(a) The schematic of the AFM tip scanning over the surface of Au and molecular islands. (b) raw STM image acquired with the conductive AFM tip in the STM mode at and . (c) Tip-sample force map acquired simultaneously. The inset of (b) shows a STM image over the molecular islands revealing the periodicity of the molecules at and . The bright and dark areas in the force map represent repulsive and attractive forces respectively. (d) Atomic steps and molecular islands are delineated with green dashed and black solid lines, respectively. (e) The molecule islands exhibit bright contrast representing a repulsive interaction between the STM tip and molecules.
(a) Raw STM image and (b) tip-sample force map simultaneously acquired at and . (c) same as (a) but at . The apparent depth of the molecular islands in (c) and normal force in (d) have clearly changed. (e) Profiles of height and normal force from (a) and (b). The compensated height is obtained by taking the raw STM height corrected by the lever deflection.
(a) The plot of normal force and apparent height measured at the molecule islands as a function of the setpoint current. Schematics of tip surface showing (b) noncontact tunneling at the current less than at the sample bias of , (c) just in contact at the current of , and (d) contact at the current higher than .
(a) Plot of the height of molecules as a function of the normal force. The height of molecule was assumed to be when the tip is out of contact from the molecules. The compensated height was calculated by taking (apparent height) subtracted by the lever deflection. Assuming the tip-sample distance at the Au is constant, the height of molecules was estimated. (b) Semilog plot of current density as a function of the height of molecules. A decay constant was found for the current passing through the film as a function of tip-substrate separation.
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