Controlled Layering of Two-Dimensional J-Aggregate of Anionic Cyanine Dye on Self-Assembled Cysteamine Monolayer on Au(111)

Abstract

An anionic cyanine dye with disulfopropyl substituents allowed its two-dimensional (2D) J-aggregate of highest-ever quality to be successfully layered on top of the self-assembled monolayer (SAM) of cysteamine (CA) on atomically flat Au(111). The chemisorption of CA from millimolar ethanolic solutions was rapid, producing an estimated density of surface amino groups of ∼4 × 1014 cm-2 well within a minute. However, sufficient ordering of the CA SAM, which we found by scanning tunneling microscopy imaging to yield a characteristic 7 × √3 superlattice structure, took a considerably long extra immersion time of a few hours at room temperature. The highest-quality 2D J-aggregate with an extraordinary sharp J-band formed preferentially on densest but still not fully ordered CA SAMs, from typically 0.1 mM dye solutions in 1:1 mixed water and ethanol solvent at considerably high adsorption temperatures of 50−65 °C. A detailed analysis of the 2D J-aggregate and its molecular linkage with the CA SAM indicated that 1.3 × 1014 cm-2 dye molecules are individually anchored to the surface amino groups in the form of ammonium sulfonates. The resultant energy barrier for the 2D J-aggregation can still be overcome at the given high temperature of dye adsorption in the case of nonrigid (only partially ordered) CA SAMs because of the strong cohesive interactions between dye molecules possibly exceeding ∼50 kJ/mol. Other important questions such as the electric charge balance in the layered assembly and the overall chemistry of 2D J-aggregation are also addressed.