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Organic molecular beam epitaxy (OMBE)

Due to their low density of defects and due the absence of dangling bonds the surfaces of layered systems like the transition metal dichalcogenides also serve as suitable substrates for the growth of organic thin films. Using the powerful VDWE concept, various organic monomers like Fullerenes [1], Perylenes [2], Naphthalenes [3], Polyacenes [4], Thiophenenes [5] or Phthalocyanines [6] have been successfully prepared as ultrathin film on weakly interacting substrates via organic molecular beam epitaxy OMBE [7]. In contrast to layered structures molecular crystals are completely built up by van der Waals forces and have therefore no preferred orientation so that growth morphologies from dense-packed clusters with commensurable periodicity to the substrate surface up to strongly localized single molecules without translational symmetry have been observed [8]. The great number of p-conjugated molecular structures enables a wide variety of geometric and electronic film properties which offer the following advantages:

  • Recent developments in chemistry allow the synthesis of molecular powders in macroscopic amounts at low costs.
  • Basic attributes like atomic setup, spectral features or transport properties can be chosen to design the physical properties of organic thin films. E.g. superconductors can be created [9] and devices can be built with efficiencies or component life times comparable to anorganic competitors [10].
  • The non-toxic properties of the major part of organic systems ensures an easy handling under cheap and simple experimental conditions.
  • The ductility of most organic films permits a combination with variously shaped substrates enabling technical applications like flexible displays or receptors with negligible degradation effects.

Figure 1: Highly symmetric setup uf the C60 molecule and closed-packed crystal structure. Fullerenes serve as electron acceptors in heterojunctions and as basic material for organic High-Tc-Superconductors (picture from S. Woedtke, Ph.D. thesis).




Figure 2: Bended planar molecular structure of TiO-Phthalocyanine and side view of the molecular crystal. Phthalocyanines are used as efficient photoreceptors and pigments (picture from S. Woedtke, Ph.D. thesis).


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