The tunneling microscope is an extremely powerful tool for studying the electronic structure of solid-state systems. Its topographical images aid in the application of chemical-specific surface analysis techniques, leading to a structural definition of the surface. You can learn about the device, functions and meaning, as well as see a photo of a tunneling microscope in this article.
Creators
Before the invention of such a microscope, the possibilities of studying the atomic structure of surfaces were mainly limited to diffraction methods using beams of x-rays, electrons, ions and other particles. The breakthrough came when Swiss physicists Gerd Binnig and Heinrich Rohrer developed the first tunneling microscope. They chose the surface of gold for their first image. When the image was displayed on a television monitor, they saw rows of precisely arranged atoms and observed wide terraces separated by steps one atom high. Binnig and Rohrerdiscovered a simple method for creating a direct image of the atomic structure of surfaces. Their impressive achievement was recognized with the Nobel Prize in Physics in 1986.
Precursor
A similar microscope called the Topografiner was invented by Russell Young and his colleagues between 1965 and 1971 at the National Bureau of Standards. It is currently the National Institute of Standards and Technology. This microscope works on the principle that the left and right piezo drivers scan the tip above and slightly above the sample surface. The central piezo-controlled server drive is controlled by the server system to maintain a constant voltage. This results in a permanent vertical separation between tip and surface. The electron multiplier detects a tiny fraction of the tunneling current that is dissipated on the surface of the sample.
Schematic view
The Tunneling Microscope Assembly includes the following components:
- scanning tip;
- controller to move the tip from one coordinate to another;
- vibration isolation system;
- computer.
The tip is often made of tungsten or platinum-iridium, although gold is also used. The computer is used to improve the image through image processing and to make quantitative measurements.
How it works
The principle of operation of the tunnelmicroscope is quite complicated. The electrons at the top of the tip are not limited to the region inside the metal by the potential barrier. They move through the obstacle like their movement in metal. The illusion of freely moving particles is created. In reality, electrons move from atom to atom, passing through a potential barrier between two atomic sites. For each approach to the barrier, the probability of tunneling is 10:4. Electrons cross it at a speed of 1013 per second. This high transmission rate means that the movement is substantial and continuous.
By moving the tip of the metal over the surface for a very small distance, overlapping the atomic clouds, an atomic exchange is performed. This creates a small amount of electrical current flowing between the tip and the surface. It can be measured. Through these ongoing changes, the tunneling microscope provides information about the structure and topography of the surface. Based on it, a three-dimensional model is built on an atomic scale, which gives an image of the sample.
Tunneling
When the tip moves close to the sample, the distance between it and the surface decreases to a value comparable to the gap between adjacent atoms in the lattice. The tunnel electron can move either towards them or towards the atom at the tip of the probe. The current in the probe measures the electron density on the surface of the sample, and this information is displayed on the picture. The periodic array of atoms is clearly visible on materials such as gold, platinum, silver, nickel and copper. vacuumtunneling of electrons from the tip to the sample can occur even though the environment is not a vacuum, but filled with gas or liquid molecules.
Formation of barrier height
Local barrier height spectroscopy provides information on the spatial distribution of the microscopic surface work function. The image is obtained by point-by-point measurement of the logarithmic change in the tunnel current, taking into account the transformation into a dividing gap. When measuring the barrier height, the distance between the probe and the sample is modulated sinusoidally using an additional AC voltage. The modulation period is chosen to be much shorter than the feedback loop time constant in a tunneling microscope.
Meaning
This type of scanning probe microscope has enabled the development of nanotechnologies that must manipulate nanometer-sized objects (smaller than the wavelength of visible light between 400 and 800 nm). The tunneling microscope clearly illustrates quantum mechanics by measuring the shell quantum. Today, amorphous non-crystalline materials are observed using atomic force microscopy.
Silicon example
Silicon surfaces have been studied more extensively than any other material. They were prepared by heating in vacuum to such a temperature that the atoms were reconstructed in a evoked process. The reconstruction has been studied in great detail. A complex pattern formed on the surface, known as Takayanagi 7 x 7. The atoms formed pairs,or dimers that fit into rows extending across the entire piece of silicon under study.
Research
Research on the principle of operation of a tunneling microscope led to the conclusion that it can work in the surrounding atmosphere in the same way as in a vacuum. It has been operated in air, water, insulating liquids and ionic solutions used in electrochemistry. This is much more convenient than high vacuum devices.
The tunneling microscope can be cooled to minus 269 °C and heated to plus 700 °C. Low temperature is used to study the properties of superconducting materials, and high temperature is used to study the rapid diffusion of atoms through the surface of metals and their corrosion.
The tunneling microscope is used primarily for imaging, but there are many other uses that have been explored. A strong electric field between the probe and the sample was used to move the atoms along the surface of the sample. The effect of a tunneling microscope in various gases has been studied. In one study, the voltage was four volts. The field at the tip was strong enough to remove the atoms from the tip and place them on the substrate. This procedure was used with a gold probe to make small gold islands on a substrate with several hundred gold atoms each. During the research, a hybrid tunneling microscope was invented. The original device was integrated with a bipotentiostat.
