Bright Beaches of Nanoscale Potassium Islands on Graphite in STM Imaging

We demonstrate, via scanning tunneling microscopy (STM) measurements performed at 48 K, the existence of “bright beaches” at the edges of K islands (diameter $\sim 5–500\mathrm{nm}$) on the graphite surface. The enhanced tunneling current is only observed in monolayer-high islands on graphite, and not in islands of similar geometry on top of a K monolayer film. First-principles density functional calculations and STM simulations suggest that this is an STM field effect, which appears as the positive tip attracts donated electrons back to the metallic K islands. The restored charge accumulates preferentially at the island edges.

Figure 1

(a) Constant current STM image ($57\mathrm{nm}\times 57\mathrm{nm}$) of K islands on graphite at a surface coverage of $\theta =0.86$ monolayers (surface bias voltage, $-2.0\mathrm{V}$; tunnel current 40 pA). (b) As (a) except the surface bias voltage is $-4\mathrm{V}$. (c) The apparent (average) heights of the edges and centers of the K islands (relative to the graphite surface) as a function of (negative) surface bias voltage. (d) STM image ($200\mathrm{nm}\times 200\mathrm{nm}$) showing a region of the surface which contains both K monolayer islands on graphite and a K monolayer island on top of an extended K monolayer (surface bias voltage, $-4.6\mathrm{V}$; tunnel current, 40 pA); inset, line profile of the second-layer island.

Figure 2

A pair of constant current STM images ($102\mathrm{nm}\times 102\mathrm{nm}$) of K islands on graphite (a) before and (b) after the tip picks up atoms (presumed K atoms) from the surface (surface bias voltage $-2.0\mathrm{V}$; tunnel current, 40 pA).

Figure 3

(a) ${\mathrm{K}}_{19}$ cluster and (b) ${\mathrm{K}}_{24}$ strip (more than one unit cell shown) on graphite. Numbers stand for the Bader charges of K atoms. Total charge for the cluster is $+7.9e$ and for the strip $+8.5e$. Projected density of Kohn-Sham states for (c) cluster and (d) strip. The Fermi-level is set to zero, and the horizontal bar denotes the bias window used in the STM simulations of Fig. 4.

Figure 4

Simulated STM images for ${\mathrm{K}}_{19}$ cluster (a) and (b) and ${\mathrm{K}}_{24}$ strip (c) and (d) without [(a), (c)] and with [(b), (d)] external electric field ($0.5\mathrm{V}/\AA$, pointing downwards). Line scans and cross sections [locations shown by lines in (a)–(d)] of the local electron density for cluster (e) and (f) and strip (g) and (h) without and with the field. Zero height of the line scans (solid black curves) is at the level of carbon atoms, and the K atoms appear at 2.95–3.15 Å. Bias voltage is $-4.0\mathrm{V}$, and the STM images and line scans are produced with electron density $n=4\times {10}^{-4}{\AA }^{-3}$.