First-principles Study On The Electron Origin Of Nano-friction In Two-dimensional Materials

According to statistics,friction consumes more than one-third of primary energy,while wear causes nearly 80%of mechanical failures.Therefore,it is an urgent problem to find a way to reduce friction to lower energy consumption and extend the service life of machinery.With the rise of atomic thin two-dimensional materials,graphene（Gr）,hexagonal boron nitride（h-BN）and molybdenum disulfide（MoS₂）are considered as the most typical,most widely studied and promising friction-reducing and abrasion materials in the field of interface lubrication,showing great potential in various nano-friction control methods.These atomic-scale friction tuning methods are continuously improved to promote the development of interface lubrication,however,the understanding at the atomic level gradually shows that it is not up to the disclosure of friction control mechanism.With the deepening of research,many sporadic studies have pointed to the essence of lubrication mechanism as"electron redistribution".Therefore,this study will attempt to establish qualitative or quantitative relations between electron density and friction performance under various control means by using first-principles simulation,so as to promote the understanding of friction control mechanism and even explore new control strategies.1.The interlayer interaction of two-dimensional layered materials at the nanoscale is the overlap of electron clouds outside the nucleus.When the interlaminar sliding occurs in layered materials,the contribution of electrons to the interlaminar interaction is the main one.Therefore,it is expected to change the interlayer interaction by the electron redistribution of the system,for the purpose of regulating the interlaminar sliding friction.Using the first principle calculation,we found that the interlaminar sliding tribological properties of the two-layer hexagonal boron nitride can be improved by introducing impurity carbon atoms to modify its electronic structure.The research shows that the variation of the energy barrier increases along the slip path and the position of the lowest energy level with the increase of the normal load have a good consistency.Namely,the strength of the interlayer interaction can be reflected by the lowest energy level position,which is reason that the fluctuation of the interlayer sliding potential energy increases with the increase of the load in all BN/BN bilayer.Therefore,our work theoretically provides support for atomic doping to improve the tribological properties of two-dimensional materials.Meanwhile,it is proved that electron redistribution is an effective method to improve tribological properties of two-dimensional structures.2.Previously,abundant researches have pointed out that the relative potential energy of the interface sliding and the nature of the band gap tuning can be traced back to the microscopic electronic structure.However,the intrinsic relationship between the bandgap and relative stability（or relative potential energy）of different stacks and the electrons distribution in bilayers has not been exposed.Using the first-principles calculation,our study shows that there is a negative correlation between the bandgap and the sliding barrier when interlayer sliding along the x-axis occurs in h-BN/h-BN bilayers.After revealing how the electrons distribution determines the shifts of energy barrier and bandgap,respectively,we conclude that the competitive distribution mechanism of electrons in interlayer and in-plane is the root cause of the negative correlation between the both:due to periodic boundary conditions,the total number of electrons in a given h-BN/h-BN bilayers system is constant.Therefore,our work unmasked the essential relationship between electrical and tribological performance of the h-BN/h-BN bilayers with a novel perspective,which will strongly promote the understanding and investigation of the complex microscopic interactions between interfaces.3.Because one dimension has reached the thickness of the nanometer,size is of undeniable importance for two-dimensional nanomaterials.In particular,the thickness of 2D nanosheet play even a decisive factor in determining its own performance,such as the interlayer binding strength and the bandgap adjustment depend on charge transfer among layers.However,neither experimental nor theoretical studies,the so-called“intralayer and interlayer”of two-dimensional structure are still two ambiguous concepts on account of the absent on well-defined boundary between them.Hence,we take the electron distribution into account to delimit the boundary between intralayer and interlayer in two-dimensional structures in this study.This section combines the first principles to calculate the electron density distribution,the full widths at half maximum（FWHM）of the two peaks on the distribution curve of the total electron density are defined as the intralayer zones（i.e.,the thickness of bottom and top atomic layer）,while the remaining between the both layers belongs to the layer-to-layer interaction zones.To verify the rationality and applicability of our theoretical method,we applied it into revealing physical origins of the negative（h-BN/h-BN bilayer）and positive（MoS₂/MoS₂ bilayer）correlations between the PES wrinkles and E_g fluctuations from the perspective of electron distribution successfully.Undoubtedly,our delimiting method would be enormous meaningful for understanding the origin of these physical issues and guiding the experimental researches in above fields,such as stack faulting energy,super-lubricity and transitions between polymorphs.4.Recently years,strain,with the costless and maneuverable characteristics,has gradually developed into a crucial method to improve the properties of two-dimensional nanomaterials（TNMs）.The incompleteness of strain transfer among adjacent layers give rise to the shifts in stacking configuration of two-dimensional atomic layers since the interlayer coupling is feeble,which formed the prevalent view at present:the incommensurate contact[registry index（RI）]is the decisive factor for strain effect in TNMs.However,to our best knowledge,the strain effect on TNMs with lossless transfer among the adjacent layers has never been reported in any literature.In this section,combining the first-principles calculations,we performed a comprehensive research on the contact strain effect.Analyses of static energetics indicated that the binding energy of vertical separation and energy barrier of lateral sliding possess opposite trend with the application of in-plane strain:compression facilitates lateral sliding while stretch avails vertical separation between layers.Further quantification results of electron redistribution confirmed that interlayer electron density（ρinter-）and its relative value（?ρinter-）are responsible for the vertical and lateral effects of strain respectively.This original theoretical research not only reveals that the strain can manipulate interlayer coupling effectively even if in the commensurate-contact state without mismatched,but also provides a novel insight into the pivotal roles of improvement of tribological properties in strain engineering of TNMs.