Molecules within single surface layers organise in balance of inter-molecular and molecule-surface forces and therefore exhibit a characteristically different arrangement than extrapolated from the 3D liquid or bulk crystal. Local structural investigations on 2D layers can be performed by Scanning Tunnelling Microscopy (STM), where atomic scale images are provided, and by X-Ray Photoelectron Diffraction (XPD), where the statistically averaged atomic environment of an atomic species can often be deduced by comparison with simulated diffractogramms.

We investigate Sub-Phthalocyanine (SubPc), a dipole-bearing molecule with a delocalised π-electron system (Fig. 1), which features complex monolayer adsorption behaviour, as revealed by STM [1–3]. The adsorp­tion of SubPc on Ag(111) has been reported to be dominated by the Ag-Cl interaction (‘Cl-down’ geometry) [1] while, in contrast to this findings, pre-dominant adsorption via the isoindol systems (‘Cl-up’ geometry) have been reported on Au(111) and Cu(100) substrates [2, 3]. A ‘2D-lattice-gas’ phase, the 2D-condensed phases (e.g. honeycomb and hcp, see Fig. 2) with different densities as well as several mixed phases with C60 [2] have been associated to the delicate balance of intermolecular and molecule-surface interactions.

Figure 1: Structure of the SubPc molecule

Additional interesting phenomena potentially arise from the thermal excitation of rotational and vibronic degrees of freedom. In the condensed phase, the orientation of individual SubPc molecules is visible; therefore molecular rotations are frozen or reduced to rotational ‘flipping’ much faster than the timescale of the STM experiment. In the 2D-gas phase one may expect the excitation of rotation and/or other internal degrees of motion. This provides the motivation for performing XPD experiments, to learn about molecular vibrations and librations, which are conclusive on a timescale much shorter than the STM, i.e. the traversal time for photoelectrons at their kinetic energy.

 
Figure 2: honeycomb- and hcp-phase of a monolayer of SubPc (taken from Ref. [1]).

This project contributes to the understanding of the complex phase behaviour of SubPc, a model system for complex multi phase self-organisation by coulomb interaction between dipole bearing molecules.

University Partner: Prof. Ph. Aebi, Université de Neuchâtel, CH

Collaboration: T. Greber, University Zürich, CH 
                       R. Fasel, EMPA Dübendorf, CH

Contact: Dr. T.A. Jung: thomas.jungpsi.ch

Funding: PSI, NSF

References:

[1]    S. Berner, M. de Wild, L. Ramoino, S. Ivan, A. Baratoff, H.-J. Güntherodt, H. Suzuki, D. Schlettwein & T. A. Jung, «Adsorption and two-dimensional phases of a large polar molecule: Sub-phthalocyanine on Ag(111)», Phys. Rev. B 68, 115410 (2003).

[2]    H. Yanagi, K. Ikuta, H. Mukai & T. Shibutani, «STM-Induced Flip-Flop Switching of Adsorbed Subphthalocyanine Molecular Arrays», Nano Lett. 2, 951 (2002).

[3]    S. Mannsfeld, H. Reichhard & T. Fritz, «LEED and STM investigation of chloro(subphthalocyaninato)boron on Au(111)», Surf. Sci. 525, 215 (2003).

[4]    M. de Wild, S. Berner, H. Suzuki, Y. Yanagi, D. Schlettwein, A. Baratoff, H-J. Güntherodt & T.A. Jung, «A new route to molecular self-assembly: Self-intermixing monolayer phases», ChemPhysChem 10, 881 (2002).

[5]    R. Bertschinger, A. Scheybal, Ch. Koitsch, Ph. Aebi & T.A. Jung, «Photoelectron Spectroscopy on Molecular Monolayer: Sub-Phthalocyanine on Ag (111)», PSI Scientific Report 2003 VII, 137 (2004).

[6]    R. Bertschinger, R. Schillinger, A. Scheybal, R. Fasel, T. Greber, Ph. Aebi & T.A. Jung, «Structure and Dynamics of 2D Molecular Monolayer Phases studied by X-Ray Photoelectron diffraction (XPD): SubPc/Ag(111)», PSI Scientific Report 2004 VII, 144 (2005).