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Stabilization of large adsorbates by rotational entropy: a time-resolved variable-temperature STM study

Research output: Contribution to Journal/MagazineJournal articlepeer-review

<mark>Journal publication date</mark>14/01/2013
Issue number1
Number of pages8
Pages (from-to)162-169
Publication StatusPublished
<mark>Original language</mark>English


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 (beta-phase) in a dynamic equilibrium with static adsorbate molecules. Furthermore, exchange between an ordered phase (a-phase) and beta-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.