Fig. 2: Material dependent image contrast, CO-tip bending, and site-specific tip-surface forces.
a–c Constant-height AFM images of Cu(111), Ag(111), and Au(111) surfaces measured with CO tips. The red circles indicate top sites. The tip-surface distances are given with respect to the tunneling gap at Itunnel = 10 pA (a, c), 18 pA (b) and Usample = 100 mV (a, c), 7 mV (b), and amplitudes of 34 pm (a), 55 pm (b), 85 pm (c). d–f 2D frequency shift vs. distance maps measured along red arrows in (a–c). The directions were chosen to be almost parallel to the bending direction of the CO tip in each case. The white lines in (d–f) indicate the lateral movements of the atomic features (caused by bending of the CO tip) with respect to the vertical dashed black lines. The red markers in (d–f) indicate the tip-surface distance for the images in Figs. 1a–c, 2a–c, and Supplementary Figs. 3a–d, 4a–d. g–i Force vs. distance curves for top (red), bridge (blue), and hollow sites (orange) that were calculated from d–f via the Sader method. The “ΔF = force(bridge site) − force(top site)” values give the force difference between top and bridge sites for the distances used in (a–c). Other parameters: Cu (see caption Fig. 1), Ag (resonant frequency ≈ 26968 Hz, quality factor ≈ 37643), Au (resonant frequency ≈ 25860 Hz, quality factor ≈ 23368).
