![]() The researchers’ setup, in particular, employs two ingredients that boost SPM resolution. The spatial resolution of this variant of SPM-known as noncontact atomic force microscopy (NC-AFM) or dynamic force microscopy-is such that it can image, manipulate, and spectroscopically analyze not just single atoms but single chemical bonds. The setup used by the Regensburg team relies on detectable shifts in the oscillation of a cantilever (which holds the tip) due to chemical interactions between the apex of the tip and the sample atoms (see Fig. Several tip-sample interactions are possible, including the quantum-mechanical tunneling of electrons exploited by a scanning tunneling microscope (STM). SPM images a sample by probing it with an atomically (or molecularly) sharp tip. Recognizing that a systematic approach to quantifying the role of coordination requires exceptionally well-defined atomic clusters, Berwanger and colleagues exploited a scanning probe microscope (SPM) to achieve the ultimate control in nanostructure fabrication: they built, from the bottom up, two-dimensional Fe clusters, comprising from 3 to 15 atoms, and used the same SPM to image the atoms and probe their reactivity. Studies of catalytic materials have established, for instance, that step edges are the most active catalysis sites and that catalytically inactive surfaces can be activated by adding single atoms or atom clusters to the surface. Other atoms, by contrast, find themselves in an environment of even lower symmetry-at atomic steps on an otherwise flat surface or at the edges of a nanoscopic cluster, for example-which enhances their chemical reactivity. The complexity deriving from this reduction in symmetry is part of the reason why Pauli famously described surfaces as being “invented by the devil.” Some parts of a surface, however, can be almost perfectly flat, with atoms that have lost their neighbors above but are still largely among “friends.” These atoms have a relatively high coordination number, and correspondingly low chemical reactivity. Crack a crystal apart to expose a surface, and those atoms that were previously in the bulk are wrenched from their neighbors, lowering their coordination numbers and producing what are known, rather descriptively, as dangling bonds. Surface scientists have long recognized the importance of the coordination number c-a measure of the number of chemical bonds formed by an atom. Their results quantify how the reactivity of atoms of iron (Fe)-an important material for catalysis-varies as a function of the atoms’ coordination. The Regensburg team probed how the neighborhood of an atom affects its propensity to react with a carbon monoxide (CO) molecule, which they guided into position around an iron cluster with subangstrom precision. This is precisely what Julian Berwanger at the University of Regensburg, Germany, and colleagues have now achieved. What has been lacking to date, however, is a direct and quantitative measurement of the variation in chemical reactivity of atoms having different degrees of coordination. This coordination concept plays a central role in our understanding of chemical and catalytic activity. ![]() Chemical reactivity, in turn, depends on the degree of atomic “isolation” a lonely atom will more readily interact and form bonds with others, as compared to its more “coordinated” counterparts that are chemically engaged with other atoms. ×Ītoms are gregarious by nature, escaping a solitary existence by forming chemical bonds with their neighbors. The results quantify the reactivity of iron atoms as a function of their coordination number. They then used the same setup to measure the strength of the chemical bond formed between a CO molecule attached to the tip and individual atoms in the cluster. Using the tip of a scanning probe microscope, the researchers built iron-atom clusters of varying size. APS/ Alan Stonebraker Figure 1: Sketch of the SPM setup used by Berenger et al. ![]()
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