Nonthermal Plasma Synthesis of Core/Shell Quantum Dots: Strained Ge/Si Nanocrystals

In this work, we present an all-gas-phase approach for the synthesis of quantum-confined core/shell nanocrystals (NCs) as a promising alternative to traditional solution-based methods. Spherical quantum dots (QDs) are grown using a single-stage flow-through nonthermal plasma, yielding monodisperse NCs, with a concentric core/shell structure confirmed by electron microscopy. The in-flight negative charging of the NCs by plasma electrons keeps the NC cores separated during shell growth. The success of this gas-phase approach is demonstrated here through the study of Ge/Si core/shell QDs. We find that the epitaxial growth of a Si shell on the Ge QD core compressively strains the Ge lattice and affords the ability to manipulate the Ge band structure by modulation of the core and shell dimensions. This all-gas-phase approach to core/shell QD synthesis offers an effective method to produce high-quality heterostructured NCs with control over the core and shell dimensions

Obtaining Structural Parameters from STEM–EDX Maps of Core/Shell Nanocrystals for Optoelectronics

Characterization efforts of core/shell and core/multishell nanocrystals have struggled to quantitatively evaluate the interface width between the core and shell materials despite its importance in their optoelectronic properties. Here, we demonstrate a scanning transmission electron microscopy (STEM) method for measuring the radial elemental composition of two spherical core/shell nanocrystal systems, Ge/Si core/shell and CdSe/CdS/ZnS core/double-shell nanocrystals. By fitting model-based radial distributions of elements to measured STEM–energy-dispersive X-ray (EDX) maps, this method yields reliable and accurate measurements of interface broadening as well as core and shell sizes, surface roughness, and the fraction of core material in the shell. The direct evaluation of the structural parameters is an important step toward improving the synthesis of core/shell nanocrystals and optimizing their optoelectronic properties