(87f) Molecular Order of Mixed Self-Assembled Monolayers Studied by Surface Vibrational Spectroscopy

Self-organization of molecules into supra-molecular structures is a promising route to fabricate high performance, low cost and flexible electronic devices such as organic transistors. Here, we present a study of mixed self-assembled monolayers (SAM) of phosphonic acid (PA) and C60 modified PA on alumina, which are possible candidates for the active channel in thin-film field effect transistors. For these devices a detailed understanding and control of the interfacial properties and molecular composition is of great importance. However, studies of the interfacial molecular structure under technically relevant conditions represent a significant experimental challenge. For that reason analytical techniques that are highly interfacial specific and applicable under non-vacuum conditions need to be developed.

Vibrational sum-frequency generation (SFG) is an inherently interface sensitive optical technique which provides information on the composition and conformation of the SAM. Detailed information on mixed SAMs is revealed by a combination of SFG, ellipsometry and electronic measurements. SFG measurements show that the molecular order of pure PA SAMs is determined by the alkyl chain length. SAMs with short alkyl chains are in an amorphous state, while a quasi-crystalline state can be achieved for longer alkyl chains[1]. Evaluation of mixed SAMs is challenging since a distinction between CH stretching bands of PA and C60 modified PA cannot be made unambiguously. However, mixed monolayers that are composed of fluorinated and C60 modified PA offer a way to determine surface coverage and molecular properties of the two species independently. CF stretching vibrations of fluorinated C12-phosphonic acids give rise to vibrational bands at 1344 cm^-1 and 1376 cm^-1 in SFG spectra while a pronounced vibrational band centered at 1465 cm^-1 is attributable to C60 vibrations of C60 modified C18-PA. The coexistence of CF stretching and C60 vibrational bands is indicative for assembly of a mixed monolayer that comprises both species. Furthermore, a pronounced maximum in SFG intensity of the C60 band is observed for coverages around 75%, which shows that C60 modified alkyl chains (C18) can be stabilized by C12-phosphonic acids. As a result, the molecular order and lateral conductivity of the mixed SAM is significantly improved.

[1]        M. Novak M.; Jäger, C. M.; Rumpel, A.; Kropp, H.; Peukert, W.; Clark, T. and Halik, M. Organic Electronics 11 2010 (1476–1482).