Under the term Scanning probe microscope There are a number of microscopes and the associated measuring methods that are used to analyze surfaces. These techniques are therefore part of surface and interface physics. Scanning probe microscopes are characterized in that a measuring probe is guided over a surface at a small distance.
What is a scanning probe microscope?
All types of microscopes in which the image is created as a result of an interaction between the probe and the sample are referred to as scanning probe microscopes. This distinguishes these methods from both light microscopy and scanning electron microscopy. Neither optical nor electron-optical lenses are used here.
With the scanning probe microscope, the surface of the sample is scanned bit by bit with the help of a probe. In this way, measured values are obtained for each individual point, which are then combined to create a digital image.
The scanning probe method was first developed and presented in 1981 by Rohrer and Binnig. It is based on the tunnel effect that arises between a metallic tip and a conductive surface. This effect forms the basis for all scanning probe microscopy methods developed later.
Shapes, types & types
There are different types of scanning probe microscopes, which differ primarily with regard to the interaction between the probe and the sample. The starting point was the scanning tunneling microscopy, which in 1982 for the first time enabled an atomically resolved representation of electrically conductive surfaces. During the following years numerous other scanning probe microscopy methods developed.
With the scanning tunneling microscope, a voltage is applied between the surface of the sample and the tip. The tunnel current is measured between the sample and the tip, which are also not allowed to touch. In 1984 optical near-field microscopy emerged. Here light is sent through the sample from a probe. In the atomic force microscope, the probe is deflected by means of atomic forces. Usually the so-called van der Waals forces are used. The deflection of the probe has a proportional relationship to the force, which is determined according to the spring constant of the probe.
Atomic force microscopy was developed in 1986. In the beginning, atomic force microscopes worked on the basis of a tunnel tip that acts as a detector. This tunnel tip determines the actual distance between the surface of the sample and the sensor. The technology makes use of the tunnel voltage that exists between the back of the sensor and the detection tip.
Nowadays, this method has largely been replaced by the detection principle, with detection using a laser beam that functions as a light pointer. This is also known as a laser force microscope. In addition, a magnetic force microscope was developed in which magnetic forces between the probe and the sample serve as the basis for determining the measured values.
In 1986 the scanning thermal microscope was also developed, in which a tiny sensor functions as a scanning probe. There is also a so-called optical scanning near-field microscope, in which the interaction between probe and sample consists of evanescent waves.
Structure & functionality
In principle, all types of scanning probe microscopes have in common that they scan the surface of the sample in a grid. The interaction between the probe of the microscope and the surface of the sample is used. This interaction differs depending on the type of scanning probe microscope. The probe is huge compared to the sample being examined, and yet it is able to determine the tiny surface features of the sample. The foremost atom at the tip of the probe is particularly relevant at this point.
With the help of scanning probe microscopy, resolutions of up to 10 picometers are possible. For comparison: the size of atoms is in the range of 100 picometers. The accuracy of light microscopes is limited by the wavelength of the light. For this reason, only resolutions of around 200 to 300 nanometers are possible with this type of microscope. This corresponds to roughly half the wavelength of light. Therefore, electron beams are used instead of light in a scanning electron microscope. By increasing the energy, the wavelength can in theory be made as short as desired. However, too small a wavelength would destroy the sample.
Medical & health benefits
With the help of a scanning probe microscope, it is not only possible to scan the surface of a sample. Instead, individual atoms can also be removed from the sample and deposited again at a specified location.
Since the early 1980s, the development of scanning probe microscopy has advanced rapidly. The new possibilities for improved resolution of far less than a micrometer were an essential prerequisite for advances in nanosciences and nanotechnology. This development has occurred particularly since the 1990s.
Based on the basic methods of scanning probe microscopy, numerous other sub-methods are nowadays divided. These take advantage of different types of interaction between the probe tip and the sample surface.
Scanning probe microscopes play an essential role in research areas such as nanochemistry, nanobiology, nanobiochemistry and nanomedicine. Scanning probe microscopes are even used to explore other planets such as Mars.
Scanning probe microscopes use a special positioning technique based on the so-called piezo effect. The apparatus for moving the probe is controlled by the computer and enables highly precise positioning. This allows the surfaces of the samples to be scanned in a controlled manner and the measurement results to be combined into an extremely high-resolution display.
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