TU Delft measurement technique shown to reveal important properties of quantum dots
Quantum dots are semiconductor crystals only a few nanometres in size and are useful building blocks for solar cells. Thanks to their tiny dimensions, they have exactly the electronic properties that are required to convert sunlight into electricity.
However, the surface area of the quantum dots also embodies a complication: the tiniest irregularities on the surface can greatly reduce the efficiency of the solar cell. This is why the nanocrystals are often encased in protective molecules. But how good is the adhesion between these molecules and the semiconductor crystal? And how many irregularities are left on the crystal surface?
To answer these questions, Dr Stephan Eijt (Radiation Science & Technology department) looked for an analysis technique that focuses specifically on the surface of the quantum dots. "We use positron annihilation techniques at TU Delft," explains Eijt. "A positively charged antiparticle, a positron, is implanted into a material. If it encounters a negatively charged electron, the particle and the antiparticle will cancel each other out. This produces a tiny amount of gamma radiation that can be observed by detectors. The technique allows us to conduct depth-dependent studies of electron distributions and hence also the electronic structure of very thin layers of material. However, in the case of quantum dots, it was not clear whether the positron annihilation technique was sensitive to the surface or the interior of the nanocrystals."
To answer this question, the scientists used a similar technique that will soon be available at TU Delft: Positron Annihilation Lifetime Spectroscopy (PALS). PALS measurements in Germany by PhD candidate Wenqin Shi demonstrated that the majority of the positrons annihilated exactly as predicted by the theoretical calculations for the outer shell of quantum dots.
"This proves that the positron annihilation technique is sensitive to that critical surface area of quantum dots," says Eijt. The findings were published in the Physical Review Letters .
Eijt anticipates that the positron annihilation technique will gain popularity in the development of improved solar cells. TU Delft is the only provider of this technique in the Netherlands.
TU Delft is developing a PALS facility as part of the
Als onderdeel van het OYSTER-project, which involves the renovation and improvement of the TU Delft research reactor and peripheral equipment. "This is a very important step in the development of these promising new solar cells," says Eijt.