Elemental fingerprinting of materials with sensitivity at the atomic limit.

By using synchrotron X-rays as a probe and a nanofabricated smart tip of a tunneling microscope as a detector, we have achieved chemical fingerprinting of individual nickel clusters on a Cu(111) surface at 2 nm lateral resolution, and at the ultimate single-atomic height sensitivity. Moreover, by varying the photon energy, we have succeeded to locally measure photoionization cross sections of just a single Ni nanocluster, which opens new exciting opportunities for chemical imaging of nanoscale materials.