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Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

Abstract : 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]
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Submitted on : Thursday, December 5, 2019 - 10:15:42 AM
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Elena Shornikova, Aleksandr Golovatenko, Dmitri Yakovlev, Anna Rodina, Louis Biadala, et al.. Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets. 2019. ⟨hal-02394926⟩

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