Simple trick lets pure blue quantum dots maintain their performance without cadmium

A simple structural tweak has dramatically enhanced the colour clarity and stability of heavy metal-free blue quantum dots. By incorporating sulfur into the eco-friendly zinc–selenium–tellurium nanocrystal lattice, researchers in China produced pure blue quantum dot LEDs (QLEDs) that rival the performance of state-of-the-art ones.

‘Quantum dots are semiconductor nanocrystals with size-dependent optical and electronic properties,’ explains Yajie Dong, an optoelectronics researcher at the University of Central Florida who wasn’t involved in the research. The physical size of these particles confines the electrons into such a small space that quantum effects dominate their behaviour, leading to tuneable optical and electronic properties that are heavily exploited in the production of coloured LEDs.

However, these semiconducting nanostructures frequently contain toxic heavy metals, such as cadmium, lead and indium, creating safety concerns around their use in consumer electronics. Zinc, selenium and tellurium-based quantum dots are a promising emerging alternative, but ‘heavy metal-free QLEDs have historically struggled with lower efficiency and stability and broader emission linewidth’, resulting in reduced colour purity, says Dong.

Now, by incorporating an additional sulfur reagent into the synthesis of ZnSeTe quantum dots, Xuyong Yang and his team at Shanghai University have overcome these limitations and produced an efficient pure blue heavy metal-free QLED with performance comparable to their cadmium counterparts.

Conventional ZnSeTe nanocrystals suffer from compositional inhomogeneity as the highly reactive tellurium precursors cause the tellurium atoms to aggregate within the structure. These pockets of tellurium introduce faults and defects into the crystal and trap electrons in the tellurium-rich regions, reducing both the stability and colour purity of the resulting quantum dot.

Yang’s team therefore incorporated a reactive sulfur-triphenylphosphite reagent into the synthesis, hoping to promote the homogeneous growth of the crystal by mediating the electronic interactions between each component. X-ray analysis revealed that addition of electronegative sulfur into the lattice inhibited the aggregation of tellurium, creating a uniform, defect-free structure. Spectral analysis comparing both crystals confirmed that this regular arrangement had narrowed the photoluminescent range, enhancing the colour purity of the modified quantum dot.

With the required homogeneous crystal in hand, the team designed a simple LED device to investigate the performance of ZnSeTeS in a practical setting. The modified structure produced a pure-blue emission and exhibited more than double the quantum efficiency (the proportion of photons emitted versus absorbed) of the original ZnSeTe lattice. ‘The 24.7% [external quantum efficiency] places ZnSeTeS QLEDs among the best-performing cadmium-free blue QLEDs,’ says Dong. ‘Their narrow 17nm emission linewidth at 460nm ensures excellent colour purity, essential for display applications.’

However, the stability and lifetime of the device still requires further optimisation before these quantum dots are a feasible alternative for commercial electronics. The 30,000-hour half-lifetime is a promising start, comments Dong, but commercial displays should still be operating at 95% luminance/capacity after this which is significantly harder to achieve.

Despite these challenges, Dong is impressed by the team’s work and optimistic about the future of heavy metal-free blue LEDs. ‘Blue QLEDs could transform full-colour displays, either by working with red and green QLEDs or by serving as a backplane technology to replace blue organic LEDs. Beyond displays, they could serve as medical light sources, where flexibility and precise spectral control are beneficial and long-term durability is not a primary concern,’ he says.