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.