Facet Selectivity Enables Near-Unity External Quantum Yields in Shell-free Silicon Quantum Dots

Abstract Core–shell quantum dots (QDs) such as epitaxially passivated CdSe, InP, and lead-halide perovskite QDs, which are often used in solar cells and as photocatalysts, are highly luminescence-efficient and tunable. However, concerns surrounding sustainability, toxicity, and material availability have spurred interest in heavy-metal-free QDs derived from Earth-abundant elements. Ensuring that shell-free colloidal QDs emit efficiently and stably─a long-standing goal that is expected to simplify syntheses and broaden technological applicability─remains particularly challenging for silicon, an indirect-bandgap semiconductor traditionally regarded as emitting light inefficiently. Herein, we synthesized shell-free silicon QDs (SiQDs) with controlled crystal sizes (1.8–2.5 nm) via a hydrogen-silsesquioxane-polymer route and created a quantitative structure–property framework. The photoluminescence quantum yields (PLQYs) of fresh SiQDs were found to be determined by quantum confinement, exciton tunneling, crystallinity, and tensile strain, with volcano-type size dependences observed. Beyond this intrinsic behavior, long-term aging was found to distinctly enhance efficiency, with PLQYs of up to 99% sustained over extended periods of time (200–450 d) in solution under ambient conditions. This enhancement is attributable to the slow, facet-selective oxidation of Si–Si back bonds on Si(111) facets. These findings identify facet-selective back-bond chemistry in the subsurface region as a critical design feature for highly efficient, shell-free SiQDs and provide a general framework for sustainable, heavy-metal-free photonic nanomaterials.