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It's a matter of transparency

22 October, 2019

It's a matter of transparency. Not in light, but in strength.

An international team led by the University of Trieste sheds light on one of the weakest forces that exist in nature.

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 field theory, to be able to fully understand its quantum origin. Only in the last 30 years has it 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 interaction of van der Waals that is practiced because of the multitude of hairs of which is equipped each finger of their legs. But these forces also 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 them unique is the fact that they are the weakest of the inter-atomic and inter-molecular forces we know of in nature and therefore remain extremely difficult to measure with great accuracy. At the same time, even the inclusion of these forces in the most modern and accurate methods of calculation has not yet found a universal solution and the different approaches used by physicists and theoretical chemists to take them into account can sometimes lead to completely conflicting results.

 

It is in this context that the work of the research group led by Prof. Alessandro Baraldi of the Physics Department of the University of Trieste has been inserted, who decided to investigate the transparency effect of particular materials at these forces. The objective 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 the graphene, "the material of wonders", the thinnest ever synthesized by man, on which the research group has a consolidated experience. Thanks to the powerful methods of experimental investigation 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. It is a unique phenomenon: in fact, it has never been observed for the much stronger interactions on which the chemical bonds are based, which take place only between materials "in direct contact". "Our results" says Franceso Presel, first author of the publication, now post-doc at the Technical University of Denmark in Copenhagen and already PhD student in Nanotechnology at UNITS, "clarify an open and debated problem for some years. Our studies have shown that, not only in the case of water, but also in the case of other atoms and molecules, graphene allows a good part of the forces of van der Waals to pass, reaching a value of almost 50%".

The results, published in the prestigious international journal ACS Nano, will be useful to clarify the processes of interaction of atoms or molecules with surfaces and to clarify some aspects of the complex and weak interactions that exist in the latest generation of materials. We are talking about heterostructures formed by layers of two-dimensional materials superimposed as sheets of paper (in addition to graphene, hexagonal boron nitride, molybdenum disulphide, etc.), each of the thicknesses of a single atom or little more than after the advent of graphene in 2004 have made a breakthrough in the panorama of materials science and today are called Materials of van der Waals.