Experimental methods

STM/STS offers the unique opportunity to examine individual nanostructures with atomic resolution and to measure their spectra. A very fine tip is brought so close to a surface that a tunnel current flows. If the tunnel current I is measured as a function of the applied voltage V at a fixed distance, a signal is obtained that is proportional to the integral over the density of states of the sample. In particularly stable microscopes, it is possible to use lock-in technology to measure a dI/dV signal that is directly proportional to the local density of states. In nanostructures, the electronic states are quantized. This leads to so-called Van Hove singularities in the density of states in one-dimensional structures and to discrete eigenstates in quantum dots. The lifetime can be measured from the broadening of the energy levels. If the tip is scanned over the surface, information about the local density of states and the structure of the surface can be obtained with atomic resolution. In our group, we have a low-temperature tunneling microscope in operation that is cooled with liquid helium and operates at T = 5 K.  It is so stable against thermal drift and vibrations that high-resolution spectroscopy can be carried out.

You can find a very good overview of the diverse measurement possibilities with an STM/STS in the book by Bert Voigtländer, Scanning Probe Microscopy, Atomic Force Microscopy and Scanning Tunneling Microscopy, Springer-Verlag Berlin Heidelberg, 2015

STM image of an Au(111) surface with atomic resolution. The reconstruction of the surface (herringbone reconstruction) is also visible.

We also use the LEED method to obtain information about periodicities on surfaces. Due to the short range of the slow electrons, the method is very surface-sensitive.

The AES method provides us with information about the chemical composition of the surface.

A short introduction to both methods can be found, for example, in the book Surface Physics, Fundamentals and Methods by Thomas Fauster, Lutz Hammer, Klaus Heinz, and Alexander Schneider, Walter de Gruyter Verlag, Berlin/Boston, 2020

LEED image of an Au(111) surface measured at about 120 eV.

In angle-resolved photoemission, the sample is irradiated with UV light of a certain energy between 10 and 50 eV. The photoelectric effect triggers electrons that pass into the vacuum. From the direction and energy of the electrons, the energy and momentum of the electrons inside the crystal can be determined with very high accuracy. This method is used to measure the band structure of solids. The line width is inversely proportional to the lifetime of the electronic states and therefore provides access to the measurement of correlation effects. The advantage over methods from classical solid-state physics is the high resolution in reciprocal space (k-space), which makes it possible to measure the coupling of individual states at specific k-space points. In contrast, classical methods such as transport or cyclotron resonance frequency integrate over areas of the Brillouin zone. The increase in effective mass due to electron-phonon coupling can be measured particularly reliably with ARPES.

A brief introduction to ARPES can be found, for example, in the book Surface Physics, Fundamentals and Methods by Thomas Fauster, Lutz Hammer, Klaus Heinz, and Alexander Schneider, Walter de Gruyter Verlag, Berlin/Boston, 2020

High-resolution photoelectron spectrometer