Full Paper Sensing with Nafion Coated Carbon Nanotube Field-Effect Transistors Alexander Star,* Tzong-Ru Han, Vikram Joshi, Joseph R. Stetter Nanomix Inc., 5980 Horton St., Suite 600, Emeryville CA 94608, USA *e-mail: astar@nano.com Received: June 30, 2003 Final version: August 15, 2003 Abstract Sequential CVD and CMOS processes were used to make a FET that has single walled carbon nanotubes to serve as the conducting source to drain channel. This structure can be decorated to provide gas and liquid responses and herein is evaluated as a humdity sensor. The Na  ,K  , and Ca 2 ion-exchanged Nafion polymer acts as the chemically sensitive layer in this electrochemical sensor. The effect of gate voltage on the charge-sensitive NT structure was found to be RH dependent over the range of 12 ± 93% RH with msec response time. Keywords: Single-walled carbon nanotubes field-effect transitors (NTFETs), Nafion membranes, Relative humidity sensor 1. Introduction Since they were first made in 1998 [1], field effect transistors made from semiconducting single-walled carbon nanotubes (NTFETs) have been found to be sensitive to various gases, such as oxygen [2], ammonia and nitrogen dioxide [3]. Such devices, together with devices based on nanowires [4], have been explored as chemical [5] and biological sensors [6 ± 8]. We have used the NTFETas the platform for the design of sensitive and selective chemical sensors. We have found that specific chemical functionalization of NTFET devices can make them responsive to a variety of gases, vapors, liquids, and ions. These individually functionalized NTFETs can be used to create large sensor arrays [5], a first step in making chemical and biological detection systems such as electronic noses and tongues [9 ± 11]. Nanotube/polymer systems have recently received much attention for solubilizing carbon nanotubes [12 ± 16]. Poly- mer coatings have also been shown to functionalize non- covalently bonded carbon nanotubes [17] and modify the characteristics of nanotube FET devices [18]. In particular, polyethylene-imine (PEI) was found to shift the FET device characteristics from p to n-type, presumably due to the electron-donating ability of amine groups in the polymer. Recently, we have exploited another attribute of a polymer coating: the creation of ionic charges on or in the vicinity of the device induced by poly-(sodium 4-styrenesulfonate) [19]. Because of their unique ion-exchange, discriminative, stability, chemical resistance, and biocompatibility proper- ties, Nafion films have been used extensively for the modification of electrode surfaces and for the construction of amperometric biosensors [15, 20, 21]. The humidity dependence of the conductivity of Nafion coated electrodes has been studied [22, 23] for different ion-exchanged materials. In this paper, Nafion membranes were construct- ed on the NTFET devices and their effect on the device characteristic is presented. The study of Nafion coated NTFET devices, including their humidity dependence at different membrane compositions is an important first step towards carbon nanotube based RH and biosensors. The commercial viability of Nafion as an ionic polymer mem- brane and the unique charge sensitivity of the robust NT structure offer an attractive opportunity to investigate a potentially highly reliable RH nanosensor. 2. Experimental 2.1. Materials Nafion 5% (w/w) solution was purchased from Aldrich and was used without additional purification. For electronic measurements, Nafion was diluted to 1wt.% in 1 mM aqueous solutions of various carbonates (Na 2 CO 3 ,K 2 CO 3 , and CaCO 3 ). Commercial Nafion is received in the hydrogen ion form with equivalent weight of 1,100. Ion-exchange of the hydrogen ions in the Nafion (  9 mM) with either Na  ,K  or Ca 2 was achieved by heating the Nafion solution with corresponding carbonate (1 mM) solution. 2.2. NTFET Fabrication Devices were prepared using lithography techniques on 100 mm wafers. Figure 1A shows a schematic diagram of the carbon nanotube devices. NTFET devices were fabricated 108 Electroanalysis 2004, 16, No. 1-2 ¹ 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200302925