Scanning tunneling microscopy (STM)

Introduction

After the invention of scanning tunneling microscopy in 1981 by Binnig and Rohrer, a flora of other probe micrcopies has emerged. However, STM is still the most important scanning probe microscopy method today. Perhaps the most fascinating feature of STM is its ability to provide local, real-space information of surfaces with atomic resolution, that is, chracterising surface structures atom by atom.

Principle of operation

The principle of a scanning tunneling microscope (STM) is extremely simple. The STM measurement is performed by raster scanning a sharp tip over the sample at a distance of few Ångström units (1 Å = 0.1 nm). Once a bias voltage between the tip and the conducting sample has been applied, a current can flow due to the quantum mechanical tunneling effect, although the tip and the sample are not in contact with each other. To a first-approximation, the tunneling current depends exponentially on the bias voltage, the tip-sample distance and the tunneling barrier height. A feedback system keeps the tip-to-sample distance constant. The feedback signal provides the information of the structural details of the surface. As a result high resolution can be obtained down to the atomic scale, i.e. about 0.5 Å laterally and 0.01 Å vertically. 

The figure below (on the left) demonstrates a constant current topographic STM image of the Au/TiO2(110)-(1x1) surface showing Au nanoclusters as bright protrusions on a TiO2(110) support (the image size is 30 nm x 30 nm). Individual Ti atom rows separated by 0.65 nm can clearly be resolved corresponding to the length of the unit cell along the [1-1 0] direction of the unreconstructed TiO2(110)-(1x1) surface (see the figure on the right, the image size is 6 nm x 6 nm).

Apart from allowing us to investigate the structural properties in great detail, STM also allows us to study the local electronic structure of the surface in a form of scanning tunneling spectroscopy (STS) measurements. STS spectra can be recorded during the constant current imaging by stopping the scan at a certain point of interest, interrupting the STM feedback loop and measuring the tunneling current as a function of the bias voltage. These I-V curves can then be correlated with the corresponding geometric features on the surface.

Applicability

In contrast to electron microscopes and many surface analytical methods using electrons, STM can be operated in various gas phases, in liquids as well as in vacuum because there are no free electrons involved in the STM measurements. The real-space information is particularly important for investigating non-periodic features of surfaces, such as defects and other structural and chemical inhomogeneties. Therefore, STM is most suitable for studying multicomponent materials, polycrystalline samples with grain boundaries, composites and nanostructured materials.