Improved Surface Passivation of Colloidal Ge1-xSnx Nanoalloys through Amorphous SiO2 Shell Growth

Ge1-xSnx alloy nanocrystals (NCs) are a class of semi-conductors that show interesting (photo) physical properties such as composition-dependent visible to near-IR energy gaps and enhanced lightmatter interactions compared to single element Ge NCs. With decreasing size and increasing Sn content, the molar absorptivity and emission efficiency increase, making these NCs attractive for optoelectronic, molecular imaging, and sensing studies. To further improve the optical stability, there is a need to passivate the Ge1-xSnx surface with a robust shell material to protect the core from oxidation and inhibit chemical exchange when exposed to extreme environments. Herein, we report a fluent synthetic method for the growth of a thin silica layer on bulk-like (14.9 +/- 1.7 nm) and quantum-confined (4.4 +/- 0.6 nm) Ge1-xSnx NCs. Physical characterization of Ge1-xSnx/SiO2 core/shell NCs suggests that the diamond cubic structure of the core is retained upon shell growth, whereas solid-state and solution-state absorption spectra confirm the composition-dependent energy gap tunability. The core-shell NCs (1.23-2.07 eV for x = 0.03-0.09) show an average energy gap increase of 0.38 eV relative to the core NCs (0.91-1.67 eV for x = 0.03-0.09) owing to minor surface etching induced by shell growth. Surface analysis of core-shell NCs suggests a notable decrease in Ge2+ species and greater dominance of Ge-0 species relative to the organically passivated Ge1-xSnx core NCs, confirming the production of robust, oxidation-resistant, and optically stable Ge1-xSnx/SiO2 alloy NCs.

Automated summary

Ge1-xSnx alloy nanocrystals exhibit composition-dependent energy gap tunability and enhanced light-matter interactions. By growing a silica layer on the surface of Ge1-xSnx, optical stability is improved and oxidation is inhibited. The Ge1-xSnx/SiO2 core/shell nanocrystals demonstrate robustness, oxidation resistance, and optical stability.