Reversible Photoswitching Function in Atomic/Molecular-Layer-Deposited ZnO:Azobenzene Superlattice Thin Films

We report new types of reversibly photoresponsive ZnO:azobenzene superlattice thin films fabricated through atomic/molecular-layer deposition (ALD/MLD) from diethylzinc, water, and 4,4′-azobenzene dicarboxylic acid precursors. In these ultrathin films, crystalline ZnO layers are interspersed with monomolecular photoactive azobenzene dicarboxylate layers. The thickness of the individual ZnO layers is precisely controlled by the number (m) of ALD cycles applied between two subsequent MLD cycles for the azobenzene layers; in our {[(Zn-O)_m+(Zn-O₂-C-C₆H₄-N=N-C₆H₄-C-O₂)]_n+(Zn-O)_m} samples, m ranges from 0 to 240. The photoresponsive behavior of the films is demonstrated with ultraviolet-visible spectroscopy; all the films are found to be photoreactive upon 360 nm irradiation, the kinetics of the resultant trans-cis photoisomerization somewhat depending on the superlattice structure. The reversibility of the photoisomerization reaction is then confirmed with a subsequent thermal treatment. Our work thus provides proof-of-concept evidence of the suitability of the ALD/MLD technology for the implementation of photoactive moieties such as azobenzene within an inorganic matrix as an attractive new methodology for creating novel light-switchable functional materials.