Article pubs.acs.org/JPCC
Spontaneous Change in Molecular Orientation at Order−Disorder Transition of Tetracene on Ag(111) Tomoki Sueyoshi,*,†,‡ Martin Willenbockel,†,‡ Michael Naboka,§ Alexei Nefedov,§ Serguei Soubatch,†,‡ Christof Wöll,§ and F. Stefan Tautz†,‡ †
Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany Jülich Aachen Research Alliance (JARA)-Fundamentals of Future Information Technology, 52425 Jülich, Germany § Institute of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany ‡
ABSTRACT: We investigate the molecular orientation of tetracene in ordered and disordered layers on Ag(111) using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Quantitative analysis of NEXAFS intensities reveals that the compact monolayer α-phase at 150 K consists of essentially flatlying molecules with an average tilt angle α < 9° with respect to the Ag(111) surface. At room temperature the tetracene monolayer is dynamically disordered with α = 15 ± 4° (α′-phase). The increase in the average tetracene tilt angle indicates that orientational degrees of freedom perpendicular to the surface drive the order− disorder transition in the tetracene monolayer on Ag(111). We argue that the phase transition can be explained by a delicate balance between the interfacial enthalpy and the orientational entropy. Furthermore, we suggest that tetracene/Ag(111) may be a good example of Onsager’s system of nonattracting rigid anisotropic objects (so-called “hard rods”) in two dimensions. On the other hand, the complex bilayer β-phase at 150 K includes tilted molecules with α ≈ 36°, in good agreement with the structure model introduced by Soubatch et al. [Phys. Rev. B 2011, 84, 195440]. This demonstrates that, under the influence of a second layer, π-conjugated molecules in contact with a metal surface can tilt out of the surface plane. The tetracene submonolayer δ-phase (
