Researches On The Surface And Interface Friction Properties Of Two-Dimensional Materials And Their Applications In The Twist-Angle Control

Since the first discovery of atomically thin graphene in 2004,the family of two-dimensional（2D）materials,represented by graphene,molybdenum disulfide（Mo S₂）,and hexagonal boron nitride（h-BN）,has flourished,which aroused strong research interest in many disciplines.People have discovered a wide range of novel physical properties of 2D materials in the fields of electronics,magnetism,optics,thermodynamics,mechanics,etc.Due to their flat and dangling-bond-free surfaces and ultra-thin thickness,2D materials have also been extensively studied in the field of nanotribology.In terms of surface friction,2D materials exhibit properties including friction anisotropy,layer-number dependence,and so on.For the interface of 2D materials,the perfect periodic lattice arrangement makes it easy to form incommensurate contact.Therefore,2D van der Waals（vd W）systems have become excellent platforms for the studies of superlubricity.Exploring the surface and interface friction properties and their influencing factors can give us insight into the mechanism of friction and wear at the microscopic level and enrich and improve the theoretical system of nanotribology.These studies can also guide us to use these properties for better lubrication in microscopic and macroscopic applications,thus extending mechanical life,reducing energy loss,and achieving sustainable development.To this end,this dissertation focuses on the theme of“researches on the surface and interface friction properties of two-dimensional materials and their applications on the twist-angle control”.Mo S₂,graphene,h-BN,and the vd W systems constructed by them are selected as the main research objects.By means of the atomic force microscope（AFM）and other techniques,we conducted our study in two parts,which are“the effects of plasma treatment and oxygen doping on the surface and interface friction properties of 2D material systems”and“using the superlubricity at the interface to achieve high-precision continuous control of the interlayer twist angle of 2D vd W systems”.In the first part,in order to investigate how the surface and interface friction properties of 2D materials,especially the superlubricity at the interface,would change in the complex external environments,so as to better guide the future macroscopic applications of superlubricity,we epitaxially grew Mo S₂on the h-BN and graphite substrates by chemical vapor deposition（CVD）,and systematically studied the effects of surface plasma treatment and oxygen doping on the surface and interface friction properties of the heterostructures.It has been found that the plasma treatment would cause an increase in the friction force between the AFM tip and the upper surface of the Mo S₂.The friction force at the interface of the Mo S₂/h-BN heterostructure also went up.With the increase of the treatment power and time,the elevation of the surface and interface friction force became more significant.However,the coefficient of friction（COF）at the interface always remained in the range of superlubricity.Characterization by scanning tunneling microscopy（STM）indicated that the surface plasma treatment locally induced the phase transition of Mo S₂ from the 2H phase to the 1T phase.The defects generated during the treatment process and the distortions of the atomic arrangement at the phase boundaries were the main reasons for the change in the surface and interface friction properties.Annealing in the sulfur atmosphere could lead to a partial repair of the friction properties of the system.Oxygen doping caused an obvious increase in the friction force between the AFM tip and the upper surface of the Mo S₂.But no significant change occurred in the superlubricity at the interface between the Mo S₂ and the graphite substrate.The STM image showed that most of the doped oxygen atoms were located on the upper surface of the Mo S₂ and the atomic arrangement of the lower surface remained almost unchanged.As a result,the oxygen doping only modulated the surface friction properties of the Mo S₂ and had no obvious effect on the superlubricity at the heterostructure interface.Studying the effects of these factors on the friction properties is essential for us to understand the friction mechanisms of the2D material surface and interface and can better guide the potential applications in the future.In the second part,we took advantage of the superlubricity at the 2D vd W interface to propose a method to achieve high-precision continuous control of the interlayer twist angle of the 2D moirésystem by an AFM tip.Vd W homostructures and heterostructures have always been the research focus of 2D materials.In recent years,the rise of magic-angle graphene has opened up a new field of 2D twistronics,making 2D moirétwisted systems become popular research objects in condensed matter physics.However,there are still some difficulties in the high-quality and high-precision preparation of 2D twist-angle samples with moirésuperlattice.Therefore,we epitaxially grew Mo S₂ on the graphene substrate and achieved a clean interface with atomic-level flatness.Then,the superlubricity at the heterostructure interface was utilized to achieve continuous control of the interlayer twist angle with high precision by an AFM tip.We performed conductive AFM（C-AFM）and spectroscopic characterizations on the samples manipulated by this method and verified the variation of moirésuperlattice,band gap,and strain with the change of the interlayer twist angle.Moreover,this method has been extended to epitaxially grown homostructures with incommensurate contact and 2D vd W systems constructed by transfer and stacking,demonstrating its convenience and universality.These studies realized the application of the superlubricity at the 2D material interface in the microscopic field and provided a feasible way for the preparation of 2D moirétwisted systems.In conclusion,we investigated the influence of external environmental factors on the friction properties of 2D material surface and interface,especially the superlubricity at the interface,and analyzed the microscopic mechanisms affecting the modulation of the friction properties.Also,by the use of the interface superlubricity,we realized the high-precision continuous control of the interlayer twist angle of 2D vd W systems and provided a new method for the preparation of high-quality samples for 2D twistronics.Our work contributed guidance for better utilization of 2D material surface and interface friction properties in the future.