Probing the Structure of Colloidal Core/Shell Quantum Dots Formed by Cation Exchange

Abstract

Cation-exchange reactions have greatly expanded the types of nanoparticle compositions and structures that can be prepared. For instance, cation-exchange reactions can be utilized for preparation of core/shell quantum dots with improved (photo)stability and photoluminescence quantum yield. Understanding the structure of these nanomaterials is imperative for explaining their observed properties and for their further development. Core/shell quantum dots formed by cation exchange are particularly challenging to characterize because shell growth does not lead to an increase in overall particle size that can easily be characterized by standard transmission electron microscopy (TEM). Here, we report on the direct observation of the PbSe/CdSe core/shell structure (formed by cation exchange) using high-angle annular dark field (HAADF) imaging and energy-filtered TEM (EF-TEM). These results are further confirmed by energy-dependent X-ray photoemission spectroscopy (XPS) data that show increasing Pb/Cd signal with increasing X-ray photon energies. High-resolution XPS at varying X-ray photon energies was also used to examine chemical speciation and reveal greater complexity in both the PbSe core-only and the PbSe/CdSe core/shell structures than previously reported. Finally, small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) methods are combined to provide further inorganic and organic structural information. All experiments agree within error, and the results are summarized as final structural models for the core and core/shell particles.

Phase transformation of PbSe/CdSe nanocrystals from core-shell to Janus structure studied by photoemission spectroscopy

“…Although the existence of additional components in the core-level spectra of Pb and Cd could be caused by the formation of an alloy structure, 49 we rule out this hypothesis. The lattice-type mismatch between rocksalt IV-VI semiconductors and zinc-blende II-VI semiconductors makes both materials very immiscible.…”

“…Such a hypothesis agrees with a recent study of the Pb 5d core-level spectrum measured for PbSe/CdSe HNCs with a complete shell that shows a shoulder at low binding energy, whereas, the PbO component was absent from the spectrum. 49 This result would suggest that Pb is not expulsed from the outer layer of the shell as fast as it was initially thought.…”

“…Therefore, fitting the Pb 5d with three components is consistent with the achieved decomposition and with the result described in a recent study. 49 Finally, we also notice a small peak at a binding energy around 25 eV [inset of Fig. 3(a)].…”

“…These results are in line with our current understanding of the surface chemistry of PbSe nanocrystals capped with oleate. [47][48][49] Indeed, we know that the nanocrystals are nonstoichiometric with a surface that is predominantly composed of lead. Thus, we expect the Se 3d core level to mainly consist of a bulk component which is supported by the decomposition of the related spectrum.…”

Excited-State Dynamics in Colloidal Semiconductor Nanocrystals

“…The very dynamic nature of the NC surface and the strong interplay between capping ligands and the inorganic core [16] are also likely important factors determining charge carrier trapping. Unfortunately, the organic surface ligands are invisible in electron microscopy and can currently be investigated only on the ensemble level using infrared absorption [249,265,266], nuclear magnetic resonance spectroscopy [249,267,268], or neutron scattering [269,270].…”