Metal Ion-Engineered Carbon Quantum Dots From Hazelnut Shell via Solid-State Synthesis for Efficient OLED Devices.

In this study, we report a scalable and green solid-state synthesis of nitrogen-doped carbon quantum dots (CQDs) from hazelnut shell biomass, using citric acid and urea as carbon and nitrogen sources. Controlled BaCl2 and ZnCl2 doping was applied to tailor nucleation, crystallinity, and surface chemistry. Structural analyses (FTIR, XRD, STEM, XPS, and DLS) revealed that BaCl2-assisted CQDs exhibited higher graphitization, narrower size distribution (7-13 nm), and fewer defects, while ZnCl2-assisted CQDs showed more amorphous and heteroatom-rich surfaces. Optical measurements indicated strong π-π* absorption (≈280 nm), bright blue emission (λem 405-412 nm), and quantum yields of 63.4% (BaCl2) and 50.1% (ZnCl2), with > 95% stability after 30 days. When used as OLED emissive layers, BaCl2-CQDs achieved a luminous efficiency of 0.75 cd A-1, nearly four times that of ZnCl2-CQDs (0.20 cd A-1), despite lower maximum luminance (48.9 vs. 308.1 cd m-2). These results highlight metal ion-assisted nucleation as an effective strategy to engineer CQD properties and enhance device performance, paving the way for sustainable, scalable OLED technologies.