Synthesis of II-VI colloidal nanoplatelets and encapsulation by spray-drying method

Les nanocristaux de semi-conducteur colloïdaux bidimensionnelles ont émergé comme une nouvelle classe de nanomatériaux du fait de leurs propriétés optiques, électroniques et mécaniques uniques. Parmi eux, les nanoplaquettes présentent un confinement unidimensionnel selon leur épaisseur, contrôlée au niveau atomique, et de ce fait un pic d’émission extrêmement fin. Ces objets apparaissent ainsi comme des candidats sérieux pour la fabrication de dispositifs optoélectroniques. Cependant, contrairement aux nanocristaux sphériques, où l’énergie de la bande interdite peut être finement ajustée en jouant sur leurs tailles, les nanoplaquettes ne présentent pas d’émission continuellement modulable. De surcroît, la croissance de coque sur ces objets, nécessaire à l’optimisation et à la stabilisation des propriétés optiques (augmentation du rendement quantique), entraine un fort décalage dans le rouge de la longueur d’onde d’émission. Ainsi, la gamme de longueur d’onde des structures résultantes allant de 615 à 700 nm ne satisfait pas aux exigences des dispositifs d’affichage pour lesquels la couleur verte est également requise. Dans le but d’observer un décalage continu dans le bleu des longueurs d’onde d’absorption et d’émission des structures finales, une stratégie de synthèse consistant à incorporer du soufre aux cœurs de séléniure de cadmium a été développée. En outre, une nouvelle méthode d'encapsulation par procédé aérosol-gel favorisant l'implémentation des nanoplaquettes au sein de dispositifs optoélectroniques est présentée. Finalement, l'évolution des propriétés optiques des nanoplaquettes nues ou encapsulées soumises à un flux de photons à haute puissance a été étudiée.Colloidal two-dimensional semiconductor nanocrystals have emerged as a new class of nanomaterials due to their unique optical, electronic and mechanical properties. Nanoplatelets have a one-dimensional confinement along their thickness, controlled at the atomic level, and thus an extremely narrow emission line width. These objects have been considered as potential candidates for the fabrication of optoelectronic devices. However, nanoplatelets have not demonstrated continuously tunable emission when compared to quantum dots, which exhibit a band gap that can be finely tuned by adjusting nanoparticle size. Moreover, shell growth on these objects, which has been show to improve their quantum yield, causes a strong red shift in the emission wavelength. Thus, the wavelength range of these structures ranging from 615 to 700 nm does not fulfill the requirements of display devices where green is also required. In order to observe a continuous blue shift of the absorption and emission wavelengths of the final structures, a new strategy consisting of incorporating sulfur into the cadmium selenide cores has been developed. In addition, a new method of encapsulation by aerosol-gel method promoting the implementation of nanoplatelets within optoelectronic devices is presented. Finally, the evolution of the optical properties of unmodified and encapsulated nanoplatelets exposed to high power photon flux was studied and compared

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate para-magnetism owing to the uncompensated spins of dangling bonds (DBSs). We reveal that by optical spectroscopy in high magnetic fields up to 15 Tesla using the exciton spin as probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the DBS and exciton spin polarization as well as by the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction can be adjusted by the nanoplatelet growth conditions. The surface of colloidal nanocrystals (NCs) greatly controls their optical and electronic properties making the surface chemistry critically important in nanocrystal research and applications. 1-3 Undercoordinated surface atoms with excess electrons, which in colloidal synthesis are often metal cations, either rearrange themselves by surface reconstruction or adsorb surfactant ligands. 4,5 The ligands are used to control the colloidal synthesis, increase the NC solubility, screen the NCs from environment, and stabilize their surface by saturating dangling bonds of the surface atoms. 6,7 They influence surface trap states and thereby control photoluminescence quantum yield. 8-11 Not every dangling bond can be passivated due to steric hindrance, as the ligand diameter typically exceeds the lattice constant of NC material and due to poor interaction of a facet with the ligands. An electron transfer from the d-shell of a surface atom to the ligand can provide surface magnetism. 12-14 Also the spins of unpassivated dangling bonds can contribute to it. The spins of surface atoms act similar to spins of magnetic impurities in diluted magnetic semiconductors, 15-17 and may influence crucially the optical, electronic and magnetic properties of colloidal NCs. 18,19 Nominally diamagnetic NCs may demonstrate paramagnetic behavior and giant magneto-optical effects. Here, we study the surface spins in colloidal quasi-two-dimensional nanoplatelets (NPLs) based on CdSe semiconductor. These emerging nanostructures have an atomically controlled thickness of a few monolayers, providing remarkable optical properties with narrow emission lines of neutral and charged excitons. 20,21 The interaction of confined excitons with the dangling-bond spins (DBSs) provides a nanoscopic tool for monitoring surface magnetism. In particular, the exciton spin polarization in magnetic field and the radiative recombination of dark excitons are strongly modified due to the exchange interaction with DBSs. We use high magnetic fields up to 15 T to measure the degree of circular polarization (DCP) of exciton photoluminescence and the exciton spin and recombination dynamics at cryogenic temperatures. In contract to a pure diamagnetic behavior, we find a strongly nonmonotonic magnetic field dependence of DCP, which sign changes for NPLs synthesized in air or argon atmosphere. This allows us to identify two mechanisms resulting from the exciton interaction with the surface spins. The first one is an additional Zeeman splitting of the exciton states, which is similar to the giant Zeeman splitting effect in diluted magnetic arXiv:1909.13700v1 [cond-mat.mes-hall