ULTRATHIN FILMS OF PERYLENEDIANHYDRIDE AND PERYLENEBIS(DICARBOXIMIDE)DYES ON (001)ALKALI-HALIDE SURFACES

Ultrathin films of two perylene dyes, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and N,N'-di-n-butylperylene-3,4,9,10-bis(dicarboxi mide (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 up on the substrate and growth conditions and that a form of layered grow th is observable in the first few monolayers of deposition for both dy es. Nucleation of these crystalline deposits at edge sites on these su bstrates appears to be central to the growth of the first monolayers o f these materials. Luminescence spectra for both PTCDA and C4-PTCDI, c aptured in situ during the growth of the first 1-4 monolayers of mater ial, show the presence of a monomer-like entity during the initial gro wth stage. For PTCDA thin films this luminescence signal decays rapidl y as nucleation of the crystalline film occurs. As film coverage is in creased, luminescence of ultrathin films of both dyes is dominated by emission from one or more excimeric states. For C4-PTCDI/KCl (001) the monomer-like emission does not completely disappear during the first monolayers of film growth, suggesting a degree of disorder sustained o n a distance scale of a few molecular units at the grain boundaries an d edges of the crystalline regions. The relative ratio of monomer/exci mer emission of C4-PTCDI on KCl (001) is strongly temperature dependen t and changes reversibly as the temperature is cycled between room tem perature and 100 degrees 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 de clines in the dark conductivity of these materials as they are taken f rom UHV to atmospheric environments.