A tunneling microscope is an extremely powerful tool for studying the electronic structure of solid state systems. His topographic images help with the application of surface methods of analysis with chemical specificity, leading to structural determination of the surface. You can find out about the device, functions and value, as well as see a photo of the 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 by diffraction methods using beams of X-rays, electrons, ions, and other particles. The breakthrough came when the 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 Rohrer discovered a simple method for creating a direct image of the atomic structure of surfaces. Their impressive achievement was recognized by the Nobel Prize in Physics in 1986.
Predecessor
A similar microscope called Topografiner was invented by Russell Young and his colleagues from 1965 to 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 surface of the sample. The central piezo-controlled server drive is controlled by the server system to maintain constant voltage. This leads to a constant vertical separation between the tip and the surface. The electron multiplier detects a tiny fraction of the tunneling current that scatters on the surface of the sample.
Schematic view
The tunnel microscope device includes the following components:
- scanning tip;
- a controller for moving the tip from one coordinate to another;
- vibration isolation system;
- a computer.
The tip is often made from tungsten or platinum iridium, although gold is also used. The computer is used to improve the image through its processing and to perform quantitative measurements.
How does it work
The principle of operation of the tunneling microscope is quite complicated. The electrons at the tip tip are not limited to the area inside the metal by a potential barrier. They move through an obstacle like their movement in a metal. The illusion of freely moving particles is created. In fact, electrons move from atom to atom, passing through a potential barrier between two atomic sites. For each approach to the barrier, the tunneling probability is 10: 4. Electrons cross it at a speed of 1013 pcs per second. This high transmission rate means that the movement is substantial and continuous.
By moving the metal tip over the surface to a very small distance that overlaps the atomic clouds, atomic exchange is carried out. This produces a small amount of electric current flowing between the tip and the surface. It can be measured. Thanks to these ongoing changes, the tunneling microscope provides information on the structure and topography of the surface. On its basis, a three-dimensional model is constructed 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 tunneling electron can move either toward them or toward 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 in the picture. A periodic atomic array is clearly visible on materials such as gold, platinum, silver, nickel and copper. Vacuum tunneling of electrons from the tip to the sample can occur even though the environment is not a vacuum, but is filled with gas or liquid molecules.
Barrier height formation
Spectroscopy of the local barrier height provides information on the spatial distribution of the microscopic function of the surface. The image is obtained by measuring the points of the logarithmic change in the tunneling current, taking into account the transformation into a separation gap. When measuring the height of the barrier, the distance between the probe and the sample is modulated according to a sinusoidal law using an additional alternating voltage. The modulation period is chosen much shorter than the time constant of the feedback loop in a tunneling microscope.
Value
This type of scanning probe microscopes allowed us to develop nanotechnologies that should manipulate objects of nanometric size (less than the wavelength of visible light from 400 to 800 nm). A tunneling microscope clearly illustrates quantum mechanics by measuring the quantum of a shell. Today, amorphous non-crystalline materials are observed using atomic force microscopy.
Silicon example
Silicon surfaces have been studied more widely than any other material. They were prepared by heating in a vacuum to such a temperature that the atoms were reconstructed in the induced process. The reconstruction has been studied in great detail. A complex pattern, known as Takayanagi 7 x 7, formed on the surface. The atoms formed pairs, or dimers, that fit into rows extending over the entire studied part of silicon.
Research
Studies of the principle of operation of a tunneling microscope have led to the conclusion that it can work in the surrounding atmosphere as well as in a vacuum. It was operated in air, water, insulating liquids and ionic resolutions used in electrochemistry. It 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.
A tunneling microscope is mainly used for imaging, but there are many other applications that have been studied. A strong electric field between the probe and the sample was used to move atoms along the surface of the sample. The effect of a tunneling microscope in various gases was studied. In one study, the voltage was four volts. The field at the tip was strong enough to remove 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. In the course of research, a hybrid tunneling microscope was invented. The original device was integrated with a bipotentiostat.