Sensors and Actuators B 139 (2009) 425–428 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb A simple method to fabricate chemical sensors using horizontally aligned clean carbon nanotubes S.M. Jung, H.Y. Jung, J.S. Suh ∗ Nanomaterials Laboratory, Department of Chemistry, Seoul National University, Kwanak-ro 599, Kwanak-gu, Seoul 151-747, Republic of Korea article info Article history: Received 1 November 2008 Received in revised form 6 February 2009 Accepted 4 March 2009 Available online 20 March 2009 Keywords: Carbon nanotubes Gas sensors NH3 gas NO2 gas Electrophoresis abstract We have developed a simple and inexpensive method to fabricate carbon nanotube-based chemical sen- sors by fabricating a horizontally aligned carbon nanotube film on a titanium/gold-coated glass plate using electrophoresis and fissure formation techniques. The sensors exhibited low noise and thus high sensitivity to the NH 3 and NO 2 molecules and the response times were relatively short. They were com- pletely recovered within 40 s when a dc voltage of 5 V was applied for less than 30 s. The fabrication processes do not require any expensive equipment like e-beam lithographs. They are relatively simple and low-cost, and the sensors themselves are mass-producible. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Sensors for gaseous molecules play an important part in mon- itoring the environment, controlling chemical processes, and in medical and agricultural applications. Considerable effort has been made to develop new and better sensory materials. Various elec- trical sensory materials such as semiconducting metal oxides [1], silicon based systems [2,3], conducting polymers [4], and carbon black-polymer composites [5–7] have been used. More recently, it has been demonstrated that carbon nanotubes (CNTs) represent a new type of chemical sensory material capable of detecting small concentrations of molecules with high sensitivity under ambient conditions [8,9]. Gas sensors based on CNTs mostly rely on moni- toring the direct change in their electrical properties in response to the interaction with gaseous molecules [10–14]. When electron- withdrawing molecules (e.g., NO 2 and O 2 ) are adsorbed on the surface of p-type semiconductor CNTs, the Fermi levels are shifted to the valence band, generating more holes and thus enhancing the conductance. The adsorption of electron-donating molecules (e.g., NH 3 ) causes the number of holes to decrease and the resistance to increase. Therefore, making good contacts between electrodes and CNTs with clean surfaces is essential for fabricating a good sensor. Various methods for fabricating gas sensors based on CNTs have been developed [15–19]. These methods can be categorized into ∗ Corresponding author. Fax: +82 2 889 1568. E-mail address: jssuh@snu.ac.kr (J.S. Suh). two methods: one is to construct electrodes on CNTs and the other is to deposit CNTs on the surface of constructed electrodes. For the former, as a sequential process after constructing the elec- trodes on the CNTs, a thick and long passivation layer is usually coated on the contacts of the nanotubes/electrodes. It can pre- vent the contacts’ exposure to air in a controlled fashion, leaving only the midsection of the nanotubes exposed. For this reason, nearly perfect ohmic contact can be made between nanotubes and electrodes so that the sensor exhibits low noise and thus high sensitivity and performance. However, up to now, the lack of a simple and reliable process to deposit electrodes in a con- trolled way has been a critical drawback. In contrast, the latter is a much simpler process. However, when CNTs are deposited on the surface of the constructed electrodes, they are adhered physi- cally to the electrodes. Consequently, their adhesion is very weak. Also, the contacts of the nanotubes/electrodes are exposed to air, causing high noise and resultant low sensitivity. This means that both methods have a crucial drawback. It is therefore necessary to develop a better method to fabricate CNT-based gas sensors. This improved method should be relatively simple, be cost-effective to perform, and create good contacts between nanotubes and elec- trodes. In this report, we present a simple and low-cost method to fabricate chemical gas sensors based on CNTs by fabricating hor- izontally aligned CNTs on titanium/gold-coated glass plates using electrophoresis and fissure formation techniques. The sensors show low noise and high sensitivity. Furthermore, mass production is possible due to the simplicity and low cost of the procedure. 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.03.016