Introduction to Scanning Tunneling Microscopy

Abstract

Scanning tunneling microscopy (STM) was invented by Binnig and Rohrer (see Fig. 2.1) [2,9]. Using the combination of a coarse approach and piezoelectric transducers, a sharp, metallic probing tip is brought into close proximity with the sample. The distance between tip and sample is only a few angstrom units, which means that the electron wave functions of tip and sample start to overlap. A bias voltage between tip and sample causes electrons to tunnel through the barrier. The tunneling current is in the range of pA to nA and is measured with a preamplifier.¹ This signal is the input signal of the feedback loop, which is designed to keep the tunneling current constant during (x, y)-scanning. The output signal is amplified (high voltage amplifier) and connected to the z-piezo. According to the feedback output voltage and the sensitivity of the piezo (typically nm/V) the tunneling tip is moved backwards or forwards and the tunneling current is kept constant during acquisition of the image. This operation mode is called constant current mode.