Fig. 1: Distance dependence of image contrast on Cu(111).
a–d Constant-height frequency shift AFM images of a Cu(111) surface measured with a CO tip. The average tip-surface distances were −411 pm (a), −441 pm (b), and −471 pm (c) with respect to the tunneling gap at Itunnel = 10 pA and Usample = 100 mV (amplitude ≈ 34 pm, resonant frequency ≈ 26960 Hz, quality factor ≈ 15791). The red circles in a are fitted to the dark regions and indicate the positions of Cu atoms (top sites). These dark features are the most pronounced image features at the tip-surface distance of −411 pm. The pink circles in a indicate very faint bright features that are already observable at the rather large tip-surface distance of −411 pm. The red circles were copied to (b–d). The orange circles in (b–d) are fitted to the dark image features and reveal the lateral shift with respect to the red circles. The image in (d) was measured after (a–c) again at −411 pm. e Frequency shift vs. distance curves above top (red), hollow (orange), and bridge sites (blue) (see markers in a). The inset in e shows a zoom-in (see dashed black box). The dashed black lines in the inset correspond to the distances used in (a–c).
