​Quantum dots are semiconductor nanostructures capable of confining electrons (and holes) in space, giving them properties close to those of an artificial atom and opening up a wide range of applications (photonics, display screens, biomedical imaging, photovoltaics, etc.). As an example, they improve the color rendering of television screens by emitting intense fluorescence across the visible spectrum.

Indium phosphide (InP) is the semiconductor used for these applications, in preference to cadmium selenide (CdSe) or lead sulfide (PbS), which contain toxic elements - the very restricted use of which is governed by the European RoHS (Restriction of Hazardous Substances) directive.

Bigger and better

Researchers at Irig, who have been working for some fifteen years on the synthesis of new types of quantum dots free from toxic heavy metals, wanted to extend the spectral range of InP quantum dots into the infrared. For in vivo imaging, the use of infrared would reduce absorption and scattering by the biological environment, enabling more in-depth analysis of tissues, while for photovoltaics, it would improve the solar cell conversion efficiency.

Until now, InP quantum dots were only 6 nanometers in size, but now the scientists have succeeded in synthesizing InP quantum dots larger than 10 nm. As the optoelectronic properties of these nanostructures can be modulated with their size, the emission wavelength can be extended from around 630 nm (red) to 730 nm (infrared).

The process involves the use of a new indium precursor, which acts both as a source of indium and as a reductant for the phosphorus precursor (of aminophosphine type). Another advantage is the narrower luminescence line width in the near infrared.

​These results were obtained as part of an ANR project involving INSA Toulouse and the company ALEDIA in Grenoble.