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High resolution images of DNA specimens obtained with an Atomic Force Microscope (AFM) operating at low temperature and under Ultra High Vacuum (UHV) conditions are presented. The microscope was completely developed in our laboratory to study biological samples. We present in this work an exhaustive study of the operating conditions to image DNA from intermittent contact mode to non contact mode. We emphasize in the common points of those methods without ignoring the differences introduced by the feedback parameters in each case. By performing spectroscopy experiments we improve the contrast when imaging DNA samples. We analyze the relation between the frequency shift and the tip-surface interactions in order to choose the optimal value of the set point when imaging in non-contact mode.
The improved results indicate to use non contact-mode imaging in order to recognize crossings and other topological characteristics of DNA molecules because the deformation effect, produced by the tip-sample interaction, could be considerably reduced. Problems related to the limitation of lateral resolution such as tip aspect ratio and piezo transducers drifting will be discussed.
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