Internal Atomic-Scale Structure Determination and Band Alignment of II-VI Quantum Dot Heterostructures

This work shows that ZnTe/CdSe core/shell quantum dots synthesized by standard literature procedures in actuality have an alloyed Cd^­_xZn_1-xTe core. We employ X-ray absorption spectroscopy (XAS) at all four K-shell ionization edges (Zn, Te, Cd, Se) and perform a global fitting analysis in order to extract the first-shell bond distances. We combine our XAS results with transmission electron microscopy (TEM) sizing and elemental analyses, which allows us to propose models of the internal particle structure. Our multimodal characterization approach confirms (1) the presence of Cd-Te bonds, (2) cation alloying in the particle core (and the absence of anion alloying), and (3) a patchy pure-phase CdSe shell. We synthesize particles of different shell thicknesses and performed synthetic control studies that allowed us to discard a ZnTe/CdTe/CdSe core/shell/shell structure and confirm the alloyed core/shell structure. Our structural analysis is extended with electronic band structure calculations and UV/vis absorption spectroscopy, demonstrating that the alloyed Cd^­_xZn_1-xTe/CdSe core/shell quantum dots exhibit a direct band gap, different from the predicted type-II band alignment of the intended ZnTe/CdSe core/shell quantum dots. This study highlights the challenges with synthesizing II-VI quantum dot heterostructures and the power of XAS for understanding the internal structure of heterogenous nanoparticles.