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Structure and stability of semiconductor tip apexes for atomic force microscopy

机译：原子力显微镜用半导体针尖的结构和稳定性

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摘要

The short range force between the tip and the surface atoms, that is responsible for atomic-scale contrast in atomic force microscopy (AFM), is mainly controlled by the tip apex. Thus, the ability to image, manipulate and chemically identify single atoms in semiconductor surfaces is ultimately determined by the apex structure and its composition. Here we present a detailed and systematic study of the most common structures that can be expected at the apex of the Si tips used in experiments. We tackle the determination of the structure and stability of Si tips with three different approaches: (i) first principles simulations of small tip apexes; (ii) simulated annealing of a Si cluster; and (iii) a minima hopping study of large Si tips. We have probed the tip apexes by making atomic contacts between the tips and then compared force–distance curves with the experimental short range forces obtained with dynamic force spectroscopy. The main conclusion is that although there are multiple stable solutions for the atomically sharp tip apexes, they can be grouped into a few types with characteristic atomic structures and properties. We also show that the structure of the last atomic layers in a tip apex can be both crystalline and amorphous. We corroborate that the atomically sharp tips are thermodynamically stable and that the tip–surface interaction helps to produce the atomic protrusion needed to get atomic resolution.
机译：尖端和表面原子之间的短程力主要由尖端顶点控制，这是原子力显微镜（AFM）中原子尺度对比的原因。因此，成像，操作和化学识别半导体表面中单个原子的能力最终取决于顶点结构及其组成。在这里，我们对最常见的结构进行了详细而系统的研究，可以在实验中使用的Si尖端的顶点进行预期。我们通过三种不同的方法来解决硅尖端结构和稳定性的确定问题：（i）小尖端顶点的第一原理模拟; （ii）Si团簇的模拟退火; （iii）对大硅尖端的最小跳变研究。我们通过在尖端之间进行原子接触来探测尖端的顶点，然后将力-距离曲线与通过动态力谱获得的实验性短程力进行比较。主要结论是，尽管对于原子尖锐的顶点有多个稳定的解决方案，但它们可以分为具有特征原子结构和特性的几种类型。我们还表明，尖端顶点中最后一个原子层的结构可以是结晶的也可以是非晶的。我们证实原子尖的尖端在热力学上是稳定的，并且尖端与表面的相互作用有助于产生获得原子分辨率所需的原子突起。

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