电场下二维材料微观摩擦特性研究与摩擦调控

摩擦在自然界和工程系统中扮演着至关重要的角色，因此深入理解摩擦机制并实现动态调节滑动接触处的摩擦力具有显著的意义。了解材料的电子性质是研究摩擦机制的核心问题之一，但目前对电子摩擦机制的理解仍然不够充分。利用电场来改变材料的电子性质，并研究和调控电子摩擦耗散过程，是一种很好的方法。二维材料由于其低摩擦、高耐磨性、卓越的电学特性以及光滑的表面和规则的原子结构，成为理想的摩擦实验研究对象。因此，本研究基于原子力显微镜技术，分别对拥有内置电场的二维Janus过渡金属硫族化合物以及通过外部电场调控载流子浓度的二硫化钼和石墨烯进行了摩擦性质研究。以下将详细介绍这两部分的具体研究内容。

在第一部分中，通过合适的方法调整过渡金属硫族化合物（TMDs）的原子构型，得到了具有面外偶极子的Janus 结构材料。对MoS₂，MoSe₂，Janus MoSSe和Janus MoSeS四种材料进行摩擦测试，观察到了面外偶极子引起的摩擦增强现象。通过粘附力和原子粘滑实验，提出了一个假设，即滑动能量势垒随偶极子大小的增加而线性增强。通过进一步地测试Janus TMDs在正向堆叠与反向堆叠下的摩擦力，发现实验结果符合提出的理论假设，也为实现摩擦的主动调控提供了一种可行方案。

第二部分研究了外电场调控单层二硫化钼与单层石墨烯载流子浓度对表面摩擦的影响，并通过扫描开尔文探针显微镜(Scanning Kelvin Probe Microscopy, SKPM)验证了摩擦测试中载流子的成功调控。实验发现二硫化钼载流子浓度对摩擦的影响与接触材质有关，而石墨烯的表面摩擦几乎不受载流子浓度变化影响。提出是由于静电作用导致的摩擦力变化。通过粘附力的测量中发现了静电作用对载流子浓度敏感，在高载流子浓度静电作用更强，低载流子浓度较弱。在导电材料如石墨烯或金属存在的系统中，静电荷产生受抑制，对摩擦影响微小。这些发现对设计和优化电场环境下的二维固体润滑系统具有重要意义。

Friction plays a crucial role in both natural phenomena and engineering systems, so gaining a deep understanding of friction mechanisms and achieving dynamic control over frictional forces at sliding interfaces is of significant importance. Understanding the electronic properties of materials is a key aspect of studying friction mechanisms, but the current understanding of electronic friction mechanisms remains incomplete. Utilizing electric fields to modify the electronic properties of materials and investigate and control the dissipation process of electronic friction is a promising approach. Two-dimensional materials are ideal subjects for experimental friction studies due to their low friction, high wear resistance, exceptional electrical properties, and smooth, regular atomic structures. Therefore, based on atomic force microscopy techniques, this study investigated the frictional properties of two-dimensional Janus transition metal dichalcogenides with built-in electric fields and monolayer molybdenum disulfide and graphene whose carrier concentrations are controlled by external electric fields. The specific research contents of these two parts are detailed below.

In the first part, Janus structured materials with out-of-plane dipoles were synthesized by adjusting the atomic configuration of transition metal dichalcogenides (TMDs). Friction tests conducted on MoS₂, MoSe₂, Janus MoSSe, and Janus MoSeS revealed an enhancement in friction attributed to the presence of out-of-plane dipoles. Through measurements of adhesion forces and atomic stick-slip experiments, we hypothesized that the sliding energy barrier linearly increases with the size of the dipole. Further testing of the frictional forces under forward and reverse stacking of Janus TMDs confirmed our theoretical assumption, offering a viable strategy for the active control of friction.

The second part investigated the effect of externally controlled carrier concentration on surface friction in monolayer MoS₂ and monolayer graphene, with successful modulation of carrier densities verified through Scanning Kelvin Probe Microscopy (SKPM) technology. It was discovered that the influence of carrier density on friction for molybdenum disulfide is dependent on the contact material, whereas the surface friction of graphene is almost unaffected by changes in carrier density. We propose that variations in frictional force are due to electrostatic interactions, which are sensitive to carrier density; stronger at high carrier densities and weaker at low densities. In systems containing conductive materials, such as graphene or metals, the generation of static charges is suppressed, minimally affecting friction. These findings hold significant implications for the design and optimization of two-dimensional solid lubrication systems in electric field environments.

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