Among the various types of chemical gas sensors, resistive gas sensors are attractive because of their high sensitivity, stability, low cost and fast response. In particular, metal oxide (MOx) semiconductors, such as SnO₂, ZnO and TiO₂, have been widely studied as active layer. However, performances of sensors and their selectivity are strongly dependent of the morphology, structuration and nature of the active material. In particular, due to the strong correlation between grain size and sensor response, nanostructured and/or heterostructured materials permit an enhancement of the gas sensing response. Such structuration requires versatile and well-controlled elaboration approach. Atomic layer deposition (ALD) thus appears as a technique of choice due to its simplicity, reproducibility, the atomic scale precision of the deposited thickness and high homogeneity of the obtained films.

Herein, it will be shown that ALD has proven to be well-suited for the elaboration of compact thin films, nanostructures and heterostructures to be applied for the detection of a variety of analytes.[1] We aim at summarizing the most significant progresses related into the literature to gas sensors based on ALD thin films, nanostructures and heterostructures in order to highlight the peculiarity of atomic layer deposition for the fabrication of the sensing layer in resistive sensors. Particular attention will be given to heterostructures based on carbon nanotubes (CNTs) coated with ALD metal oxides investigated as sensing layers. We will also shortly discuss the nanostructure properties in parallel to the sensing mechanisms in order to try to develop clear structure–property correlations.

Indeed, with their high surface area, good thermal and electric conductivity and mechanical as well as chemical stability, CNTs provide ideal properties as a support for a second material that can be deposited onto their surface either as particles or as a thin film. Due to the small dimensions, interactions between the deposited material and the tubes at the interface can significantly alter the properties of the composite. This is specifically the case for semiconducting materials when the dimensions are in the range of the Debye length. Using a non-aqueous sol-gel ALD approach and controlled surface functionalization, metal oxides such as V₂O₄, TiO₂ and SnO₂ have been precisely deposited as film or particles onto CNTs. On the one hand, combining semiconductor oxides with a conductive support permits to reduce the overall resistance of the sensitive layer. On the other hand, due to the formation of a p-n heterojunction between the p-type conductive support and the n-type thin film, an enhancement of the gas-sensing response is observed depending on the oxide morphology. In particular, V₂O₄-, TiO₂-, ZnO- and SnO₂-ALD coated nanotubes have been tested as active component in gas-sensing devices.[2-4]

[1] C. Marichy, N. Pinna

[2] C. Marichy, N. Donato, M.-G. Willinger, M. Latino, D. Karpinsky, S.-H. Yu, G. Neri, N. Pinna, Advanced Functional Materials 2011, 21, 658-666.

[3] M. G. Willinger, G. Neri, A. Bonavita, G. Micali, E. Rauwel, T. Herntrich, N. Pinna, Physical Chemistry Chemical Physics 2009, 11, 3615-3622.

[4] C. Marichy, N. Donato, M. Latino, M. G. Willinger, J. P. Tessonnier, G. Neri, N. Pinna, Nanotechnology 2015, 26, 024004.