STM imaging of cyanine dye J-aggregates formed on carboxyl-terminated self-assembled monolayers

We report on the preparation and STM imaging of J-aggregates of 1,1'-diethyl-2,2'-cyanine (PIC) dye formed on a self-assembled monolayer of 11-mercaptoundecanoic acid (HS-(CH2)(10)-COOH). The formation of J-aggregates on the COOH surface was verified by reflection absorption measurements; however, the molecular assemblies were not bound strongly enough to the SAM to withstand the lateral forces applied during STM imaging. We found that pretreating the COOH surface with a solution of 1 mM AgNO3 before application of the PIC dye solution greatly improved the stability of the J-aggregates. We suggest that the reason for this is the formation of a silver chelate with the carboxyl head groups of the SAM which permits the subsequent adsorption of bromide counterions from the dye solution. The formation of a counterion monolayer has been shown to be important in the formation of stable J-aggregates. STM images of these samples revealed long, essentially linear aggregates (with a packing structure similar to the proposed "staircase structure") extending over hundreds of nanometers with an interaggregate spacing of 30 Angstrom. We compare these structures with those formed by the same dye applied directly on silver (111). In addition to the reported brickwork packing structure of the dye monolayer which is in direct contact with the metal substrate(1) we also observed a previously unreported bilayer structure. The second layer of dye molecules comprises linear aggregates with a similar packing arrangement to those formed on the pretreated COOH SAM.