The Tip-Sample Interaction in Atomic Force Microscopy and its Implications for Biological Applications

Author: Baselt, David Randall

Year: 1993

Degree: Dissertation (Ph.D.)

Advisor: Baldeschwieler, John D.

Committee Members: Goddard, William A., III; Baldeschwieler, John D.; Okumura, Mitchio; Revel, Jean-Paul

Option: Chemistry

DOI: 10.7907/5ZMM-7Q64

Abstract

This thesis describes the construction of an atomic force microscope and its application to the study of tip-sample interactions, primarily through the use of friction and hardness (elasticity) imaging.

Part one describes the atomic force microscope, which consists of a scanned-cantilever stage (chapter 2); a versatile digital signal processor-based control system with self-optimizing feedback, lock-in amplifier emulation (for hardness imaging), and macro programmability (chapter 3); and image processing software (chapter 4).

Part two describes a number of results that have helped to characterize the tip-sample interaction and the contact imaging modes used for its study. Meniscus forces act laterally as well as normally, and that they vary with position (chapter 5). Friction measurements couple with scanner position quid feedback, and the meniscus effects friction images (chapter 6). Sliding of the tip over the sample surface introduces slope-dependence into hardness measurements (chapter 7). Dull tips can create prominent topography artifacts even on very flat surfaces (chapter 8).

In an investigation of collagen fibrils, AFM has revealed die characteristic 65 nm banding pattern, a second, minor banding pattern, and microfibrils that run along the fibril axis. The distribution of proteoglycans along the fibrils creates a characteristic pattern in friction images. Although imaging in water reduces interaction forces, water can also make biological samples more sensitive to force. However, for robust biological samples imaged in air, tip shape presents a greater obstacle than tip-sample interaction forces to obtaining high-resolution images. Tip contamination increases tip-sample friction and can occasionally improve resolution (chapter 9).

For a separate project I have designed a general-purpose nearfield scanning optical microscope (chapter 10).

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