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

Attention to semiconductor nanostructures with a narrow band gap energy and low production cost has increased in recent years, due to practical demands for use in various optoelectronics 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 preprepared 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 ringlike 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. These intriguing processes were corroborated by a theoretical estimation of the relative diffusion length of the individual elements at the reaction temperature. The nanostructures which were comprised of low-toxicity elements were endowed with optical tunability and chemical stability, which lasted more than one month at ambient conditions.