Ultrahigh-vacuum scanning force microscopy: Atomic-scale resolution at monatomic cleavage steps

The surfaces of in situ cleaved NaF crystals have been studied by scanning force microscopy (SFM) in ultrahigh vacuum (UHV). Operating the UHV force microscope in the contact mode, the surface structure with extended terraces interrupted by steps of typically 0.25 nm and 0.5 nm in height is revealed. Atomic-scale resolution is achieved on the terraces as well as at monatomic steps of the ionic crystal. The square surface lattice shows a periodicity of 0.45±0.04 nm that corresponds well to the spacing between equally charged ions along the 〈100〉 directions in the bulk phase. From the SFM data at cleavage steps the imaging area of the sensing tip is estimated to be less than 1 ${\mathrm{nm}}^2$. Some aspects of the contrast mechanism in the lateral and normal force images are discussed. The SFM data are complemented by low-energy-electron-diffraction measurements on NaF(001) carried out in the same compact UHV system.