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From scanning tunneling microscopes to atomic force microscopes: Why are they so powerful?
Scanning probe microscopy (SPM) is a branch of microscopy that forms images by scanning the surface of a sample with a physical probe. SPM has advanced rapidly since the invention of the scanning tunneling microscope in 1981, an instrument capable of imaging surfaces at the atomic level. The successful experiments of Gerd Binnig and Heinrich Rohrer marked the beginning of this field, the key of which was to use a feedback loop to regulate the distance between sample and probe.
Scanning probe microscopes use piezoelectric actuators to make atomic-scale or finer movements under electronic command, which allows them to efficiently acquire data, usually in the form of a two-dimensional grid of data, and then transmit it to a computer. The colors of the image are visualized.The resolution of scanning probe microscopy varies between different techniques, but some probe techniques are capable of achieving quite impressive atomic resolution.
Different Types of Scanning Probe Microscopes
Within the field of SPM, there are many established techniques, such as atomic force microscopy (AFM), chemical force microscopy (CFM), scanning tunneling microscopy (STM), and many other variants. These technologies have their own characteristics and can be selected according to different application requirements.
Scanning probe microscopy data are often displayed as heat maps, which produce the final image.
Image formation technology
Scanning probe microscope images are usually generated using raster scanning technology. The probe is drawn over the sample surface and a specific value is recorded at each scan point. The values recorded during this process may vary depending on the specific operating mode.
Differences between models and technologies
Two common operation modes include constant interaction mode and constant height mode. In constant interaction mode, the distance between the probe and the sample is adjusted through a feedback loop to maintain a stable interaction. In constant height mode, the z-axis of the probe does not move, which increases the risk of collision between the probe and the sample.
Probe design and characteristics
The shape and material of the SPM probe depends on the specific technique used, and the shape of the probe tip is critical to the resolution of the microscope. The finer the probe, the higher the resolution, and to achieve atomic resolution, the tip of the probe must be a single atom.
Advantages and Disadvantages of Scanning Probe MicroscopyDuring microscopy imaging, the tip of the probe may not be able to achieve the expected resolution, which may be caused by excessive probe bluntness or multiple peaks.
The main advantage of scanning probe microscopy is its diffraction-free resolving power, but this feature is also its limitation because of the long scanning times required. The spatial information during the scanning process is embedded in the time series, which may lead to measurement uncertainty.
Scanning Photocurrent Microscopy (SPCM)
As a member of scanning probe microscopes, SPCM uses a focused laser beam as a local excitation source to study the optoelectronic properties of materials. This technology is particularly important for the study of semiconductor nanostructures.
Through SPCM, key parameters such as current characteristic length, recombination kinetics, and doping concentration can be analyzed.
Data visualization and analysis software
The data generated by scanning probe microscopy often needs to be analyzed and presented using professional visualization software. There are a variety of commercial and free software options available on the market that allow users to better understand the data obtained.
Developments in scanning tunneling microscopes and atomic force microscopes continue to drive advances in nanotechnology, but does this mean we will face more challenges in the future?
