Observation of Atomic-Level Objects

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Assuming that a voltage is applied between a metal probe with a sharp end and the surface of a conductor or semiconductor sample, if the distance between the probe and the sample becomes very close and converges, current flows in a non-contact state due to the quantum mechanical tunneling effect. In this situation, when the change in the distance between the probe and the sample surface is not zero on an atomic scale, the change in the magnitude of the current occurs within a small error range. By this principle, the distribution of atoms can be grasped, and thus atomic-level objects can be observed. This observer is called the Scanning Tunneling Microscope (STM).

Quantum Mechanical Tunneling of Electrons

In classical mechanics, particles cannot cross a higher energy barrier than they have. The nucleons constituting the nucleus are strongly bound together by the action of the nuclear force. This is referred to as a potential barrier. According to classical mechanics, it is impossible for a nucleon to escape out of an atom because it has less energy than the potential barrier creates. However, in reality, alpha particles in radioactive materials leave the nucleus with less energy than the nuclear force, and this is called the tunnel effect (quantum mechanical tunneling).

This allows electrons to move from one side to the other through the energy barrier of the vacuum by applying a bias voltage between the tip and the sample while the conductive tip is very close to the sample surface in the STM. To observe the surface, the tunneling current flowing between the tip and the sample is observed on the sample surface at the tip end, where the resulting tunneling current is determined by the location of the tip, the applied voltage, and the sample’s local state density (LODS).

Required degree of vacuum

Gas molecules continuously move due to interference, and there is a very high probability of causing interference by affecting collisions due to high density and randomness. As a specific cause of interference in the STM, observation of the sample surface is hindered by attaching to the surface of the sample to be observed, reacting with the surface, or covering the surface. In experiments using STM for easy observation, it is necessary to block contact between the sample to be observed and gas molecules as much as possible, so a vacuum state is ideal.

Vacuum refers to a state in which gas pressure is lower than atmospheric pressure, and gas pressure and vacuum are inversely proportional. When the temperature inside the vacuum cylinder is constant and only one type of gas molecules exist, the gas pressure inside the cylinder is proportional to the number of gas molecules floating around per unit volume, regardless of the type of gas molecules. Therefore, the gas pressure inside the vacuum canister can be lowered by extracting the gas molecules from the vacuum canister or immobilizing them inside the vacuum canister.

The time it takes for gas molecules floating inside the vacuum tank to attach to the surface of the sample to be observed and form a film is called the monomolecular layer formation time. The higher the collision frequency of gas molecules per unit area, the shorter it is. In addition, according to gas kinetic theory, at a fixed temperature, the larger the mass of gas molecules or the lower the pressure of the gas, the longer the monomolecular layer formation time.

In the case of nitrogen, the monomolecular layer formation time is 3 × 10 -9 seconds at 20 ° C and 760 Torr atmospheric pressure, but increases to approximately 2,500 seconds when the pressure is reduced to 10 -9 Torr on the same temperature line. For this reason, it can be obtained that a scanning tunneling microscope usually requires an ultra-high vacuum state of 10 −9 Torr or less in order to secure an observable time of the sample.

Ion pump

In order to practically implement ultra-high vacuum, an ion pump is generally used. The ion pump is composed of a permanent magnet, a cylindrical anode made of metal, and a cathode in the form of a plate made of titanium. A magnetic field induced by a magnet is hung in the axial direction of the cylindrical anode, and a high voltage of 2 to 7 kV is applied between the anode and cathode.

Under the influence of the high voltage applied between the anode and the cathode, the electrons emitted from the cathode move to the anode while drawing a complex trajectory under the influence of the magnetic field. In this process, the electrons emitted from the cathode collide with the surrounding gas molecules and separate the gas molecules into their components, positive ions and electrons. Here, the magnetic field increases the distance electrons travel to the anode compared to when there is no magnetic field, thereby increasing the frequency of collisions between electrons and gas molecules. Positive ions generated in this process are pulled to the cathode by the electric force and become embedded in the cathode, making them unable to move. This is called the primary pumping action.

When the positive ions hit the cathode, the titanium breaks off and sticks around the impact point. Due to its high chemical reactivity, the attached titanium easily reacts with various gas molecules and adsorbs the floating gas molecules. Since this has the effect of reducing the number of floating gas molecules, this is called secondary pumping. Therefore, an ultra-high vacuum state can be created by the action of the first and second ion pumps.

Origins and References

Origin: September (Mock)
et cetera:

1. June 2012 (Mock Test) – History and types of vacuum tubes (1/2)
2. “STM References - Annotated Links for Scanning Tunneling Microscope Amateurs”.
3. US4,343,993　Priority number(s): CH19790008486 19790920.
4. Jayong Koo, Dalhyun Kim. (1992). Measuring principle and application of scanning tunneling microscope. Journal of the Korean Society of Mechanical Engineers, 32(5), 420-428.
5. C. Julian Chen (1993). 《Introduction to Scanning Tunneling Microscopy》 (PDF). Oxford University Press. ISBN 0-19-507150-6.
6. September 2007 (Mock) – Introduction to Photography (All)

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