Designing 2D colloidal nanocrystals for photonic applications across the visible and near infrared spectral range

Iwan Moreels1

1Department of Chemistry Ghent University, Krijgslaan 281-S3, 9000 Gent, Belgium

 

 qdots

Figure 1: Sketch depicting the continuous tuning of the fluorescence spectrum of 2D CdSe/ZnS nanocrystals, and increase in the fluorescence quantum efficiency.[3]

Colloidal nanocrystals are solution-based materials that are well suited for low-cost and scalable lighting and energy applications. When synthesized as 2D nanoplatelets, they result in nanocrystals that typically have a thickness less than 2-3 nm. This yields a strongly confined vertical direction, which allows to control the emission color, in combination with larger in-plane sizes that determine optical properties such as the fluorescence lifetime and nonradiative recombination rate. As such, they form excellent candidates for a next generation of light emitting or energy harvesting devices.

 

Their opto-electronic properties can be engineered toward specific applications by control over the material dimensions, synthesis of core/shell architectures and inclusion of heterovalent dopants. In this presentation, I will focus on the wide possibilities of bottom-up synthesis, by discussing CdSe 2D nanocrystals, their heterostructures and doping with silver ions. 

First, despite a growth mechanism[1] that strongly favors the synthesis of 2D nanocrystals with a limited thickness, we were able increase this parameter by including chloride ions in a two-step synthesis that switches from a lateral 2D to a more isotropic 3D growth.[2] As a result, we obtained CdSe nanoplatelets with emission up to 625 nm, while maintaining a 9-13 nm emission band width, resulting in excellent color purity. To further increase their fluorescence quantum efficiency, we also prepared CdSe/ZnS core/shell heterostructures.[3] Despite the type-I band alignment, we observed that the band edge red shifts significantly upon shell growth. This unique property allows to continuously tune the band gap across the visible spectrum.We finally developed a protocol to dope CdSe nanoplatelets and reach near-infrared emission, with silver ions in a post-synthesis cation exchange.[4] At the expense of a longer emission lifetime, CdSe:Ag nanoplatelets are excellent emitters when a large Stokes shift is required to suppress self-absorption, in for instance solid-state lighting or luminescence solar concentrators.

Our results demonstrate that the opto-electronic properties in 2D materials can be tuned to a high degree via a combination of quantum confinement, electron-hole Coulomb interactions, and nanocrystal doping. By targeted material synthesis, we can optimally design 2D heterostructures toward different photonic applications, such as light-emitting or energy harvesting devices. Insights gained on the prototypical CdSe nanoplatelets should also be readily translatable to other 2D nanomaterials, offering prospects for a new generation of spectrally narrow, highly efficient colloidal nanocrystal phosphors. 

Acknowledgments
I.M. acknowledges funding from the European Research Council (grant agreement no. 714876 PHOCONA).

References

[1]   A. Riedinger et al., Nature. Mater. 2017, 16, 743–748.

[2]  S. Christodoulou et al., Nano Lett.2018, 18, 6248–6254.

[3]  A.Polovitsyn et al., Chem. Mater. 2017, 29, 5671–5680.

[4]  A.H. Khan et al., Chem. Mater.2019, doi 10.1021/acs.chemmater.8b05334.