Ge–GeSn Core–Shell Nanowires Under The Lens: Effects Of Sn Alloying On Morphology And Bandgap Transitions Via High–Resolution Transmission Electron Microscopy

Abstract

This study investigates the impact of Sn alloying on the structural, electronic, and optical properties of core-shell Ge-GeSn nanowires, with Ge as the core and GeSn as the shell. Using advanced transmission electron microscopy (TEM) techniques, including High-Resolution Scanning TEM (STEM) with High-Angle Annular Dark Field (HAADF) imaging, Energy-Dispersive X-Ray Spectroscopy (EDS), and Electron Energy-Loss Spectroscopy (EELS), we achieve high spatial, energy, and momentum resolution. These methods reveal complex morphological changes and bandgap transitions within the Ge_(1-x)Sn_x nanowires. The study demonstrates that defect-free Ge-Ge_(1-x)Sn_(x) core-shell nanowires can be successfully synthesized by overcoming challenging growth conditions, achieving stable structures even with varying Sn content. Detailed sub-angstrom investigations reveal that these nanowires maintain stability and defect-free characteristics despite the presence of strain, which is alleviated by their core-shell morphology. Our findings show that increasing Sn content from 8 to 18 at.\% leads to a notable transition from an indirect to a direct bandgap, with the bandgap energy decreasing to approximately 0.2 eV at high Sn concentrations. This research highlights the significant role of Sn alloying in altering the characteristics of Ge-Ge_(1-x)Sn_(x) core-shell nanowires and confirms the transition to a direct bandgap with increased Sn content.