The formation of core/shell structures has become an established approach to passivate the surface and enhance the photoluminescence quantum yield of semiconductor nanocrystals, quantum dots. However, lattice mismatch between the core and the shell materials results in surface reconstructions at the core/shell interface and compressive or tensile strain in the core and the shell. Concomitantly formed surface traps can have a negative impact on the emission properties. Growing buffer layers with intermediate lattice constants or using alloys to tune the lattice constant is often considered to reduce the reconstruction-induced strain. We present a study that quantitatively relates strain and shell composition in the case of InP/(Zn,Cd)­Se core/shell quantum dots. We apply Raman spectroscopy to quantize strain and find that adjusting the composition of the (Zn,Cd)Se shell tunes the strain from compressive to tensile. The transition between both regimes is found at shell compositions where the bulk lattice constants of InP and (Zn,Cd)Se match, which confirms that matching lattice constants is a viable strategy to achieve strain-free core/shell nanocrystals.

How can ion-exchange process occur in nanocrystals without the size and shape changing and why is the ion transport much faster than in classical interdiffusion processes in macrocrystalline solids? We have investigated these processes at the molecular level by means of high-resolution and analytical electron microscopy in temperature-dependent kinetic experiments for several model reactions. The results clearly show a diffusion process that proceeds exclusively through the interstitial lattice positions with a subsequent "kick out" to remove individual ions from lattice sites without the formation of vacancies. This mechanism has not been observed in nanocrystalline systems before.

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Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminescence intensity of the triplet trion is more than four times larger with circular excitation than with linear excitation. We interpret our results as a consequence of a large dynamic polarization of resident electrons using circular light.