Research Experience for Undergraduates

Atomic Force Microscopy (AFM) is a form of scanning probe microscopy which can reach resolutions on the order of angstroms. In AFM, a cantilever tip is scanned across the surface of a sample at a close distance. The force interactions between the surface and tip cause the tip to deflect in accordance to Hooke's law. These deflections are detectable by shining a laser on the cantilever and monitoring the laser movement with a photodiode. An example experimental setup is shown in figure 1. Common surface forces that cause the cantilever to deflect primarily include electrostatic forces, Van Der Waal's forces, and chemical bonding forces [1].

In non-contact mode atomic force microscopy (NC-AFM), the cantilever is oscillated at its eigenfrequency (frequency modulation) and a user-defined amplitude (amplitude modulation). Interactions between the surface and the tip cause shifts in the eigenfrequency and amplitude, which can be monitored with very high accuracy using feedback loops [1]. This information can be then used to construct high resolution images. Our particular AFM setup uses an ultra high vacuum environment to take atomic resolution images. Below, figure 2 shows a topographic image of the KBr crystal.