Site-Dependent Evolution of Electrical Conductance from Tunneling to Atomic Point Contact

Using scanning tunneling microscopy (STM), we investigated the evolution of electrical conductance between a Pb tip and Pb(111) surface from tunneling to atomic point contact at a site that was defined with atomic precision. We found that the conductance evolution depended on the contact site, for instance, on-top, bridge, or hollow (hcp and fcc) sites in the Pb lattice. In the transition from tunneling to contact regimes, the conductance measured at the on-top site was enhanced. In the point contact regime, the hollow sites had conductances larger than those of the other sites, and between the hollow sites, the hcp site had a conductance larger than that of the fcc site. We also observed the enhancement and reversal of the apparent height in atomically resolved high-current STM images, consistent with the results of the conductance traces. Our results indicate the importance of atomic configuration in the conductance of atomic junctions and suggest that attractive chemical interactions have a significant role in electron transport between contacting atoms.

Figure 1

(a) Schematic of the conductance measurements as a function of the tip displacement $\mathrm{\Delta }z$. (b) Conductance traces measured at on-top (black), bridge (green), hcp (red), and fcc (blue) sites on a flat Pb(111) surface, which were obtained from each 10 points marked in the inset STM image ($1.2\times 1.2{\mathrm{nm}}^2$, $V_S=3.8\mathrm{mV}$, $I_t=30\mathrm{nA}$) and averaged. The right panel is a zoom of the dotted area in the left panel. All 100 traces (gray) taken in the inset image are also plotted. The circles labeled “Cr” and “Br” indicate the regions in which the crossover and branching of the conductance traces occur, respectively. The two vertical lines correspond to the distances at which the conductance mappings shown in Figs. 2 and 2 were taken.

Figure 2

Spatial mappings of the conductance at various tip displacements (a) topographic image ($V_S=3.8\mathrm{mV}$, $I_t=30\mathrm{nA}$, $3.0\times 3.0{\mathrm{nm}}^2$) taken simultaneously with $64\times 64$ conductance traces. Dotted boxes indicate the area analyzed in Fig. 1. (b) $\mathrm{\Delta }z=-32\mathrm{pm}$ [between “Br” and “Cr” in Fig. 1] (c) $\mathrm{\Delta }z=-50\mathrm{pm}$ (contact regime) (d) histogram of the conductance at $\mathrm{\Delta }z=-50\mathrm{pm}$. The arrows correspond to the averaged conductance values taken at the four sites. The solid red line is a visual guide.

Figure 3

(a) High-current STM images ($V_S=9.3\mathrm{mV}$, $5.0\times 5.0{\mathrm{nm}}^2$). The set current was changed from 100 nA (bottom) to 600 nA (top) in increments of 100 nA. The offset height of each section was adjusted so that the atomic contrast could be clearly seen. (b) Cross sectional plots along the lines drawn in each section of the STM image (a). Long-range corrugation due to the moiré was eliminated by FFT filtering. Each plot was offset for clarity. (c) Conductance traces measured at the on-top and hcp sites prior to the STM imaging shown in (a). The trace of the hollow site was shifted by $-2\mathrm{pm}$ in order to compensate for the height difference at $\mathrm{\Delta }z=0$ ($V_S=9.3\mathrm{mV}$, $I_t=100\mathrm{nA}$). The horizontal lines correspond to the conductances at which the STM images were taken. (d) The height difference between the on-top and hcp sites obtained from the STM images and conductance traces. Positive (negative) values mean higher (lower) contrast at the on-top site than at the hollow site.

Figure 4

Schematics showing the configuration of the tip and surface atoms from tunneling to point contact at on-top (upper panel) and hollow (lower panel) sites. The atoms whose positions are relaxed by the chemical interaction are colored red. Note that the amount of the tip displacement is exaggerated and not in scale.