpose this gas discrimination strategy to the electrodes–CNTs junctions. Various metals can easily be deposited to top-contact the CNT arrays by evaporation without needing an extra step involving an electrodeposition process of metal NPs [29]. Several dies of the CNT sensors discussed here are manufactured with Pt, Pd and Au, respectively, as the electrode metal top-contacting CNT mats to investigate their influence on the gas sensor response (see Section 3.1). A thin layer of Cr or Ti is evaporated first to improve the top-contact metal adhesion on the substrate. Gold and palladium are chosen for their similar work function (∼5.1 eV) while platinum has a higher one (∼5.65 eV). Each die comprises chemiresistors fabricated with various designs and architectures. The CNT arrays are grown with “channel” lengths between the metal contacts ranging from 4 to 7 ␮m. Similarly, the CNT mat width is alternately 60, 72 or 84 ␮m. Moreover, the CNT arrays are integrated whether in a continuous (Fig. 3a) or dashed manner (Fig. 3b). This means that the 60 ␮m, 72 ␮m or 84 ␮m width may be split in multiple CNT arrays of smaller width: 12 ␮m, 6 ␮m and 2 ␮m wide, respectively. Finally, two types of device architecture are implemented: with two electrodes and one CNT array bridging the gap (Fig. 3a) or interdigital electrodes with 15 arrays bridging the gaps between the parallel digits (Fig. 3b), respectively. 2.2. Device localized passivation Despite the demonstrated performances of CNTs for gas sensing, no consensus has been reached regarding the underlying sensing principle. Many mechanisms have been proposed [30,31]. Dai’s group proposed chemical gating of the SWCNT by NO2 gas molecules. The adsorbed analytes shift the Fermi level of

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Fig. 3. (a) Pd-contacted, two electrode sensor with a continuous, 84 ␮m wide, and 7 ␮m long CNT array. (b) Pt-contacted, interdigitated electrode sensor with 15 dashed, 10 ␮m × 6 ␮m wide, and 7 ␮m long CNT arrays.

semiconducting CNTs. For ammonia, the same group suggested indirect analyte binding through the hydroxyl groups on the SiO2 substrate. The neutralization of charged groups on the oxide by adsorbed molecules generates an electrostatic gating [9]. Analyte binding on the CNT inducing direct charge transfer to or from the nanotube has also been proposed [32,33]. All these mechanisms affect the CNT channel conductivity. Modulation of the Schottky barrier (SB) formed at the metal–CNT junction has also been reported as a sensing principle [34,35]. For this mechanism charged or polarized gas molecules vary the work-function difference at the metal–CNT interface. This in turn modifies the carrier transport. The debate on whether the CNT channel or the (contact) metal–CNT interface is the sensitive part of the sensor to gases remains open. Various reports advocate each hypothesis. To investigate the underlying principle of the presented discrimination strategy and clarify the sensitive area of the sensor (metal–CNT interface or CNT channel), a dedicated die of sensors is passivated with 50 nm of conformal Al2 O3 (Fig. 5a) [36]. This

Fig. 4. Diagram of the gas sensing experimental setup.

Fig. 5. (a) SEM picture of a cleaved device: cross-section of a device electrode fully covered and passivated by a 50 nm thick, conformal ALD-Al2 O3 layer. (b) SEM picture from an interdigitated CNT channel passivated sensor. Due to the ALD-Al2 O3 passivation layer, the CNT arrays below appear blurred. In contrast, electrodes free from Al2 O3 are clearly visible.

aluminum oxide layer is deposited by atomic layer deposition (ALD) at 200 ◦ C in N2 atmosphere. It is then wet etched in hot phosphoric acid (H3 PO4 ) over specific areas of the sensor by means of a patterned e-beam polymer mask. Phosphoric acid is preferred to buffered hydrofluoric acid due to its selectivity with respect to the materials composing the sensor: SiO2 in particular. Four device categories, each one comprising several sensors, have been designed as illustrated in Fig. 6: fully passivated sensors, sensors with contact passivation only, sensors with the CNT channel passivated only (Fig. 5b), and sensors with no passivation. The gas measurements on these devices are discussed in Section 3.2. 2.3. Gas sensing procedure The setup employed for gas measurements is described in Fig. 4. The devices are wire-bonded to a dual inline package which serves as a plinth for the small gas test chamber (∼3 cm3 ) directly mounted on top. During two 15 min steps (unless specified otherwise), CNT sensors are exposed to low concentrations of H2 , NH3 , NO2 , toluene, ethanol vapors (which are nonetheless well above the sensor detection limits). A low flow of analyte gas is mixed with a synthetic dry air buffer (N2 80%, O2 20%). The mixture is then sent to the chamber inlet at a constant flow rate of 1 L/min. In the remainder of the text, such a gas mixture is referred as exposure to gas in dry conditions since no moisture is injected in the chamber. However, this does not discard the presence of minimal water residues in the gas flow, due to gas cylinder impurities for instance. All measurements are performed at atmospheric pressure and room temperature. Measurements in wet conditions, involving relative humidity (RH) are achieved by bubbling a N2 flow through

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Fig. 7. Responses to hydrogen in wet conditions of three interdigitated Pt-sensors with 84 ␮m wide CNT arrays of respective lengths: 5, 6 and 7 ␮m.

Fig. 6. Diagrams showing (a) sensors fully passivated with ALD-Al2 O3 ; (b) sensors with only the contacts passivated; (c) sensors with passivation of the central CNT channel only; and (d) sensors with no passivation.

water. Operation of the sensors in ambient conditions is meant to exploit the potential of CNTs to achieve low power devices, a feature that is highly demanded for prospective applications. Although rising the operating temperature has been reported to enhance the CNT sensors response to gases, active heating of the devices is energy consuming and mitigates the advantages of CNT sensors with respect to metal oxide semiconductor sensing technology for example. For each die (with Pt, Pd and Au top-contact, respectively), 14 devices with various designs are measured in parallel. The data are acquired through a National Instruments USB module and adequate Labview programming (Fig. 4). A load resistance is connected in series with each sensor in a voltage divider configuration. By applying a constant 200 mV bias, the sensor resistance R is extracted at any time by continuously monitoring the voltage drop over the load resistance. The load resistance value is similar to the sensor resistance so as to optimize the measurement resolution. The sensor “normalized” response to a gas is defined and plotted as the ratio R/R0 where R0 is the initial resistance of the device in air, prior to gas exposure.

likely originates from an averaging effect of the 15 CNT arrays put in parallel. A single exception is observed. When exposed to H2 in dry conditions, a two-electrode Pt-sensor shows a 2% response similar to interdigitated electrode device response (Fig. 8a). For the same concentrations of hydrogen under 50% RH, both architectures exhibit higher responses. However, the two-electrode resistive shift is twice as high as that of the interdigitated ones: 6% versus 3% (Fig. 8b). Lundström et al. [37] described extensively the reactions occurring at the metal surface during hydrogen exposure. H2 adsorbs and dissociates on catalytic Pd or Pt surfaces in atomic hydrogen which diffuses inside the metal crystal. Unlike what would happen within an inert argon atmosphere, they reported that the presence of oxygen facilitates hydrogen desorption through water-forming reactions. Adsorbed oxygen reacts with hydrogen to form hydroxyl groups. This is the rate limiting reaction. A hydroxyl group then reacts with adsorbed atomic hydrogen to produce water. Water

3. Results and discussion 3.1. Enabling gas discrimination through various metal–CNT interfaces For every die with a given top-contact metal, 14 non-passivated sensors are exposed to gases. Their responses R/R0 are recorded and compared. All devices exhibit very similar traces regardless of their particular design configuration. For instance, Fig. 7 shows the response of interdigitated Pt-sensors with increasing CNT lengths being exposed to hydrogen in 50% RH conditions. This demonstrates the high design robustness and the device-to-device reproducibility of the sensing capabilities. Therefore, individual sensor calibration is not necessary. It also highlights that the design parameters (CNT mat length and width among others) have little or no influence on the device performances. Surprisingly, the influence of the device architecture is not very significant. Two electrode sensors exhibit noisier responses to gases but similar to the responses of interdigitated electrode devices (Fig. 8a). The lower response noise in interdigitated structures most

Fig. 8. Responses to H2 of a Pt-contacted, 84 ␮m wide CNT array, two-electrode device (orange) and interdigital-electrode device (green), respectively, in (a) dry conditions and (b) wet conditions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 9. Responses in wet conditions of 84 ␮m wide, 7 ␮m long CNT array sensors top-contacted with different metals (a) to H2 : Pt-sensor shows an overall 3% shift, Pd-sensor a 1% response and Au-sensor shows no sensitivity; (b) to NH3 : Pt-sensor shows a total 2.5% shift, Au and Pd-sensors a 1% response.

eventually desorbs in gas phase. The higher responses in humid conditions are possibly ascribed to the hindrance of the hydroxyl group reactions by the water adsorbed on the metal surface. The adsorption rate of hydrogen at the internal surface of the metal is favored. As a result, the same concentration of H2 introduced in dry conditions generates a greater response in humid conditions. The architecture influence in humid conditions remains unclear at present. The beginnings of an explanation could be the higher metal surface available per CNT array in the two electrode architecture. Petersson et al. [38,39] and Harris et al. [40] reported that the hydrogen atoms on the metal surface have a temperature activated lateral mobility prior diffusion inside the metal. If this mobility is large enough, the electrochemically active catalytic surface per CNT mat may be greater for two electrode architecture devices. They also have shorter paths and scattering for the diffusion of hydrogen in the metal. Next, the responses R/R0 of identical configuration sensors with Pt, Pd and Au electrodes, respectively, are compared. In comparison to when device design or architecture is changed, the sensor response to different gases significantly changes depending on the top-contact metal. Hydrogen generates a higher response from Pt-contacted sensors than Pd-sensors in both dry and humid conditions. Au-sensors do not react at all. Fig. 9a exemplifies the sensor behavior exposed to H2 in 50% RH. Pt-sensors react with a total 3% resistance shift-up, Pd-sensors ∼1% and Au-sensor resistance remains unchanged. All sensors are sensitive to ammonia in both dry and wet conditions. The quantitative responses are different depending on the metal. In 50% RH conditions, Pd and Au sensors both have a total 1% resistive shift and Pt sensors show a 2.5% response (Fig. 9b). Similarly, Pd and Pt-sensors exhibit no response to toluene while Au-sensors show a negative resistive shift. Only Pt-sensors react to ethanol in dry conditions. Table 1 summarizes, for the three metals, the qualitative responses (positive, negative or no resistance shift) to gases in dry conditions and to relative humidity. This table highlights the predominance of the top

Fig. 10. Response of Pt-sensor (a) to ammonia well fitted by a double exponential model; (b) to hydrogen partially fitted by a double exponential model; and (c) to water vapor (relative humidity) showing no exponential component.

contact metals in the sensor response to gases. The use of different electrode metals generates distinct sensor behaviors upon interaction with H2 , toluene, ethanol or NH3 . The combined data of a CNT sensor array, composed by a sensor of each metal, create an analyte-specific pattern and enables discriminative detection. The gas discrimination strategy proposed is suitable candidate for “electronic nose” applications [41]. But in order to develop a viable application for “real-world” analyte detection with interfering gas species and confounding factors [42], the multiplex discrimination strategy presented here should be next tested with gas mixtures Table 1 Sensor resistive responses (shift up: +, down: −, none: 0) to gases in dry conditions and to relative humidity depending on the top contact metal. Sensor metal

Pt Pd Au

Gases H2

Ethanol

Toluene

NH3

H2 O

+ + 0

+ 0 0

0 0 −

+ + +

+ + 0

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Fig. 11. Response of differently passivated Pt-sensors to (a) H2 , 1000 ppm in dry conditions; (b) NO2 , 500 ppb in 50% relative humidity; (c) NH3 , 100 ppm in dry conditions; and (d) NH3 , 100 ppm in 50% relative humidity.

and the selectivity of each sensor composing the array should be assessed quantitatively toward several gas species (relevant to a given application such as air quality monitoring or breath testing) injected at a fixed partial pressure over vapor pressure ratio as in [43]. 3.2. Sensing mechanism We now consider the possible sensing mechanism of the CNT array sensor. The above measurements show that, when exposed to gases, the sensor resistance modulation is dependent on the material used for the top-contact. Bondavalli et al. [44] discussed the underlying mechanism of CNT FETs having different contact metals upon exposure to DiMethyl-Methyl-Phosphonate (DMMP). They attributed the modification of the transfer characteristics to the interaction between gas and metal. The gas molecules adsorbing on the sensor are polarized. They form a dipole layer that shift the metal work function, characteristic of each metal. The Schottky barrier height formed at the metal–CNT junction is consequently modified in a metal specific way. This Schottky barrier modulation is unlikely to be the sole sensing mechanism at stake for the Au, Pt and Pd CNT sensors and gases tested here. Au and Pd have similar work functions but the sensors exhibit very distinct responses to hydrogen, toluene and water (Table 1). The dimensions of the CNT arrays are observed to have little influence on the sensor response. On the other hand, we found that the top-contact metal cannot account alone for the response and the distinct behaviors observed. This fact is demonstrated by patterning a simple platinum metal line as a resistor. When exposed to H2 , NO2 and NH3 in both dry and wet conditions (50% RH), the

structure exhibits no significant response as compared to the noise. Another consideration is the very low contribution of the metal electrodes to the sensor resistance. Pd [37,45] and Pt [46,47] are commonly used as catalysts for hydrogen sensing. H2 adsorbing on the metal surface dissociates in atomic hydrogen. These atoms diffuse swiftly inside the metal. It is reported that the metal resistivity increases up to maximum of 80% for Pd when saturated with hydrogen [48,49]. Simple calculations of the electrode resistance demonstrate the resistivity shift is too low to account for the totality of the sensor response measured. Furthermore, the response kinetics of an interdigitated Ptsensor to NH3 , H2 and RH are studied in Fig. 10. NH3 is observed to obey a double exponential model (Fig. 10a). Hydrogen exhibits a “steep” but non-exponential rise in the first minute followed by a single exponential response (Fig. 10b). Finally response to relative humidity exhibits a fast changing (step-like) resistance rise with no exponential component (Fig. 10c). Besides providing potential for gas identification on the transient sensor response [50], the distinct response kinetics suggests different sensing mechanisms for every gas. The long response times observed in Fig. 10, several tens of minutes, originate from the adverse effect of contamination on the CNTs, metal surfaces and at metal/CNT contact interfaces. Literature reported that cleaning the sensor surface, nanotubes in particular, from their multiple contaminants, in order to obtain a pristine and accessible surface on which gas molecules can readily adsorb, enhances the sensor performances [51,52]. In further research, the contamination could be reduced through liquid-phase purification with acids [53]. Another option is to tackle the problem at its source by optimizing the CVD process feedstock gas with hydrogen and ammonia [54,55] or water [56] in order to grow directly pristine CNTs.

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Fig. 12. I–V traces of a Pt-contacted CNT sensor in synthetic air and when exposed to H2 (5000 ppm) in synthetic air.

Au, Pt and Pd noble metals are frequently used as catalysts for analyte detection [57,58]. We hypothesize that the different metals may catalyze specific reactions when interacting with a gas and indirectly favor its detection at the metal/CNT interface. As described in Section 2.3, Pt-sensors locally passivated with aluminum oxide are exposed to H2 , NO2 and NH3 in dry and wet conditions (relative humidity 50%). The absence of response of fully passivated devices during the whole gas sequence evidences the correct impermeability of the passivation layer to the gases tested [59]. Fig. 11a shows the passivated device responses to 1000 ppm of hydrogen. CNT channel-passivated devices and sensors with no passivation respond similarly while contact-passivated sensors (with only the CNT arrays exposed) show no reaction. This observation agrees with the low interaction and binding energy reported for H2 on bare CNTs [60] and suggests that hydrogen is sensed at the metal–CNT interface. Fig. 12 shows the I–V characteristic of an interdigitated Pt-sensor under a flow of synthetic air with and without addition of 5000 ppm of hydrogen. In synthetic air, the device shows a linear trace with a 53.3  resistance. This characteristic is observed for all devices in room conditions regardless of the top-contact metal. (Ohmic behavior is coherent with the majority of metallic, double-walled CNTs composing the arrays reported in previous works [23,61].) Under H2 though, the I–V trace exhibits higher resistance (56.3 ) and a slight non-linearity emerges. This is representative of a barrier formation at the metal–CNT contact which is ascribed to the response of the low percentage of semiconducting CNTs within the array to hydrogen. This observation complies with the hydrogen sensing mechanism already reported extensively in literatures [17,37,62]. Being energetically favorable, H2 molecules are first dissociated into atomic hydrogen adsorbed on the catalytic Pt surfaces. Part of these atoms diffuse quickly through the metal thin film and are adsorbed at the metal–CNT interface. These hydrogen atoms are polarized. They form a dipole layer which shifts the energy levels at the metal–CNT interface. The Pt work function is lowered. The barrier height at the interface is modified and electrons to transfer from metal to CNTs. As a result, hole density is decreased in the p-type CNT and the resistance increases. Gold does not act as a catalyst for H2 thus Au-sensors do not react to hydrogen. Pd catalyzes the same dissociation of hydrogen as Pt. However, its weaker interaction and lower binding energy with H2 and CNTs could reasonably explain the lower response of Pd-sensors [60]. Palladium lattice constant is also known to increase upon H2 exposure [48,49]. This generates a compressive stress at the metal–CNT interface and in the CNT arrays which electronic properties are very sensitive to. The investigation of this phenomenon to the sensor response is complex in

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Fig. 13. I–V traces of a Pt-contacted CNT sensor in synthetic air and when exposed to NH3 (50 ppm) in synthetic air.

the present CNT arrays device. For instance, the effect of stress on a carbon nanotube band gap depends on its chirality. Contact passivated sensor response to 500 ppb of NO2 (Fig. 11b) exhibits clear inflexion points at the gas introduction and stop while channel passivated devices exhibit barely none. These observations support that NO2 sensing occurs by interaction with the CNT channel [59]. The twice weaker response observed for channel passivated devices is ascribed to the CNT surface uncovered close to the metal–CNT contact (as can be seen in Fig. 5b). Several effects taking place at the CNT channel could account for the observed responses. The electron withdrawing behavior of NO2 adsorbed on CNTs may cause hole doping. It increases carriers in p-type CNTs and explains the device resistance [32,33]. In the same line of explanation, adsorption of NO2 molecules at the interstitial sites between the CNTs forming the array could also be another possible mechanism [31]. This intertube modulation affects both metallic and semiconducting CNTs [21]. The response to 100 ppm of ammonia (Fig. 11c) yields less conclusive results. All sensors respond to NH3 although contact passivated devices exhibit responses five times weaker than channel passivated sensors. This would support a predominant sensing mechanism at the metal–CNT interface at room temperature as reported by Peng et al. [63]. I–V characteristic of an interdigitated Pt-sensor is measured under a flow of synthetic air with and without addition of 50 ppm of ammonia (Fig. 13). The resistance increases from 53.3 to 54.8  but both traces are apparently linear. Therefore, Schottky barrier formation can a priori be discarded as a sensing mechanism. Au, Pt and Pd are all three promoting ammonia reaction [58]. The higher response with Pt-sensors (Fig. 9b) is believed to originate from different catalytic reactions and different energy configuration with the analyte [64]. The enhancement of the sensor response depends on the charges or dipoles produced by the reactions [65]. Pt has been reported to catalyze the decomposition of ammonia producing a negatively charged amino ion by deprotonation [64]. Nevertheless, further investigation is necessary to fathom with certitude the possible underlying mechanisms. The environmental conditions, such as the relative humidity effects, have to be studied: humidity enhances the Pt-sensor response to NH3 , as illustrated in Fig. 11d. The water solubility of this chemical compound plays certainly a role in this. Konvalina et al. [66] links the influence of humidity with the polar nature of the analyte. They suggest a competitive adsorption between water and chemically similar, small, polar compounds. Conversely, the sensing of water and non-polar molecules is suggested to be additive. Nevertheless, considering in particular the sensor response to non-polar hydrogen in humid conditions, the effects of humidity and its latent mechanisms appear to be more complex. Similarly the role of

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oxygen [9] as a possible intermediary in the analyte sensing should be addressed. Finally, the effects of the passivation process on CNTs presenting defects [36] should be investigated. Some devices have been negatively affected and presented higher noise. The above measurements highlight that overall distinct gas sensing mechanisms are involved depending on the gas and electrode metal. They validate the concept of the gas discrimination strategy proposed. 4. Conclusion Gas sensing resistors have been fabricated by direct integration of horizontal CNT arrays between metal electrodes through a directional and selective catalytic CVD growth process. Different electrode metals: Pt, Pd and Au have been deposited to top-contact the CNT arrays providing sensors with three distinct metal–CNT junctions. The CNT sensor response when exposed to gases (such as H2 , NH3 , toluene or ethanol) depends drastically on the top-contact metal. Direct electrode metal contribution is insignificant as sensing mechanism in our devices. Locally passivated, Pt-contacted CNT sensors evidence distinct sensing areas and mechanisms according to the analyte gas. H2 is sensed at the metal–CNT interface through a Schottky barrier modulation while NO2 is sensed on the CNT channel possibly via charge transfer. Finally, NH3 seems predominantly sensed at the metal–CNT junction but formation of a barrier is discarded by the persisting ohmic behavior of the device. These findings support that a thoughtfully designed CNT sensor array, taking advantage of the specific interaction that each metal/CNT/gas exhibits, offers a great potential for gas discrimination at room temperature. Funding sources This research is part of the Integrated Project e-BRAINS ICT25748 funded by the European Union 7th Framework Program. Conflict of interest The authors declare no competing financial interest. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Acknowledgments This research is part of the Integrated Project e-BRAINS (ICT257488) funded by the European Union 7th framework program. We thank the CMI-EPFL staff for fruitful discussions and their valuable help with the fabrication. References [1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58, http://dx.doi.org/10.1038/354056a0. [2] A. Javey, J. Guo, D.B. Farmer, Q. Wang, D. Wang, R.G. Gordon, et al., Carbon nanotube field-effect transistors with integrated ohmic contacts and high-k gate dielectrics, 2003, ArXivcond-Mat0312389. http://arxiv.org/abs/ cond-mat/0312389 (accessed 13.03.13). [3] A. Javey, J. Kong, Carbon Nanotube Electronics, Springer, 2009, http://books. google.ch/books?hl=fr&lr=&id=68b8clF7HvAC&oi=fnd&pg=PR5&dq=carbon+ nanotube+electronics+springer+javey&ots=kRqWUAV1Sw&sig= srqslQX3JN2yYh dcWdjne SYrk (accessed 18.03.13). [4] M.S. Dresselhaus, G. Dresselhaus, J.C. Charlier, E. Hernández, Electronic, thermal and mechanical properties of carbon nanotubes, Philos. Trans. R. Soc. Lond. Ser. Math. Phys. Eng. Sci. 362 (2004) 2065–2098, http://dx.doi.org/ 10.1098/rsta.2004.1430.

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Biographies Hoël Guerin earned an Engineering Master’s Degree from Ecole Centrale de Lille and a Master’s Degree in Nanoscience from Keio University (Tokyo) in 2010, respectively. He is currently pursuing a Ph.D. at École Polytechnique Fédérale de Lausanne (EPFL) under the direction of Prof. Ionescu. His research focus is on the use as quantum dots and the transport properties of gold nanowires as well as on the development of gas sensors based on carbon nanotube arrays. Hélène Le Poche graduated in 1999 with an Engineer Degree from “Graduate School of Physics and Chemistry of Bordeaux” and with a Master degree in Polymer Science from University of Bordeaux I (France). She earned her Ph.D. in Material Science from University of Bordeaux I in 2003. She joined CEA LETI in Grenoble in 2003 for a postdoctoral position devoted to carbon nanofiber integration in field emission devices. Since 2005, she has a permanent position at CEA LITEN in Grenoble. Her work focuses on the development of CNT growth processes by CVD and CNT integration by in situ growth into devices for electronic or energy applications. Roland Pohle received the Ph.D. degree in physics from the Technical University, Munich, Germany, in 2000. Since 1998, he has been with the Corporate R&D of Siemens AG and is engaged in the development of gas sensors based on metal oxides and FET transducers and on the realization of applications using gas sensors. His research interests range from investigations on surface chemistry of semiconducting metal oxides and other gas sensitive materials to the application of work function methods for the realization of low cost gas sensing devices. Elizabeth Buitrago received her B.Sc. degree in chemical engineering from the University of California San Diego (UCSD), in La Jolla, California and her M.Sc. degree in process engineering from the Eidgenössische Technische Hochschule Zürich (ETHZ) with an emphasis in particle technology. Currently, she is a Ph.D. Student-Researcher at the Ecole Polytechnique Fédérale de Lausanne (EPFL), in Switzerland, developing vertically stacked silicon nanostructure devices for biosensing applications. Through her internship and work experiences as a Process Engineer at AMI Semiconductor and Micron Technology in the United States, she became highly interested in the semiconductor industry. ˜ Badía received her M.Sc. degree in telecommuMontserrat Fernández-Bolanos nication engineering from the University of Seville, Seville, Spain, in 2005, and her Ph.D. degree in microsystems and microelectronics from the Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, in 2010. Since receiving the Ph.D. degree, she worked as a Scientific Collaborator in the Nanoelectronic Device Laboratory at EPFL and since June 2013 she joined IBM Research Zurich as a senior researcher. The focus of her research is in the field of NEM relays for ultra-low power logic applications as well as RF MEMS switches and tunable filters for airborne applications. Her present research interests include the open challenges of MEMS/NEMS devices such as reliability and 3-D heterogeneous integration with RF ICs. Jean Dijon graduated from “Institut National Polytechnique of Grenoble” (specialty physics) in 1979 with an Engineer degree. He earned his Ph.D. “Doctor es Science” in 1984. He joined CEA in 1984 where he was involved in liquid crystal displays. In 1989 he built a team devoted to laser damage threshold at LETI. Since 2000 he is in charge of Carbon Nanotubes work. His first interest was for field emission applications and now he is focused on CNT for electronic and energy applications. He joined the LITEN Nano Material Department in 2006. He is the director of Research at CEA since 2002. He is the author of around 80 papers and 40 patents.

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Adrian Mihai Ionescu received his Ph.D. degree from the National Polytechnic Institute of Grenoble in France. He is a full Professor at the Swiss Federal Institute of Technology, Lausanne (EPFL) in Switzerland. He has held staff and/or visiting positions at LETI-CEA, Grenoble, LPCS-ENSERG, and Stanford University during 1998 and 1999. His research interests focus on micro- and Nanoelectronic devices

aimed at integrated circuit design, particularly process development, modeling, and electrical characterization. He has published more than 250 articles in international journals and conference proceedings. He is the Director of the Laboratory of Micro/Nanoelectronic Devices (Nanolab) at EPFL.

Please cite this article in press as: H. Guerin, et al., Carbon nanotube gas sensor array for multiplex analyte discrimination, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.10.117