Future Outlook Toward Heavy-Metal-Free Metal Chalcogenide Nanocrystals for Optoelectronic Applications.

Colloidal nanocrystals (NCs) represent a transformative class of materials for next-generation optoelectronic devices, including photodetectors (PDs), photovoltaics (PVs), catalysis, and light-emitting diodes (LEDs). Historically, research in this field has been dominated by toxic heavy-metal-based systems (Cd, Pb, Hg), but it is now undergoing a critical shift toward sustainability, driven by health concerns and RoHS regulations. This review provides a comprehensive overview of heavy-metal-free (HMF) chalcogenide NCs, including binary, ternary, and multinary systems. We analyze the fundamental principles governing their performance, specifically size-dependent quantum confinement and compositional tuning that allow these "green" alternatives to achieve photoluminescence quantum yields exceeding 70%-90% and power conversion efficiencies of 8%-12%. This review further explores the unique structure-property relationships of HMF chalcogenide NCs, highlighting their superior biocompatibility, low toxicity profiles, and defect-tolerant electronic structures. Diverse applications are evaluated, ranging from broadband infrared photodetection to high-purity color emission for QLEDs. Special emphasis is placed on the challenges of surface engineering and ligand exchange, focusing on the transition from time-consuming layer-by-layer processing to efficient one-step solution-phase strategies. Finally, future research directions are outlined, including mitigation of ions in HMF chalcogenide NCs and development of self-powered sensing platforms for the Internet of Things (IoT) and wearable electronics.