title: Stabilization of large adsorbates by rotational entropy: a time-resolved variable-temperature STM study.
abstract: Investigating the dynamics in an adlayer of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4'-BTP) on Ag(111) by fast scanning tunneling microscopy (video-STM), we found that rotating 2,4'-BTP adsorbates coexist in a two-dimensional (2D) liquid phase (β-phase) in a dynamic equilibrium with static adsorbate molecules. Furthermore, exchange between an ordered phase (α-phase) and β-phase leads to fluctuations of the domain boundary on a time scale of seconds. Quantitative evaluation of the temperature-dependent equilibrium between rotating and static adsorbates, evaluated from a large number of STM images, gains insight into energetic and entropic stabilization and underlines that the rotating adsorbate molecules are stabilized by an entropy contribution, which is compatible with that derived by using statistical mechanics. The general validity of the concept of entropic stabilization of rotating admolecules, favoring rotation already at room temperature, is tested for other typical small, mid-size and large adsorbates.
authors.affiliation: Institute of Surface Chemistry and Catalysis, Ulm University, 89069 Ulm, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Waldmann
authors.forename: Jens
authors.initials: J
authors.lastname: Klein
authors.forename: Harry E
authors.initials: HE
authors.lastname: Hoster
authors.forename: R Jürgen
authors.initials: RJ
authors.lastname: Behm