Highly Luminescent Solid‐State Carbon Dots Embedded in a Boric Acid Matrix

Aggregation‐caused quenching of carbon dots (CDs) has limited their application for optoelectronic devices. In this work, we reported a highly luminescent solid‐state CDs through a freeze‐drying assisted hydrothermal method, using citric acid, urea and boric acid as raw materials. The hydrothermal process triggers the formation of the carbonaceous structures, and freeze‐drying treatment results in the dehydration and crossing linking reactions between the carbonaceous structures and boric acid, forming CDs embedded in the matrix of boric acid. Steady‐state fluorescence spectra and time‐resolved PL decay curves, together with the XPS data suggest the significant promotion of photoluminescence quantum yield after processing CDs from solution to powder state is attributed to surface passivation and activating of new emissive states caused by the attraction of boron. The CDs shows a high photoluminescence quantum yield of 80.1 %, with an excellent thermal stability, which are further used as phosphor for fabricating LED. A white LED is obtained after combining blue emissive CDs with orange emissive Cu nanocluster with UV‐LED chips, achieving a high color rendering index of 89, and a Commission Internationale de L'Eclairage chromaticity coordinates of (0.33, 0.32).