Figure 1
(a–c) Illustration of the measurement cycle in the experiment. After preparing and characterizing the tip on Cu(111) (a), the tip is transferred to a \(\hbox {CaF}_{2}\)(111) sample for the measurements (b), and then again back to a Cu(111) sample to verify that the tip did not change during the whole experiment (c). The COFI images (d,g) recorded before and after the measurement on \(\hbox {CaF}_{2}\)(111) show that the tip remained stable for the complete measurements. Imaging height: 70 pm retracted from the STM setpoint height on the bare Cu(111) surface, (\({-10}\hbox { mV}\), \(-100\hbox { pA}\)). (b) A CO-terminated tip has been modeled with a negative point charge q. Graph: Plot of the calculated force contrast \(\Delta F\) and responsible tip-sample interactions as a function of tip-sample distance z above \(\hbox {CaF}_{2}\)(111). (e,f) Side and top view of the \(\hbox {CaF}_{2}\)(111) surface, consisting of neutral F\(^-\)–Ca\(^{2+}\)–F\(^-\) triple layers32,39.
