Current colloidal synthesis methods for CdSe nanoplatelets (NPLs) routinely yield samples that emit, in discrete steps, from 460 to 550 nm. A significant challenge lies with obtaining thicker NPLs, to further widen the emission range. This is at present typically achieved via colloidal atomic layer deposition onto CdSe cores, or by synthesizing NPL core/shell structures. Here, we demonstrate a novel reaction scheme, where we start from 4.5 monolayer (ML) NPLs and increase the thickness in a two-step reaction that switches from 2D to 3D growth. The key feature is the enhancement of the growth rate of basal facets by the addition of CdCl, resulting in a series of nearly monodisperse CdSe NPLs with thicknesses between 5.5 and 8.5 ML. Optical characterization yielded emission peaks from 554 nm up to 625 nm with a line width (fwhm) of 9-13 nm, making them one of the narrowest colloidal nanocrystal emitters currently available in this spectral range. The NPLs maintained a short emission lifetime of 5-11 ns. Finally, due to the increased red shift of the NPL band edge photoluminescence excitation spectra revealed several high-energy peaks. Calculation of the NPL band structure allowed us to identify these excited-state transitions, and spectral shifts are consistent with a significant mixing of light and split-off hole states. Clearly, chloride ions can add a new degree of freedom to the growth of 2D colloidal nanocrystals, yielding new insights into both the NPL synthesis as well as their optoelectronic properties.

The approaches to stabilize the perovskite structure of formamidinium lead iodide (FAPI) commonly result in a blue-shift of the band gap, which limits the maximum photo-conversion efficiency.Here, we report the use of PbS colloidal quantum dots (QDs) as stabilizing agent, preserving the original low band gap of 1.5 eV,. The surface chemistry of PbS plays a pivotal role, by developing strong bonds with the black phase but weak ones with the yellow phase. As a result, stable perovskite FAPI black phase can be formed at temperatures as low as 85°C in just 10 minutes, setting a record of concomitantly fast and low temperature formation for FAPI, with important consequences for industrialization. FAPI thin films obtained through this procedure reach an open circuit potential (Voc) of 1.105 V -91% of the maximum theoretical Voc-and preserve the efficiency for more than 700 hours. These findings reveal the potential of strategies exploiting the chemi-structural properties of external additives to relax the tolerance factor and optimize the optoelectronic performance of perovskite materials.

Using semianalytical models we calculate the energy, effective Bohr radius, and radiative lifetime of neutral excitons confined in CdSe colloidal nanoplatelets (NPLs). The excitonic properties are largely governed by the electron-hole in-plane correlation, which in NPLs is enhanced by the quasi-two-dimensional motion and the dielectric mismatch with the organic environment. In NPLs with lateral size L 20 nm the exciton behavior is essentially that in a quantum well, with super-radiance leading to exciton lifetimes of 1 ps or less, only limited by the NPL area. However, for L < 20 nm excitons enter an intermediate confinement regime, hence departing from the quantum well behavior. In heterostructured NPLs, a different response is observed for core-shell and core-crown configurations. In the former, the strong vertical confinement limits separation of electrons and holes even for type-II band alignment. The exciton behavior is then similar to that in core-only NPL, albeit with weakened dielectric effects. In the latter, charge separation is also inefficient if band alignment is quasi-type-II (e.g., in CdSe/CdS), because electron-hole interaction drives both carriers into the core. However, it becomes very efficient for type-II alignment, for which we predict exciton lifetimes reaching microseconds.

Strain in colloidal heteronanocrystals with non-centrosymmetric lattices presents a unique opportunity for controlling optoelectronic properties and adds a new degree of freedom to existing wavefunction engineering and doping paradigms. We synthesized wurtzite CdSe nanorods embedded in a thick CdS shell, hereby exploiting the large lattice mismatch between the two domains to generate a compressive strain of the CdSe core and a strong piezoelectric potential along its c-axis. Efficient charge separation results in an indirect ground-state transition with a lifetime of several microseconds, almost one order of magnitude longer than any other CdSe/CdS nanocrystal. Higher excited states recombine radiatively in the nanosecond time range, due to increasingly overlapping excited-state orbitals. k̇p calculations confirm the importance of the anisotropic shape and crystal structure in the buildup of the piezoelectric potential. Strain engineering thus presents an efficient approach to highly tunable single- and multiexciton interactions, driven by a dedicated core/shell nanocrystal design.

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