(a) A constant current STM topograph of a Ag thin film on a Si(111)-7 × 7 surface. STS measurements were made on the spots, marked by circles (also marked by numbers), which are of different thicknesses; (b)A selected area height distribution of the Ag islands in (a). The peak at 0.0 nm corresponds to the wetting layer. The peak between 0.9 and 1.0 nm corresponds to a 4-AL thick film on the wetting layer; (c) Line profiles of the islands marked in (a) with corresponding colors. The thickness at spot “5” is 4-AL on the wetting layer (i.e., 5-AL on Si).
Tunneling current (I) vs. sample bias voltage (V) characteristics at the marked positions in Fig. 1 . The vertical black marks indicate the NDR onset voltages. In the topmost panel, an I-V curve, obtained with a clean unmodified W tip on a 5-AL height island on Si, is shown. This does not show NDR.
(a) Tunneling conductance (dI/dV) vs. sample bias (V) voltage plot, obtained by numerical derivatives of the I-V data, clearly shows the NDR effect; (b) positions of minima in tunnelling conductance in (a), marked by red arrows, are shown as a function of film thickness; (c) NDR onset voltages and peak positions in Fig. 5(a) , calculated from DFT, are plotted with the film thickness.
The tunneling mechanism for NDR is schematically shown for the negative sample bias voltages. (a), (b), and (c) show the sample-tip LDOS near the Fermi energy with different bias voltages; (d) corresponding I-V characteristics for situations (b) and (c) are schematically shown.
(a) Electronic DOS for Ag thin films of different thicknesses (1-5atomic layers), as obtained from DFT calculations. Green and black arrows show the corresponding shift of peak position. (b) Peak position vs. thickness plot of the particular energy state marked by green arrows in (a).Theoretical model for DFT calculation representing the sample ground state geometry [for 3-AL Ag on Si(111)] is shown in the inset of (b).
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