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Photonics in Flatland: Band Structure Engineering of 2D Excitons in Fluorescent Colloidal Nanomaterials

Periodic Reporting for period 4 - PHOCONA (Photonics in Flatland: Band Structure Engineering of 2D Excitons in Fluorescent Colloidal Nanomaterials)

Reporting period: 2022-04-01 to 2022-09-30

In the PHOCONA project, we developed a new class of highly fluorescent 2D and quasi-2D colloidal nanomaterials for solution-processed light sources. The materials consist of suspended semiconductor nanoplatelets with a thickness below 5 nm, and transition-metal dichalcogenide monolayers. Prepared by colloidal chemistry, they take advantage of both quantum confinement through their nanoscale dimensions, as well as dielectric confinement via the dielectric mismatch between nanomaterials and surrounding host. With smart nanocrystal design, the nanomaterials will be shaped toward efficient and fast single exciton, multiexciton and stimulated emission.

As conclusion of the action, we managed to synthesized a wide variety of 2D nanocrystals, both Cd-based and Cd-free, covering a range of emission wavelengths from the UV to the near-infrared. The photophysics investigations allowed us to improve our understanding of the exciton and biexciton optoelectronic properties, which will help future research efforts steering this class of materials toward novel photonics applications.
The research focused on the chemical synthesis and photophysics of 2D colloidal nanocrystals, in particular CdSe nanoplatelets and transition metal dichalcogenide nanosheets.

For CdSe nanoplatelets, we were able to cover the emission range from the UV to the near-infrared, by developing several highly fluorescent 2D nanocrystals. In particular, we developed protocols to synthesize CdSe nanoplatelets with variable thickness and emission ranging from the UV (400 nm) to the red (625 nm). In addition, by incorporating silver into the CdSe nanoplatelets, we extended this to the near-infrared (880 nm).

The growth of heterostructures resulted in an engineering of the band structure, yielding fluorescence upconversion of red and infrared excitation light into green emission, as well as efficient stimulated emission from blue-emitting nanoplatelets.

The photophysics of excitons and biexcitons in 2D nanocrystals where investigated with transient absorption spectroscopy, revealing their oscillator strength, binding energy, lifetime and, for highly excited nanoplatelets, the optical gain coefficient an lifetime. Results yielded both new insights into the 2D nanocrystal optoelectronic properties and demonstrated that they are efficient gain materials.

Regarding Cd-free alternatives, we developed a library of synthesis protocols for MX2 (M=Mo, W; X= S, Se) transition-metal dichalcogenide nanosheets, with a thickness down to a single monolayer, a prerequisite for obtaining a direct-gap semiconductor. As a result, MoX2 monolayers already displayed fluorescence from 4K up to room temperature.

Results where disseminated in scientific journals and conferences, as well as on social media and via press releases.
Efforts will continue to develop Cd-based and Cd-free 2D nanocrystals, with a particular focus on increasing the fluorescence efficiency and gain characteristics, decreasing the lateral dimensions to obtain single-photon emission, and exploring RoHS-free fluorescent 2D nanocrystals.
Fluorescence of a Series of CdSe Nanoplatelets with Varying Thickness
Depiction of the Emission from CdSe Nanoplatelets