Solid-state synthesis of Si1−xGex nanoalloys with composition-tunable energy gaps and visible to near infrared optical properties

Abstract

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.