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GaAs Quantum Dots Assemble Themselves

Quantum dots can self-assemble at the apex of a GaAs/AlGaAs (gallium arsenide/aluminium gallium arsenide) core/shell nanowire interface. This breakthrough could bolster quantum photonics and solar cell efficiency
Scientists from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) and other labs have demonstrated a process where quantum dots can self-assemble at optimal locations in nanowires.

This breakthrough could improve solar cells, quantum computing, and lighting devices.

Quantum dots are tiny crystals of semiconductor a few billionths of a metre in diameter. At that size they exhibit beneficial behaviours of quantum physics such as forming electron-hole pairs and harvesting excess energy.

The researchers demonstrated how quantum dots can self-assemble at the apex of the GaAs/AlGaAs core/shell nanowire interface.

Crucially, the quantum dots, besides being highly stable, can be positioned precisely relative to the nanowire’s centre. That precision, combined with the materials’ ability to provide quantum confinement for both the electrons and the holes, makes the approach a potential game-changer.

Electrons and holes typically locate in the lowest energy position within the confines of high-energy materials in the nanostructures. But in the new demonstration, the electron and hole, overlapping in a near-ideal way, are confined in the quantum dot itself at high energy rather than located at the lowest energy states. In this case, that’s the GaAs core. It’s like hitting the bulls-eye rather than the periphery.

The quantum dots, as a result, are very bright, spectrally narrow and highly anti-bunched, displaying excellent optical properties even when they are located just a few nanometres from the surface - a feature that even surprised the scientists.

“Some Swiss scientists announced that they had achieved this, but scientists at the conference had a hard time believing it," says NREL senior scientist Jun-Wei Luo, one of the co-authors of the study.

Luo got to work constructing a quantum-dot-in-nanowire system using NREL’s supercomputer and was able to demonstrate that despite the fact that the overall band edges are formed by the gallium arsenide core, the thin aluminium-rich barriers provide quantum confinement both for the electrons and the holes inside the aluminium-poor quantum dot. That explains the origin of the highly unusual optical transitions.

Several practical applications are possible. The fact that stable quantum dots can be placed very close to the surface of the nanometres raises a huge potential for their use in detecting local electric and magnetic fields. The quantum dots also could be used to charge converters for better light-harvesting, as in the case of photovoltaic cells.

This work is described in detail in the paper, “Self-assembled Quantum Dots in a Nanowire System for Quantum Photonics," by M. Heiss et al in Nature Materials, (2013). DOI:10.1038/nmat3557

The team of scientists working on the project came from universities and laboratories in Sweden, Switzerland, Spain, and the United States.

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