Unfortunately, the technique, which works by scanning the AFM tip, kept at a constant height, across a surface sample, is only suitable for flat or nearly flat molecules and not bulky, 3D ones. Both AFM and STM scanning tunnelling microscopy were invented in the s and both make use of a sharp tip that scans the sample surface to produce an image of it.
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STM measures the tunnelling current that flows between the tip and the sample while AFM exploits the force that the surface exerts on the tip. Here, the tip that is attached to one prong of an oscillating quartz tuning fork is scanned across the surface of a molecule. This leads to small changes in the resonant frequency of the tuning fork that can be monitored.
Repulsive interactions between the CO tip and the atoms of the molecules being imaged produce positive frequency shifts and bright image contrast, while attractive interactions lead to negative frequency shifts and dark contrast. In this way, an image reflecting the bond structure of the molecule can be obtained. While the method is perfectly suited to analyse flat molecules, it falls short when imaging bulky, non-planar ones.
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The reason is straightforward: only the top of the 3D object can be imaged since the frequency shift is measured at a constant height and the tip is simply too far from areas underneath to collect a useful signal. Previous attempts to do this, however, have involved complex procedures and additional apparatus.
Ebeling and colleagues have now found a way around this stumbling block by exploiting the constant tunnelling current mode of an STM instead of operating the AFM at a constant height.
The new technique yields the same information as the classic bond-imaging technique for flat surfaces, he adds. In their experiments, the researchers began by studying a flat molecule, 2-iodotriphenylene ITP C18H11I deposited on a silver Ag substrate. They then removed one of the iodine atoms on the molecule, which made it form a radical with a complex 3D structure.
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In constant-current mode, however, it is possible to image it and even determine its tilting angle, thanks to the tip-sample feedback signal. Right: By measuring the cantilever vertical displacement, with the feedback signal on, an image equivalent to the derivative of the AFM image is obtained. This image is particularly useful to detect sudden variations of the topography, such as steps on the top of the terraces.
This is a real 3-dimensional representation of the data, which can be rotated to reveal features not observed in a given orientation.
Because of the digital character of AFM data, linescans can be generated from any part of the image, and it is also possible to measure the angle between features. In the present case, the angle between the sections marked by the red arrows is Other parameters that can be automatically obtained from AFM analysis include peak-to-valley distance, roughness and image surface area.
AFM images of carbon nanotubes. A position-sensitive photo diode PSPD can be used to track these changes. Thus, if an AFM tip passes over a raised surface feature, the resulting cantilever deflection and the subsequent change in direction of reflected beam is recorded by the PSPD.go site
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The raised and lowered features on the sample surface influence the deflection of the cantilever, which is monitored by the PSPD. By using a feedback loop to control the height of the tip above the surface—thus maintaining constant laser position—the AFM can generate an accurate topographic map of the surface features.
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