Optoelectronic and Structural Properties of Vacuum-Deposited Crystalline Organic Thin Films
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QE systems are considerably different from previously studied lattice-mismatched physisorbed atomic and molecular systems [12,13] in that the interlayer compressibility of molecular heterointerfaces is far greater than the intralayer compressibility due to the spatial extent of the molecules. This is in contrast to atomic vdW systems where these quantities are comparable [13]. As will be shown, it is this asymmetry in elasticity which leads to layer orientation in crystalline organic thin film systems. In Sec. II, we will summarize our theoretical models of the QE growth of the archetype, nonpolar, planar molecules; 3,4,9,10 perylenetetracarboxylic dianhydride (PrCDA, C2406H8) and 3,4,7,8 naphthalenetetracarboxylic dianhydride (NTCDA, C 1406H4). Many aspects of the model have been experimentally verified previously. In Sec. III we present structural data obtained via scanning tunneling microscopy (STM) and scanning electron microscopy that relate to the theoretical studies. In Sec. IV we consider the growth of a highly polar molecule: 2methoxy-4'-nitro-(E)-stilbene (MNS), and compare the growth of that molecular species with results for the nonpolar molecules which are the focus of our theoretical work. In Sec. V, conclusions are presented. I1. MODELING OF STRUCTURE OF NON-POLAR ORGANIC MOLECULAR THIN FILM INTERFACES While models have been previously developed to understand epitaxial growth (e.g. the "rigid lattice" model [14]), epitaxial systems are inherently different from QE systems. The primary difference lies in the incommensurability of QE layers with the substrate. Whereas one can model epitaxial growth by a harmonic potential with a period equal to the atomic spacing of the substrate, incommensurate lattices cannot be treated as such since the potential between overlayer and substrate is anharmonic. The analytical solutions which are attained for epitaxial systems are therefore replaced by computationally intensive models. The choice of an approximate, "ellipsoidal potential" greatly simplifies the problem of modeling QE, therefore allowing us to rapidly determine the layer structure which is achieved for even the most complex planar molecular structures. Extending the model to other molecules where the long-range Coulomb potential plays a role, as is the case MNS, significantly complicates the problem, and is not considered here. However, the methods employed, in principle, can be extended to include these and other bonding forces. We therefore consider purely vdW-bonded molecular layers as model systems whose study enables the development of a qualitative picture of the primary factors involved in determining how crystalline order in QE layers can be obtained. The primary requirement for QE is that there exists a range over which a surface molecule can be translated relative to the substrate without significantly changing the system energy [15]. If the potential between molecules within a layer is 4itra, and between molecules in different layers is +inter, then this condition is related to the magnitudes
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