Influence of water on supra-molecular assembly of 4, 4'-dihydroxy azobenzene on Ag(111)

We explore co-deposition of water and 4, 4'-dihydroxy azobenzene on Ag(111) by low-temperature scanning tunneling microscopy at different water-to-azobenzene ratios. At all ratios, the water interacts with the hydroxyl end groups of the molecule replacing the direct hydrogen bonding. The change in bonding reduces the azobenzene density as compared to the one in the closed-packed waterless azobenzene structure. At intermediate water-to-azobenzene ratios, pores are formed in the azobenzene layer at nanometer distance from the water. At high water-to-azobenzene ratios, a water superstructure with a 1.4 nm × 1.4 nm unit cell develops. Our results point to a method to vary the density of an organic layer by tuning the amount of an inorganic additive.