In this work we present and demonstrate a method for the chemical identification of individual surface atoms on multi-elements systems using atomic force microscopy (AFM).

The chemical identification of single atoms and molecules at surfaces has been pursued from the invention of both the scanning tunneling microscope (STM) and the AFM (almost 25 years ago), because it could multiply the already outstanding capabilities of these techniques. The intrinsic detection nature of the STM and the AFM has hindered, until now, most of these efforts, and single atom chemical identification still remains a challenge.

On this quest for single-atom chemical identification, dynamic force microscopy (DFM) has an advantage since the imaging mechanism is based on detecting the short-range forces associated with the onset of the chemical bonding between the outermost atom of the AFM tip and the atoms at the surface.

Forces associated with a chemical bonding between two atoms are related to the nature of the atomic species involved. Thus, the short-range chemical forces we are measuring over the different surface atoms when exploring a heterogeneous surface with DFM should contain information about these surface atoms’ chemical nature. However, to extract this information is not trivial at all because, as we demonstrate in this paper, these short-range chemical forces present a strong variability upon the atomic structure and chemical termination of the AFM tip used to probe the surface.

We have found a magnitude that remains nearly constant independently of the AFM tip termination we used. This magnitude is the relative interaction ratio of the minimum values of the short-range chemical forces measured over two different atomic species probed with the same tip (relative interaction ratio for short in the following). In this publication, we demonstrate that the relative interaction ratio is connected to the relative strength these pairs of surface atoms have for making a chemical bond with the outermost atom of the AFM tip. This property makes it possible to use the relative interaction ratio as a fingerprint for the chemical identification of atoms at surfaces.

The identification method we have proposed here consists on measuring the short-range chemical interaction force over each of the atoms in a surface area using the same AFM tip, and then compare the ratio of the minimum force values between pairs of atomic species with the previously tabulated relative interaction ratio for the expected atoms to be contained at the surface.