UCLA

We prepared titanyl phthalocyanine (TiOPc) monolayers on sapphire-prism supported Au{111} substrates through solution deposition and characterized their photoresponses using a custom-built laser-assisted scanning tunneling microscope under ambient conditions. Two types of lattice structures (hexagonal and rectangular) were observed, and the distributions of photoinduced charges were measured under evanescently introduced 633 and 780 nm laser illumination. The distributions of photoexcited electrons in molecules in hexagonal lattices match theoretically calculated charge density changes in TiOPc molecules upon excitation. However, the photoresponses of TiOPc molecules in rectangular lattices are different than those predicted and TiOPc molecules in these arrangements may have lower excitation probabilities at 633 and 780 nm. Our results suggest that the photocarrier generation efficiency of TiOPc molecules is related to their packing arrangements in monolayers and local environments. The growth mechanisms of the monolayers and their connections to different polymorphs of TiOPc are worth to be further explored so that the photogenerated electronic property changes that we observed at the molecular level can provide more accurate guidance to the application of TiOPc crystals and assemblies.