Atom-Resolved Analysis of an Ionic KBr(001) Crystal Surface Covered with a Thin Water Layer by Frequency Modulation Atomic Force Microscopy

An ionic KBr(001) crystal surface covered with a thin water layer was observed with a frequency modulation atomic force microscope (FM-AFM) with atomic resolution. By immersing only the tip apex of the AFM cantilever in the thin water layer, the Q-factor of the cantilever in probing the solid–liquid interface can be maintained as high as that of FM-AFM operation in air, leading to improvement of the minimum detection of a differential force determined by the noise. Two types of images with atom-resolved contrast were observed, possibly owing to the different types of ions (K+ or Br–) adsorbed on the tip apex that incorporated into the hydration layers on the tip and on the sample surface. The force–distance characteristics at the solid–water interface were analyzed by taking spatial variation maps of the resonant frequency shift of the AFM cantilever with the high Q-factor. The oscillatory frequency shift–distance curves exhibited atomic site dependence. The roles of hydration and the ions on the tip and ...