Thermal and electronic fluctuations of flexible adsorbed molecules: Azobenzene on Ag(111)

Reinhard J. Maurer, Wei Liu, Igor Poltavsky, Thomas Stecher, Harald Oberhofer, Karsten Reuter, Alexandre Tkatchenko, Phys. Rev. Lett., 116, 146101 (2016)

Ab Initio molecular dynamics simulation of the free energy of desorption reveals collective electronic and thermal fluctuations that define the finite-temperature energetics of complex adsorbates.

We investigate the thermal and electronic collective fluctuations that contribute to the finite-temperature adsorption properties of flexible adsorbates on surfaces on the example of the molecular switch azobenzene C12H10N2 on the Ag(111) surface. Using first-principles molecular dynamics simulations we obtain the free energy of adsorption that accurately accounts for entropic contributions, whereas the inclusion of many-body dispersion interactions accounts for the electronic correlations that govern the adsorbate binding.
We find the adsorbate properties to be strongly entropy-driven, as can be judged by a kinetic molecular desorption prefactor of 1024 s-1 that largely exceeds previously reported estimates. We relate this effect to sizable fluctuations across structural and electronic observables. Comparison of our calculations to temperature-programmed desorption measurements demonstrates that finite-temperature effects play a dominant role for flexible molecules in contact with polarizable surfaces, and that recently developed first-principles methods offer an optimal tool to reveal novel collective behavior in such complex systems.

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