Atomic resolution in scanning force microscopy:: Concepts, requirements, contrast mechanisms, and image interpretation

nature and the physical principles of dynamic force microscopy (DFM) performed in ultrahigh vacuum with fixed amplitudes much larger than the closest tip-sample distance are analyzed with a focus on the question which physical properties of the sample are actually measured. In a first part, we review conditions which essentially determine the achievable resolution in a scanning probe-based type of microscope. Then, the imaging process in a scanning force microscope is evaluated in the light of these conditions. Approximation of the nonlinear problem with a simple analytical model reveals that. the frequency shift hf which is recorded during DFM experiments is proportional to Deltaf proportional to V-int(D)/root lambda (D), where V-int(D) represents the tip-sample potential at the point of closest approach D, and lambda (D) a length which can be interpreted as decay length or range of V-int(z). The high accuracy of the derived relationship is demonstrated by comparison with other methods. Finally, we show why large oscillation amplitudes potentially enhance the stability of the measurement in comparison with very small amplitudes.