Structural and Optical Properties of Crystalline Organic Thin Films Grown by Organic Molecular Beam Deposition.

The optical properties of organic molecular crystals (OMCs) are of great interest from the perspective of pure research as well as due to their potential use in photonic applications. An OMC's optical properties are dominated by excitons. Recently, it has been shown that exciton properties can be modified by growth of ultrathin layers of an archetype organic compound: 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA). In this work we examine the structural and optical properties of ultrathin layers of several crystalline organic materials deposited by the ultrahigh vacuum process of organic molecular beam deposition. We examine the evolution of growth of PTCDA from monolayer to ~1000A thickness with reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). From RHEED measurements, we observe a crystalline, layer-by-layer growth of the organic material in a structure similar to its natural bulk configuration. In addition, STM images of a monolayer of PTCDA on graphite indicate that the crystal structure of a monolayer is different from the bulk structure. These observations are supported by theoretical calculations. We also study the optical spectra of two sets of multilayer stacks of ultrathin (~10A -500A) films of alternating crystalline organic materials: PTCDA with 3,4,7,8-naphthalenetetracarboxylic dianhydride (NTCDA), and 3,4,9,10-perylenetetracarboxylic bis-benzimidazole (PTCBI) with NTCDA. In PTCDA multilayer stacks, we observe an increase in the ground state vibrational frequency and a blue shift of the absorption and fluorescence spectra with decreasing layer thickness. These results are ascribed to confinement in ultrathin layers of the relatively large PTCDA exciton (~13A). In addition, reasonable agreement is found between these results and the solution of a Hamiltonian of an exciton-phonon coupled system. In contrast, no spectral shifts are observed in PTCBI multilayer stacks, which is attributed to the relatively small radius of the PTCBI exciton. Further analyses of the differences observed between these two sets of multilayer stacks are presented. (Copies available exclusively from Micrographics Department, Doheny Library, USC, Los Angeles, CA 90089-0192).