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Translucency of Graphene to van der Waals Forces

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 materials to exist. 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 (vdW), 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 though it took more than a century, with the new discoveries of quantum field theory, to be able to fully understand its quantum character and its relation to the vacuum energy and Casimir force. And only in the last 30 years it has been realized 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 vdW forces, which are enhanced 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 it remains 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 experiment 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: this would be a unique behaviour as most other interatomic forces only affect close neighbours. The choice fell on graphene, the thinnest material ever synthesized. Thanks to the experimental methods developed over the years at the SuperESCA beamline of 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 partially transparent to the forces of vdW 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 systems, 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 across the interface 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 different regions of the carbon network close to (about 1.9̊Å) and far from (about 3.1̊A) the metal substrate. This strategy, therefore, allows to determine the adsorption energy at different adsorbate-metal distances. 
Adsorption energies of molecules and atoms for the two different systems were measured by employing the Temperature Programmed-XPS technique, where core level spectra (C1s in the case of CO, see Fig. 1(a)) from the adsorbed species are collected during a temperature ramp (Fig. 1(b)). The determination of the spectral intensity as a function of temperature (Fig. 1(c)) allows to extract with high accuracy the adsorption temperature and therefore the interaction with the surface.

Figure 1.     CO desorption from Gr/Ir(111). (a) Selected spectra of the uptake corresponding to θCO=0.08 ML (bottom) and 0.30 ML (top). (b) TP-XPS C 1s core level spectra showing its evolution during thermal desorption of CO from Gr/Ir(111). (c) Comparison of CO coverage evolution as a function of temperature for selected CO initial coverages. 

 

The measured adsorption energies have then been 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. The results are shown in Figure 2(a) for the case of both CO molecules (red values) and Ar atoms (green values). 
 

Figure 2.     (a) Interaction of CO (red value, on the left) and Ar (green value, on the right) adsorbates for three different adsorption configurations (A: on flat graphene; B and C: in the valleys and hills (respectively) of corrugated graphene) with the graphene/metal system (I) and with the graphene (G) and metal (S) alone. (b) Dependence of the interaction on adsorbate-metal distance for carbon monoxide molecules (red) and argon atoms (green) in the different configurations discussed in (a). The blue ticks indicate the results of the best fit using the equation I=G+kS, where k is the factor accounting for the translucency of graphene to the vdW interactions. The gray scale of the vertical bars shows the expected interaction as a function of translucency, from complete blocking (black region) to total transparency (white region) of Gr.


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 were shown to be partially transmitted by graphene. The total interaction I of the adsorbates with the graphene/metal system could therefore be described using the simple equation 

I = G + kS

where G is the interaction with the free-standing graphene layer, S is the interaction with the metal substrate in absence of graphene, 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 a complete blocking of the vdW interactions by graphene, while k = 1 to a total transparency of graphene to vdW interactions. 
Figure 2(b) shows the contributions of graphene and of the substrate as a function of the translucency parameter k, for both CO (red dots) and Ar (green dots) on flat graphene and on corrugated graphene. 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, respectively showing a net dipole moment or not.


 

This research was conducted by the following research team:

Francesco Presel 1,2, Alfonso Gijon 3, Eduardo R. Hernandez 3, Paolo Lacovig4, Silvano Lizzit 4, Dario Alfè 5,6,7,and Alessandro Baraldi1,4,8


Physics Department, University of Trieste, Trieste, Italy 
DTU Physics, Technical University of Denmark Lyngby, Denmark 
Instituto de Ciencia de Materiales de Madrid - ICMM-CSIC, Madrid, Spain

Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy 
Department of Earth Sciences, Department of Physics and Astronomy, TYC@UCL 
London Centre for Nanotechnology, University College London, London, United Kingdom

Dipartimento di Fisica Ettore Pancini, Università di Napoli Federico II, Napoli, Italy

IOM-CNR, Laboratorio TASC, Trieste, Italy



Contact persons:

Alessandro Baraldi, email: alessandro.baraldi@elettra.eu

 

Reference

Francesco Presel,  Alfonso Gijon,  Eduardo R. Hernandez, Paolo Lacovig, Silvano Lizzit, Dario Alfè  and Alessandro Baraldi, “Translucency of Graphene to van der Waals Forces Applies to Atoms/Molecules with Different Polar Character” ACS Nano 13, 12230 (2019);   DOI: 10.1021/acsnano.9b07277

 
Last Updated on Tuesday, 03 December 2019 12:26