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Non-additivity of molecule-surface van der Waals potentials from force measurements.

Wagner C, Fournier N, Ruiz VG, Li C, Müllen K, Rohlfing M, Tkatchenko A, Temirov R, Tautz FS - Nat Commun (2014)

Bottom Line: The experiment allows testing the asymptotic vdW force law and its validity range.We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules.Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction.

View Article: PubMed Central - PubMed

Affiliation: 1] Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany [2] Jülich Aachen Research Alliance (JARA)-Fundamentals of Future Information Technology, 52425 Jülich, Germany.

ABSTRACT
Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system-size scaling. The difficulty to isolate and measure vdW forces on a single-molecule level causes our present understanding to be strongly theory based. Here we show measurements of the attractive potential between differently sized organic molecules and a metal surface using an atomic force microscope. Our choice of molecules and the large molecule-surface separation cause this attraction to be purely of vdW type. The experiment allows testing the asymptotic vdW force law and its validity range. We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules. Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction.

No MeSH data available.


Related in: MedlinePlus

Schematic sketch illustrating the experiment.Within each experiment a single, isolated molecule is contacted at one of the reactive carboxylic oxygen atoms, detached from the surface, lifted about 2 nm further and brought back into the adsorbed state. While repeating this cycle up to 45 times, the force gradient dFz/dz is constantly recorded. The tip-surface and molecule-surface distances ztip and zmol, as well as the distance zi between each atom i and the surface are indicated. The chemical structures of the three investigated molecules are shown on the right (from left to right: NTCDA, PTCDA and TTCDA).
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f1: Schematic sketch illustrating the experiment.Within each experiment a single, isolated molecule is contacted at one of the reactive carboxylic oxygen atoms, detached from the surface, lifted about 2 nm further and brought back into the adsorbed state. While repeating this cycle up to 45 times, the force gradient dFz/dz is constantly recorded. The tip-surface and molecule-surface distances ztip and zmol, as well as the distance zi between each atom i and the surface are indicated. The chemical structures of the three investigated molecules are shown on the right (from left to right: NTCDA, PTCDA and TTCDA).

Mentions: We have carried out our experiments on three π-conjugated poly-naphthalene derivatives, namely 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), and its perylene and terrylene counterparts PTCDA and TTCDA (Fig. 1). The investigation of this series of structurally related molecules allows us to gain insight into size-dependent effects. In contrast to previous force–distance measurements1718 that were designed to determine vdW and Casimir interactions between macroscopic bodies, we perform force gradient (dFz/dz) versus distance (z) measurements, using a commercial qPlus quartz tuning fork sensor19 in a combined CREATEC scanning tunnelling/non-contact atomic force microscope2324. It was recently demonstrated that qPlus sensors yield very precise force gradient spectra and images202122.


Non-additivity of molecule-surface van der Waals potentials from force measurements.

Wagner C, Fournier N, Ruiz VG, Li C, Müllen K, Rohlfing M, Tkatchenko A, Temirov R, Tautz FS - Nat Commun (2014)

Schematic sketch illustrating the experiment.Within each experiment a single, isolated molecule is contacted at one of the reactive carboxylic oxygen atoms, detached from the surface, lifted about 2 nm further and brought back into the adsorbed state. While repeating this cycle up to 45 times, the force gradient dFz/dz is constantly recorded. The tip-surface and molecule-surface distances ztip and zmol, as well as the distance zi between each atom i and the surface are indicated. The chemical structures of the three investigated molecules are shown on the right (from left to right: NTCDA, PTCDA and TTCDA).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4263323&req=5

f1: Schematic sketch illustrating the experiment.Within each experiment a single, isolated molecule is contacted at one of the reactive carboxylic oxygen atoms, detached from the surface, lifted about 2 nm further and brought back into the adsorbed state. While repeating this cycle up to 45 times, the force gradient dFz/dz is constantly recorded. The tip-surface and molecule-surface distances ztip and zmol, as well as the distance zi between each atom i and the surface are indicated. The chemical structures of the three investigated molecules are shown on the right (from left to right: NTCDA, PTCDA and TTCDA).
Mentions: We have carried out our experiments on three π-conjugated poly-naphthalene derivatives, namely 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), and its perylene and terrylene counterparts PTCDA and TTCDA (Fig. 1). The investigation of this series of structurally related molecules allows us to gain insight into size-dependent effects. In contrast to previous force–distance measurements1718 that were designed to determine vdW and Casimir interactions between macroscopic bodies, we perform force gradient (dFz/dz) versus distance (z) measurements, using a commercial qPlus quartz tuning fork sensor19 in a combined CREATEC scanning tunnelling/non-contact atomic force microscope2324. It was recently demonstrated that qPlus sensors yield very precise force gradient spectra and images202122.

Bottom Line: The experiment allows testing the asymptotic vdW force law and its validity range.We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules.Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction.

View Article: PubMed Central - PubMed

Affiliation: 1] Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich, 52425 Jülich, Germany [2] Jülich Aachen Research Alliance (JARA)-Fundamentals of Future Information Technology, 52425 Jülich, Germany.

ABSTRACT
Van der Waals (vdW) forces act ubiquitously in condensed matter. Despite being weak on an atomic level, they substantially influence molecular and biological systems due to their long range and system-size scaling. The difficulty to isolate and measure vdW forces on a single-molecule level causes our present understanding to be strongly theory based. Here we show measurements of the attractive potential between differently sized organic molecules and a metal surface using an atomic force microscope. Our choice of molecules and the large molecule-surface separation cause this attraction to be purely of vdW type. The experiment allows testing the asymptotic vdW force law and its validity range. We find a superlinear growth of the vdW attraction with molecular size, originating from the increased deconfinement of electrons in the molecules. Because such non-additive vdW contributions are not accounted for in most first-principles or empirical calculations, we suggest further development in that direction.

No MeSH data available.


Related in: MedlinePlus