Artifacts in Atomic Force Microscopy Images of Fine Particle and Protein Two-Dimensional Crystals as Evaluated with Scanning Electron Microscopy and Simulations

Abstract

With the aid of simulations and a scanning electron microscope equipped with a cold field emission source (FE-SEM), we evaluated artifacts in atomic force microscopy (AFM) images by using two-dimensional (2D) crystals of four types of particles that ranged in size from 12 to 144 nm. The AFM images of these 2D crystals were taken in air using an atomic force microscope equipped with a grown carbon tip whose apex was 15-20 nm in diameter. The SEM images obtained for the same 2D crystals indicated discernible spheres that were hexagonally arranged for all crystals. We took these SEM images as references for evaluating the AFM images. The AFM images for the 2D crystals of the 144 nm particles were identical to the SEM images. However, in the AFM images, we identified artifacts that gradually increased as the particle size decreased. Thus, we found that one cause of the artifacts that often occur in fine topography is the use of a probe whose tip diameter is comparable in size to the topography of the objects. In our simulations ofthe AFM imaging, we assumed four types of hexagonal lattices of particles and a spherical probe tip of 20 nm diameter. Our simulations reproduced the same phenomenon seen in the actual AFM images, namely, the increase in artifacts as the particle size decreases. Nevertheless, in these simulations, the image of the 2D crystals of 12 nm particles was still identified. Furthermore, in the same simulations, when random oscillations of either the tip or the particles were assumed to occur during the scanning over a range of a few nanometers, the simulated images were very similar to the real AFM images. Based on our results, we concluded that the artifacts identified in the real AFM images are produced by such random displacements (or oscillations) and by the effect of the finite size of the probe tip