Molecular arrangements in polymorphous monolayer structures of carbocyanine dye J-aggregates

► We conducted subangstrom accuracy AFM height measurements of J-aggregate monolayers. ► J-aggregate polymorphism is caused by a coexistence of two molecular arrangements. ► Ladder rather than brickwork molecular arrangement is observed in J-aggregates. ► Carbocyanine dye molecules are antiparallelly stacked in J-aggregate monolayers. ► J-aggregate monolayers are labile.