Molecular orientation

The exploration and understanding of the growth and the orientation of organic molecules is a very important issue fundamental as well as applied research worldwide. It has been shown that the ordering and orientation of organic molecules affects significantly electronic and transport properties. But not only is the orientation of the film as a whole important, the interface is crucial to determining the device properties and thus the orientation of the first layer is in particular crucial. Experiments in single crystalline model substrates are essential for the uderstanding of the growth of organic molecules. We note, however, that the situation becomes different in many cases on relatively ill defined, more technically relevant substrates.

At the example of some metal phthalocyanines, we have shown that the substrate roughness is an important parameter for the growth. Although a high degree of ordering for phthalocyanines is observed in many thin films, molecules stand on the rough substrates and lie on the single crystalline substrates. In some cases, in particular for CuPc on polycrystalline gold, differently oriented (“lying”), buried interfacial layers of few monolayers were observed. Importantly, the orientation of molecules directly at the metal-organic interface is related to electronic interface properties.

Methods:

Polarization dependent X-ray absorption spectroscopy (XAS), also called near edge x-ray absorption fine structure (NEXAFS) spectroscopy, represents an ideal tool to investigate the geometry of molecular adsorbates and thin films [36] especially on disordered substrates. The basic principle is illustrated in the figure below.

Figure: Probing the molecular orientation using p-polarized light (electric field vector parallel to the beam – sample normal plane): At grazing beam incidence excitations into π* orbitals are observed, whereas at normal incidence excitations occur into σ* levels.

Related publications:

Order on disorder: copper phthalocyanine thin films on technical substrates H. Peisert, T. Schwieger, J. M. Auerhammer, M. Knupfer, M. S. Golden, J. Fink, P. R. Bressler, M. Mast J. Appl. Phys., 90 (2001) 466.

Orientation and electronic properties of phthalocyanines on polycrystalline substrates (Feature article) H. Peisert, I. Biswas, M. Knupfer, T. Chassé physica status solidi (b), 246 (2009) 1529-1545.