Ultrathin Films of Perylenedianhydride and Perylenebis(dicarboximide) Dyes on (001) Alkali Halide Surfaces

Ultrathin films of two perylene dyes, 3,4,9,10-perylenetetracarboxylicdianhydride (PTCDA) and N,N′-di-n-butylperylene-3,4,9,10-bis(dicarboximide) (C4-PTCDI), have been grown on the (001) faces of freshly cleaved single crystal, NaCl, KCl, and KBr. Tapping mode AFM studies of these materials show that their crystalline motifs vary widely depending upon the substrate and growth conditions and that a form of layered growth is observable in the first few monolayers of deposition for both dyes. Nucleation of these crystalline deposits at edge sites on these substrates appears to be central to the growth of the first monolayers of these materials. Luminescence spectra for both PTCDA and C4-PTCDI, captured in situ during the growth of the first 1-4 monolayers of material, show the presence of a monomer-like entity during the initial growth stage. For PTCDA thin films this luminescence signal decays rapidly as nucleation of the crystalline film occurs. As film coverage is increased, luminescence of ultrathin films of both dyes is dominated by emission from one or more excimeric states. For C4-PTCDI/KC1 (001) the monomer-like emission does not completely disappear during the first monolayers of film growth, suggesting a degree of disorder sustained on a distance scale of a few molecular units at the grain boundaries and edges of the crystalline regions. The relative ratio of monomer/excimer emission of C4-PTCDI on KCl (001) is strongly temperature dependent and changes reversibly as the temperature is cycled between room temperature and 100°C. The luminescence intensity for thin films of both dyes on all of the substrates investigated is enhanced in the presence of atmospheric oxygen, consistent with previously reported declines in the dark conductivity of these materials as they are taken from UHV to atmospheric environments.