The Kirkendall Effect: Main Growth Mechanism for a New SnTe/PbTe/SnO2 Nano-heterostructure

Attention to semiconductor nanostructures with a narrow bandgap energy and low production cost has increased in recent years, due to practical demands for use in various opto-electronics and communication devices. Colloidal nanostructures from the IV-VI semiconductors, such as lead and tin chalcogenides, seem to be the most suitable materials platform; however, their poor chemical and spectral stability has impeded practical applications. The present work explored the mechanism for formation of new nanostructures, SnTe/PbTe/SnO₂, with a core/shell/shell heterostructure architecture. The preparation involved a single-step post treatment for the pre-prepared SnTe cores, which simultaneously generated two different consecutive shells. The process followed a remarkable Kirkendall effect, where Sn ions diffused to the exterior surface from a region below the surface and left a ring-like vacancy area. Then, Pb ions diffused inward and created a PbTe shell, filling the Sn-deficient region. Finally, the ejected Sn-ions at the exterior surface underwent oxidation and formed a disordered SnO₂ layer. The reaction stages were followed by applying high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis with energy-dispersive X-ray (EDX) measurement. These intriguing processes were corroborated by a theoretical estimation of the relative diffusion length of the individual elements at the reaction temperature. The nanostructures produced were endowed with a relatively low toxicity, optical tunability, and chemical stability, which lasted more than a month at ambient conditions.