TY - JOUR

T1 - Stabilization of large adsorbates by rotational entropy

T2 - a time-resolved variable-temperature STM study

AU - Waldmann, Thomas

AU - Klein, Jens

AU - Hoster, Harry E.

AU - Behm, R. Juergen

PY - 2013/1/14

Y1 - 2013/1/14

N2 - 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.

AB - 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.

KW - adsorption

KW - dynamics

KW - rotational entropy

KW - scanning tunneling microscopy

KW - two-dimensional liquids

KW - LARGE ORGANIC-MOLECULES

KW - SINGLE-MOLECULE

KW - METAL-SURFACES

KW - PYROLYTIC-GRAPHITE

KW - FILMS

KW - SUBSTRATE

KW - ROTOR

KW - AG

KW - TRANSISTORS

KW - ASSEMBLIES

U2 - 10.1002/cphc.201200531

DO - 10.1002/cphc.201200531

M3 - Journal article

VL - 14

SP - 162

EP - 169

JO - ChemPhysChem

JF - ChemPhysChem

SN - 1439-4235

IS - 1

ER -