Local Optoelectronic Properties of Zinc-Porphyrin/Gold Molecular interfaces

This research consists in designing a series of experiments to determine the molecular orbital energy levels of zinc-porphyrin molecule when vertically attached to Au(111) substrate. To study the zinc-porphyrine molecular orbitals we use visible light of different wavelengths. Thiolated zinc-porphyrin oligomer molecules link to Au(111) surface, embedded within an 1-octanethiol self-assembled monolayer. Current-Voltage characterization technique allow us to determine the electronic orbital structures of different zinc-porphyrin oligomer single molecules via scanning tunneling microscope. Coupling lasers of different wavelengths and the tunneling junction, illumination effect on the molecular orbital energy levels of zinc-porphyrin molecules is investigated. The results indicate that the experimental zinc-porphyrin orbital energy levels are qualitatively consistent with previous calculations and experiments of similar porphyrin molecules. With illumination at given wavelengths, HOMO-LUMO gaps decreases for zinc-porphyrin molecules, and under dark condition the dimer zinc-porphyrin molecule shows a larger HOMO-LUMO gap than the monomer counterpart. We propose a charged molecule model to explain the light illumination effect, and we attribute the larger HOMO-LUMO gap in dimer molecule to a mixing of face-to-face bundling and tilting of the molecules.