Exciton Confinement in Ultrathin Crystalline Organic Films Grown by Organic Molecular Beam Deposition
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ABSTRACT We use the low temperature fluorescence spectra of organic multiple quantum well samples consisting of the archetype materials, 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA); 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA), and 3,4,9,10 perylenetetracarboxylic-bis-benzimidazole (PTCBI), to study the effects of quantum confinement on the lowest energy excited and ground electronic states of these molecular crystals. Both the Franck-Condon and the dominant ground state vibronic mode energies are observed to undergo significant shifts with decreasing PTCDA layer thicknesses (ranging from 500A to 10A) in PTCDA/NTCDA MQWs, while no such effects are observed for PTCBI/NTCDA MQWs. These results are interpreted in the context of confinement of spatially extended excitons in ultra thin PTCDA layers, whereas the considerably smaller radius PTCBI excitons are not affected over the range of layer thicknesses investigated. These results unambiguously rule out previous hypotheses suggesting that binding of small radius excitons to interfaces results in the blue shifts previously observed in the absorption spectra of PTCDA-based MQWs. Introduction The physics of excitons in organic molecular crystals has been the subject of intense debate over the past 30 years. In particular, it has been suggested that excitons in such materials are either Frenkel or charge transfer (CT)-like[l]. Frenkel excitons are excited molecular states where the electron and hole are localized on a single molecule in the crystal. In contrast, C-T states are excited states with one charge carrier localized on a single molecule, whereas the opposite charge carrier type is on a nearby molecule. This picture is considerably different for inorganic semiconductors such as Si or GaAs, where spatially extended Wannier-Mott excitons predominate. A departure from the conventional picture of excitons in molecular crystals came in 1980 with the theoretical work of Bounds and Siebrand [2]. They concluded that C-T states in closely packed, planar stacking molecules such as anthracene were in some respects Wannier-like, since high order excited states could be modeled as extended hydrogenic states whose sizes and binding energies were determined by the spatially averaged dielectric constant of the crystal, rather than by the anisotropic dielectric tensor characteristic of such molecules. This picture of the exciton was confirmed in 1981 by Sebastian, et al. [3] through the use of electroabsorption measurements made on the higher order (S3 ) states of such anthracene-like molecules as pentacene. In that work, they showed that electroabsorption could be used to distinguish between the Stark effect and polarization (or C-T) effects in organic molecules by the presence of the first and second derivatives, respectively, of the absorption coefficient with respect to the optical energy. More recently, with the development of the ultrahigh vacuum process of organic molecular beam deposition (OMBD), allowing for the monolayer control of layer thickness [4j,
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