Atomically Resolved Chemical Reactivity of Small Fe Clusters

Small metal clusters have been investigated for decades due to their beneficial catalytic activity. It was found that edges are most reactive and the number of catalytic events increases with the cluster’s size. However, a direct measurement of chemical reactivity of individual atoms within the clusters has not been reported yet. We combine the high-resolution capability of CO-terminated tips in scanning probe microscopy with their ability to probe chemical binding forces on single Fe atoms to study the chemical reactivity of atom-by-atom assembled Fe clusters from 1 to 15 atoms on the atomic scale. We find that the chemical reactivity of individual atoms within flat Fe clusters does not depend on the cluster size but on the coordination number of the investigated atom. Furthermore, we explain the atomic contrast of the investigated Fe clusters by relating the force spectra of individual atoms with atomic force microscopy images of the clusters.

Figure 1

Short-range force vs distance $F_{z,\mathrm{SR}}(z,0)$ spectra acquired with three different CO tips (CO tips 1–3) above the center of a single Fe adatom [see Fig. S2 for corresponding energy $E_{\mathrm{SR}}(z,0)$ curves [27] ]. While the green curve (CO tip 1) shows two pronounced minima [local physisorption force minimum ($-8\mathrm{pN}$) and global chemisorption force minimum ($-363\mathrm{pN}$) [23] ] separated by a repulsive barrier, CO tip 3 exhibits only a shallow but more attractive physisorption force minimum ($-26\mathrm{pN}$) and a less attractive chemisorption force minimum ($-231\mathrm{pN}$) without a repulsive barrier in between. In the case of CO tip 2, the chemisorption force minimum ($-448\mathrm{pN}$) is most attractive, while the physisorption force minimum flattened out completely. The inset shows the differences of the $F_{z,\mathrm{SR}}$ curves of CO tip 2 (3) and CO tip 1 for $100\le z\le 500\mathrm{pm}$. Note that all presented experimental curves within the main text and Supplemental Material are acquired with a step size of $\mathrm{\Delta }z=5\mathrm{pm}$ and plotted as connected lines for clarity. The discrete data points are plotted in Fig. S9 [27].

Figure 2

(a) $F_{z,\mathrm{SR}}(z,c^{*})$ curves above the center of a single adatom (black curve) and above the center of the individual atoms of clusters consisting of 3–6 and 10 Fe atoms acquired with CO tip 2. The atomically resolved frequency shift $\mathrm{\Delta }f(x,y)$ images of the investigated clusters, acquired in constant-current mode, are shown in the inset. The atomic positions (circles) and the ($x,y$) position of the $F_{z,\mathrm{SR}}(z,c^{*})$ spectra (crosses) are overlaid (see Fig. S4 for nonoverlaid images [27]). Hereby, the circles and crosses are color coded with respect to the reduced coordination number $c^{*}$ of each atom (see SP1 [27]). (b) Chemisorption force minimum $F_{z,\mathrm{SR}}(z_{\mathrm{chem}},c^{*})$ normalized with respect to the chemisorption force minimum of the single adatom $F_{z,\mathrm{SR}}(z_{\mathrm{chem}},0)$ as a function of $c^{*}$ for CO tips 2–4. (c) Physisorption energy minimum $E_{\mathrm{SR}}(z_{\mathrm{phys}},c^{*})$ as a function of $c^{*}$ for CO tips 2–4 [see Fig. S3 for all corresponding $F_{z,\mathrm{SR}}(z,c^{*})$ and $E_{\mathrm{SR}}(z,c^{*})$ curves [27] ]. The error bars in (b) and (c) indicate the standard deviation with respect to $c^{*}$.

Figure 3

(a) $F_{z,\mathrm{SR}}(z)$ curves acquired on various positions on an atom-by-atom assembled 15 atom cluster. Enlargement of the area indicated by the red square is shown in the inset. (b) Atomically resolved vertical force image $F_{z,\mathrm{SR}}(x,y)$ at $z=335\mathrm{pm}$ [corresponding $\mathrm{\Delta }{f}_{\mathrm{SR}}(x,y)$ image and nonoverlaid images are depicted in Fig. S7 [27] ]. (c),(d) Simultaneously recorded frequency shift $\mathrm{\Delta }f(x,y)$ and topographic $z(x,y)$ images of the same spot as shown in (b) acquired in constant-current mode using $V_{\mathrm{tip}}=-1\mathrm{mV}$ and $⟨I⟩=110\mathrm{pA}$ (the same data without overlays shown in Fig. S4(j) [27]). The ($x,y$) positions of the $F_{z,\mathrm{SR}}(z,c^{*})$ curves (colored crosses) shown in (a), the atomic positions (colored circles), and the ($x,y$) location of the ring appearance (white arrow) of the top atom ($c^{*}=2$) in (c) are overlaid in panels (b)–(d). The $z$ range in which edge atoms appear as tori in (c) is indicated by $z_{\text{ring appearance}}$ in (a).