Chemical identification of individual surface atoms by atomic force microscopy

Abstract

Scanning probe microscopy is a versatile and powerful method that uses sharp tips to image, measure and manipulate matter at surfaces with atomic resolution1,2. At cryogenic temperatures, scanning probe microscopy can even provide electron tunnelling spectra that serve as fingerprints of the vibrational properties of adsorbed molecules3,4,5 and of the electronic properties of magnetic impurity atoms6,7, thereby allowing chemical identification. But in many instances, and particularly for insulating systems, determining 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 resolution8,9,10, by detecting and precisely measuring11,12,13 the short-range forces that arise with the onset of chemical bonding between the tip and surface atoms14,15 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 recognition, even at room temperature. We illustrate the performance of this approach by imaging the surface of a particularly challenging alloy system and successfully identifying the three constituent 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 topographic measurements impossible.

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Figure 1: Dynamic force microscopy with atomic resolution.
Figure 2: Probing short-range chemical interaction forces.
Figure 3: Single-atom chemical identification.

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Acknowledgements

We thank F. J. Giessibl and M. Reichling for their comments on the manuscript, and T. Namikawa and K. Mizuta for technical assistance. This work was supported by the Handai FRC, the JST, the 21st Century COE programme, and the MEXT of Japan. The work of P.P. and R.P. is supported by the MCyT, the Juan de la Cierva Programme, the CCC-UAM (Spain), and the FORCETOOL project (EU). The work of P.J. is supported by the MSMT and GAAV.

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Correspondence to Óscar Custance.

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Supplementary Information

This file contains Supplementary Notes, supplementary Figures S1-S2 with Legends, Supplementary Tables I-IV and additional references. This file contains: the determination of the relative interaction ratio for a single-atomic layer of In grown on a Si(111) substrate, the acquisition parameters for all the sets of force curves and images presented, and a discussion regarding the fundamentals behind the almost complete independence of the relative interaction ratio from the tip-apex chemical termination. (PDF 527 kb)

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Sugimoto, Y., Pou, P., Abe, M. et al. Chemical identification of individual surface atoms by atomic force microscopy. Nature 446, 64–67 (2007). https://doi.org/10.1038/nature05530

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