scanning tunnelling microscopy abbr., STM (rus. микроскопия, сканирующая туннельная abbr., СТМ) — one of the techniques of scanning probe microscopy involving measuring the density of atoms of a surface by alternating the tunnelling current. The technique is used in studies of surfaces of conducting substances and materials at the atomic level and for building three-dimensional images of surfaces. Additionally, it represents one of the technologies that allows creating synthetic nanostructures on the surfaces of substances (materials) by moving certain atoms.

Description

The development of this method in early 1980s earned its inventors, Gerd Binnig and Heinrich Rohrer, the Nobel Prize in Physics in 1986. For this technique, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. The technique can be used not only in ultra high vacuum but also in air, gases and liquids, as well as at temperatures ranging from near zero Kelvin to almost 1000 K.

The scanning tunnelling microscopy (STM) technique is based on the concept of quantum tunnelling. Probe tips are usually manufactured from metal wire (e.g., W, Pt–Ir, Au). The preparation of the atom-tip probe usually includes primary ex situ treatment of the tip (e.g., abrasive polishing, chipping or electrochemical etching) and subsequent in situ treatment in an ultra-high vacuum chamber. When the microscope’s tip is brought very near to the surface to be examined, wave functions of the nearest tip atom and atoms of the sample surface overlap. This happens when the tip-sample spacing reaches 0.5 to 2.0 nm. When voltage is applied between the tip and the sample, tunnelling current will occur in the gap.

Surfaces are scanned using a thin metallic probe that may, in ultimate cases, have just one atom on its tip end. Piezoelectric actuators lower the probe tip to the surface of a conducting object. Piezoelectric two-dimensional manipulators move the probe above the sample surface, following a raster pattern in much the same way as an electron microscope does. Parallel lines of the raster may be spaced by some fractions of a nanometre. The probe moves up and down, repeating the surface relief, controlled by the feedback mechanism that senses when the tunnelling current starts changing and changes the voltage supplied to the third manipulator. The third manipulator moves the probe vertically in such a way that the tunnelling current value remains unchanged, i.e. preserving a constant distance between the probe and the sample. The computer measures these changes in voltage and generates a three-dimensional image of the surface. The microscope’s resolution reaches the atomic level, meaning that the instrument makes it possible to see individual atoms sized ~0.2 nm.

The STM technique allows one or more atoms to be attracted to the probe and makes it possible to move or relocate these atoms by applying a somewhat higher voltage between the sample surface and the probe than in case of regular scanning. Applying certain voltage to the probe, researchers can make atoms move along the surface or separate several atoms from a molecule.

Illustrations

Atomic structure of MoS2 triangular nanoclusters on the golden substrate Au(111) surfa

Atomic structure of MoS2 triangular nanoclusters on the golden substrate Au(111) surface. Author: Mr. Jakob Kibsgaard (University of Aarhus, Denmark). Quoted from SPMage Prize portal, www.icmm.csic.es/spmage/


Authors

  • Gusev Alexander I.
  • Saranin Alexander A.

Sources

  1. Gusev A. I. Nanomaterials, Nanostructures, and Nanotechnologies (in Russian) // Fizmatlit, Moscow (2007) - 416 pp.
  2. Oura K. et al. Surface Science: An Introduction // Springer, 2010 - 452 pp.

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