Fig. 1
Experimental set-up and AFM images of two degenerate water tetramers with a CO-terminated tip. a Schematic of a qPlus-based nc-AFM with a CO-tip. The cantilever oscillates at an amplitude of A and the tip-sample force-induced frequency shift of the cantilever from its natural resonance frequency (f0) is Δf. b, c Top and side view of the water tetramer adsorbed on the NaCl(001) surface, respectively. H, O, Cl, and Na atoms are denoted as white, red, green, and purple spheres, respectively. d–h, i–m Water tetramers with clockwise and anticlockwise H-bonded loops, respectively. d, i Calculated electrostatic potential map of the water tetramers in a plane 60 pm above the outermost H atom. e, j Constant-current STM images acquired at (100 mV, 20 pA) and (100 mV, 30 pA), respectively. f, k, g, l, h, m Experimental Δf images recorded at the tip heights of 100 pm, 10 pm, −40 pm, respectively. The tip height is referenced to the STM set point on the NaCl surface (100 mV, 50 pA). The oscillation amplitude is 100 pm. The STM/AFM imaging was done on two different tetramers with two different CO-tips. The size of the images is 1.2 nm × 1.2 nm
