J aggregates of a pseudoisocyanine dye (PIC) at a mica/solution interface were in situ observed in solution for the first time by atomic force microscopy (AFM), confirming formation of the aggregates showing a narrow J absorption band (peak, ∼580 nm; line width (HWHM), ∼120-125 cm -1 ) at room temperature. The J aggregates possessed the three-dimensional island structure with the height of about 3-6 nm, consisting of at least ∼10 6 PIC molecules, but not a two-dimensional monolayer structure. The growth processes of the J aggregates were discussed on the basis of the morphological changes of the islands with a dye concentration (i.e., Volmer-Weber type growth process), while the optical properties such as the absorption peak position, line width, or fluorescence lifetime (∼50 ps) were unchanged. These results indicated that the physical aggregation size observed was much larger than the coherence size of the J aggregate.

We investigated the J-aggregation process of pseudoisocyanine chloride at a mica/solution interface by using tapping mode atomic force microscopy (TM-AFM) and polarization absorption measurements. At a mica/solution interface, pseudoisocyanine chlorides form J aggregates that have three-dimensional leaf-like island structures. The islands are anisotropically oriented with respect to the lattice of a mica substrate. This anisotropic alignment may result from the epitaxial interaction between the positively charged N atoms of the molecules and the regularly aligned negative holes left by the dissociating K+ ions. On the basis of both the orientation of islands to the lattice of a mica and the epitaxial interaction, we proposed that in the internal structure of the J aggregate, the long axis of the dye molecules is parallel to the long axis of the islands.

By using tapping-mode atomic force microscopy (AFM), we revealed for the first time the structure of J aggregates of pseudoisocyanine chloride(PIC-Cl) dyes at a mica/solution interface in an aqueous PIC-Cl solution. The J aggregates were leaf-like islands in multilayers of PIC-Cl molecules. The minimal size of the aggregate necessary for the J band was also revealed. Dye molecules grow into the J aggregates at a mica/solution interface by a Volmer-Weber type growth process.

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