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(Color) (a) topograph of the monolayer, second layer (bilayer), and third layer (trilayer) regions on the Au(111) surface. (b) spectra measured on the monolayer (black curve) and bilayer (blue curve). NDR regions in bilayer can be seen for . The curves are scaled by normalizing to the maximum. Tip stabilization: , (monolayer) and , (bilayer).
(Color) (a) curves acquired at three different tunnel junction resistances on the bilayer. Each curve is normalized to the respective local current maximum located at . Tip stabilization: ; , 50, for the black, red, and blue curves, respectively. (b) topograph and energetically resolved maps for bilayer taken at voltages just below and above . The tip height was stabilized at , for each map. Superimposed white circles show the locations of individual bilayer molecules. The color scale applies only to the maps.
(a) Schematic of tunnel junction model used in simulating bilayer . The STM tip is separated by a distance from the bilayer and the tip density of states is assumed to be constant. Splitting in the bilayer LUMO (L) and resonances is not shown in the sketch. (b) Simulated curves corresponding to the three experimental curves shown in Fig. 2. NDR occurs for due to increased effective barrier for tunneling into LUMO resonances. Increase in NDR with tip-sample separation is well reproduced in this model.
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