In a piece of reverse science, German researchers have made nanocrystals luminesce more strongly by enclosing them in a crystalline shell.

Luminescent nanocrystals can be used in monitors and LEDs and also as markers to help investigate biomolecules, but only if they luminesce with a high intensity. In the past, the problem with the crystals has been that the excitation energy passes rapidly from atom to atom, only to disappear in the surroundings and extinguish luminescence.

Researchers from the University of Hamburg have intensified the luminescence from nanocrystalline lanthanoid materials by enclosing them in a shell of large band-gap material, through which the energy cannot escape.

They enclosed crystals of terbium-doped cerium phosphate, which emit a green light when excited by UV light, in a lanthanum phosphate shell. The shell binds seamlessly to the nanocrystals because it has a similar crystal lattice structure, the researchers claim.

According to Markus Haase, who led the research team, the excitation energy can pass between cerium atoms but is not transferred to the shell. He claims that the new material has a quantum yield of 70 per cent.

Steve Kelly from the University of Hull's chemistry department, described the quantum yields achieved by the German researchers as 'extraordinarily high'.

'It is very unusual to find high quantum yields for isolated solid nanocrystals,' he notes, and suggests that the crystals could possibly be used in hybrid inorganic/organic light-emitting diodes.

Emma Davies