ULTRAHIGH-VACUUM SCANNING FORCE MICROSCOPY - ATOMIC-SCALE RESOLUTION AT MONATOMIC CLEAVAGE STEPS

The surfaces of in situ cleaved NaF crystals have been studied by scan ning force microscopy (SFM) in ultrahigh vacuum (UHV). Operating the U HV force microscope in the contact mode, the surface structure with ex tended terraces interrupted by steps of typically 0.25 nm and 0.5 nm i n height is revealed. Atomic-scale resolution is achieved on the terra ces as well as at monatomic steps of the ionic crystal. The square sur face lattice shows a periodicity of 0.45+/-0.04 nm that corresponds we ll to the spacing between equally charged ions along the [100] directi ons 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 nm2. Some aspe cts of the contrast mechanism in the lateral and normal force images a re discussed. The SFM data are complemented by low-energy-electron-dif fraction measurements on NaF(001) carried out in the same compact UHV system.