Fig. 2: Formation of oxalate anion on Cu(100) surface.

a Large-scale STM image of the 4 K⁺ ∙ C₂O₄^2- clusters on Cu(100) surface (−200 mV, 50 pA). The yellow and green arrows denote two types of tetramers. b–f Constant-height AFM images (\(\Delta f\)) (b), simulated AFM images (c, d) and the structural models (e, f) of type-Ⅰ and type-Ⅱ tetramers. K, Cu, C, O atoms are denoted by purple, yellow, grey and red spheres, respectively. The white arrows in (b) denote the CO molecules adsorbed on Cu(100). The CO molecules adsorbed around the tetramer could stabilize the tetramer, protecting it from disturbances induced by the tip during scanning. The tip heights marked in (b) are referenced to the STM set point on the Cu substrate (100 mV, 50 pA). The tip heights marked in (c, d) are defined as the vertical distance between the apex atom of the metal tip and the outermost atom of the Cu substrate. g Frequency shift (\(\Delta f\)) measured as a function of bias above the K, C and O atoms in the 4 K⁺ ∙ C₂O₄^2- tetramer (amplitude: 100 pm, tip height: 100 pm).