A low cost fabrication method for fast response gas sensor based on DWCNTs resistors

Abstract

As the demand is increasing for the detection of specific gases and vapours for public safety, environment and medicine, there is a large research effort to develop cheap and easy-to-use gas sensors. Meanwhile, carbon nanotubes (CNTs), because of their outstanding physical and chemical properties, have also been investigated for such applications. Most of the research involving CNTs for gas sensing is based on the use of field-effect transistors where semi-conductor CNTs are integrated on silicon wafers. However, the synthesis processes of CNTs do not permit to achieve uniform samples of semiconducting CNTs exhibiting all the same electronic properties (bandgap). This can be obtained by sorting approaches which remain costly, even more than the realisation of the FET-based sensors. In our research, we developed a very practical and low-cost method to integrate double walled carbon nanotubes (DWCNTs) in a simple resistor configuration due to the high proportion of metallic nanotubes in the as-synthesized powder. With this strategy, no sorting is required and the detection does not rely on single CNTs having to be connected individually, but on much larger assemblies, making the system both much easier to produce and robust. The DWCNTs used in our research are synthesized on the gram-scale by catalytic chemical vapour deposition (CCVD) [1]. Stable and low-concentration DWCNTs suspensions (<0.1wt%) are prepared with addition of carboxymethyl cellulose (CMC) as a dispersing agent. The fabrication process is described in figure 1. An array of Au/Ti microelectrodes is first produced by standard photolithography and lift-off procedure on oxidised silicon wafers. Air-plasma is used in our work to activate the surface of the microelectrodes array. A PDMS stencil is then aligned on the array of microelectrodes and, simply by using a pipette, all the cavities opened in the stencil are filled with DWCNTs suspensions. By heating at 60°C, the solvent is evaporated and the DWCNTs remain, deposited onto a gap section in the microelectrodes' array. This liquid stencil deposition process can be repeated several times without removing the stencil and, thereby, allows to obtain different conductivities depending on the quantities of deposited DWCNTs. Removal of the dispersing agent is performed by washing the device with deionised water for 15min, after peeling off the PDMS stencil. This rinsing process removes some loosely attached DWCNTs, and also turns out to improves the conductivity of the CNTs' film, as attested by the decrease of the resistance of the devices after rinsing. The main reason for this improvement of conduction might be the rearrangement of the CNTs due to the capillary forces exerted at the film surface when the liquid water meniscus passes over the device, entailing a better connexion with the electrodes. On each chip (1cm×2cm) several devices can be integrated and due to the very simple pipetting based deposition, each device can be equipped with a different CNT suspension harbouring different surface chemical functionalizations. This low cost fabrication process, despite its low-cost and simplicity is therefore fully compatible with the multiplexing required in gas sensing for being able to discriminate specifically different gas species. This is a key element of our strategy, which consists in analysing simultaneously a given vapour with different sets of electrodes working in parallel, each electrode exhibiting a different behaviour towards the compound of interest because of the different functionalisation of the DWCNTs (f-DWCNTs). Figure 2 shows a typical DWCNTs-based gas sensor device from our research, integrating seven sensing elements connecting either different quantities of CNTs or different chemical functionalizations.