It's a matter of transparency to van der Waals forces

The van der Waals interactions between light adsorbates with polar (Carbon Monoxide) and non-polar (Argon) character and a metal surface are partially screened by graphene which shows a translucency parameter equal to 0.507. 

F. Presel et al., ACS Nano 13, 12230 (2019)
If in the infinitely large it is the gravitational force that determines the evolution in space and time of planets, stars and galaxies, when we focus our observation on the atomic scale other are the forces that allow the formation of matter. These are forces that, like a "special glue", allow atoms and molecules to aggregate to form living and non-living systems. Among them we find one that, although discovered 150 years ago by Johannes Diderik van der Waals, still carries with it some aspects of ambiguity. Van der Waals was the first to reveal its origin and to give a first and simple analytical description, even if it took more than a century, with the new discoveries of the field theory, to be able to fully understand its quantum character. Only in the last 30 years it has been realized, however, how much this force pervades the natural world. One of the wonders is represented by the geckos who use these forces to climb vertical and smooth walls thanks to the contribution of van der Waals forces that is practiced because of the multitude of hairs present in each finger of their legs. These forces are also known to affect the stability of the double helix of the DNA and are also responsible for the interactions between different groups of amino acids.
What makes the vdW force unique is the fact that it is the weakest of the inter-atomic and inter-molecular forces present in nature and therefore remain extremely difficult to measure with great accuracy. At the same time, even the inclusion of these force in the most accurate methods of calculation has not yet found a universal solution and the different approaches used by theoretical physicists and chemists to take them into account can sometimes lead to conflicting results.
The goal of the research project was to evaluate how this type of force, which is exerted for example between a surface and a molecule, can propagate at a distance even when another ultra-thin material is interposed between them. The choice fell on graphene, the thinnest material ever synthesized. Thanks to the experimental methods developed over the years at the SuperESCA beamline in Elettra and the comparison of the experimental results with the theoretical ones obtained by the research groups coordinated by Prof. Dario Alfè of the University College London and the University Federico II of Naples and by Dr. Eduardo Hernandez of the CSIC of Madrid, it was understood that graphene is only partially transparent to the forces of van der Waals and therefore has a character of translucency. 
The strategy was based on the comparison of the adsorption energy of Carbon Monoxide molecules and Argon atoms deposited in different concentrations on two different interfaces, namely graphene grown on Ir(111) – where graphene is considered almost fully decoupled from this substrate  – and graphene on Cobalt/Ir(111), where the interaction is quite strong. The great advantage of this approach is that the presence of a single-layer of cobalt atoms below an extended and high-quality graphene monolayer grown on Ir results in the formation of a corrugated structure with regions of the carbon network close to (about 1.9 Å) and far from (about 3.1 Å) the metal substrate. This strategy allows to not only determine the adsorption energy at different adsorbate-metal distances, but also to probe the effects of the graphene corrugation. 
Adsorption energies of molecules and atoms for the two different systems were performed by emploing the Temperature Programmed-XPS technique, where core level spectra from the adsorbed species (C1s and Ar 2p) are collected during a temperature ramp. The determination of the spectral intensity as a function of temperature allows to extract with high accuracy the desorption temperature.
The measured adsorption energies  have been than used as benchmark to understand which vdW functional (between the D3, Langreth and DF2 formalisms) was the most accurate to theoretically describe the experimental findings. 


To investigate the role of graphene in screening the direct interactions between the adsorbates and the substrate, we separately calculated the adsorption energy for each adsorbate (CO and Ar) placed either on (i) the real systems, (ii) a graphene sheet with the substrate removed, or (iii) on the substrate alone, without graphene, for both the flat graphene on Iridium and corrugated graphene layer on Cobalt/Iridium. 
Our results show that while the adsorption energy is certainly affected by the chemical composition of the supporting substrate and by the corrugation of the carbon lattice, the van der Waals interactions between adsorbates and the metal surfaces are partially screened by graphene. 
The total interaction I of the adsorbates with the graphene/metal system was obtained using the simple equation 

I = G + kS

where G is the interaction with the free-standing Gr layer, S is the interaction with the metal substrate in absence of Gr, but at the same distance, and 0 ≤ k ≤ 1 accounts for the screening of the vdW interactions by graphene. The case k = 0 would correspond to complete blocking of the vdW interactions by graphene, while k = 1 to a total transparency of Gr to vdW interactions. 
We have shown that the contributions of Gr and of the substrate as a function of the translucency parameter k, for both CO and Ar on flat Gr and on corrugated Gr. The adsorption energy of all the systems is described with remarkable accuracy with translucency parameter k of 0.507 ± 0.034. 
These findings show that dispersion interactions, which are of a longer range than covalent bonding, can be transmitted, although partially screened, through a graphene monolayer, thus suggesting that the concept of graphene translucency, which has been put forward in the case of water droplets, can be applied also in the case of single molecules and atoms, which show a net dipole moment or not. 

Retrieve article
Translucency of Graphene to van der Waals Forces Applies to Atoms/Molecules with Different Polar Character

Francesco Presel,  Alfonso Gijon,  Eduardo R. Hernandez, Paolo Lacovig, Silvano Lizzit, Dario Alfè  and Alessandro Baraldi

ACS Nano 13, 12230 (2020)

Last Updated on Sunday, 09 October 2022 16:18