QD Products

Heavy-metal-free Quantum Dots and Nanocrystals by “Green” Synthesis

 

Semiconductor nanocrystals (NCs) or quantum dots (Qdots) are promising new materials for applications in photovoltaics, displays, solid state lighting, biomolecular imaging, and etc. While presenting exciting luminance properties, “traditional” chalcogens-based Qdots are of concerns due to the use of highly toxic heavy metal elements, including cadmium, mercury, and lead. Compounds containing these elements are well documented for their toxicity. Cd can accumulate in the kidney, liver, lung with low leaving rates. They can weaken the bone, induce birth defects, and are known carcinogens. Thus the production, manipulation, and waste treatment of heavy metal materials are expensive and deleterious to the environment. These limitations are inherently associated with “traditional” Qdots and can seriously affect the cost and environmental regulation approvals of their end products. In fact, due to their toxicity, the uses of Cd, Pb, Hg, and Sn materials are now under the regulation of “zero tolerance police” in the European Union (Official Journal of the European Union, 12.2.2003 L 37/19, 2003)

Another issue for “traditional” Qdots is their synthesis procedure which typically requires the use of pyrophoric and highly toxic reagents, such as tributylphosphine, trioctylphosine, trioctylphosphine oxide,vdiethyl zinc, bis(trimethylsilyl) sulfide ((TMS)2S). The use of these non eco-friendly reagents is not only toxic for operators, but also increasing the expenses of Qdots production.

In Mesolight we aim to develop heavy-metal-free NCs using “green” procedures, which means that NCs are free of any heavy metal composition and the synthetic procedure for them does not use pyrophoric reagents instead air-stable precursors are exclusively used. Based on our proprietary techniques, we have successfully synthesized high quality heavy-metal-free Qdots emitting in the visible (Figure 1).

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Figure 1.The violet, green, & red emitting cadmium-free QDs developed in Mesolight LLC

Two series of heavy-metal-free fluorescent NCs are available from Mesolight:

 

Figure 2.

Violet-blue emitting cadmium-free Qdots. (top) optical images of the emission and (bottom) fluorescence emission spectral in 370-442 nm excited at 350 nm.

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Figure 3.Comparison of lattice constants and bandgap energies of bulk semiconductors used in quantum dots syntheses


Green-emitting cadmium-free Qdots. These Qdots are composed of core/shell structure semiconductor materials (Figure 4). They emit bright emissions from the light-green to deep red range, with full-width-at-half-maximum falls in 35-80 nm range.

 

Figure 4.Green- and red- emitting cadmium-free Qdots. (top) optical images of the emission and (bottom) fluorescence emission spectral in 470-650 nm.


The chromaticity coordinates of these heavy metal-free NCs are shown in
Figure 5.

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Figure 5.Chromaticity coordinates of heavy-metal-free Qdots on CEI 1976 u’v’ color space.
Comparing with cadmium based or other heavy metal based Qdots, these cadmium-free NCs offer distinct advantages:

 

Cd-free composition. Zinc based NCs has much lower toxicity than Cd-QDs. The acute oral median lethal dose (LD50) of ZnSe is >5000mg/kg in rats, while for Cd that is ~100-300 mg/kg. With less costs in production and waste treatment, the cost of Cd-free QDs and LEDs can be reduced. As a consequence, these eco-friendly QDs will be inherently more competitive than current Cd-QDs based LEDs

 

Eco-friendly precursors. Significantly different from Cd-QDs preparation, these Cd-free “green” QDs are not only “greener” in the semiconductor cores, but also “green” in the use of precursors. “Green” QDs will be prepared with eco-friendly precursors, e.g. zinc state, zinc oleic, and selenium dioxide, sulfur powder, without the uses of pyrophoric and highly toxic reagents.

 

Sharp and strong emissions. Cd-QDs typically have narrow FWHM values of ~30 nm, one of the advantages over organic molecules in LEDs. Alloy ZnCdS QDs used for blue color lighting have FWHM values as narrow as 14 nm and QY 60-90%. For “green” QDs to be developed in this project, these spectral advantages will be pertained, with further improvement in QY%. For instance, our preliminary results from ZnSe QDs have demonstrated the best ever 40-66% QY%.

 

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