One-dimensional Photonic Crystals Based on Functional Ordered Mesoporous Films

M. CECILIA FUERTES; PAULA C. ANGELOMÉ; FRANCISCO J. LÓPEZ-ALCARAZ; VITTORIO LUCA; HERNÁN MÍGUEZ; GALO J. A. A. SOLER-ILLIA

Congreso; 2007 Material Research Society Spring Meeting; 2007

The possibility of obtaining optical quality materials presenting well defined and tuned pore arrays with controlled functional surfaces and precise spatial location in several length scales has opened the path towards “intelligent surfaces”. A number of strategies have been developed to create mesoporous films (MPF) with a variety of oxide frameworks, compositions, organic functions, etc.(1) The field is mature enough to reproducibly create monolayer MPF with well defined desired features in the mesoscopic or molecular scales. MPF represent thus “functional building blocks” for the construction of complex architectures, which can give access to new properties arising from order in space at different length scales. An interesting example is to combine order at the mesoscale and at the submicronic scale, which can lead to new optical responsive materials. We demonstrate that ordered mesoporous thin films can be used as building blocks to construct one-dimensional photonic crystals, also known as optical Bragg reflectors, of high optical quality and with a selective optical response to the environment.(2) These structures are made by alternate deposition of mesoporous slabs of two different compositions and combine properties due to the periodic structuring at different length scales. The highly controlled porosity stemming from periodicity in the mesoscale permits to tailor the interaction with small molecules that can adsorb or even condense in the surface or pore system of the inorganic matrix, leading to a sensitive and reversible change in the optical Bragg diffraction position. Optical quality and the effect of functionalisation are assessed by optical transmission and reflection measurements in the visible range of the ordered mesoporous Bragg reflectors exposed to different treatments and environments. The optical response of these coloured mesoporous multilayers in the presence of molecules that adsorb on the pore surface can be tuned through the selective introduction of functional groups in just one type of the building blocks of these structures. Furthermore, the introduction of MPF with adequate thickness/refraction index in particular positions in the stack leads to controlled defects and the appearance of allowed states in the gap. We demonstrate that combination of the controlled deposition of templated films and selective surface functionalisation results in complex hierarchical materials with photonic crystal properties, which are potentially useful for optical sensing or smart windows with environment-modulable properties.