Friction and adhesion of graphene nanoribbons on gold: an MD investigation

The frictional motion of contacting bodies is an ubiquitous phenomenon in physics. It encompasses a vast range of time, energy, and length scales, ranging from the macroscale of earthquakes, down to the nanometer scale of atomically flat surfaces and Atomic Force Microscopy (AFM) experiments [1, 2]. Despite the impressive amount of data that has been obtained through the centuries concerning frictional phenomena, a complete theory of friction is still lacking. The main reason is the fact that the frictional response of an interface is determined by a large number of factors, such as the specific nature and chemistry of the surfaces in contact, the operational conditions, the ageing and the sliding history of the contact [3, 4], just to mention a few of them. Moreover, the presence of out-of-equilibrium and highly non-linear processes often occurring at ill-characterized frictional contacts makes a complete understanding of friction even more challenging [5, 6].In this context, new opportunities have been brought about by advances in nanotechnology, where the invention of scanning tip instruments of the AFM family [7] enabled a systematic study of friction for well-characterized materials and surfaces at the nanoscale [8, 9, 10, 11]. Progresses in both the computer modeling of interatomic interactions, that made atomistic sim-ulations of nanostructured materials more powerful and reliable [12, 13], and the development in the theory of non-linear processes [5, 14] o\u2d9ered new theoretical tools to understand frictional phe-nomena. In particular, the di\uffculty of dealing with extremely complex systems where friction is determined by collective phenomena involving many degrees of freedom, gave a strong impulse to the search of simplified 1D- and 2D-models, such as the Prandtl-Tomlinson (PT) [15, 16, 17] and generalized Frenkel-Kontorova (FK) models [14, 18, 19, 20] to capture the essential ingredients of friction. To summarize, understanding frictional phenomena at the nanoscale seems of great importance now, since dealing with experimentally well-defined interfaces makes the fundamental mechanisms of friction easier to identify, possibly with the aim of merging the existing gap between nano-and macroscale friction, where many di\u2d9erent physical actors are at play. Technological advances in this endeavor are to be considered, since controlling friction could limit wear and dissipation, with possible impacts in improving the performances of nano- and micromachines [21], as well as biological motors [22]. In this general framework, the ongoing experimental and theoretical research on nanoscale fric-tion is covering three central topics. First of all, the study of electronic and quantum effects in friction, which happen whenever a tip or a moving agent dissipates energy by exciting local currents in the sample [2, 3]. Secondly, the study of trapped optical systems, such as driven ion chains and colloids in confined configurations [26, 27], that turned out as elegant experimental realizations of the Prandtl-Tomlinson and the Frenkel-Kontorova model. These systems are now attracting a lot of interest because of the possibility of tuning the various parameters of the experimental setup, such as temperature, substrate corrugation and degree of commensurability between the sample and the substrate, almost freely. At last, a very important field of research is represented by the frictional properties of layered and 2D materials, where evidence of a large reduction of friction (up to a factor 10) has been obtained in many graphene-based systems, grown on both amorphous [23, 24, 25] and metallic substrates [28, 29, 30]. The interest for graphene in this field is stimulated by its peculiar tribological features: the decrease of friction by increasing the number of graphene layers [23, 28, 31, 32], the increase of friction upon decreasing the normal load when graphene is chemically modified [33] and the use of graphene as solid lubricant [25]. In this Thesis, we are interested in gaining a deeper insight in the molecular mechanisms of dissipation in a specific type of graphene/metal interface, the graphene nanoribbons (or GNRs) on gold, inspired by the works of Kawai et al. [34, 35] regarding their structural and dynamical properties under the action of an external driving. The papers that represent the basis and core of the Thesis are: L. Gigli, N. Manini, A. Benassi, E. Tosatti, A. Vanossi and R. Guerra, Graphene nanoribbons on gold: understanding superlubricity and edge e\u2d9ects, 2D Materials, 4, 4 (2017); \u2022 L. Gigli, N. Manini, E. Tosatti, R. Guerra and A. Vanossi, Lifted graphene nanoribbons: from smooth sliding to multiple stick-slip regimes, Nanoscale, 10, 2073-2080 (2018); L. Gigli, S. Kawai, R. Guerra, N. Manini, R. Pawlak, X. Feng, K. M\ufcllen, P. Ru\uffeux, R. Fasel, E. Tosatti, E. Meyer, A. Vanossi, Detachment dynamics of graphene nanoribbons on gold, in press in ACSNano (2018). The Molecular Simulations that have been used for the papers [36, 37] show that the GNR/gold interface has a very low friction, with basically zero average increase upon increasing the GNR length. This property, called superlubricity (see chapter 3), is experimentally very rare and due to the interplay between the large in-plane sti\u2d9ness of graphene and the incommensurability between the GNR and the gold substrate structure (see chapter 4). We will show that this system is suitable to obtain a dynamical transition between low-friction smooth sliding states and violent stick-slip regimes by lifting one edge of the GNR at increasing heights, thus changing the e\u2d9ective GNR out-of-plane softness. Furthermore, an external driving of the GNRs along their longitudinal axis directly provides an asymmetric response of the system against pulling/pushing of the edge (see chapter 5). In the last part of the Thesis, we will analyze the dynamical behavior of the GNRs against vertical pulling and show that an Atomic Force Microscope is able to unveil the detailed structure of the system by producing unilateral detachment of its individual unit cells. A good agreement between the experimentally recorded vertical force traces and the results of the molecular simulations shows that the GNR vertical dynamics is characterized by discrete detachments, accompanied by slips of the tail, which are responsible for the complex double-periodicity observed in the vertical force profile (see chapter 6)

Loss of the arabidopsis protein kinases ANPs affects root cell wall composition, and triggers the cell wall damage syndrome

The Arabidopsis NPK1-related Protein kinases ANP1, ANP2 and ANP3 belong to the MAP kinase kinase kinase (MAPKKK) superfamily and were previously described to be crucial for cytokinesis, elicitor-induced immunity and development. Here we investigate the basis of their role in development by using conditional β-estradiol-inducible triple mutants to overcome lethality. In seedlings, lack of ANPs causes root cell bulging, with the transition zone being the most sensitive region. We uncover a role of ANPs in the regulation of cell wall composition and suggest that developmental defects of the triple mutants, observed at the cellular level, might be a consequence of the alterations of the pectic and cellulosic cell wall components. Lack of ANPs also induced a typical cell wall damage syndrome (CWDS) similar to that observed in plants treated with the cellulose biosynthesis inhibitor isoxaben (ISX). Moreover, anp double mutants and plants overexpressing single ANPs (ANP1 or ANP3) respectively showed increased and reduced accumulation of jasmonic acid and PDF1.2 transcripts upon ISX treatment, suggesting that ANPs are part of the pathway targeted by this inhibitor and play a role in cell wall integrity surveillance

WoT Store: una piattaforma per l'interoperabilità semantica in contesti IoT e WoT

Partendo da un'analisi dell'Internet of Things (IoT) e del Web of Things (WoT), delle loro criticità e delle possibili soluzioni, viene proposta la realizzazione di una piattaforma: WoT Store che, rispettando gli standard del W3C WoT, permetta una semplice e ampia operabilità su applicazioni e Thing su base semantica. Per quanto riguarda le applicazioni il WoT Store ne effettua la distribuzione, la ricerca e l'installazione. Permette inoltre la ricerca e la visualizzazione interattiva delle Thing disponibili e la selezione delle applicazioni compatibili con esse. Il lavoro è da considerarsi in una fase dinamica, non solo in quanto necessita di continuo adeguamento con gli standard W3C, ma anche perchè si può prevedere l'implementazione della compra-vendita delle applicazioni e dei dati delle Thing tramite criptovaluta

Modeling the ferroelectric phase transition in barium titanate with DFT accuracy and converged sampling

The accurate description of the structural and thermodynamic properties of ferroelectrics has been one of the most remarkable achievements of Density Functional Theory (DFT). However, running large simulation cells with DFT is computationally demanding, while simulations of small cells are often plagued with non-physical effects that are a consequence of the system's finite size. Therefore, one is often forced to use empirical models that describe the physics of the material in terms of effective interaction terms, that are fitted using the results from DFT, to perform simulations that do not suffer from finite size effects. In this study we use a machine-learning (ML) potential trained on DFT, in combination with accelerated sampling techniques, to converge the thermodynamic properties of Barium Titanate (BTO) with first-principles accuracy and a full atomistic description. Our results indicate that the predicted Curie temperature depends strongly on the choice of DFT functional and system size, due to the presence of emergent long-range directional correlations in the local dipole fluctuations. Our findings demonstrate how the combination of ML models and traditional bottom-up modeling allow one to investigate emergent phenomena with the accuracy of first-principles calculations and the large size and time scales afforded by empirical models.Comment: 15 pages, 10 figure

Room temperature Bloch surface wave polaritons

Polaritons are hybrid light-matter quasi-particles that have gathered a significant attention for their capability to show room temperature and out-of-equilibrium Bose-Einstein condensation. More recently, a novel class of ultrafast optical devices have been realized by using flows of polariton fluids, such as switches, interferometers and logical gates. However, polariton lifetimes and propagation distance are strongly limited by photon losses and accessible in-plane momenta in usual microcavity samples. In this work, we show experimental evidence of the formation of room temperature propagating polariton states arising from the strong coupling between organic excitons and a Bloch surface wave. This result, which was only recently predicted, paves the way for the realization of polariton devices that could allow lossless propagation up to macroscopic distances

Blockchain and Web of Things for Structural Health Monitoring Applications: A Proof of Concept

Interoperable and secure data management techniques are fundamental for most of large-scale Structural Health Monitoring (SHM) systems. Indeed, given the relevance of SHM critical measurements, data integrity must be protected against tampering or falsifications. In this paper, we propose a four-layer SHM architecture that allows to build an effective data pipeline from sensors to consumer applications, passing through the cloud. The architecture is built on top of the MODRON platform and exploits the recent advances of the W3C Web of Things (WoT) standard for interoperability. We then discuss how third-party services can take benefit of the W3C WoT architecture to retrieve the SHM critical data and to publish them on the Ethereum Blockchain through an SHM-specific Smart Contract, for data protection and traceability purposes. We test the effectiveness of the Smart Contract implementation in terms of latency and costs under simulated workload

Ultrafast flow of interacting organic polaritons

The strong-coupling of an excitonic transition with an electromagnetic mode results in composite quasi-particles called exciton-polaritons, which have been shown to combine the best properties of their bare components in semiconductor microcavities. However, the physics and applications of polariton flows in organic materials and at room temperature are still unexplored because of the poor photon confinement in such structures. Here we demonstrate that polaritons formed by the hybridization of organic excitons with a Bloch Surface Wave are able to propagate for hundreds of microns showing remarkable third-order nonlinear interactions upon high injection density. These findings pave the way for the studies of organic nonlinear light-matter fluxes and for a technological promising route of dissipation-less on-chip polariton devices working at room temperature.Comment: Improved version with polariton-polariton interactions. 13 pages, 4 figures, supporting 6 pages, 6 figure