Just as the surface of a material has very different properties from the bulk, the atoms at the end of a one-dimensional structure should behave differently to the other atoms in the structure. However, these zero-dimensional "end states", which are predicted to be localised to a single atom, had never been seen until now.

Jason Crain and David Pierce at the National Institute of Standards and Technology (NIST) in Boulder created one-dimensional chains of gold atoms by placing small amounts of gold onto a slightly mis-cut silicon surface at high temperature. Each chain contained between three and nine atoms. By making the chains on a semi-insulating surface, as opposed to the metal surfaces used in previous experiments, Crain and Pierce were able to observe the end states with scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS).

In STM a sharp tip is scanned over the surface while keeping the tunnelling current between tip and sample constant at a given bias voltage. This maintains the tip at constant height and provides an image of the sample’s surface topography. For STS, the tip is held fixed and the bias voltage between the tip and sample is varied. The change in the tunnelling current with the bias voltage gives a measure of the local density of states for the electrons in the sample.

The NIST team found that electrons in the end states had lower energies than those in the inner atoms. Moreover, they were localised to the end atoms, as predicted by theory (see figure).

"Because electrons confined in one-dimensional chains interact strongly with each other, very interesting and unusual properties are expected," Crain told PhysicsWeb. "Our detailed study of the electronic structure is a first step toward understanding the electronic properties in one dimension. Ultimately, it may be possible to create atom wires for applications in nanoelectronics."

The duo now plans to explore how the quantum states in the chains change as the temperature is lowered to absolute zero.