Height measurement mode image

A Digital Instruments Scanning Probe Microscope was used in the tapping mode for all AFM measurements as seen schematically in Figure 3.2-3. A silicon nitride tip was driven with a ratio of 3.0V (RMS) to 1.8V (Set point). At least three height and phase images were generated on a 5.0 im, 2.0 im, or a 1.0 tm square scale for each sample listed in Table 3.2-3. In addition, the difference in elevation for two specifically selected points on each polymer brush sample were measured. 

In the classical contact mode (Fig. 6a) AFM measures the hard-sphere repulsion forces between the tip and the sample. As a raster-scan drags the tip over the sample surface, the detector measures the vertical deflection of the cantilever, which indicates the local sample height. A feedback loop adjusts the position of the cantilever above the surface as it is scanned and monitors the changes in the surface height, generating a 3D image—a decisive advantage of AFM over TEM [3]. 

Different topologies of dsDNA on amino-terminated mica have been imaged using the tapping mode and ambient conditions [48]. Fig. 8 presents linear A-DNA, non-twisted circular DNA and circular supercoiled DNA. The measured width and height of dsDNA are 3-7 nm and around 1 nm, respectively. 

Fig. 8. Surface structure of a single crystal of perfluorododecyleicosane F(CF2 )12(CH2)20H measured by contact mode SFM on the (a) micrometer and (b) nanometer scale. The constant height image in (a) shows variation of the normal force, while the constant force image in (b) displays variation in height caused by densely packed CF3 end groups protruding at the surface...

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