Internal Atomic-Scale Structure Determination and Band Alignment of II–VI Quantum Dot Heterostructures
Cecilia Gentle, Yuanheng Wang, Tyler N. Haddock, Conner Dykstra, Renske M. van der Veen
Abstract
This work shows that\nZnTe/CdSe core/shell quantum dots synthesized by a standard literature\nprocedure in actuality have an alloyed Cd<i><sub>x</sub></i>Zn<sub>1–<i>x</i></sub>Te core. We employ X-ray\nabsorption spectroscopy (XAS) at all four K-shell ionization edges\n(Zn, Te, Cd, and Se) and perform global fitting analysis to extract\nthe first-shell bond distances. We combine our XAS results with transmission\nelectron microscopy (TEM) sizing and elemental analyses, which allows\nus to propose models of the internal particle structure. Our multimodal\ncharacterization approach confirms (1) the presence of Cd–Te\nbonds, (2) cation alloying in the particle core (and the absence of\nanion alloying), and (3) a patchy pure-phase CdSe shell. We synthesize\nparticles of different shell thicknesses and performed synthetic control\nstudies that allowed us to discard a ZnTe/CdTe/CdSe core/shell/shell\nstructure and confirm the alloyed core/shell structure. Our structural\nanalysis is extended with electronic band structure calculations and\nUV/vis absorption spectroscopy, demonstrating that the alloyed Cd<i><sub>x</sub></i>Zn<sub>1–<i>x</i></sub>Te/CdSe\ncore/shell quantum dots exhibit a direct band gap, different from\nthe predicted type II band alignment of the intended ZnTe/CdSe core/shell\nquantum dots. This study highlights the challenges with synthesizing\nII–VI quantum dot heterostructures and the power of XAS for\nunderstanding the internal structure of heterogeneous nanoparticles.