Contrast artifacts in tapping tip atomic force microscopy

Abstract

When recording images with an atomic force microscope using the resonant vibrating cantilever mode, surprising strange results are often achieved. Typical artifacts are strange contours, unexpected height shifts, and sudden changes of the apparent resolution in the acquired images. Such artifacts can be related to the dynamical properties of the cantilever under the influence of the force between the tip and the sample. The damping of the cantilever oscillation can be either due to attractive interaction between the tip and the sample or due to a combination of attractive and repulsive interaction. The oscillating cantilever will be in a specific swing mode according to which type of interaction is dominating, and it is the switching between these modes that is responsible for a range of artifacts observed during image acquisition. This includes the artifact often referred to as "contrast reversal".The resonant vibrating cantilever mode [1, 2] of the atomic force microscope (AFM) is being increasingly used, e.g. in imaging of soft matter, a growing field. The vibrating cantilever mode is also particularly useful for the imaging of loose lying objects on substrate surfaces [3]. The main advantage of this mode of operation is the reduction of lateral forces between the tip and the sample when compared to operation in contact mode [2]. Moreover, there are now several reports about achievement of atomic resolution using the AFM under ultra high vacuum (UHV) conditions in the resonant vibrating cantilever mode [4,5]. This is an indication that detection using the high-Q resonant circuit constituted by the cantilever can be extremely sensitive. Additionally, the stiff vibrating cantilever allows the use of almost all kinds of vertical forces for detection including adhesive forces such as capillary forces and forces due to chemical bonding.When the vibrating cantilever is oscillating at amplitudes which are high compared to the extension of the tip-sample interaction potential, the tip and the sample are interacting only during a fraction of each swing cycle. Accordingly, roughly speaking, the effective force gradient experienced by the cantilever depends alone on the total amount of work done during the interaction period and on the oscillation amplitude and not on the specific functionality of the interaction potential. This does not imply, however, that the shape of the potential does not influence the motion of the cantilever. On the contrary, recent reports [6,7] have shown that the presence of long range attractive forces and short range repulsive forces generally facilitates two modes of operation where (1) the tip and sample interact purely attractively and (2) the interaction is both attractive and repulsive. In this article we show how this effect can influence imaging and result in a number of artifacts, including the often discussed "contrast reversal".All experiments were carried out under ambient conditions using a commercial microscope and commercial cantilevers [8]. The shown images were recorded using ...

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