Optical Anisotropy of Ultra-thin Organic Films

Monitoring the adsorption and growth of organic molecules on surfaces in-situ and in real time is important for controlling the deposition of high quality organic thin films used for optoelectronic devices. The surface analytical tools available to characterize simultaneously the adsorption kinetics, molecular interaction, orientation, conformation, as well as the structure, morphology and the optical properties of the growing film are, however, limited. We plan to establish the application of an optical method - reflectance difference spectroscopy (RDS) - as a new and extremely sensitive method for the investigation of adsorption and growth of organic molecule thin films on metal surfaces. RDS has been established in our lab as a powerful tool to characterize the anisotropy of metal surfaces, the adsorption kinetics of inorganic atoms and molecules, the structure and morphology of adlayers and thin films. Our preliminary studies reveal that the RDS signal from adsorbed organic molecules is particularly strong and extremely sensitive to the orientation, interaction, and conformation of organic molecules contained in the growing films. The enhanced sensitivity originates from the anisotropic molecular electronic structure (intra- molecular optical transitions) as well as the anisotropic molecular interactions (exciton states). The detection limit of the adsorbed organic molecules is well below 1% of a monolayer of lying molecules - far beyond the detection limit of conventional optical techniques. To establish the RDS as a powerful tool to characterize the growth of organic thin films, a fundamental study on the RDS response to molecular aggregates as a function of size and configuration on the surfaces is essential. We propose to study selected model systems at low temperature (down to 6 K) in ultra high vacuum (UHV) by RDS and to compare the results to those obtained with complementary analytical techniques such as, scanning tunneling microscopy (STM), low energy electron diffraction (LEED), atomic force microscopy (AFM), X ray diffraction (XRD), angle resolved ultraviolet photoelectron spectroscopy (ARUPS) and temperature programmed desorption (TPD). The systems to be studied are para-sexiphenyl (p-6P) and pentacene on Cu(110), Cu(110)-(2x1)O and Cu(110)- (2x3)N. The different rigidity of p-6P and pentacene allows the systemic analysis of molecular conformation, whereas the choice of the different substrates offers the possibility to tune the corrugation and electronic structure of the substrate surfaces and, thus, to control the adsorption and growth behavior of organic molecules. Besides, depositing organic molecules on the surfaces with two fold symmetry allows the fabrication of thin films with single molecular orientation resulting in a low density of domain boundaries and strongly polarized absorption/emission characteristics. These properties should also improve the performance of related optoelectronic devices. The investigation will be performed in the temperature range from 6 K to the desorption temperatures of the molecules. The systemic variation of the substrate temperature allows to extract the energetic and kinetic parameters which are of fundamental importance for the understanding of the nucleation and growth of organic thin films.