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

• Published in: Journal of physical chemistry. C Vol. 116; no. 6; pp. 3968 - 3978
• Main Authors: Abel, Keith A, FitzGerald, Paul A, Wang, Ting-Yu, Regier, Tom Z, Raudsepp, Mati, Ringer, Simon P, Warr, Gregory G, van Veggel, Frank C. J. M
• Format: Journal Article
• Language: English
• Published: Columbus, OH American Chemical Society 16-02-2012
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Materials science; Nanocrystalline materials; Nanoscale materials and structures: fabrication and characterization; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures; Physics; Structure of solids and liquids; crystallography; X-ray diffraction and scattering; X-ray scattering (including small-angle scattering); Particle size; Core shell structure; Photoluminescence; Small angle X ray scattering; Quantum yield; Structural models; Nanoparticles; Transmission electron microscopy; Small angle neutron scattering; Experimental design; Imaging; Growth mechanism; Ion exchange; Nanostructured materials; X-ray photoelectron spectra
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp2112928

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.