Atomic Force and Scanning Near-Field Optical Microscopy Study of Carbocyanine Dye J-aggregates

Atomic force microscopy (AFM) and scanning near-field optical microscopy (SNOM) imaging of fibrillar J-aggregates of carbocyanine dye 3,3’-bis(2-sulfopropyl)-5,5’,6,6’-tetrachloro-1,1’- dioctylbenzimidacarbocyanine (C8S3) have been conducted on mica and glass substrates at a room temperature. High resolution AFM imaging reveals that thick fibrils consist of narrow subfibrils with the cross diameter in the nanometer range. A model of the "elementary fibril" with the cross-dimension of 3x4 nm2 was proposed as the aggregate of two J-ladders oriented face-to-face by their hydrophobic sides while the dye molecules are oriented normaly to the long fibril axis. The SNOM measurements have been conducted in the contact mode indicating the large mechanical strength on the fibrillar J-aggregate morphology