Colloidal atomic layer deposition: a versatile method to synthesize core-shell nanocrystals in the infrared

For the application of quantum dots (QDs) in the infrared, PbX (S,Se,Te) chalcogenide QDs proved to be very effective for photodetection, multiple exciton generation and solar energy conversion. However, long radiative lifetimes and 8 fold degeneracy of HOMO-LUMO states associated with PbX chalcogenides makes them less suitable for light emission applications, thus paved way to look for other alternative materials. In this context, HgX (X=Se,Te) chalcogenides are quite promising. HgX is a semi-metal in the bulk yet when the crystal dimensions are reduced to nanometer size, a bandgap gets opened up, which is tunable throughout the infrared range. Shorter radiative lifetimes and a lower degeneracy of their band-edge states in comparison with PbX chalcogenides makes them suitable for light emission applications. A decisive step in this respect is the formation of core-shell QDs. As such, QDs are regarded as hybrid materials consisting of an inorganic core, which predominantly determines the opto-electronic properties and the organic ligands that keep QD colloids from aggregating. However, these organic ligands cannot completely passivate the quantum dot surface, with the remaining mid-gap states leading to poor photoluminescence quantum yields. Such issues could be addressed by growing a shell material on top of the inorganic core resulting in core/shell nanocrystals with a better surface passivation. So far, QD shelling is predominantly based on high temperature layer-by-layer or seeded-growth reactions. In the case of PbX and HgX, such conditions, however, prove incompatible with the need to preserve the original core QD. Fast Cd for Pb cation exchange prevents the growth of a CdX shell around PbX QDs whereas HgX QDs degrade by excessive Ostwald ripening. HAADF image of PbS-CdS core-shell QDS Here, we demonstrate that a room temperature colloidal atomic layer deposition (c-ALD) overcomes these issue and enables PbS/CdS, HgSe/CdX (X=S, Se) and CdS/PbS core/shell QDs to be made by additive shelling. TEM analysis on QDs indicate an increase in overall diameter by 0.45 nm/cycle, implying the shell growth in a monolayer fashion. HAADF further corroborates the formation of core-shell QDs. By combining this layer-by-layer growth with optical spectroscopy, we can delineate the core/shell boundary even in the case of overall non-stoihiometric nanocrystals and we present maps of charge carrier (de)localization as a function of core and shell dimensions. In the widely studied case of PbS/CdS, for example, we find that delocalization starts once PbS cores are smaller than 4 nm, a result corroborated by effective-mass calculations. In the case of HgSe/CdSe core-shell QDs, XRD result indicates a strain free core-shell system in line with the corresponding lattice parameter. Additionally, we show that both CdSe and CdS shells undo the n-type doping associate with larger HgSe QDs. Introduction of PbS shell growth starting from CdS core QDs result in CdS/PbS core-shell QDs with an inverted type-I structure that has tunable emission in the infrared. In this way, these novel core-shell heterostructures paves way to understand their fundamental properties and their applications for infrared opto-electronics