First-Principles Calculations On Interfacial Sliding Friction Properties Of Two-Dimensional Heterostructure In₂Se₃/graphene

When two objects in contact move relative to each other,a force called friction that hinders the movement will be formed on the contact surface.Friction produce inevitable energy loss in daily life and affect the economy.Therefore,it is one of the most important problems in fundamental physics to explicitly illustrate the basic principles and efficiently reduce friction.In the macroscopic scale,the friction force can be described by the law of Da Vinci-Amontons,the friction force F increases with the increase of the normal pressure N,that is,It can be expressed as F=?N,where?is the friction coefficient.When the research scale enters the field of nanometer,the friction appears many exotic phenomena,such as super lubrication(??0),negative friction(?<0).In this thesis,based on first-principles theoretical calculations,we investigate the friction properties of two heterojunctions(p?,p?)composed of two-dimensional ferroelectric materials In₂Se₃ and graphene,In₂Se₃/graphene.It is predicted that the friction coefficient of In₂Se₃/graphene oscillates from?>0 to?<0with the increase of load.Based on the analysis of the interlayer interaction and the intralayer electronic structure of In₂Se₃,it is found that the coupling of electrostatic interaction between the interlayer and deformation of In₂Se₃ leads to the interconversion of friction coefficients.The formation of?<0 interval is closely related to the codirectional coupling of In₂Se₃.The simulation of intrinsic In₂Se₃ shows that the surface charge increases with the increase of pressure in region of?<0,which enhances the electrostatic interaction between In₂Se₃ and graphene,reduces the fluctuation of interlayer spacing,and finally reduces the sliding barrier.In addition,this thesis has further studied how to adjust the electronic structure of In₂Se₃ by applying an electric field to change the sliding barrier.This series of work provides a new research direction for understanding microscopic friction,and propose a new adjustable ferroelectric solid lubricant candidate.The chapter content of this thesis is arranged as follows:The first chapter is introduction.We briefly introduce the history and development trend of friction,from the law of Amontons to the emergence of nano-scale super-lubrication,from lubricants to the study of two-dimensional super-lubricant nano solid lubricants.Then,we introduce the development history of In₂Se₃ and graphene used in this paper model,and analyze the feasibility that to be used in controllable solid lubricant field.In chapter two,we introduce the theoretical methods involved in this study,the development of molecular simulation,the basic knowledge of first principles and the some softwares used in this paper.In chapter three,we in-depth studied the interlayer friction characteristics of the heterojunction.First,we introduce two different heterojunction models,and then propose a method to simulate the pressure exerted on atoms and sliding method between layers at the molecular level.By using this method to study the friction characteristics of the two heterojunctions,we find that the friction between the layers has the oscillation phenomenon of?>0 and?<0 with respect to load.The analysis of the interaction between the layers and the structural deformation of this phenomenon,shows that the deformation of In₂Se₃ and the electrostatic force between the layers have two different couplings.It is demonstrated that the periodic deformation fluctuation of In₂Se₃ under external force and the coupling of these two factors lead to the appearance of?>0 and?<0.Subsequently,the electronic structure analysis shows that the change of heterojunction layer spacing is related to the surface electron redistribution of In₂Se₃under external force.In chapter four,we investigate the friction characteristics of the heterojunction under the condition of an external electric field.It is found that the friction force of the two heterojunctions has different responses to the external electric field.Reasonable explanation is given by studying the transformation of two coupling mechanisms under external electric field.These findings provide a new and feasible scheme for the regulation of nano-friction.The last chapter is the summary and prospect of thesis.This discovery is expected to play an important role in the future design of adjustable two-dimensional solid lubricant.