Real-Time Imaging of K Atoms on Graphite: Interactions and Diffusion

Scanning tunneling microscopy (STM) at liquid helium temperature is used to image potassium adsorbed on graphite at low coverage ($\approx 0.02$ monolayer). Single atoms appear as protrusions on STM topographs. A statistical analysis of the position of the atoms demonstrates repulsion between adsorbates, which is quantified by comparison with molecular dynamics simulations. This gives access to the dipole moment of a single adsorbate, found to be $10.5\pm 1\mathrm{D}$. Time-lapse imaging shows that long-range order is broken by thermally activated diffusion, with a 30 meV barrier to hopping between graphite lattice sites.

Figure 1

Topography of the HOPG graphite surface prior to potassium deposition. (a) Large scale image showing the clean surface. (b) Atomic resolution image of the area highlighted in (a). The tunneling bias and current are 100 mV and 100 pA.

Figure 2

Topographic image of the graphite surface after the deposition of potassium, showing isolated potassium atoms. The tunneling bias and current are $-2\mathrm{V}$ and 5 pA. (b) The height profile is taken along the dotted line drawn in (a). (c) Landscape (full height 200 pm) of a single potassium atom.

Figure 3

Study of the spatial organization of the diluted phase of the potassium atoms on graphite. (a) Autocorrelation image $C(x,y)$ averaged on about 30 images similar to Fig. 2a. The dark area around the central peak evidences the repulsive interaction between the potassium atoms. (b) Radial distribution function $C(r)$, i.e., average over the angle of $C(x,y)$. (c) Pair distribution function, i.e., probability of having two atoms separated by a given distance. The empty circles are the experimental data; dotted lines are molecular dynamics simulations for evenly spaced dipole moment $\{6.2\mathrm{D},\dots ,16.3\mathrm{D}\}$, with a solid line for 10.5 D; dashed line is the expected distribution for a system without interaction, i.e., setting the dipole moment to 0 in the simulations. The inset shows the data and the best fit on a larger scale to show the effect of the finite size of the image.

Figure 4

Diffusion of potassium atoms on the graphite surface. (a) Initial topographic STM image and trajectory of several selected potassium adsorbates (for clarity) over 2 h (solid lines). (b) Evolution of the standard deviation $\sigma (t)$ of the distribution as a function of time. The solid line is a fit with $\sigma (t)=\sqrt{2Dt}$, used to extract $D$. Panel (c) shows the maximum value of the distribution $P(0,t)$ as a function of time with a fit with $P(0,t)=\mathrm{erf}(\frac{r_0}{\sqrt{4Dt}})$. (d) Dependence of the diffusion coefficient on the diffusion barrier at 11 K.