Quantifying the phononic contribution to friction at the single-atom scale

在全球范围内，20%的能源被用来克服摩擦。尽管摩擦力很重要，但它通常用经验公式来解释。为了超越这一点，朝着预测摩擦行为的未来目标前进，需要从基本物理原理开始描述能量耗散途径。其中一种途径是通过声子耗散能量。即使材料没有磨损，声子也会从滑动表面转移能量。研究声子摩擦效应的一种方法是比较以氢为末端的表面和以氘为末端的表面的摩擦特性。由于氢(H)和氘(D)除了质量外非常相似，摩擦的差异归因于通过声子的能量耗散的差异。在真空中，可以制备干净和原子平坦的硅表面，并使H或d饱和。尽管这两种表面之间的摩擦有所不同，但关于这种差异是由于声子差异还是由于不同数量的不饱和表面键，文献中一直存在争论。这是因为没有实验既研究表面又用原子分辨率测量横向力。在这个项目中，我将侧向力显微镜应用于这个突出的问题，并对两个样品系统进行测量，以确定原子精确尖端在H端和d端表面上横向移动时的能量耗散。利用低温非接触侧向力显微镜，可以在单原子水平上对材料的表面和尖端进行表征。通过测量单个原子上的横向尖端振荡的阻尼，我们将精确地表征这种能量耗散。这些测量将使我们不仅能够回答正在进行的声子差异与悬垂键密度的问题，而且，通过应用最先进的原子力显微镜技术，可以在原子尺度上表征声子摩擦。

On a global scale, 20% of the energy produced is used to overcome friction. Despite the importance of friction, it is most often explained with empirical formulas. To move beyond this, towards the future goal of predicting frictional behavior, a description of the energy dissipation pathways is required starting from basic physical principles. One of these pathways is energy dissipation via phonons. Even without material wear, phonons transfer energy away from the sliding surfaces. One method to study the effect of phononic friction is to compare the frictional properties of surfaces terminated in hydrogen to surfaces terminated with deuterium. As hydrogen (H) and deuterium (D) are very similar apart from their mass, differences in friction are attributed to differences in energy dissipation via phonons. In vacuum, clean and atomically-flat silicon surfaces can be prepared and saturated with H or D. Although a difference in friction between these two surfaces was reported, there is an ongoing debate in the literature as to whether this difference is due to phononic differences or due to a different number of unsaturated surface bonds. This is because no experiment has both studied the surface and measured lateral forces with atomic resolution. In this project, I apply lateral force microscopy to this outstanding question and perform measurements on two sample systems to determine the energy dissipation as an atomically-precise tip moves laterally over H- and D-terminated surfaces. With a low-temperature non-contact lateral force microscope, both the surface and tip apex can be characterized at the single atom level. By measuring the damping of the lateral tip oscillation over individual atoms we will characterize this energy dissipation precisely. These measurements will allow us not only to answer the ongoing question of phononic differences versus a dangling bond density but moreover, by applying this state-of-the-art atomic force microscopy technique, to characterize phononic friction at the atomic scale.