The general principle behind STM is quite elegant. A piezo-electric scanner is used to accurately position an atomically sharp tip above a sample. Changing the position in the lateral (x,y) plane allows to scan continuously across the sample surface and changing the vertical (z) position allows to maintain desired tip-sample distance. If that distance becomes small enough (10 - 0.1 Angstroms) and voltage is applied between the sample and the tip, the tunneling current (1 - 0.1 nA) can be observed. This current depends exponentially on the tip-sample separation. Therefore if a feedback loop is used to adjust the vertical position to keep the current constant (constant current scanning mode) tip-sample separation can be kept constant with great precision. Alternatively the z-coordinate can be held constant and the tunneling current recorded. Since the current is essentially proportional to the density of electronic states in the sample, the first method maps constant density of states contours and the second method maps the actual density of states.
Schematic of Basic STM Setup. Atomically sharp tip is mounted on 3 piezo crystals that allow precise positioning in 3 directions. Moving in (x,y) plane scans the tip across the sample, z piezo determines the tip-sample distance.
The sample surface in the above schematic is represented by a 3D view of an actual
STM image of
7x7 reconstruction on
Silicon (111) surface.
Prof. Ellen Williams and her University of Maryland group present a more detailed introduction to STM and Si(111) surfaces.
The pioneers of STM technique at IBM have more information and links on the STM and other scanning probe techniques as well as some great images on their Nanotechnology page.
