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Chimie moléculaire Paris Centre

Control of the composition and the surface chemistry of II-VI semiconductor nanoplatelets

Séminaire du Dr Sandrine Ithurria (ESPCI)

Abstract :

2D colloidal II-VI semiconductor nanoplatelets (NPLs) are a unique class of nanoparticles with optical properties comparable to quantum well. Their thicknesses, the unique confined direction, are defined at the atomic scale all along the nanoparticles without roughness which provide them optical properties without inhomogeneous broadening. In a synthesis all the NPLs present exactly the same number of monolayers. An effort has been dedicated to the growth of cadmium chalcogenides NPLs and their heterostructures. Indeed due to their shapes, these NPL can either be grown parallel to the confined direction leading to core/shell or perpendicular to the confined direction leading to core/crown NPLs. In order to extend the optical properties of the NPLs, cation exchange has been proposed as a strategy to synthesis mercury chalcogenides nanoplatelets. Compare to spherical quantum dots emitting in the same range of wavelength, the HgTe NPLs present a similar PL efficiency (10%) but with a far narrower emission (60meV for an emission at 880nm) and shortest decay rate. In addition of the control of their compositions and quantum confinement, a control of the surface chemistry is another way to control the optical properties. Indeed NPLs can be stabilized by halides that act as X-L-type ligands, costabilized by amines which provide colloidal stability in nonpolar solvents. The hydrogen from the amine can participate in a hydrogen bond with the lone pair electrons of surface halides. In the case of CdSe, the improved surface passivation leads to an increase in the fluorescence quantum efficiency of up to 70% in the case of bromide. This ligand exchange is associated with a decrease in stress that leads to unfolding of the NPLs, which is particularly noticeable for iodide-capped NPLs.


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