Scanning Probe Microscopy
In Scanning Probe Microscopy (SPM), the sample surface to be examined is scanned point by point with a nanoscale probe. This interacts with the sample. By using different probes, a wide variety of sample properties can be measured, e.g. topography, friction coefficients, magnetic properties, hardness, electronic properties etc.
From microstructures to the imaging of individual molecules and atoms - areas that were difficult or impossible to access in the 1980s until the advent of Scanning Tunneling Microscopy (STM) became visible (Fig. 1).
The advantages of scanning probe microscopy over other methods, such as Scanning Electron Microscopy (SEM), are not only high resolution, but also the fact that this is achieved with comparatively small devices. These are (as a rule) neither bound to a vacuum nor to enlarging or focusing components. The ability to use the scanning probe microscopes both in air and under liquids, provides a variety of applications.
Atomic Force Microscope (AFM)
As a probe, the AFM guides a fine tip (Fig. 2) over the sample. The tip is mounted on a lever spring (a.k.a. cantilever). As the tip approaches the sample surface, force interactions cause the cantilever to deflect. A laser beam directed towards the cantilever is reflected onto a 4-quadrant photodiode, thus detecting its deflection and torsion. The electrical signal is used for image formation (Fig. 3).
Ananlysis is not limited to conductive surfaces. Insulating or organic materials can also be inspected.
The types of imaging can be subdivided into "contact" and "non-contact" depending on the forces acting between the tip and the sample:
- Non-contact mode
Attractive forces between the tip and the sample are mainly the result of van-der-Waals attraction and, in case of contaminated samples, by capillary forces. In this mode, the cantilever is vibrated at a certain distance from the sample (at resonance frequency), and the damping of the vibration is measured. This procedure is mainly used for soft samples.
- Contact mode
With further approach to the sample, repulsive forces appear, which are used in this mode for image formation.
Two devices of TopoMetrix (now Bruker AXS) are available at the institute: TMX 1000 and TMX 2010 (Fig. 4). The TMX 2010 can be used as both a tunnel and a scanning force microscope.
Spatially resolved IV characterization is possible, as well as measurement in different media.
The sample size is limited to a diameter of about 10 mm and a height of max. 5 mm. The surface roughness of the samples must not exceed a certain value (depending on the scanner used). Areas of about 1 μm to 70 μm lateral length can be examined.
The analysis software offers various tools for analyzing the measured values.
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- R. Wiesendanger: Scanning probe microscopy and spectroscopy: Methods and applications, Cambridge (u.a.): Cambridge Univ. Press, 1998.
- Tien T. Tsong: Fifty Years of Seeing Atoms, Phys.Today 59,3 (2006), 31-37.
- J. Loos: The Art of SPM: Scanning Probe Microscopy in Materials Science, Adv.Mater. 17,15 (2005), 182-183.
- P. Eaton and P. West: Atomic Force Microscopy, Oxford: Oxford University Press, 2018.