Figure 1: The organic self-assembled monolayer occupies the gap between the metallic tip and the conductive gold substrate and modulates the tunneling current during the STM experiment. The elevation of the tip over the substrate is principally affected by the conductance of the material in the gap, the voltage applied between the tip and substrate, and the setpoint for the tunneling current. (a) If the two first parameters are constant, the tunneling current becomes the relevant parameter to prevent incursion of the tip into the monolayer. (b) The STM tip probes the outer surface of SAMs of thicknesses of up to 2.0 nm when currents of the order of 1 pA are used, allowing end groups to be differentiated and their packing inspected. (c) Molecules in the monolayer evidently "assist" the propagation of electrons because the tip moves towards the surface when it crosses a thinner region in the monolayer in order to maintain the tunneling current. Hence, STM topographs recorded on monolayers are positives of the monolayer topography.