High-resolution scanning force microscopy of gold nanoclusters on the KBr (001) surface

In this study we use a combination of dynamic scanning force microscopy experiments and first-principles simulations to study the imaging process of gold nanoclusters adsorbed on the (001) surface of KBr. In previous experiments atomic resolution was readily obtained on the KBr substrate. However, it was not possible to resolve atoms within the clusters themselves. This correlates with imaging simulations we present here using several different probable tip models: measurable contrast was readily achieved on the KBr surface and on the gold (001) surface, but simulations on the clusters demonstrated poor contrast for all tips. We further consider the role of cluster charging in the tip-surface interaction, and the role that surface defects play in the properties of adsorbed clusters.

Figure 1

(Color online) (a) ${\mathrm{Au}}_{13}$ tip and (b) ${\left(\mathrm{K}\mathrm{Br}\right)}_6$ tip used in imaging simulations. ${\mathrm{K}}^{+}$ ions are represented by the green (darker) spheres.

Figure 2

(Color online) ${\mathrm{K}}^{+}$-terminated ${\left(\mathrm{K}\mathrm{Br}\right)}_6$ tip imaging the 25-atom Au cluster on KBr surface. The cluster has a mean lateral size of about $d=1.1\mathrm{nm}$ and a height of $h=0.8\mathrm{nm}$. ${\mathrm{K}}^{+}$ ions are represented by the green (darker) spheres.

Figure 3

(Color online) Au(001) surface structure with $(1\times 5)$ reconstruction. The surface used in calculations was two times larger, having 24 atoms at the top layer oriented towards the tip.

Figure 4

(Color online) Force-distance curves over different KBr surface sites, imaged with Au and KBr tips. ${\mathrm{Br}}^{-}$ and ${\mathrm{K}}^{+}$ tips denote ${\mathrm{Br}}^{-}$- and ${\mathrm{K}}^{+}$-terminated ${\left(\mathrm{K}\mathrm{Br}\right)}_6$ tips respectively. The lines are fitted averages to the calculated points of the same color. Legend (see also the inset): Red (middle gray) disks: over Br ion. Blue (dark gray) squares: over K ion. Black triangles: bridging position between K and Br. Green (light gray) circles: over the cell center.

Figure 5

(a) Experimental constant height image showing black disk with a white rim contrast. The image was additionally straightened and slightly Fourier filtered. ($f_0=267.9\mathrm{kHz}$, average $\Delta f=-55.0\mathrm{Hz}$, $k=32\mathrm{N}∕\mathrm{m}$, $A_{\mathrm{p}-\mathrm{p}}=13\mathrm{nm}$, $U_{\mathrm{dc}}=-0.02\mathrm{V}$, contrast in $\Delta f=3.4\mathrm{Hz}$, image size $1.9\mathrm{nm}\times 1.4\mathrm{nm}$). (b) Simulated KBr surface imaged with a ${\mathrm{Br}}^{-}$-terminated KBr tip at a constant height of $4.9\mathrm{\AA }$. The parameters of the tip oscillation ($f_0$, $\Delta f$, $k$, $A_{\mathrm{p}-\mathrm{p}}$) in the simulation were the same as in the experiment.

Figure 6

(a) A typical experimental constant frequency change image of the KBr surface showing white disk contrast. The image was additionally straightened and is not filtered ($f_0=293.2\mathrm{kHz}$, $\Delta f=-83.8\mathrm{Hz}$, $k=48.5\mathrm{N}∕\mathrm{m}$, $A_{\mathrm{p}-\mathrm{p}}=4.2\mathrm{nm}$, $U_{\mathrm{dc}}=0.056\mathrm{V}$, contrast in $z=39\mathrm{pm}$, image size $2.0\mathrm{nm}\times 1.5\mathrm{nm}$). (b) Simulated KBr surface imaged with a ${\mathrm{K}}^{+}$-terminated KBr tip with constant frequency change. Bright spots (attraction above ${\mathrm{Br}}^{-}$ ions) are clearly separated. The parameters of the tip oscillation ($f_0$, $\Delta f$, $k$, $A_{\mathrm{p}-\mathrm{p}}$) in the simulation were the same as in the experiment.

Figure 7

Topography image of the KBr(001) surface after the deposition of 0.07 ML of gold. Note that the density of clusters directly scales with the density of steps which is lower in the lower part of the image than in the region on the top of the image ($400\times 400{\mathrm{nm}}^2$, $293.2\mathrm{kHz}$ tip, $\Delta f=-23.9\mathrm{Hz}$, $k=48\mathrm{N}∕\mathrm{m}$, $A_{\mathrm{p}-\mathrm{p}}=13\mathrm{nm}$, $U_{\mathrm{dc}}=5.0\mathrm{V}$, contrast in $z=6\mathrm{nm}$).

Figure 8

(Color online) Topography $\left(z\right)$ and detuning $\left(\Delta f\right)$ images of a gold cluster taken at different distances (top→bottom: distance decreases). $z$-value given for each image denotes the corrugation in topography images. In detuning images the contrast is up to 15% of $\Delta f$. ($18\times 12{\mathrm{nm}}^2$, $293\mathrm{kHz}$ tip, $k=48.5\mathrm{N}∕\mathrm{m}$, $A_{\mathrm{p}-\mathrm{p}}=10\mathrm{nm}$, $U_{\mathrm{dc}}=0.06\mathrm{V}$, (a) $\Delta f=-7.7\mathrm{Hz}$, $z=1.0\mathrm{nm}$, (b) $\Delta f=-17.2\mathrm{Hz}$, $z=1.3\mathrm{nm}$, (c) $\Delta f=-23.2\mathrm{Hz}$, $z=1.5\mathrm{nm}$, (d) $\Delta f=-28.1\mathrm{Hz}$, $z=1.2\mathrm{nm}$, (e) $\Delta f=-32.3\mathrm{Hz}$, $z=1.2\mathrm{nm}$, (f) $\Delta f=-37.9\mathrm{Hz}$, $z=0.9\mathrm{nm}$.)

Figure 9

(Color online) Tip-cluster forces, above two surface sites of the Au cluster. In the inset Au denotes the site directly above a gold atom, and H the hollow site between atoms. Note that the ${\mathrm{Br}}^{-}$ tip curves have been raised by a constant force of $0.1\mathrm{eV}∕\mathrm{\AA }$ for clarity. For all three tips the force contrast between the two surface sites is small, except for the ${\mathrm{Br}}^{-}$ tip at $4\mathrm{\AA }$ separation, where atomic jumps hinder imaging.

Figure 10

(Color online) Force-distance curves above three surface sites of the reconstructed $(1\times 5)$ Au(001) surface shown in Fig. 3, imaged with the ${\mathrm{Br}}^{-}$ terminated tip. With typical experimental imaging parameters the force contrast between most attractive and least attractive sites corresponds to a small $0.25\mathrm{\AA }$ contrast in topographic images.

Figure 11

(Color online) Charging in the ${\mathrm{Au}}_{25}$ cluster over an $F$-center (Br vacancy). Negative (blue −) charging in the atoms close to the surface, positive (red +) charging in the upper layers of the Au cluster. The $F$-center is below the lowest negative gold atom.