Hand-built caesium-based ‘artificial atoms’ used to create ‘synthetic’ benzene

‘Artificial atoms’ that form molecular orbitals reminiscent of those in well-known organic molecules have been produced by researchers in the Netherlands using strategically positioned caesium atoms on the surface of a semiconductor. The artificial atoms should allow researchers to measure the energies of molecular systems that would be unstable in real life, and thereby potentially gain greater insights into chemical energetics.

The scanning tunneling microscope’s ability to manipulate individual atoms  was famously demonstrated in 1989 when Don Eigler and Erhard Schweizer at IBM Almaden in California spelt out ‘IBM’ using 35 xenon atoms on a nickel substrate. In 1993, Eigler and two colleagues then looked at quantised electronic resonances from iron atoms arranged in a circle on copper, and others have subsequently created more complex 2D arrangements akin to atoms and molecules. However, ubiquitous metal substrates have proved a problem. ‘You do create something like an artificial molecule on the surface,’ explains Daniel Wegner  of Radboud University in Nijmegen, ‘but it is still strongly coupled to the substrate, and that basically kills a lot of the properties that are intrinsic to what you actually build.’ Removing the conductive substrate, however, is tricky because scanning tunnelling microscopy works by measuring the variation in the quantum tunnelling of electrons between the tip and the surface under an applied bias voltage. If the substrate itself does not conduct electrons, the tunnelling current becomes zero, which makes the tip unable to detect the substrate and thus renders scanning tunnelling microscopy impossible.