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

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

Reporting period: 2019-04-01 to 2020-09-30

In the PHOCONA project, we aim to develop a new class of highly fluorescent 2D and quasi-2D colloidal nanomaterials for solution-processed coherent light sources and ultrafast single-photon emitters. The materials consist of suspended semiconductor nanoplates 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 multiexciton and stimulated emission, as well as stable, blinking-free single-photon emission. The intrinsic opto-electronic properties will be further modified toward ultrafast exciton recombination by coupling the nanomaterials to small mode-volume plasmonic nanocavities, hereby placing them in a local photon density-of-states that will lead to a strong Purcell enhancement.
The research during this period focused on the chemical synthesis and photophysics characterization of 2D colloidal nanocrystals, in particular CdSe nanoplatelets and transition metal dichalcogenide nanosheets. For CdSe nanoplatelets, we were able to extend the emission range from the UV to the near-infrared, by developing several 2D nanocrystal materials. In particular, we developed protocols to synthesize CdSe nanoplatelets with emission ranging from the UV (400 nm) to the red (625 nm). By incorporation of silver into the CdSe nanoplatelets, we extended this to the near-infrared (880 nm). The CdSe nanoplatelets show stimulated emission at blue, green and red wavelengths, opening prospects for efficient, solution-processed coherent light sources.
Starting from CdSe nanoplatelets, the growth of complex heterostructures resulted in an engineering of the band structure toward fluorescence upconversion of red and infrared excitation light into green emission with a fast fluorescence decay time.
Regarding Cd-free alternatives, we developed synthesis protocols for MoS2 and MoSe2 transition-metal dichalcogenide nanosheets, with a thickness down to a single monolayer, a prerequisite for obtaining a direct-gap semiconductor that displays efficient fluorescence.
For the next period, efforts will be continued to further 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 coupling them to plasmonic structures.
Fluorescence of a Series of CdSe Nanoplatelets with Varying Thickness
Depiction of the Emission from CdSe Nanoplatelets