Identical quantum dots created by multinational team

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Physicists at the Paul-Drude-Institut für Festkörperelektronik (PDI) in Berlin, along with collaborators at NTT Basic Research Laboratories in Japan and the US Naval Research Laboratory have used a scanning tunnelling microscope to create quantum dots with identical, deterministic sizes. The perfect reproducibility of these dots is said to enable quantum dot architectures completely free of uncontrolled variations.

Quantum dots, says the team, are often regarded as artificial atoms because, like real atoms, they confine their electrons to quantised states with discrete energies. However, while atoms are identical, quantum dots usually have variations in their size, shape and properties. In its work, the team assembled quantum dots on an atom-by-atom basis using a scanning tunnelling microscope (STM), with an atomically precise surface template define where atoms could be placed. The template was the surface of an InAs crystal, which has a regular pattern of indium vacancies and a low concentration of native indium adatoms adsorbed above the vacancy sites. Kiyoshi Kanisawa, a NTT physicist and expert in crystal growth, used molecular beam epitaxy to fabricate an InAs surface. The adatoms – ionised +1 donors – can be moved with the STM tip by vertical atom manipulation. The team assembled quantum dots consisting of linear chains with 6 to 25 indium atoms. Stefan Fölsch, a PDI physicist who led the team, said: "The ionised indium adatoms form a quantum dot by creating an electrostatic well that confines electrons normally associated with a surface state of the InAs crystal. The quantised states can then be probed and mapped by scanning tunnelling spectroscopy measurements of the differential conductance." Because the indium atoms are confined to the regular lattice of vacancy sites, every quantum dot with N atoms is essentially identical, with no intrinsic variation in size, shape or position. This means that quantum dot 'molecules' consisting of several coupled chains will reflect the same invariance."