Subatomic Resolution in Noncontact Atomic Force Microscopy: Electron Cloud Interactions or Harmonics Processing Artifacts?

In 2004 Hembacher et al. [Science 305, 380–383 (2004)] reported higher harmonics AFM images of a graphite surface acquired using a tungsten tip which revealed subatomic features. These features were interpreted as the signature of electron bonding lobes at the tip apex atom. We recently applied a computational method based on density functional theory to simulate the images of Hembacher et al. and found that features of subatomic size can indeed be observed under ideal conditions. However, a number of important questions remain open, the most significant of which concerns signal processing. Here we continue our bottom-up analysis by exploring the effects of harmonics processing on the images obtained. Our simulations suggest that there are no imaging artifacts inherent to the filtering process used but that harmonics averaging is not an appropriate method for enhancing subatomic contrast due to variations in the harmonics ratios across the surface. Instead, a promising approach may be the individual mapping of the first two harmonics, which are expected to dominate the contrast under the conditions studied by Hembacher et al.