The term scanning probe microscope covers a range of microscopes and associated measurement techniques that are used to analyze surfaces. As such, these techniques fall under surface and interfacial physics. Scanning probe microscopes are characterized by passing a measuring probe over a surface at a small distance.
What is a scanning probe microscope?
The term scanning probe microscope covers a range of microscopes and their associated measurement techniques that are used to analyze surfaces. Scanning probe microscope refers to all types of microscopes in which the image is formed as a result of interaction between the probe and the sample. Thus, these methods differ from both optical microscopy and scanning electron microscopy. Here, neither optical nor electron-optical lenses are used. In the scanning probe microscope, the surface of the sample is scanned piece by piece with the aid of a probe. In this way, measured values are obtained for each individual spot, which are finally combined to produce a digital image. The scanning probe method was first developed and presented by Rohrer and Binnig in 1981. It is based on the tunnel effect that occurs between a metallic tip and a conductive surface. This effect forms the basis for all scanning probe microscopy techniques developed later.
Shapes, types, and styles
Several types of scanning probe microscopes exist, differing primarily in the interaction between the probe and the sample. The starting point was scanning tunneling microscopy, which first allowed atomic-resolution imaging of electrically conductive surfaces in 1982. During the following years, numerous other scanning probe microscopy techniques developed. In scanning tunneling microscopy, a voltage is applied between the surface of the sample and the tip. The tunnel current between the sample and the tip is measured, and they must not touch each other. In 1984, optical near-field microscopy was first developed. Here, light is sent through the sample starting from a probe. In the atomic force microscope, the probe is deflected by means of atomic forces. As a rule, the so-called Van der Waals forces are used. The deflection of the probe shows 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 acting as a detector. This tunnel tip determines the actual distance between the surface of the sample and the sensor. The technique makes use of the tunnel voltage that exists between the back of the sensor and the detection tip. In modern times, this technique has been largely superseded by the detection principle, where detection is done using a laser beam that acts 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 acts as a scanning probe. There is also a so-called scanning near-field optical microscope, in which the interaction between probe and sample consists of evanescent waves.
Structure and operation
In principle, all types of scanning probe microscopes have in common that they scan the surface of the sample in a grid. This takes advantage of the interaction between the probe of the microscope and the surface of the sample. This interaction differs depending on the type of scanning probe microscope. The probe is huge compared to the sample being examined, yet capable of detecting the minute surface features of the sample. Of particular relevance at this point is the foremost atom at the tip of the probe. Using 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 about 200 to 300 nanometers are possible with this type of microscope. This corresponds to approximately half the wavelength of light.Therefore, a scanning electron microscope uses electron radiation instead of light. By increasing the energy, the wavelength can be made arbitrarily short in theory. However, a wavelength that is too short would destroy the sample.
Medical and health benefits
Using a scanning probe microscope, it is not only possible to scan the surface of a sample. Instead, it is also possible to pick individual atoms from the sample and place them back at a predetermined location. Since the early 1980s, the development of scanning probe microscopy has progressed rapidly. The new possibilities for improved resolution of much less than one micrometer represented a major prerequisite for advances in nanoscience as well as nanotechnology. This development occurred especially since the 1990s. Based on the basic methods of scanning probe microscopy, numerous other sub-methods are subdivided nowadays. These make use of different types of interaction between the probe tip and the sample surface. Thus, scanning probe microscopes play an essential role in research fields such as nanochemistry, nanobiology, nanobiochemistry, and nanomedicine. Scanning probe microscopes are even used to explore other planets, such as Mars. Scanning probe microscopes make use of a special positioning technique based on the so-called piezoelectric effect. The apparatus for displacing the probe is controlled from a computer and enables highly accurate positioning. This allows the surfaces of the samples to be scanned in a controlled manner and the measurement results to be assembled into an enormously high-resolution image.
