Solid-state synthesis of Si _{1− x} Ge _x nanoalloys with composition-tunable energy gaps and visible to near infrared optical properties

Si_1-xGe_x alloy nanocrystals (NCs) are a class of benign semiconductors that show size and composition-tunable energy gaps and promising optical properties because of the lattice disorder. The random distribution of elements within the alloys can lead to efficient light-matter interactions, making them attractive for Si-compatible optoelectronic devices, transistors, charge storage, and memory applications. However, the fabrication of discrete, quantum-confined alloys has proved a challenging task. Herein, we report solid-state co-disproportionation of a hydrogen silsesquioxane (HSQ)/GeI₂ composite precursor to produce homogeneous Si_1-xGe_x NCs with control over the diameter (5.9 ± 0.7-7.8 ± 1.1 nm) and composition (x = 0-14.4%) with strong size confinement effects and visible to near IR absorption and emission properties. As-synthesized alloys show an expanded diamond cubic Si structure, a systematic red-shift of Si-Si Raman peak, and emergence of Si-Ge/Ge-Ge peaks with increasing Ge, consistent with the admixture of isovalent elements. Surface analysis of alloys reveals Si⁰/Ge⁰ core and Siⁿ⁺/Geⁿ⁺ surface species and efficient surface functionalization with alkyl ligands via thermal hydrosilylation and/or hydrogermylation. Alloy NCs exhibit absorption onsets (2.26-1.92 eV), indirect (1.53-1.80 eV) and direct (2.88-2.47 eV) energy gaps, and photoluminescence (PL) maxima (1.40-1.27 eV) that can be tuned by manipulating the diameter and/or composition. The experimental PL energies are consistent with those predicted by density functional theory (DFT), suggesting that the PL originates from NC core electronic transitions. The facile low-temperature solid-state synthesis and control over physical properties realized in this study will allow discrete Si_1-xGe_x NCs to emerge as low to nontoxic, earth-abundant, and Si-compatible nanostructures for a broad range of electronic and photonic technologies.