| Fig 4 Force spectroscopy measurement between tip and sample (A) schematic of the proposed tip model imaging the surface and a small molecule. The main contributions to force are macroscopic vdW interactions, local vdW interactions, and midrange electrostatic interactions33. (B) curves obtained with analytical expressions for the long-range force, the shortrange force, and the total force to illustrate their dependence on the absolute tip-surface distance34. (C) constant frequency shift image of the Si (111)7 × 7 surface. (D) total force (red line with symbols) and short-range force (yellow line) determined above the adatom site labeled 2 in (C). in the inset, the measured short-range force is compared with a first-principles calculation (black line with symbols)35. (E) dynamic force microscopy topographic images of a single-atomic layer of Sn grown over a Si (111) substrate. (F) sets of short-range force curves obtained over structurally equivalent Sn and Si atoms. The curves in each set are now normalized to the absolute value of the minimum short range force of the Si curve (|FSi (set)|). (G) topographic image of a surface alloy composed by Si, Sn and Pb atoms blended in equal proportions on a Si (111) substrate. Blue, green, and red atoms correspond to Sn, Pb and Si, respectively. (H) distribution of maximum attractive total forces measured over the atoms in (G)34. color online |
