Bottom‐Up Synthesis of Carbon Quantum Dots With High Performance Photo‐ and Electroluminescence

• Published in: Particle & particle systems characterization Vol. 35; no. 7
• Main Authors: Kim, Hong Hee, Lee, Yeon Ju, Park, Chanho, Yu, Seunggun, Won, Sung Ok, Seo, Won‐Seon, Park, Cheolmin, Choi, Won Kook
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.07.2018
• Subjects: bottom‐up two step synthesis; Brightness; Carbon; carbon quantum dots; Carbonization; carbonization of citric acid; Citric acid; Cost effectiveness; Crystal defects; Crystal structure; Crystallinity; Diodes; Electric potential; Electroluminescence; Emission analysis; inverted light emitting diode; Light emission; Optical properties; Organic light emitting diodes; Photoluminescence; Quantum dots; Quantum efficiency; surface ligand exchange
• ISSN: 0934-0866; 1521-4117
• DOI: 10.1002/ppsc.201800080

Although carbon quantum dots (CQDs) are of great interest because of cost effectiveness and environmental compatibility with the facile tunability of their optical properties, poor photo‐ and electroluminescence (EL) of CQDs limits further implementation. Here, a novel bottom‐up synthetic route for fabricating highly crystalline CQDs suitable for high‐brightness blue light‐emitting diodes is demonstrated. The two‐step solution process is based on time‐controlled thermal carbonization of citric acid, followed by ligand exchange of the CQDs with oleylamine (OA) in solution. Carbonization allows for the nucleation and growth of crystalline CQDs, while OA treatment disperses the CQDs and stabilizes the solution, giving rise to CQDs with low structural defects and uniform sizes. The systematic study reveals the origin of the light emission of OA‐treated CQDs by photoluminescence (PL) analysis, which yields a high quantum efficiency of ≈30%. The photoluminescence‐optimized OA‐treated CQDs exhibit excellent blue EL performance with a low turn‐on voltage of ≈4 V and high brightness of 308 cd m−2; a negligible voltage‐dependent color shift when they are employed to an inverted light‐emitting diode. Highly crystalline carbon quantum dots are synthesized by a solution‐processed, bottom‐up process involving time‐controlled thermal carbonization of citric acid, followed by ligand exchange. A light emitting diode with an internal quantum efficiency of ≈30% exhibits excellent blue electroluminescence with a turn‐on voltage and maximum luminance of ≈4 V and 308 cdm–2, respectively, for emerging eco‐friendly lightings, sensors, and displays.