Dye aggregates and organic superlattices formed by organic-inorganic molecular beam epitaxy

We describe the formation of ordered ultrathin films of chloroindium phthalocyanine (InPcCl) on thin films of the layered semiconductor SnS₂, both of which are deposited by a molecular beam epitaxy process to form InPcCl/ SnS₂/mica assemblies. The resultant thin films show markedly narrowed linear absorbance spectra versus those seen previously for Pc thin films. Low energy electron diffraction and reflection high energy electron diffraction data collected for the first few monolayers of InPcCl deposited on the SnS₂ substrate suggest that three (equally populated) ordered Pc domains are formed, with the Pc rings parallel to the SnS₂ surface, aligned along the principle axes of this hcp SnS₂ substrate. The red-shift and peak shape of the absorbance spectra of these thin films (about 758 nm, full width at half maximum of about 60 nm) are consistent with the type of splitting of the exciton energies expected of Pc aggregates in which the adjacent Pc rings are cofacial, but staggered in both x and y directions by about half a molecular diameter. This is the structure determined from related trivalent metal Pc's by single crystal X-ray diffraction. Loss of the flat-lying configuration of Pc in these ultrathin films is immediately apparent from the surface electron diffraction data.