First-principles Study Of Nanofriction In Several Typical Two-Dimensional Systems

Friction is closely linked with human life, and the research on the friction phenomena has a long history. Tribology is an interdisciplinary course of the basic theory and practice, which studies friction, wear and lubrication between the interaction surfaces with relative moving, and their mutual relations. Tribology is very important to modern machines involving the sliding and rolling surfaces. Large to space vehicle, and small to using a pen to write, all without exception are closely related to friction and wear. Approximately one-third of the world's energy resources in present use are consumed as friction in one form or another. Therefore, reducing friction and controlling wear are the needs of economic growth and sustainable development. The purpose of research on tribology is understandably the minimization or elimination of losses resulting from friction and wear at all levels of technology where the surface frictions are involved. However, the frictions that people are usually exposed to belong to macro frictions. Long-term practices show that dry frictions of macro systems(no lubricant on the interfaces) follow the three classical tribology laws whose forms are very simple. In 1980s', on the one hand, with the development of nanotechnology, designing nanodevices with the excellent performance prompted people to investigate the frictions and wears of various nanomaterials; on the other hand, the new instruments invented in this period were used to measure at the micro and nano scales. Based on these two reasons, an upsurge of research on nanofriction has been emerging in the world. The emergence of nanotribology marked a new stage in the development of tribology. However, the surface effects, the quantum size effects and the tunneling effects of materials, which are peculiar to the nanometer scale, lead to the properties of nanofriction different from macro friction. Therefore, although nanofriction has been extensively studied as a new field, the friction at nanoscale is still not clearly understood because nanofriction is extremely sensitive to environment, and the findings are lack of reliability and difficult to repeat. And with the exponential growth of computing power, as a bridge of communicating experiment and theory, the simulating researches of nanofriction are also in constant development. Now, the numerical method has been successfully used in the studies on nanofriction properties.In this paper, using first-principles method based on the density functional theory, we investigated the properties of nanofriction of some monolayers and molecules adsorbed on several typical two-dimensional systems, such as the ultrathin Pb(111) films, h-BN, Mn/W(110) surface and graphene. Our findings have important implications for tuning nanofriction, understanding the dissipation mechanisms of nanofriction and producing graphene with the high qualities. The research content of this paper is divided into four parts:1) The friction can be affected dramatically by quantum size effects(QSEs) and edge effects at nanoscale. The modulations of QSEs on nanofriction of rare gas(RG) monolayer sliding on Pb(111) ultrathin films are investigated by using the first-principles approach within density functional theory(DFT) including van der Waals(vdW) interaction correction. Our findings reveal that there exist even-odd oscillations in the friction with the thickness of Pb(111) substrate and the friction can be tuned up to 30% by the different thicknesses of Pb(111) films. Moreover, such modulation is more obvious for the RG adatoms with larger radius. The underlying physics is that the oscillations of the electronic density of states at Fermi level induce different interactions and energy barriers between RG and Pb(111) films with different thicknesses. Overall, we here propose an approach to tune friction and a way to identify the electronic contribution to friction via the different thicknesses of substrates at nanoscale.2) The nanofriction characteristics between two atomically sheets are investigated by Density functional theory(DFT) calculations including dispersion correction. We find that hexagonal boron nitride(h-BN) system exhibits anisotropic interlayer friction, which is different from the isotropic friction of grapheme system. These results can be understood by the difference of electronic structures between the two systems. The charge transfer driven by the difference in electronegativity of B and N atoms leads to charge depletion and accumulation around B and N atoms, respectively, which enlarges the disparity of interaction energy among different stackings. We further confirme the results by the heterostructured models of h-BN on graphene and hydrogenated graphene. The investigations provide an insight into the role of electronegativity in the friction behaviors and a strategy to tune nanofriction of low dimensionally polar materials.3) The van der Waals(vdW) corrected first-principles approach within density functional theory(DFT) is performed to investigate the mechanism of direct exfoliation of graphite into graphene by pyrene-based molecules. Our results show that the interaction energies between pyrene-based molecules and graphene are larger than the interlayer inaction energy of bilayer graphene. However, the frictions of pyrene-based molecules on graphene are lower than the interlayer friction of bilayer graphene. The comparisons of adsorption energies and frictions for different molecules on graphene indicate that the size of friction can be affected by the length and type of the additional chain of pyrene-based molecule. These studies illustrate that the PCA can bond to graphene from adsorption energy view, and can slide on the graphene easily, which explains the experiment very well, and provides a few of alternative molecules to produce the aqueous dispersions of graphene flakes according to different demands.4) Employing first-principles approach within density functional theory(DFT), spin friction properties of Co monolayer sliding on Mn/W(110) substrate are investigated. The magnitude of spin friction is obtained by calculating the friction difference between the spin-polarized and non-spinpolarized cases. Our results reveal that the magnetic system exhibits significantly lower friction than the non-magnetic system, which can be explained by an electronic level mechanism of spins on friction properties. Additionally, this study provides an approach to estimate the spin friction under the formwork of DFT.