Site-Specific Polarization Screening in Organic Thin Films

Systematic local spectroscopy of the affinity levels, by means of a scanning tunneling microscope, in highly ordered molecular semiconductor films of tetracene reveals strong energy level shifts by up to similar to 1.0 eV from molecule to molecule. This final state effect can be traced back to the site specificity of the polarization energy in organic materials with complex unit cells, caused by a combination of different molecular environments, the intrinsically anisotropic molecular polarizability, and the influence of the substrate.