Gas sensing using single wall carbon nanotubes ordered with dielectrophoresis

Gas sensing using single wall carbon nanotubes ordered with dielectrophoresis

Sensors and Actuators B 111–112 (2005) 181–186 Gas sensing using single wall carbon nanotubes ordered with dielectrophoresis M. Lucci a,c , P. Regoli...

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Sensors and Actuators B 111–112 (2005) 181–186

Gas sensing using single wall carbon nanotubes ordered with dielectrophoresis M. Lucci a,c , P. Regoliosi a,c , A. Reale a,c,∗ , A. Di Carlo a,c , S. Orlanducci b,c , E. Tamburri b,c , M.L. Terranova b,c , P. Lugli d , C. Di Natale a,e , A. D’Amico a,e , R. Paolesse b,e a

c

Department Electronic Engineering, University Rome Tor Vergata, Via del Politecnico, 1 00133 Rome, Italy b Department Chemical Sciences and Technologies, University Rome Tor Vergata, Rome, Italy Interdisciplinary Micro and Nano-structured Systems laboratory (MINAS), University Rome Tor Vergata, Rome, Italy d Lehrstuhl f¨ ur Nanoelektronik, TU M¨unchen, M¨unchen, Germany e CNR – Institute Microelectronic Microsystems, Rome, Italy Available online 8 August 2005

Abstract We demonstrate efficient NH3 detection in single wall carbon nanotubes (SWCNT) ordered by mean of dielectrophoretical process. The employed approach was to disperse the nanotubes, treated following a specific protocol, in CHCl3 and to distribute the suspension between the tracks of multifinger Au electrodes (40 ␮m spacing) on SiO2 /Si substrates. The control of arrangement and alignment of the SWCNT bundles was achieved by applying an alternate voltage (frequency 1 MHz, 10 Vpp ) during the solvent evaporation. The sensitivity for NH3 detection resulted to be strongly enhanced by the degree of SWCNT alignment between the electrodes. The sensitivity resulted enhanced also by increasing up to 80 ◦ C the temperature of the devices. We investigated also the effect induced on the NH3 absorption/desorption processes by a gate voltage applied to the Si substrate beneath the interdigitated electrodes on the NH3 . The results indicate that the sensitivity of the SWCNT-based sensor can be increased applying a negative gate voltage. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Gas sensing; Dielectrophoresys; NH3

1. Introduction Single-walled carbon nanotubes (SWCNT), formed by a graphene sheet wrapped around along a lattice vector [1], represent the last generation of carbon nanomaterials. The increasing interest in studying such tubular graphitic nanostructures is motivated by the remarkable properties of SWCNT, which make them attractive for a number of potential technological applications ranging from fuel cells to nanotransistors [2–3]. A special attention [4–10] has also been devoted to the development of gas sensors, mainly for detection of O2 , NO2 , or NH3 . Gas adsorption on carbon nanotubes and nanotube bundles is indeed an important issue for both fundamental research and technical applications, thanks ∗

Corresponding author. Tel.: +39 06 72597372; fax: +39 06 2020519. E-mail address: [email protected] (A. Reale).

0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.033

to the high specific surface area (1580 m2 /g2 ) [11], that allows a high capability of interaction between gas molecules and SWCNT. Experiments have shown that the nanotube sensors can detect ppm levels of gas molecules at room temperature, and this opens a possibility of developing nanotube operating at room temperature. Chemically induced perturbations on the resistance of nanotubes can give direct information easy to read-out, and the system can be interfaced with conventional electronic architectures. This may also provide the best chances of high integration for lab-on-a-chip applications. In particular, several research groups [7–10] have demonstrated that the electrical conductance of the semiconducting SWCNTs can change significantly upon exposure to O2 , NO2 , or NH3 gases. In the present research, purified SWCNTs are used as sensing material in an interdigitated electrode platform for NH3 detection. The sensor response is found

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Fig. 1. Experimental setup.

dependent on different parameters and conditions of operation: (a) the relative orientation of the nanotubes and their organization between the electrodes; (b) the temperature of the substrate; (c) the voltage applied to a back gate contact. Ordered networks of SWCNT bundles can be achieved using dielectrophoresis [12,13] to move and position nanotubes along preferential directions between patterned electrodes. The degree of nanotube aggregation is controlled using different solvents and treatments.

2. Sample preparation In this work, commercially available Carbolex SWCNTs (purity 50–70%) were used. The as-received samples were purified using HNO3 2M solution. Following this chemical treatment, a solid fraction (no-functionalized nanotubes) and a suspension of carbon nanotubes (functionalized nanomaterials) were obtained. The solid fraction was characterized by field emission scanning electron microscopy (FESEM) and Raman spectroscopy in order to check phase purity and structural integrity of the material. The use of infrared spectroscopy (FTIR) allowed to evidence that the nanotubes were not functionalised [14]. Finally a controlled amount of SWCNTs treated following the abovementioned protocols were dispersed in a CHCl3 solution and sonicated for 30 min before using. A controlled volume of this dispersion was deposited onto an interdigitated electrode platform (gold electrodes, with 40 ␮m spacing, evaporated on SiO2 layer grown on Si substrate). A set of SWCNT coated multifinger electrodes, with different degree of order, were produced by means of a dielectrophoretic process, where the electrokinetic motion of dielectrically polarized materials in non-uniform electric fields is induced by an alternate electrical field applied to the solution of SWCNT dispersed in a proper solvent during the deposition and evaporation processes. After many systematic studies of the more efficient conditions for alignment of SWCNT between our interdigitated electrodes with 40 ␮m of spacing [15], we obtained optimal results with an AC field having a frequency of 1 MHz and 10 Vpp .

A back gate contact was prepared on the substrate, to improve the control of the interaction of gas molecules with the SWCNTs. We prepared a specific set up (see Fig. 1) in order to investigate the dependency of sensitivity on different parameters: (a) the degree of order (alignment); (b) temperature; (c) gate voltage applied. Both ordered systems obtained by dielectrophoretic process and randomly placed SWCNTs were used for the detection of NH3 . Under the influence of an inhomogeneous electric field the nanotubes in suspension are found to align and move, with a motion depending upon the relative dielectric constant of nanotubes and solvent [16,17]. Single nanotubes and bundles moving along a direction parallel to the field stick each others, and providing therefore more reliable contacts between the gold fingers. The deposition procedures with and without applied field were repeated several times in order to check the reproducibility. A FESEM was used to check the effective difference in the morphological structure obtained following the two different procedures. Fig. 2 shows a FESEM image of the SWCNT bundles ordered

Fig. 2. FESEM image of the SWCNT ordered by mean of dielectrophoresis. The SWCNT bundles follow the electric field lines between the two electrodes.

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resistance changes xi  R  xi = R0 c=ci

Fig. 3. FESEM image of the random placement of SWCNT without electric field applied between interdigitated electrodes.

and aligned along direction of the applied electric field. A completely different feature is obtained when SWCNT are deposited between electrodes without electric field applied. Fig. 3 shows that only a random placement of SWCNT bundles can be obtained in this case.

3. Experimental results: detection of NH3 For the gas sensing an aqueous solution with concentration 1:10 of NH3 (30% vol.) was put in an bubbler in parallel with a flux of N2 . Five different concentration were obtained with five different N2 fluxes in the bubbler, exactly: 2/200, 4/200, 6/200, 8/200 and 10/200 Sccm. The resistance change was monitored (until the saturation condition was reached) varying the NH3 concentration. Fig. 4. shows the resistance increase in our SWCNT based sensor for three increasing concentrations of NH3 , ranging from 150 to 750 ppm. The sensitivity of the sensor is calculated as follows: we first determine the adimensional relative

Fig. 4. Resistance change in the SWCNT based sensor for three increasing concentrations of NH3 .

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(1)

for each ammonia concentration ci , with respect to the equilibrium value R0 . We then plot xi , as a function of the concentration of NH3 . The slope of the linear fit of xi (ci ) express the sensitivity of the sensor in (R/R)/ppm. The presence of NH3 (electron-donor) influences the SWCNT conductivity, and its reduction reveals their p-type behaviour [18–21]. Theoretical calculations based on the density functional method [20] show indeed that physical explanation of this process can be given in terms of a mechanism of charge transfer from the NH3 molecules to the SWCNT. It is important to note that the capability of SWCNT to interact with the environment depends on many physical and chemical parameters, concerning either intrinsic properties of the SWCNT (like metallic or semiconductor behaviour, morphology and alignment, functionalisation, etc.), either external conditions (such as temperature, pressure, etc.). We have investigated the effect of some of these properties, that we believed where of more stringent interest. In particular we analyzed the effect of the morphological order of the SWCNT between the electrodes through dielectrophoretical alignment via ac electric field, the effect of temperature through the use of a controlled heater beneath the substrate, and the effect of polarization of the substrate of the SWCNT through biasing the back gate contact. Fig. 5 shows the different behaviour at room temperature, with no gate voltage applied, of the device with aligned and disordered SWCNT. The sensitivity of multifinger with aligned SWCNT is double with respect to disordered SWCNT (see Fig. 6). This effect is probably induced by the fact that the ordered SWCNTs are more uniformly exposed to the interaction with NH3 molecules, than the case of placement of SWCNTs in form of a random network, where part of the SWCNT remain inaccessible to the gas molecules. The morphological analysis shown in Figs. 2 and 3 reveals

Fig. 5. Relative resistance change of the aligned (diamonds) and disordered (triangles) SWCNT, as a function of NH3 concentration. Solid lines represent linear fits.

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Fig. 6. Sensitivity of aligned (right) and of disordered (left) SWCNT.

indeed the striking structural difference of the two cases, that might be related to the increased sensitivity of the aligned nanotubes. We investigate also the resistance change varying the temperature (23 and 80 ◦ C). The different temperatures were obtained mounting the multifinger on a controlled heather. Fig. 7 shows the better behaviour of the device at the higher temperature for five different NH3 concentration. In Fig. 8 it is possible to see the increased sensitivity for the heated device. To investigate the reasons of such behaviour of the sensor, we have also observed how the conductance of the unexposed SWCNT modifies with temperature. We have found that resistance reduces with temperature, as found by other authors [21]. These observations suggests that the increased sensitivity of the sensor at higher temperature is due to a different scaling with temperature of R0 with respect of Ri . In other words, the interaction of NH3 with the nanotubes is less dependent on temperature than the conductance of the bulk SWCNT device, in the temperature range we considered. We studied also the resistance change obtained by varying the voltage applied between the back gate contact and one of the electrodes. We checked three different value: −20 V;

Fig. 7. Relative resistance change of the room temperature (diamonds) and heated (triangles) SWCNT, as a function of NH3 concentration. Solid lines represent linear fits.

Fig. 8. Sensitivity of heated (right) and of room temperature (left) SWCNT.

Fig. 9. Relative resistance change induced in the SWCNTs when the back gate voltage is equal to −20 V (squares), 0 V (triangles), +20 V (circles), as a function of NH3 concentration. Solid lines represent linear fits.

0 V; + 20 V. Fig. 9 shows the relative resistance change induced in the SWCNTs when the back gate voltage is equal to −20 V (squares), 0 V (triangles), +20 V (circles), as a function of NH3 concentration. Solid lines represent linear fits. In Fig. 10 it is shown the sensitivity obtained by varying the

Fig. 10. Sensitivity of SWCNT as a function of the back gate voltage: −20 V (left), 0 V (centre), +20 V (right).

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voltage applied, and it is visible the different behaviour of the sensor with positive and negative value of the back gate voltage. The increased sensitivity obtained in the case of negative voltage applied to the back gate contact might be justified in our opinion by the fact that the adsorption of NH3 (electrondonor) is assisted by the electrostatic interaction controlled by the gate potential [19]. Moreover, some authors [22,23] have proven that conductance of random networks of SWCNT can indeed be modulated by a back gate potential, so that a p-type behaviour of the SWCNT is revealed. The effect of the negative voltage applied to the gate is then twofold, since acts on the bulk resistance Ro of the SWCNT, and on the resistance change Ri .

4. Conclusions A new strategy to assemble gas sensor based on aligned SWCNTs has been undertaken. We demonstrated that the sensitivity of our SWCNT sensor device with respect to NH3 can be controlled and optimized using aligned SWCNTs, applying negative voltage to the back gate contact, and heating the sample. Aiming to extend the research to the detection of other gases species (especially NOX ) we are currently investigating the different chemical and physical parameters that can optimise gas sensing response, with a special attention to the optimization of the time response of the sensor. Concerning these issues we are testing various treatments able to functionalise SWCNTs in order to improve chemical selectivity, checking the temperature range, voltage and morphological layout of the SWCNT that gives the better performances.

Acknowledgement This work has been performed in the frame of the project MIUR, FIRB RBNE019TMF.

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Biographies M. Lucci is PhD student in “Sensorial Systems” at Tor Vergata University, Rome (Italy). He works in the “Minas Laboratory” and “Thin Film Deposition Laboratory”. Presently his research activity is mainly focused on “Carbon Nanotube for Gas Sensing”, “Plastic Solar Cells Application” and “Deposition and Electronic Spectroscopy of Superconductive Materials”. M. Lucci is author of some papers and of 1 patent and of 2 patents pending. Pietro Regoliosi has taken is degree in Physics in 2002 at University of Rome “La Sapienza”. Since then he is pursuing his PhD work in the Electronic Engineering Department of University of Rome, Tor Vergata, where he studies the sensing application of carbon nanotubes and their composites. He is also involved in photoconductivity measurements on semiconductor devices (principally GaAs and GaN based ones) to measure their thermal behaviour. Dr. A. Reale received the PhD in Microelectronics and Telecommunications in 2001. His main topic of interest include the experimental study of carbon nanotubes for their sensoristic applications for strain and pressure sensors, as well as for gas sensing. His interests also include the theoretical and experimental analysis of the optical, electro-optical and electrical properties of heterostructure devices based on nitrides, and the study, design, characterization of the linear and non-linear optical properties of active and passive devices for telecommunications. Dr. Reale has 24 publications on international journals, and 2 patent pending. Aldo Di Carlo received the physics degree from the University of Rome, Rome, Italy, in 1991, and the PhD degree from the Walter Schottky Institute of the Technical University, Munich, Germany, in 1995. He is currently an associate professor in the Department of Electronic Engineering, University of Rome “Tor Vergata,” Rome, Italy, where his research interests include the theoretical study of optical and transport processes in semiconductor nanostructures, devices, and organic materials. Silvia Orlanducci was born on 5 February 1974 in Rome. Education: PhD in Chemistry in 2003 at “Tor Vergata” University of Rome. Title of thesis “Synthesis and Characterisation of Nanostructured Carbon Mate-

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rials” under the supervision of Prof. M.L. Terranova. In 1999 Degree in Chemistry. Research activity: synthesis of diamond films for photoemission. Synthesis and characterisation of single wall carbon nanotubes, post-synthesis treatments of CNTs: purification, functionalization, polymer composite preparation, self-assembled systems. CNTs based sensors device assembling. She is co-author of 26 papers in international journals with referee, 12 conference proceedings and more than 70 conference presentations. Emanuela Tamburri studied Chemistry at the “La Sapienza” University of Rome (Italy) completing her MSc in 2003 on the area of specialization: Synthesis and Characterization of Materials, Inorganic Chemistry. In the years 2003–2004 she joined scientific activities in the Department of Chemical Sciences and Technology at the “Tor Vergata” University of Rome (Italy) where in 2004 she was awarded a research scholarship to undertake a PhD in Chemistry. Her research interests are in: synthesis and characterization of electrically conducting and electroactive ␲-conjugated polymers; synthesis and characterisation of polycristalline diamond and single wall carbon nanotubes; post-synthesis treatments of carbon nanotubes: purification, functionalization, polymer composite preparation, self-assembled systems; assembling of carbon nanotubes based sensor devices. Prof. Maria Letizia Terranova is professor of “Nanostructured Materials”, “Lab of Solid State Chemistry” and “General Chemistry for Physics” at Tor Vergata University, Rome (Italy). Chief of the “Laboratory of Film Deposition” and Coordinator of “Interdisciplinary Micro and Nanostructured System Lab” (MINASlab). Experience in coating technologies and material science. Presently her research activity is mainly focused on synthesis, post-synthesis treatments, chemical–physical processing and functional characterizations of carbon-based materials and nanostructures for applications in electronics, optoelectronics and sensing. Coordinator of

national projects on production and applications of nanomaterials. Author of 160 papers and of 3 patents, editor of 2 books. Paolo Lugli was born in Carpi, Italy, in 1956. He received the Laurea degree in physics from the University of Modena, Modena, Italy, in 1979, and the PhD degree in electrical engineering from Colorado State University, Fort Collins, in 1985. He is currently a full professor at the Lehrstuhl f¨ur Nanoelektronik, TU M¨unchen, M¨unchen, Germany, and was formerly full professor of optoelectronics at the University of Rome “Tor Vergata,” Rome, Italy. His current research interests include the theoretical study and numerical simulation of semiconductor nanostructures and devices. Corrado Di Natale is an associate professor of electronics at the Faculty of Engineering of the University of Rome “Tor Vergata”. His main research interests are in the fields of chemical sensors for taste and olfaction sensor systems, molecular electronics, and multicomponent analysis. He authored more than 300 papers on peer reviewed journals and international conferences. Arnaldo D’Amico is full professor of electronics at the Faculty of Engineering of the University of Rome “Tor Vergata”. His main research interests are in the fields of chemical sensors for taste and olfaction sensor systems, micro and nano-systems, and low voltage analog electronics. He authored more than 450 papers on peer reviewed journals and international conferences. Since 1999 he serves as chairman of the Eurosensors Conferences steering committee. Roberto Paolesse is an associate professor of inorganic chemistry at Faculty of Engineering of the University of Rome Tor Vergata. His main research activity is concerned with the design and the synthesis of pyrrolic macrocycles and their characterization and application as chemical sensors. He authored more than 200 papers on peer reviewed journals and international conferences.