Tip convolution effects

More difficult is the differentiation of tip convolution effects if there is no hidden symmetry. In the images shown in Fig. 2.46, the tip was altered in the course of an experiment. In fact, the images show the same area of a vertically stretched... 

Fig. 3.38 Contact mode AFM height image of egg PC vesicles adsorbed on glass captured with an imaging force of 30 pN left) and 50 pN right). The halo in the fast scan direction right to left) indicates that the tip can no longer track the surface features accurately, when imaging force and noise of the deflection signal become comparable ( 30 pN in this case). When the imaging forces are increased to 50 pN, the surface is tracked better. The asymmetry of the features can be explained by tip convolution effects (asymmetry of the probe tip) [87]...

In semicrystalline polymers, fillers may act as reinforcement, as well as nucle-ation agents. For example in PP, nanoscale silica fillers may nucleate the crystallization resulting in spherulites that show enrichment in particles in the center of the spherulite (Fig. 3.64). For a quantitative analysis of, e.g., filler sizes and filler size distributions, high resolution imaging is necessary and tip convolution effects [137-140] must be corrected for. The particles shown below are likely aggregates of filler particles considering the mean filler size of 7 nm [136]. 

We have seen that electron microscopy and scanning probe microscopies are very complementary techniques to characterize the structure and the morphology of supported clusters. The internal structure can only be resolved by HRTEM while the surface atomic structure can be only revealed by STM or AFM. TEM gives accurate diameter measurements and height can only be measured in profile view that needs special sample preparation. STM or AFM give accurate height measurements but diameters can be obtained only after correction from the tip-sample convolution effect. 

A new class of artificial membranes, based on lipid bilayers supported on porous alumina, were recently studied by Hennesthal and Steinem using AFM [58], In this case, tip-sample convolution effects affecting the detection of pores (see Section 2.3) were clearly demonstrated by the authors on comparing the average pore size of the alumina support as obtained by SEM (60 nm) and by AFM (50 nm). 

It should be noted that the force measured in AFM is proportional to the inverse of the tip-sample distance to the power of n. This is less sensitive than the exponential variation of tunneling current as a function of tip-sample distance in STM. In addition, due to the restricted sharpness of the tip apex of an AFM probe (presently, the finest commercial probes have a radius 10 nm), the convolution effect of the tip shape may be... 

As tip-sample convolution occurs due to the finite size of the probe, it is obvious that the effect would be reduced by further miniaturizing the probe s dimensions. With some success, recent attempts to achieve this goal have utilized single-walled carbon nanotubes attached to the apexes of regular AFM probes. Commercially available sharpened silicon oxide probes can also help improve the attainable level of resolution. 

Figure 12. Schematic diagrams showing the geometrical convolution of a tip of finite width with a corrugated surface. Note the existence of "forbidden space, inaccessible to the tip, and the consequent effect on the image. One reason for "tip hop" is al.so shown.

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