Atomic force microscopy studies of mesoscopic structures

grantor: University of TorontoThe atomic force microscope (AFM) allows both qualitative and quantitative images of a sample's topography. Unfortunately, the imaging process suffers from distortions due to geometric interactions when the sample size range is comparable with that of the AFM tip, specifically from 2 to 500 nm, which is typical for colloids and biological systems. This thesis identifies these distortions, presents some examples, and provides some solutions to the predicament. Two simple numerical procedures for separating the probe tip geometry from images obtained by AFM are presented. The procedures described are of general applicability: they make no assumptions about the tip geometry and require no pre-treatment of the data, such as filtering. The importance of this is twofold: first, it provides a way of visualizing the probe tip in a nondestructive manner, which is useful in eliminating artifacts, and second, it enables the deconvolution of the probe tip from the image, resulting in a more accurate picture of the sample. Examples of deconvolution using both an idealized tip and a real tip are given. The efficacy of such schemes is explored with the use of simulations of the tip-sample interaction using simple geometric considerations. Images taken of paired helical filaments (PHF) using AFM have shown the apparent existence of both left and right-handed filaments. Computational simulations of the images obtainable have been carried out to explain these findings and to confirm them as instrumental artifacts. When carried out using ideal spherical tips, the images of the structures are distorted in their lateral dimensions but retain their helicity. The fine structure of the sample is often lost for tips which are slightly oblong, resulting in beaded structures. In some instances, the image distortion is such that the handedness of the structures appears to change. These findings illustrate the applicability or lack thereof to determine the helicity of a sample using AFM. A digitized representation of the tip, similar to that afforded by deconvoluting AFM images, can allow calculations of the long range van der Waals forces between the tip and surface plane. By approximating the volume of the tip as an arrangement of rectangular columns, various geometries were investigated and compared to analytic solutions. The results indicate that this columnar representation of the tip can generate adequate results for most models considered. Measurement of the elasticity of collagen fibers during their initial stages of growth by extending the AFM to function as a force/displacement transducer proved unsuccessful due to the limitations imposed by the length scales involved.Ph.D