Ken-Ichi Saitow

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.

Mechanochemistry in planetary ball mills is a transformative and sustainable chemical process by which mechanical impact is converted into reaction‐driving energy. High‐energy collisions between balls, analogous to meteorite impacts on Earth, generate transient extreme pressures (∼10 GPa) and temperatures (∼1500°C) and supercritical water in microscale “hot spots,” allowing reactions once restricted to high‐temperature or solvent‐intensive laboratory or industrial conditions to proceed. This platform achieves hydrogen evolution efficiencies comparable or superior to electrolysis and even realizes a new phenomenon—room‐temperature thermochemical water‐splitting cycles—without CO ₂ emissions, oxygen separation systems, or external heaters. Furthermore, the mechanochemical activation of TiO ₂ yields photocatalysts with markedly enhanced absorption from the UV to the near‐infrared through defect and polymorph engineering. Beyond energy applications, the direct halogen‐free, HF‐free synthesis of alkoxysilanes provides a green, scalable route to value‐added chemicals with the coproduction of hydrogen at room temperature. These processes exploit abundant or waste materials, operate in compact setups, and consume very little energy, suggesting their potential for distributed fuel generation and sustainable materials manufacturing. Planetary ball milling can therefore offer a generalizable framework for green chemistry, bridging solid‐state reaction engineering with energy conversion and functional materials synthesis to provide practical routes toward low‐carbon, scalable technologies.

Silicon quantum dots (SiQDs) are an emerging class of high‐performing, sustainable, environmentally safe luminescent nanomaterial. They offer opportunities for next‐generation displays, solid‐state lighting, medical applications, and quantum technologies. Here, we highlight recent breakthroughs in colloidal SiQD synthesis and photophysics, comparing eight synthetic strategies. Among these, we focus on the hydrogen silsesquioxane (HSQ) polymer route, a simple and cost‐effective hot‐injection‐free method that yields highly crystalline, ultrabright, and stable SiQDs with photoluminescence quantum yields approaching 80%. We also describe how solvent engineering realizes SiQD light‐emitting diodes (LEDs) with record external quantum efficiencies (EQEs, >16%), >700‐fold‐increased lifetimes, and far‐red emissions to rival state‐of‐the‐art perovskite QD LEDs. Moreover, rice husk‐derived SiQD LEDs illustrate the potential for low‐waste circular material cycles. Thus, SiQDs are a sustainable platform for plant growth technologies, photodynamic therapy, and beyond.

Most quantum dots (QDs) contain either toxic elements, which are health and environmental hazards, or costly precious metals. In contrast, as nanocrystals consisting mainly of an earth‐abundant, light element, silicon QDs (SiQDs) have attracted attention as cost‐effective biomedical, display, and solid‐state lighting materials. However, unlike heavy‐metal or perovskite QDs, SiQDs have not yet been used to create high‐performance optoelectronics or long‐lifetime light‐emitting diodes (LEDs). Herein, the fabrication via solvent engineering of SiQD LEDs with record‐breaking external quantum efficiency (16.5%) and lifetimes up to 733 times longer than the previous record is reported. Furthermore, the far‐red (750 nm) luminance is comparable to that of state‐of‐the‐art perovskite QD LEDs. Dispersing the SiQDs in octane yields particularly efficient LEDs owing to negligible SiQD aggregation, and Joule heating minimization realizes long‐term stability (lifetime >200 h). Thus, solvent engineering is harnessed to break four QD LED performance records—for efficiency, luminance, voltage, and operational lifetime—using a more sustainable QD material, and the mechanisms underlying these performance improvements are unveiled. Thus, a new solvent‐engineering approach for developing efficient, stable, and sustainable far‐red SiQD LEDs, which are valuable light sources for applications including plant growth acceleration and photodynamic therapy, is highlighted.

We report the accidental discovery and subsequent investigation of room-temperature thermochemical water splitting in a ball mill. High-purity H₂ (>99%) was produced using water and/or seawater feedstocks in a compact (∼50 cm) low-power reactor.

Multicolor, bright silicon quantum dots (SiQDs)—SiQDs with photoluminescence in a range of colors and quantum yields (PLQYs) of >90%—are promising heavy-metal-free light sources for full-color displays, lighting, and biomedical imaging. Colloidal SiQDs can be used to manufacture devices via printing and roll-to-roll processing. Furthermore, the in vivo use of biodegradable SiQDs and Si nanomaterials, for imaging cancer cells and as drug delivery systems, has been demonstrated. However, a large body of research demonstrates that the photoluminescence (PL) wavelength and PLQY of colloidal SiQDs are dependent not only on the SiQD particle size but also on the methods and/or procedures and chemical reagents used to synthesize them. This is because SiQDs are quite sensitive to both the intrinsic properties of Si and external factors. These intrinsic and external factors can be respectively linked to different PL mechanisms: the quantum confinement effect, which produces a slow-decaying “S”-band PL signal, and surface ligand effects, corresponding to fast-decaying “F”-band PL. This review focuses on mechanistic insights into the relationships linking the structures, ligands, and optical properties of SiQDs. Synthesis methods and the application performance of bright multicolor colloidal SiQDs, based on excellent state-of-the-art experimental and theoretical studies, are also reviewed.