Probing Ge-Induced Strain and Emission in Rutile Sn_1-xGe_xO₂ Nanocrystals

Rutile-phase GeO₂ and its alloys, such as Sn_xGe_1-xO₂, are promising semiconductors due to their ultrawide-bandgaps (4.4-4.7 eV), high carrier mobilities, ambipolar dopability, and excellent thermal conductivity. However, GeO₂ preferentially crystallizes in the α-quartz phase under ambient conditions, limiting the exploration of its rutile form. Alloying with rutile SnO₂ offers a route to stabilize this metastable phase. Here, we report the nanoscale characterization of rutile-Sn_xGe_1-xO₂ nanocrystals grown on Zn₂GeO₄ nanowires, achieving Ge incorporation well beyond its solubility limit while preserving the rutile structure. Exploiting advanced X-ray nanoprobe techniques, we achieve a correlative view of the composition, structure, and optical properties within individual Sn_xGe_1-xO₂ nanostructures. Our results reveal preferential Ge accumulation along the faceted sides, generating local strain gradients without disrupting the rutile phase. Spatially resolved luminescence measurements show enhanced defect-related orange emission (∼2.1 eV) at the faceted nanocrystal edges. First-principles calculations confirm that while Ge incorporation modulates the bandgap of the alloy, the primary factors governing luminescence are the presence of oxygen vacancies and the increased surface-to-volume ratio at the crystallite edges. These findings establish a robust multimodal nanoscale approach for characterizing and engineering next-generation oxide nanomaterials with tunable optoelectronic properties.