CVD techniques for surface functionalization: ALD potentiality for long-range periodic structures

The surface of a solid material and the regions closest to it represent the 'element of dialogue' with the surrounding environment, so conditioning the physical and chemical properties of the material itself. In this context, Material Science is fundamental to properly functionalize the material surface through a suitable bottom-up approach in order to induce a targeted behavior. Chemical Vapor Deposition (CVD) is an attractive technique for the synthesis of opportunely designed nanostructured thin films. The possibility to modify several physical and chemical growth parameters makes CVD a challenging process to tailor material composition, microstructure, and morphology, so influencing the final functional properties. In particular, Atomic Layer Deposition (ALD), a recent variant of CVD process, is a powerful and unique technique which allows the deposition of high quality thin films with atomic level control of composition and thickness. The capability to produce periodic metal oxide structures with both a long range order and a good control of dimension at micro- and nano-metric level is of great scientific interest. These surfaces, thanks to their high surface-to-volume ratio, may find wide application in processes based on surface reactions, such as heterogeneous catalysis, sensors and photovoltaics. In fact, although structuring micro- and nano-materials may also be obtained with conventional templating and lithographic techniques, an alternative and strategic approach is found exploiting the self-assembly of polymeric materials (to set the template patterns up) combined with the ALD process (to conformally cover the polymeric templates). Here, a case study is reported in order to show the ALD potentiality by focusing the attention on the optimization of hybrid organic-inorganic methodology to prepare 2D-microporous TiO2 with honey-comb (HC) morphology. The hybrid methodology first employs the preparation of a polystyrene template with a specific pattern, which is later used for the subsequent deposition of TiO2 via ALD at low temperature. Finally, high temperature post-deposition thermal treatments on PS/TiO2 composite material is carried on in order to decompose and extract the PS templates and in the meantime to crystallize TiO2 in anatase phase, maintaining the HC structure.