Indium phosphide quantum dots are semiconductor nanostructures used to improve the color rendering of television screens, as they emit intense fluorescence throughout the visible spectrum. Beyond this, the near-infrared range is used for applications such as biomedical imaging, photodetectors and solar cells.
For
in vivo imaging the use use of infrared enhances the signal-to-noise ratio, by minimizing absorption and scattering by the biological medium enabling more in-depth analysis of tissues (Fig. left).
Capturing photons over an extended range from the visible to the near infrared enhances the efficiency of solar cells. (Fig. right).
The main advantage of quantum dots is that their optical properties can be adjusted down to a few nanometers in size. The band gap that separates the valence band from the conduction band in indium phosphide (InP) is 1.35 eV; this energy "gap" corresponds to a light wavelength of around 920 nm.
Until now processes enable InP quantum dots to be synthesized only to a maximum size of 6 nm limiting the emission wavelength to around 630 nm (red).
For the past fifteen years, researchers at IRIG [collaboration] have been working on the synthesis of new types of quantum dots free from toxic heavy metals. They have succeeded in synthesizing InP quantum dots largerer than 10 nm, in order to achieve infrared luminescence up to 730 nm (infrared). The process involves the use of a new indium precursor wich acts both as a source of indium and as a reductant for the precursor (of aminophosphine type). Another advantage is the narrower luminescence line width in the near infrared, as it is less sensitive to size variations.
Thanks to the development of their optical properties, indium phosphide quantum dots are becoming a promising material in the visible and infrared ranges. These heavy-metal-free nanoparticles are used in preference to cadmium selenide and lead sulfide which contain toxic elements restricted by the
European RoHS (Restriction of Hazardous Substances) directive.
Figure on the left: for in vivo imaging, the reduced absorption between 650 and 900 nm makes this spectral range particularly suitable, notably due to the reduced absorption of hemoglobin (Hb, green), oxyhemoglobin (HbO2, red) and water (blue). Credit CEA
Figure on the right: overlap between the solar emission spectrum (yellow) and the absorption spectra of InP quantum dots: threshold at 630 nm for sizes up to 6 nm according to the classical synthesis (light grey) and according to the Irig synthesis, the threshold increases to 730 nm for 10 nm quantum dots (dark grey). Credit CEA
Collaboration: Irig-MEM for structural analysis. These results were obtained as part of an ANR project (18-CE09-0039-01 FLUO), involving the LPCNO Laboratory of Physics & Chemistry of Nano-Objects at INSA Toulouse, and the ALEDIA company in Grenoble.
RoHS (Restriction of Hazardous Substances) European Parliament Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment (2011).