Quasi-two-dimensional (2D) semiconductor nanocrystals (NCs) are captivating due to their improved optical properties, as compared to their spherical counterparts, such as extremely narrow emission line widths, suppressed Auger recombination, and giant oscillator strength. However, procedures for the direct synthesis of 2D nanoplatelets (NPLs) are not yet developed for a broad range of materials, and even well understood systems, such as CdSe, are still limited due to limited thicknesses that are readily available. At the same time, extending the optical properties into the near-infrared region (NIR) is desirable for applications in telecommunications, photovoltaics, and photodetectors, where light sources with photoluminescence (PL) inside the optical windows of glass fibers, as well as active light-absorbing and charge-transporting layers are necessary to produce high-performance components at the low cost associated with solution-processing techniques. To capitalize on the research success of the CdSe system, one can employ postsynthetic cation exchange reactions to obtain narrow band-gap materials such as lead and mercury chalcogenides, which show optical activity in the NIR. Control of the chemical composition as well as the size of the NPLs allows for broad and fine tuning of their absorption and emission spectra.

In this presentation, our recent results on the synthesis, characterisation, and application of NIR NPLs will be summarized. Two main cation exchange reactions Cd²⁺-to-Hg²⁺ and -to-Pb²⁺ have been employed to synthesize corresponding Cd_xHg_1–xSe and PbSe NPLs. Already small inclusions of mercury ions into template CdSe NPLs result in a pronounced shift of their PL to longer wavelengths.[1] A more extended partial Cd²⁺-to-Hg²⁺ cation exchange yields Cd_xHg_1–xSe NPLs emitting in the range of 700–1100 nm with quantum yields reaching 55%.[2] The resulting NPLs possess broad PL spectra due to inhomogeneous distribution of HgSe domains within CdSe cores. By optimizing the synthesis conditions, we achieved much narrower PL tunable from 1300 to 1500 nm with bandwidths down to 102 meV for Cd_xHg_1–xSe/Zn_yCd_1–yS core/shell NPLs, thus, covering the first and the second telecommunication windows.[3] A further shift of the PL can be realised by complete Cd²⁺-to-Pb²⁺ cation exchange, depending on the thickness of initial CdSe NPLs.[4] Resulting PbSe NPLs exhibit extremely narrow PL spectra with bandwidths down to 80 meV with quantum yields reaching 15%. Thus synthesized NPLs were successfully applied in photodetectors [5] and efficient NIR LEDs [3].