Low-Temperature Scanning Tunneling Microscopy

The scanning tunneling microscope (STM) has revolutionized surface science since its invention in 1982 (Binnig and Rohrer, 1982) by providing a means to directly image atomic scale spatial and electronic structure. Using the combination of a coarse approach and piezoelectric transducers, a sharp, metallic tip is brought into close proximity with the sample. The distance between tip and sample is less than 1 nm, which means that the electron wave functions of tip and sample start to overlap. A bias voltage is applied between tip and sample that causes electrons to tunnel through the barrier. The tunneling current is a quantum mechanical effect: tunneling of electrons can occur between two electrodes separated by a thin insulator or a vacuum gap and the tunneling current decays on the length scale of one atomic radius. The tunneling current is in the range of picoamperes to nanoamperes and is measured with a preamplifier. In an STM, the tip is scanned over the surface and electrons tunnel from the very last atom of the tip apex to single atoms on the surface, providing atomic resolution. The exponential dependence of the tunneling current on the tip–sample distance can be exploited to control the tip–sample distance with high precision. There are four basic operation modes for any STM: constant current imaging, constant height imaging, spectroscopic imaging, and local spectroscopy. Their interpretation and realization will be briefly discussed below. For details about other modes and a comprehensive introduction to electron tunneling and STM see Wiesendanger (1994).