A First-principles Study Of Charge Transfer Mechanism Of Atomic-scale Friction

Friction is a ubiquitous phenomenon.Friction theory has been a subject of research and exploration by scholars from ancient times to the present.According to classical friction theory,the friction force is directly proportional to the loads and is independent of contact area.The advent of Amundon's law is a great progress of the macro friction theory.With the development of experimental conditions and computer technology,researchers have gradually focused on friction at the micro and even nanoscale.A number of micro-friction theories have been proposed in which the relatively wellknown theory is the energy dissipation mechanism.It indicates that sliding needs to overcome the energy barrier in the forward direction.The larger the barrier,the greater the friction.How does the potential barrier affect the friction,or what is the essence of the potential barrier affecting friction? Little is known about it! Some scholars have suggested that the redistribution of charges during the friction process may be one of the reasons.However,there is always a lack of systematic quantitative means to establish a relationship with potential energy,and hence predict frictional properties.Based on it,we investigated a variety of friction systems systematically through the first-principles to explore the essential relationship between friction,potential energy and charge transfer.The main contents and conclusions are as follows:1.Atomic-scale friction adjustment enabled by doping-induced modification in graphene nanosheet.As a typical atomically-thin solid lubricant,graphene is widely utilized and investigated.However,the existence of defects in engineering graphene seriously damage its lubricating property.In this work,we theoretically reported a new method to repair graphene defects and thus improve its frictional properties by doping with boron and nitrogen.The results show that,compared with boron-doped graphene,nitrogen-doped graphene has a better doping effect and exhibits more excellent frictional properties,which is ascribe to the lower the interlayer interactions caused by the electrostatic repulsion between layers.We also clarify that van der Waals and electrostatic interactions hold a special distribution ratio toward the frictional properties of different doped species.In addition,by observing the charge transfer of different doping systems,the inherent characteristics of charge introduction are clarified,which is the essential reason that affects the atomic-scale friction.2.First-principles theory of atomic-scale friction explored by an intuitive charge density fluctuation surface.Atomic-scale friction theory,even superlubricity,is inseparable from charge redistribution,but lacking of a bridge to establish the potential link between electrical and tribological properties.Here,we performed a DFT calculation on a series of bilayer systems of different bonding materials,including metal bonds,covalent bonds,and van der Waals bonds,during which we first reported a quantified charge density fluctuation surface(CDFS)that reflecting the origin of friction.By analyzing the CDFSs,it is found that a key physical quantity that controlling friction is the charge density fluctuation between layers during friction process.Such charge density fluctuation surface(CDFS)and potential energy surface(PES)hold perfect anti-corrugated features,especially for van der Waals bonding materials,an mutual recognition between CDFS and PES can be achieved by defining the conversion factor K.The combination research between them can carry out in-depth micro-friction exploration in many aspects from the appearance and the essence.3.Atomic-scale rolling friction theory: An integrated study of physical deductions and DFT simulations.We have already studied the interlayer charge transfer mechanism of micro-sliding friction system very deeply,so is the atomic-scale rolling friction still related to the charge transfer at the contact interface? Using DFT simulations,as well as a simplified physical model arising from the one-dimensional Prandtl-Tomlinson(PT)model,we investigated the atomic-scale motions of nanotubes on a graphene substrate.Such a comprehensive study shows that rolling and sliding on the nanoscale exhibit distinct frictional behaviors.We found superlubricity in rolling mode while it not appear in sliding systems.Through a combined study of the PESs and CDFSs,it indicates that the rolling mode of the nanotube exists directional locking characters,thereby achieving a perfect rolling process.Finally,we clarify that the key physical quantity that determines the atomic-scale frictional properties,whether it is rolling or sliding,is the charge density fluctuations in the contact area of a given friction system.