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

在全球范围内，20%的能源用于克服摩擦。尽管摩擦的重要性，它是最常见的解释与经验公式。为了超越这一点，朝着预测摩擦行为的未来目标，需要从基本物理原理开始描述能量耗散途径。这些途径之一是通过声子的能量耗散。即使没有材料磨损，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氢终止表面与氘终止表面的摩擦性质。由于氢（H）和氘（D）除了质量之外非常相似，摩擦的差异归因于通过声子的能量耗散的差异。在真空中，可以制备干净且原子级平坦的硅表面，并用H或D饱和。虽然这两个表面之间的摩擦差异的报道，有一个正在进行的辩论，在文献中是否这种差异是由于声子的差异，或由于不同数量的不饱和表面键。这是因为没有实验既研究了表面，又测量了原子分辨率的横向力。在这个项目中，我应用横向力显微镜这个悬而未决的问题，并进行测量两个样品系统，以确定能量耗散作为一个原子级精确的尖端横向移动的H-和D-终止表面。利用低温非接触侧向力显微镜，可以在单原子水平上表征表面和尖端。通过测量单个原子的横向尖端振荡的阻尼，我们将精确地描述这种能量耗散。这些测量将使我们不仅能够回答目前的问题，声子的差异与悬挂键密度，但此外，通过应用这种最先进的原子力显微镜技术，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.