Measuring and understanding forces on atomic length scales with the atomic force microscope

Most microscopes can be used with little understanding of how they work--much can be learned looking through a light microscope without ever knowing what a photon is or who Maxwell was--and the Atomic Force Microscope (AFM) is no exception. Many AFM images don't look much different from a mountainous landscape, and much is learned interpreting them as such; however, to really push a microscope to its limits means understanding the interactions creating the contrast in the picture. For a Scanning Electron Microscope, this means understanding how electrons interact with matter, for an AFM it means understanding forces. The focus of this thesis is understanding the forces acting (especially in liquids) between tip and sample in AFM and a better understanding the instrument itself.;Chapters I, II and VI involve better characterizing and improving the most important part of the AFM, the tiny cantilever used to measure forces. Chapter I describes a solution to one of the most basic problems that must be solved before forces can be accurately measured--measuring the stiffness of these cantilevers. Many limitations in AFM are set by physical characteristics of the cantilever itself, such as resonance frequency, spring constant, and quality factor. If an external force can be applied to the cantilever, feedback can be used to improve these characteristics. Chapter II shows how to do this using a magnetically applied external force, which has the advantage of working in liquids. These physical characteristics also change drastically when the cantilever is immersed in fluid. The resonance frequency of common cantilevers drops by as much as a factor of six in going from air to water. Chapter VI studies these changes and shows how further miniaturization of cantilevers can improve imaging speeds and signal-to-noise ratio.;Early in its career, the AFM was heralded as having atomic resolution, but as the field matured researchers realized that the contact area between tip and sample was usually much larger than just one atom. Chapter III is a study of forces between crystals and the AFM tip in water to improve our understanding of when "true" atomic resolution is possible. Finally Chapters IV and V use what I once considered a nuisance--thermal noise of the cantilever--to make a measurement of oscillatory hydration potentials due to layering of the fluid molecules at a crystal-liquid interface.