Being able to detect photons emitted from single atoms would be of great help for characterizing nanoscale structures and devices. Previous work in this field has largely focused on using electron energy loss spectroscopy (EELS) to detect single lanthanide metal atoms and light atoms like carbon. However, EELS can only be applied to certain elements because of the high-energy beams employed in this method that can damage samples. Nobel metals, such as gold and platinum, are also difficult to detect with high sensitivity using EELS – a major drawback when it comes to investigating meteorites, catalytic clusters or anticancer drugs, where only a very small number of noble metals are looked at in any given sample.

Energy-dispersive X-ray spectroscopy (EDX) is a good way to chemically characterize a wide range of materials, but researchers have been reluctant to use the technique to detect single atoms because of the difficulties involved in obtaining good photoemission spectra. Kazu Suenaga of the Nanotube Research Center at AIST in Tsukuba and colleagues at JEOL Ltd in Akishima and Kyushu University in Fukuoka are now saying that they have successfully used EDX to sense single atoms of erbium thanks to advanced excitation and detection apparatus.

Metallofullerene peapods

Suenaga and colleagues studied metallofullerene peapods in their experiments and in particular, erbium peapods (Er@C82) – so-called because the atoms are lined up in rows like peas in a pod. Each peapod is made up of a single erbium atom inside a carbon-82 cage, supported in a carbon nanotube. The advantage of looking at such a sample is that the structure is well ordered, with each metal atom separated from its neighbour by around 1 nm. The atoms can thus easily be distinguished in the resulting X-ray spectra.

The team obtained its results by using a finely focused electron beam (down to a few angstroms) to excite single Er atoms in an electron microscope so that they emitted X-ray photons. A newly developed large-sized (around 100 mm2) silicon drift detector (SDD) was also employed to collect as many X-rays as possible from the sample.

"X-rays are typically emitted in all directions, so a normal-sized detector misses a lot of them and only a few percent can be collected," says Suenaga. "Our new large SSD greatly improves on collecting efficiency – by at least several times," he told

"Being able to perform X-ray spectroscopy on single atoms in this way will be of great help in nano-optics research," he added.