Controlled Movements in Superlubric MEMS

Structural superlubricity(SSL)refers to a state where the friction and wear between two directly contacted solid surfaces are virtually zero.The realization of microscale SSL in 2012 rapidly explored SSL technologies which hold great potential in the development of reliable and energy⁃efficient micro devices.A key to a successful superlubric device is to control the movements of the superlubric slider.To solve this challenge,here two general principles are shown to guide and control the motion of the slider,i.e.,by minimization of interfacial energy and minimization of electrostatic energy.When the shapes of the slider and substrate are designed appropriately,the excess interfacial energy of the contact⁃pair provides restoring and constraining forces to the slider.Similarly,tunable driving and constraining forces are enabled by the electric fields induced by the electrodes buried in the substrate.These concepts are demonstrated on the design of a superlubric resonator whose natural frequency of the lateral translational mode is well⁃defined and unfavorable rotation is constrained.The above design principles should be applicable to superlubric devices in general and help the development of future applications of structural superlubricity.

Recent understanding of solid-liquid friction in ionic liquids

Ionic liquids(ILs)-solid interfacial phenomenon is undoubtedly one of the topic issues in energy related fields,especially bringing great challenges in the development of chemical engineering.Solid-ILs friction under microscale shows outstanding properties such as widely studied electro-controllability by surface potential or electric field,and promising performance of superlubricity.Vast applications,such as boundary lubrication and supercapacitors,have been benefiting from these studies.In this review,we summarized the recent advances of the measurements of solid-ILs friction,and focused on the microscale and fascinating findings.A brief tutorial for this topic,containing both experimental and theoretical methods,was also provided.It was anticipated that this review would help researchers grasp the emerging field and stimulate the advance of new ideas and methods in ILs on solids.

Temperature and velocity dependent friction of a microscale graphite-DLC heterostructure

One of the promising approaches to achieving large scale superlubricity is the use of junctions between existing ultra-flat surface together with superlubric graphite mesas.Here we studied the frictional properties of microscale graphite mesa sliding on the diamond-like carbon,a commercially available material with a ultra-flat surface.The interface is composed of a single crystalline graphene and a diamond-like carbon surface with roughness less than I nm.Using an integrated approach,which includes Argon plasma irradiation of diamond-like carbon surfaces,X-ray photoelectron spectroscopy analysis and Langmuir adsorption modeling,we found that while the velocity dependence of friction follows a thermally activated sliding mechanism,its temperature dependence is due to the desorption of chemical groups upon heating.These observations indicate that the edges have a significant contribution to the friction.Our results highlight potential factors affecting this type of emerging friction junctions and provide a novel approach for tuning their friction properties through ion irradiation.

Elasticity-based-exfoliability measure for high-throughput computational exfoliation of two-dimensional materials

Two-dimensional(2D)materials are promising candidates for uses in next-generation electronic and optoelectronic devices.However,only a few high-quality 2D materials have been mechanically exfoliated to date.One of the critical issues is that the exfoliability of 2D materials from their bulk precursors is unknown.To assess the exfoliability of potential 2D materials from their bulk counterparts,we derived an elasticity-based-exfoliability measure based on an exfoliation mechanics model.The proposed measure has a clear physical meaning and is universally applicable to all material systems.We used this measure to calculate the exfoliability of 10,812 crystals having a first-principles calculated elastic tensor.By setting the threshold values for easy and potential exfoliation based on already-exfoliated materials,we predicted 58 easily exfoliable bulk crystals and 90 potentially exfoliable bulk crystals for 2D materials.As evidence,a topology-based algorithm indicates that there is no interlayer bondingtopology for 93%predicted exfoliable bulk crystals,and the analysis on packing ratios shows that 99%predicted exfoliable bulk crystals exhibit a relatively low packing ratio value.Moreover,literature survey shows that 34 predicted exfoliable bulk crystals have been experimentally exfoliated into 2D materials.In addition,the characteristics of these predicted 2D materials were discussed for practical use of such materials.