Atom tracking AT 5
Dynamically track local maxima or minima and compensate for drift in 3D
The Atom tracking module adds atom tracking capability to the Nanonis base package. It is designed to track topographical features (not only atoms) dynamically and can therefore measure and compensate for thermal drift and sample tilt. A fully automated procedure automatically calculates drift velocity and sample tilt in both X and Y-directions as well as drift in Z-direction and compensates for these. This module is of particular interest when the tip position has to follow a local extremum (e.g. an atom or molecule, maximum or minimum) between point-spectroscopy or when scanning a small scan area where drift is highly noticeable.
Combined with custom spectroscopy scripts the atom tracking module is a powerful tool. It ensures that measurement sequences lasting several hours can be performed without losses in postitioning accuracy. Its tight integration with the Nanonis software simplifies this otherwise cumbersome operation significantly.
The atom tracking module works according to the following principle: by applying a sinusoidal modulation to the x and y-coordinate, the tip defines a rotational plane. The z-position of the tip is then demodulated with two lock-in detectors using this modulation as a reference, leading to the slope along x and y directions, that is ∂z/∂x and ∂z/∂y. These signals are then used as the input for the I-controller feedback loop, which adjust the tip x/y position in order to minimize the slopes, i.e. the tilt of the plane in which the tip rotates. In this way, the tip x/y position is locked on top of a local extremum which can be an atom, molecule, island, pit or any other local feature