Multifrequency techniques for tuning fork based atomic force microscopes: Lateral resolution and additional separate control knobs.

在动态原子力显微镜中存在多种不同的操作模式，其允许以不同的方式激发悬臂。特别是最近研究了所谓的多频技术。在2004年的原始多频模式中，悬臂在两个频率下同时被激励，这两个频率对应于悬臂的第一和第二本征模式。与标准的单模原子力显微镜相比，多频技术提供了更高的信息深度，并且振荡的每个本征模式可以用于不同的目的。通常，第一本征模用于跟踪样品形貌，而第二本征模提供额外的和增强的材料对比度。这可以通过分别调整和优化每个本征模的振荡参数来实现。同时，多频概念适用于不同环境（空气、液体和真空）中的不同样品系统，事实证明，多频操作在横向分辨率和信噪比方面是有益的。然而，迄今为止，对基于音叉的原子力显微镜还没有进行系统的研究。音叉主要用于需要最大横向分辨率的低温特高压应用。在这个项目中，我们将分析哪些成像参数将提供最佳的分辨率/灵敏度时，使用多频技术。接下来，将研究哪种操作模式最合适，以及在图像分辨率方面提供了哪些好处。

In dynamic atomic force microscopy multiple different operation modes exist, which allow to excite the cantilever in different ways. In particular the so-called multifrequency techniques have been studied recently. In the original multifrequency mode from 2004 the cantilever is excited simultaneously at two frequencies, which correspond to the 1st and 2nd eigenmodes of the cantilever. In contrast to standard single mode atomic force microscopy the multifrequency technique offers a higher information depth and each eigenmode of the oscillation can be used for a different purpose. Usually the 1st eigenmode is used for tracking the sample topography while the 2nd eigenmode delivers additional and enhanced material contrast. This can be achieved by separately adjusting and optimizing the oscillation parameters of each eigenmode. Meanwhile the multifrequency concept was adapted for different sample systems in different environments (air, liquid, and vacuum) and it was proven that multifrequecy operation is beneficial in terms of lateral resolution and signal-to-noise ratio. A systematic study for tuning fork based atomic force microscopes, however, has not been performed so far. Tuning forks are mainly used for applications in UHV at low temperatures where the maximum lateral resolution is needed. In this project we will analyze which imaging parameters will offer the best resolution/sensitivity when using the multifrequency technique. Successively, it will be studied which operation modes are most suitable and what benefits are provided with regard to the image resolution.