Periodic Reporting for period 1 - MolNanoTribology (Tuning molecular friction and adhesion by atomic/chemical design)
Berichtszeitraum: 2018-04-01 bis 2020-03-31
Friction is a phenomenon that is in our everyday life although we tend to remember it only when it is nearly absent such as in the comical popular staple of slipping on a banana peel. Its presence across disparate length scales (earth-quakes, car engines down to molecular machines) reminds us of its ubiquity which endows friction of an utmost practical importance and therefore attempts to control it are almost as old as civilization. Through the works of da Vinci, Amonton and Coulomb the fundamental laws of friction were established and although they hold remarkably well for a wide set of conditions they do not provide any insight into the underlying principles that govern friction. This lack of fundamental understanding has proven to be the Achilles heel of tribology as it hinders our predictive power, e.g. “it is impossible to predict a priori the friction coefficient for a given system”. In a renewed attempt to control friction by atomistic design and aiming to bridge the gap across different length scales, the fields of nanotribology and nanomanipulation were established. Powered by major advances in nanotechnology, they allowed for a nanometer scale control of sliding interfaces, e.g. through Ultra-High-Vacuum (UHV) and Low-Temperature (LT) experiments, which resulted in major advances on our understanding of friction at an atomic scale. Interestingly, during the past decades we have also witnessed a growing desire in miniaturization of devices down to the nanometer scale. Examples such as auto-assembling of molecular structures capable of meeting specific needs (e.g. synthetic photosynthesis, catalysis and molecular electronics) up to molecular-machines (recipient of 2016 Nobel prize in Chemistry) are showing to be promising avenues with a huge technological and economic interest in the decades to come. “Special problems occur when things get small […] and it might turn out to be advantages if we knew how to design for them”, said Feynman when discussing the prospects of building “infinitesimal machinery”. The unavoidable downsizing technological roadmap shall certainly call for novel paradigms to mitigate energy dissipation in such small devices. Hand-in-hand with major advances of synthetic organic chemistry allowing us to build nanoarchitectures almost in a atom-by-atom fashion, we urgently need to explore how energy dissipation can be controlled at molecular scale so to guide these efforts into more efficient/greener technologies. In this project we set up a road-map to address this challenge by combining state-of-the-art atomistic Molecular Dynamics (MD) simulations with the unparalleled resolution provided by LT-UHV experiments. Through carefully designed molecules we have shown that friction and adhesion at single molecule can be tuned via chemical design.
In the course of this project we have published over 10 publications in international peer-reviewed journals (most of winch top 10% in the respective research fields) shown in over 12 prestigious international conferences (via invited and contributed talks). The invited talks and numerous high-impact publications not only reflect the excellence of this work, inline with the stringent criteria set by MSCA, but also manifests the broad interest and productivity of this research line. Sill, as in any ambitious undertaking of this sort, some of the obtained results are still unpublished, although they are expected to be publicly available in the year following the end of this project.
During this action we have studied the friction and adhesion of different molecules over surfaces. Given the large scale of these systems (10nm) and also the large time-span intrinsic to the manipulation of these objects over surfaces (>100ns) one is faced with the difficulty of the impossibility to simulate such process using accurate quantum-mechanical methods and the lack of reliable inter-atomic interactions (quintessential in large scale modeling schemes such as molecular dynamics simulations). Throughout this project we have developed a method which now allow us to bridge this divide. This methodological development required a significant amount of effort and time of this project. Thanks to this effort we were able to properly describe molecular vibrations using large scale classical molecular dynamics simulations. Then using two model systems, namely single-stranded DNA molecule and pyrene chains (used in a wide range of nanoarchitectures) we have show how a detail balance of molecular vibrations are key to understand adhesion and friction processes on-surfaces. In fact this breakthrough allowed us among other results to measure for the first time the deformation of a single-strand-DNA, but also to detect in real time rotations around a single C-C bond. The former, is of utmost importance as very often defect on DNA are positioned not in the nucleotide but instead on the backbone. Most sequencing techniques rely on identification of nucleotide thus completely missing any damage present on DNA backbone. With this approach, we demonstrate the possibility accurately measure the elasticity of the backbone at a single base level, which provides a direct mean to accurately detect the presence of the aforementioned defects. Concerning the detection of the twisting around single C-C bonds, this is a long standing issue, on how to detect clock/anticlockwise rotation in what otherwise is a perfectly symmetric system. Depending on the orientation of the rotation one may control the chirality of the molecule. Here the symmetry is broken thanks to the presence of the surface, and moreover we demonstrate how this weak twist force can dramatically affect friction and on-surface motion of molecules. At last, still in the context of controlling friction/motion of molecules over surfaces we proposed simple molecular decorations which allowed us to control its motion with temperature ultimately resulting in a supra-molecular assembly with a thermal expansion coefficient that is over 100 times larger than conventional materials.
During this action we have studied the friction and adhesion of different molecules over surfaces. Given the large scale of these systems (10nm) and also the large time-span intrinsic to the manipulation of these objects over surfaces (>100ns) one is faced with the difficulty of the impossibility to simulate such process using accurate quantum-mechanical methods and the lack of reliable inter-atomic interactions (quintessential in large scale modeling schemes such as molecular dynamics simulations). Throughout this project we have developed a method which now allow us to bridge this divide. This methodological development required a significant amount of effort and time of this project. Thanks to this effort we were able to properly describe molecular vibrations using large scale classical molecular dynamics simulations. Then using two model systems, namely single-stranded DNA molecule and pyrene chains (used in a wide range of nanoarchitectures) we have show how a detail balance of molecular vibrations are key to understand adhesion and friction processes on-surfaces. In fact this breakthrough allowed us among other results to measure for the first time the deformation of a single-strand-DNA, but also to detect in real time rotations around a single C-C bond. The former, is of utmost importance as very often defect on DNA are positioned not in the nucleotide but instead on the backbone. Most sequencing techniques rely on identification of nucleotide thus completely missing any damage present on DNA backbone. With this approach, we demonstrate the possibility accurately measure the elasticity of the backbone at a single base level, which provides a direct mean to accurately detect the presence of the aforementioned defects. Concerning the detection of the twisting around single C-C bonds, this is a long standing issue, on how to detect clock/anticlockwise rotation in what otherwise is a perfectly symmetric system. Depending on the orientation of the rotation one may control the chirality of the molecule. Here the symmetry is broken thanks to the presence of the surface, and moreover we demonstrate how this weak twist force can dramatically affect friction and on-surface motion of molecules. At last, still in the context of controlling friction/motion of molecules over surfaces we proposed simple molecular decorations which allowed us to control its motion with temperature ultimately resulting in a supra-molecular assembly with a thermal expansion coefficient that is over 100 times larger than conventional materials.
This MSCA has enabled us to develop a novel theoretical approach to study friction a nanoscale. Whilst most of the works thus far have focused on a more classic view on friction of two rigid objects sliding past each other, here we go beyond this paradigm by harnessing molecular vibrations to control on-surface motion. To achieve this ambitious goal, a major effort of developing accurate modeling scheme had to be developed and also challenged against state of the art experiments. The results achieved in this endeavor: detecting stiffness of DNA backbone down to single base, detection of weak intra-molecular rotations and how these may control on-surface motion, a novel supra-molecular material with a thermal expansion coefficient over 100 times larger than conventional materials … among other findings; show how promising is to build upon chemical intuition to control motion and energy dissipation at nanoscale. The socio-economic implications of a fundamental research such as the one considered in this proposal is rather difficult to access in the short term. Nevertheless, it seems sensible to assume that a better control of on-surface motion and energy dissipation at nanoscale, as shown here, will have profound implications on a wide range of industries, being catalysis at this point the one with more immediate implications.