Ultrafast sensors for volatile organic compounds

Nanocomposites containing metallic filler in a dielectric matrix exhibit very interesting electronic properties. For instance, the conductivity varies from insulating to metallic as function of the metal concentration and drops over many orders of magnitude near the percolation threshold. Below but near the percolation threshold, charge transport is governed by thermally activated hopping from nanoparticle to nanoparticle. In this regime near the metal-insulator transition, the conductivity depends exponentially on the cluster separation. This can be used for the detection of volatile organic compounds.

The principle is based on the swelling properties of polymer films by the incorporation of organic vapors which increases the separation of the metal nanoparticles. Since the electron tunneling probability depends exponentially on the width of the tunneling barrier and also on the barrier properties, this gives rise to a change in the resistivity.

In our sensors, 2D nanocomposites with only a few nm thickness are used which are fabricated by embedding of metallic nanoparticles into polymer films. Thus the diffusion length of the organic vapor is extremely short, and the sensors exhibit ultrafast response.

The sensors were prepared by taking advantage of the high process control in the deposition of metal clusters via thermal evaporation or sputtering on top of a thin polymer film. In situ resistivity measurements ensure control of the coverage at a reproducible value close to the percolation threshold. These sensors show a reversible signal, which is sensitive to the kind of organic vapor present in the surrounding as well as to the amount (vapor pressure) of the vapors. The figure shows an example for different organic vapors. One notes that the response to the vapors is different for every organic compound. This is based on the fact that different organic vapors have different solubilities in a given polymer. Hence, by choosing the suitable polymer matrix, one can tune the sensor to exhibit maximum sensitivity towards a specific vapor. In addition, by combining several different polymer matrices in a sensor array, one can build a selective sensing device taking the response ratios of the different sensors as a fingerprint for a particular vapor.

Response of a Au-Nylon nanocomposite sensor to different organic vapors. The different resistance changes are related to the different solubility parameters of the vapors. This can be explored for selective sensor arrays. Ultrafast response is obtained due to the extremely short diffusion length in the 2D nanocomposites as indicated in the sketch on the left.

Selected publications

Hanisch, C.; Ni, N.; Kulkarni, A.; Zaporojtchenko, V.; Strunskus, T.; Faupel, F.: Fast electrical response to volatile organic compouns of 2D Au nanoparticle layers embedded into polymers, Journal of Material Science 46 (2010) 438.

Faupel, F.; Zaporojtchenko, V.; Strunskus, T.; Elbahri, M.: Metal-Polymer Nanocomposites for Functional Applications, Advanced Engineering Materials Vol. 12, Issue 12 (2010) 1177-1190.

Hanisch, C.; Kulkarni, A.; Zaporojtchenko, V.; Faupel, F.: Polymer-metal nanocomposites with 2-dimensional Au nanoparticle arrays for sensoric applications, Journal of Physics: Conference Series 100 (2008) 52043.