Atomic/Molecular Layer Deposition of Photoresponsive Azobenzene-Containing Thin Films

Owing to their rapid and reversible photoisomerization, azobenzenes are efficient molecular photoswitches that could potentially provide effective control over various properties of their host surroundings. This unique ability has opened up a wide array of potential applications spanning across diverse fields, from electronics and medicine to environmental monitoring and security. This thesis aims to unveil new possibilities for the application of the atomic/molecular layer deposition (ALD/MLD) thin-film technique to incorporate fully functional azobenzene moieties into an inorganic matrix. Several new ALD/MLD processes with azobenzene-4, 4'-dicarboxylic acid (AzoBDC) as the source of the azobenzene moiety were developed. Three types of novel thin-film materials were fabricated: amorphous zinc-azobenzene hybrids, zinc oxide:azobenzene superlattice (SL) structures, and metal-organic framework (MOF) type structures with azobenzene as the linker. In zinc-azobenzene hybrids (Zn-O-C14H9N2-O4-), diethyl zinc (DEZ) was used as the source of zinc in combination with AzoBDC. The fabrication route developed for the Zn-AzoBDC hybrid films was then combined with the DEZ/H2O ALD process for zinc oxide thin films to grow SL structures where single azobenzene layers are sandwiched between thin crystalline ZnO blocks. The ratio of the ALD-ZnO and MLD-(Zn-O-C14H9N2-O4-) cycles was varied between 199:1 and 1:1. The photoreactivity was confirmed for all the films upon 360 nm irradiation, and the kinetics of the resultant trans−cis photoisomerization was found to somewhat depend on the SL structure. Crystalline photoresponsive MOFs were prepared by using FeCl3, Ca(thd)2, Li(thd), Na(thd), or Sr(thd)2 as the metal precursor in combination with AzoBDC. Isomerization of azobenzene units within the strictly crystalline, highly porous environment of Fe-AzoBDC, Li-AzoBDC, and Ca-AzoBDC films was confirmed despite their different crystal structures. Furthermore, the gas absorption and release capability of the films was demonstrated. The ALD/MLD synthesis of Fe-AzoBDC MOFs was also modified by using in parallel with AzoBDC terephthalic acid (TPA) as another organic precursor to obtain structures with two types of organic linkers. These films were deposited by mixing the FeCl3 + AzoBDC and FeCl3 +TPA deposition cycles on a 1:1 frequency to deposit Fe-AzoBDC–Fe-TPA films. It was shown the dilution of the photoactive AzoBDC linkers with the nonactive TPA linkers increased the stability of the crystal structure.