White Light-emitting Diode Based On Single-phase Carbon Dots Phosphor

Light emitting diode (LED) has attracted great attention from scientific andindustrial community owing to low energy consumption, high efficiency andlong lifetime. Research on single-phase phosphor for white LED is scientificallyand industrially significant. Single-phase phosphor for white LED is based onrare earth, colloidal quantum dots, and carbon dots (CDs). CDs have becomehotspots of single-phase phosphor owing to high quantum yield and stability aswell as low toxicity.In order to enhance quantum yield (QY) and blue-to-red light emitting,single-phase CDs phosphor was hydrothermally prepared for6h at variousreaction temperatures using citric acid monohydrate as carbon source,ethylenediamine, thiourea and L-cysteine as additives, obtaining nitrogen-dopedCDs (N-CDs) and nitrogen and sulphur-doped CDs (NS-CDs). OptimizedN-CDs4and NS-CDs9with highest QY and blue-to-red light emitting wereselected to fabricate white LEDs. Chromaticity coordinate and correlated colortemperature (CCT) of white LEDs were characterized.The results are as follows:(1) Preparation and characterization of N-CDs. QY of N-CDs increasedinitially and declined thereafter with increasing ethylenediamine addition. In160-200℃, QY of N-CDs exhibited temperature-independent behavior.Blue-to-red light emitting of N-CDs was independent of ethylenediamineaddition and reaction temperature. N-CDs with the highest QY (i.e., N-CDs4) were obtained with molar ratio of citric acid to ethylenediamine1:4and reactiontemperature200℃. QY and blue-to-red light emitting of N-CDs4was67.06%and53.01%, respectively, which were higher than those reported by others. Twofluorescence centers were discovered in N-CDs with lifetime17.0ns and11.1ns,separately. Energy transfer was measured from fluorescence center with lifetime17.0ns to that with lifetime11.1ns. QY of N-CDs was promoted owing tointroduction of C-N group.(2) Preparation and characterization of NS-CDs. QY of NS-CDs with C=Ssurface group increased initially and decreased subsequently with increasingthiourea addition. In160-200℃, QY of NS-CDs with C=S surface group isproportional to increasing temperature. Blue-to-red light emitting of NS-CDswith C=S surface group is inversely proportional to increasing thiourea additionand reaction temperature. NS-CDs with the highest QY (i.e., NS-CDs3) wereobtained with molar ratio of citric acid to thiourea1:1and reaction temperature200℃. QY and blue-to-red light emitting of NS-CDs3was9.80%and47.82%,respectively. With the average fluorescence lifetime8.88ns, NS-CDs exhibitedthree fluorescence centers. The lifetime of CDs was delivered by fitting thedecay traces with a triexponential functional model. QY and blue-to-red lightemitting of NS-CDs9with C-S surface group was65.37%and61.93%,respectively, indicating that C=S surface group antagonized promotion of QYcompared with C-S surface group. Blue-to-red light emitting of NS-CDs wasefficiently boosted by introduction of sulfur.(3) Fabrication and optical properties of CDs-based white LED. N-CDs4andNS-CDs9were employed as single-phase phosphor. Chromaticity coordinateand CCT of white LED based on N-CDs4were (0.38,0.42) and4290K,respectively. Chromaticity coordinate and CCT of white LED based onNS-CDs9were (0.32,0.36) and5822K, respectively. Red shift of CDs emission spectra was discovered owing to optical encapsulant and polyvinyl alcoholhydrogel, leading to emission covering the entire visible range and direct whiteLED.