Exploitation of ALD potentiality for the realization of long-range ordered structures

Atomic Layer Deposition (ALD) is a powerful and unique technique which allows the deposition of high quality thin films with atomic level control of composition and thickness, and with a high conformal coverage even on complex shaped surfaces. It can also work at low temperature (LT-ALD), so allowing the employment of temperature-sensitive substrates [1]. The capability to realize periodic metal oxides 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 this regard, ALD is a key technology for the surface modification of complex nanostructured materials [2]. In fact, although structuring micro- and nano-materials may also be possible with conventional templating and lithographic techniques, an alternative and strategic approach is based on the self-assembly of polymeric materials (to set the template patterns up) combined with the ALD process (to conformally cover the polymeric templates). The aim of this work is to investigate the potentiality of ALD process by focusing the attention on the optimization of hybrid organic-inorganic methodologies to prepare two different long-range ordered spatial organizations. In particular, 2D-microporous TiO2 with honey-comb (HC) morphology and 3D-anatase TiO2 hollow nanospheres with Direct Opal (DO) arrangement are here presented. The hybrid methodology first employs the preparation of polystyrene templates with a specific pattern, which are later used for the subsequent deposition of TiO2 via ALD at low temperature. Finally, high temperature post-deposition thermal treatments on PS/TiO2 composite materials are carried on in order to decompose and extract the PS templates and in the meantime to crystallize TiO2 in anatase phase, maintaining the HC and DO structures. Samples were characterized by SEM, XRD and UV-Vis analyses. As demonstrated by SEM analysis, the ALD growth produces films with optimal infiltration features and conformal coverage thus preserving the original structure of the starting templates.