LETTERS Chemical identification of individual surface atoms by atomic force microscopy Yoshiaki Sugimoto 1 , Pablo Pou 2 , Masayuki Abe 1,3 , Pavel Jelinek 4 , Rube ´n Pe ´rez 2 , Seizo Morita 1 &O ´ scar Custance 1 Scanning probe microscopy is a versatile and powerful method that uses sharp tips to image, measure and manipulate matter at surfaces with atomic resolution 1,2 . At cryogenic temperatures, scanning probe microscopy can even provide electron tunnelling spectra that serve as fingerprints of the vibrational properties of adsorbed molecules 3–5 and of the electronic properties of magnetic impurity atoms 6,7 , thereby allowing chemical identification. But in many instances, and particularly for insulating systems, determin- ing the exact chemical composition of surfaces or nanostructures remains a considerable challenge. In principle, dynamic force microscopy should make it possible to overcome this problem: it can image insulator, semiconductor and metal surfaces with true atomic resolution 8–10 , by detecting and precisely measuring 11–13 the short-range forces that arise with the onset of chemical bonding between the tip and surface atoms 14,15 and that depend sensitively on the chemical identity of the atoms involved. Here we report precise measurements of such short-range chemical forces, and show that their dependence on the force microscope tip used can be overcome through a normalization procedure. This allows us to use the chemical force measurements as the basis for atomic recog- nition, even at room temperature. We illustrate the performance of this approach by imaging the surface of a particularly challen- ging alloy system and successfully identifying the three constitu- ent atomic species silicon, tin and lead, even though these exhibit very similar chemical properties and identical surface position preferences that render any discrimination attempt based on topo- graphic measurements impossible. The chemical identification of single atoms and molecules at sur- faces has been pursued since the invention of both the scanning tunnelling microscope and the atomic force microscope (AFM). Particularly promising in this quest is dynamic force microscopy, which achieves true atomic imaging resolution 8–10 by detecting the short-range forces associated with the onset of the chemical bond between the outermost atom of the tip apex and the surface atoms being imaged 14,15 (see Fig. 1 for schematic illustration of the method and imaging examples). Moreover, dynamic force spectroscopy 11–13 makes it possible to quantify these forces. Figure 2a shows five sets of dynamic force spectra measured on a single atomic layer of Sn grown on a Si(111) substrate. Each set of force curves was obtained over an Sn atom and an Si atom having the same local surface configuration as the corresponding atoms highlighted in the topographic image shown in Fig. 1d, always using identical acquisition and analysis protocols (see Methods). However, the sets were collected over multiple measurement sessions, using tips that had different apex terminations. These tip apexes presumably differ in both structure and composition (Sn or Si), as sometimes slight tip–surface contacts were intentionally produced before the acquisition of each set of force curves. The sets seem to share only one feature: curves measured over the Si atoms are characterized by a stronger attractive interaction force. Given the high degree of stability, lateral positioning accuracy, and reproducibility provided by our acquisition protocol 12,16 , we attribute the variability seen in the data in Fig. 2a to a strong tip dependence of both the registered 1 Graduate School of Engineering, Osaka University, 2-1 Yamada-Oka, 565-0871 Suita, Osaka, Japan. 2 Departamento de Fı ´sica Teo ´rica de la Materia Condensada, Universidad Auto ´noma de Madrid, 28049 Madrid, Spain. 3 PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan. 4 Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka ´ 10, 1862 53, Prague, Czech Republic. 1 –3 –2 –1 0 2 3 4 5 Tip–surface distance (Å) Van der Waals force Short-range chemical force Total force Force (nN) 6 7 8 9 a c b d e Figure 1 | Dynamic force microscopy with atomic resolution. Schematic illustration of AFM operation in dynamic mode (a), and of the onset of the chemical bonding between the outermost tip atom and a surface atom (highlighted by the green stick) that gives rise to the atomic contrast 14,15 (b). However, the tip experiences not only the short-range force associated with this chemical interaction, but also long-range force contributions that arise from van der Waals and electrostatic interactions between tip and surface (though the effect of the latter is usually minimized through appropriate choice of the experimental set-up). c, Curves obtained with analytical expressions for the van der Waals force, the short-range chemical interaction force, and the total force to illustrate their dependence on the absolute tip–surface distance. d–e, Dynamic force microscopy topographic images of a single-atomic layer of Sn (d) and Pb (e) grown, respectively, over a Si(111) substrate. At these surfaces, a small concentration of substitutional Si defects, characterized by a diminished topographic contrast 20 , is usually found. The green arrows indicate atomic positions where force spectroscopic measurements were performed (see Fig. 2). Image dimensions are (4.3 3 4.3) nm 2 ; for the acquisition parameters see the Supplementary Information. Vol 446 | 1 March 2007 | doi:10.1038/nature05530 64 Nature ©2007 Publishing Group