TiO2(110): a flexible playground for thin film growth of aromatic molecules

DIPC Seminars

Speaker

Luca Floreano, IOM-CNR, Laboratorio TASC, Trieste, Italy

When

2012/07/13
13:00

Place

Donostia International Physics Center (DIPC).Paseo Manuel de Lardizabal, 4 (nearby the Facultad de Quimica), Donostia

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**TiO 2(110): a flexible playground for thin film growth of aromatic molecules** _Luca Floreano_ _IOM-CNR, Laboratorio TASC, Trieste, Italy_ Until now, much effort has been devoted to the study of organic semiconductors on metal substrates. Good control of the interface structural and electronic properties has been achieved on these hybrid junctions, whereas dielectrics have been hitherto regarded as inert substrates where the structure, morphology and electronic transport of the organic overlayer are simply dictated by the intermolecular van der Waal's attraction. However, when the device miniaturization is pushed down to the thickness of a few molecular layers, the interaction with the dielectric, although weak, cannot be neglected any longer. In the past few years, the TiO2(110) rutile surface emerged as the most relevant dielectric capable of supporting the oriented growth of molecular overlayers thanks to its large anisotropic corrugation. The large spacing (6.5 Å) between the Oxygen rows protruding out of the surface, is well matched to host the aromatic rings of organic molecules organized into periodic arrays, such as the high symmetry C60 molecule [1]. The uniaxial pentacene molecule also forms planar phases, that can be stabilized for a few layer thickness into an artificial geometry that mimicks the bulk-like electronic structure [2]. In addition, TiO2(110) offers the opportunity of playing the dual role of either conductive or insulating substrate. This surface displays the natural tendency to loose Oxygen atoms upon mild annealing. in vacuum The excess of charge associated with the formation of an Oxygen vacancy gives rise to the emergence of a new electronic state in the gap, effectively making conductive the surface. Very recently, we have deomonstrated by a novel technique, resonant photoelectron diffraction [3], that the defect state, which is known to display a clear Ti 3d character, is associated with a charge redistribution among multiple Ti sites in the subsurface layers [4]. Most importantly, we have shown that the localization of the excess of charge is an intrinsic property of TiO2(110), irrespective of the mechanism of charge injection [5]. This observation paves the way to the formation of an hybrid junction where charge transfer can take place, like on metal electrodes. While we have observed no charge transfer between TiO2 and either pentacene (donor) or C60 (acceptor), preliminary results indicate that the TiO2(110) defect state is effectively quenched upon growth of a commensurate monolayer of a specific perylene-derivative (PTCDI, acceptor). From comparison with the monolayer phase of the bare perylene molecule (donor), we have seen by resonant photoemission that the substrate excess of charge is transferred (statically) from the Ti sites to the C and N backbone of the PTCDI molecules. **References** [1] C. Sanchez-Sanchez, V. Lanzilotto, C. Gonzalez, A. Verdini, P. de Andres, L. Floreano, M.F. Lopez, and J.A. Martin-Gago, " _Weakly interacting molecular layer of Spinning C60 molecules on TiO2(110) surfaces_ ", **Chem. Eur. J.** _18_ (2012) 7382. [2] V. Lanzilotto, C. Sanchez-Sanchez, G. Bavdek, D. Cvetko, M.F. Lopez, J.A. Martin-Gago, and L. Floreano, " _Planar growth of pentacene on the dielectric TiO2(110) surface_ ", **J. Phys. Chem C** _115_ (2011) 4664. [3] A. Verdini, P. Krueger and L. Floreano, " _Resonant Photoelectron Diffarction_ ", in _Surface Analytical Techniques_ , ed. by G. Bracco and B. Holst, Springer Series in Surface Science (Springer-Verlag, Berlin, in press), chapter VIII. [4] P. Krueger, S. Bourgeois, B. Domenichini, H. Magnan, D. Chandesris, P. Le Fevre, A.M. Flank, J. Jupille, L. Floreano, A. Cossaro, A. Verdini, and A. Morgante, " _Defect states at the TiO2(110) surface probed by resonant photoelectron diffraction_ ", **Phys. Rev. Lett.** _100_ (2008) 055501. [5] P. Krueger, J. Jupille, S. Bourgeois, B. Domenichini, A. Verdini, L. Floreano, A. Morgante, " _Intrinsic nature of the excess electron distribution at the TiO2(110) surface_ ", **Phys. Rev. Lett.** _108_ (2012) 126803.