Hydrogen and humidity sensing properties of C60 thin films

Hydrogen and humidity sensing properties of C60 thin films
G. Sberveglieri a,b, G. Faglia a, C. Perego a, P. Nelli al*, R.N. Marks ‘, T. Virgili ‘, C. Taliani ‘,
R. Zamboni ’
a Department of Chemistry and Physics for Materials, INFM, via Valotti 9, I-25133 Brescia, Italy
b Department of Physics, INFM, via Paradiso 12, I-44100 Ferrara, Italy
’ CNR-Institute of Molecular Spectroscopy, via P. Gobetti 101, I-40129 Bologna, Italy



Abstract

We discuss the sensing properties of C,a thin films towards reducing gases and humidity. C,a thin films were sublimed onto alumina substrates with platinum interdigitated contacts, and a platinum heating element was deposited on the substrate back face. The electrical responses of the C6a films towards 1000 ppm of hydrogen, ethyl alcohol and carbon monoxide were measured both in dry and humid air, in the temperature range between 50 and 300 “C. At 300 “C we observe a reversible increase of the current in the presence of hydrogen, while the water vapour causes a reversible decrease in the current. We suggest that the response to hydrogen may be due to a chemical reaction between the oxygen molecules trapped in the film and the hydrogen, reducing the concentration of oxygen-based electron traps in the CGO,
hence increasing the conductivity. The reduction in conductivity in the presence of humidity is probably due to the formation of extra trap states by water molecules in the film.

1. Introduction
In the past few years, fullerenes have attracted a great deal of attention due to their novel electronic properties. They are semiconducting molecular solids with an optical bandgap of
1.7 eV [ 11; when suitably doped (e.g. with alkali metals) they show metallic or super conducting properties. Cc0 can be sublimed in vacuum in the form of thin polycrystalline films, which have been used in devices such as field-effect transistors [ 2,3] and photodiodes [ 4-61.
CeO crystals are face-centred cubic ( f.c.c.) with large interstitial voids which allow the efficient diffusion of gases through them, and make this material an interesting candidate
for gas sensing applications. In this paper we describe the detection properties of C6,, thin films towards some reducing gases.