Bottom-tracking: the possibilities and physical meaning of keeping the bottom of the frequency shift in atomic force microscopy

In ultrahigh vacuum atomic force microscopy, the frequency of an oscillator holding the scanning tip takes a typical Lennard-Jones like curve with a local minima just before contact. We demonstrate here the application of a control scheme to keep this local minima, or the bottom of the frequency shift curve (FC) as the working point for imaging in UHV, and discuss its physical meaning and possible applications. Tip-sample distance modulation and Lock-in detection were used to obtain the gradient of the FC, where null control of the gradient signal resulted in implementing the designated control. Atomic resolution was confirmed for Si(111) and solder. The histogram of minimum frequency shift on the apex of atomic features on solder showed two to four peaks, implying the ability of the method to map characteristic differences of the depth of the FC per site. The method is an alternative to the existing constant-frequency-shift mode and constant-height mode, with the possibility to access chemical information on-the-fly.