Scanning tunnel spectroscopy

The scanning tunneling spectroscopy ( English scanning tunneling spectroscopy , STS ) is a method of scanning tunneling microscope . It can be used to measure the local (near-surface) density of states (LDOS) of electrons (or holes , i.e. missing electrons).

functionality

The scanning tunnel spectroscopy is carried out with a scanning tunnel microscope . When a DC voltage is applied between a fine tip and the surface, a small current flows if the distance is sufficiently small. This is created by the tunneling of electrons from occupied states through a barrier, which is created by the distance between the tip and the surface. If the tip scans over the surface, a topography of constant electron density can be recorded. As a first approximation, the tunnel current depends on the distance between the tip and the surface and on the electron density of the surface below the tip. Because the topography is primarily of interest in scanning tunneling microscopy, a DC voltage is used to record an image, which promises a good image result. In contrast, in scanning tunneling spectroscopy, changes are made in a targeted manner, while the tip rests at a constant height on one point. The measured current response is a function of the electron energy, which provides information about the density of states. ${\ displaystyle V}$ ${\ displaystyle V_ {T}}$ ${\ displaystyle I_ {T}}$

{\ displaystyle I \ propto LDOS _ {\ text {tip}} \ int _ {0} ^ {eV} LDOS _ {\ text {sample}} (E) \ mathrm {d} E}

Measurement modes

Variation of the tunnel voltage during imaging

If you want to measure the influence of the tunnel voltage on the tunnel current, you can measure the same surface several times with different tunnel voltage. Alternatively, the voltage can also be changed after each line if it can be assumed that there is a periodic structure (e.g. single crystal).

Point spectroscopy

However, if individual points on the surface are of interest, the tip can be placed specifically over them and then point spectroscopy can be carried out. For this purpose, an alternating voltage is applied to the tip and this is shifted systematically with a DC offset. If one plots, one obtains the density of states of the electrons at this point. It is often difficult to check whether the tip is exactly over the desired location. ${\ displaystyle \ mathrm {d} I / \ mathrm {d} V}$

Image spectroscopy

Another approach is to take pictures and curves at different tunnel voltages. It should be ensured that the distance between the tips and the sample is constant despite the change in the tunnel voltage. Otherwise, data analysis is difficult due to the overlapping of electronic and geometric effects. ${\ displaystyle \ mathrm {d} I / \ mathrm {d} V}$

literature

DP Woodruff, TA Delchar: Modern Techniques of Surface Science . 2nd Edition. Cambridge University Press, Cambridge, GBR 1994, ISBN 978-0-511-62317-2 ( eblib.com [accessed October 26, 2018]).
J. Tersoff, DR Hamann: Theory of the scanning tunneling microscope . In: Phys. Rev. B . tape 31 , no. 2 , 1985, pp. 805-813 , doi : 10.1103 / PhysRevB.31.805 .

Individual evidence

^ Christian Hess: Introduction to Scanning Tunneling Microscopy and Spectroscopy. In: Superconductivity II. IFW Dresden, accessed on October 26, 2018 (English).

[1] Christian Hess: Introduction to Scanning Tunneling Microscopy and Spectroscopy. In: Superconductivity II. IFW Dresden, accessed on October 26, 2018 (English).