Multi-color carbon dots from cis-butenedioic acid and urea and highly luminescent carbon dots@Ca(OH)2 hybrid phosphors with excellent thermal stability for white light-emitting diodes

Multi-color carbon dots (CDs) were obtained from cis-butenedioic acid (C-BA) and urea by a simple one-step solvothermal route. This is the first report about using C-BA as the raw material to synthesize CDs. The fluorescent properties of as-prepared CDs vary with the ratio of C-BA to urea. And the sample with 1:1 of C-BA and urea exhibits the optimal yellow emission and highest photoluminescence quantum yield (PLQY) of 16.17%. Further investigations indicate that the variation of PL properties comes from the different content of surface functional groups such as C--C, C--O, -COOH, and graphitic N. Both of C--O and graphitic N cause the PL redshift, whereas the C--C and -COOH groups lead to the blueshift. Based on the above, a solid-state luminescent CDs-based phosphor was obtained by adding alkaline Ca(OH)2 powder to as-prepared yellow CDs solution. In this case, PL intensity is increased to 1.78 times, the corresponding emission redshifts from 553 to 568 nm, and PLQY is improved from 16.17% to 35.12%. Simultaneously, CDs@Ca(OH)2 hybrid phosphor exhibits good thermostability. Finally, a white light-emitting diode (LED) was successfully fabricated by combining the yellowemission CDs-based phosphor with a blue LED chip. It exhibits a high CRI of 86.9, the CCT of 5388 K, and a luminous efficacy of 15.12 lm/W. The CIE coordinate locates at (0.3341, 0.3075) which is very close to the pure white light of (0.33, 0.33). Thus, as-prepared CDs-based solid-state phosphor is a potential candidate for practical lighting devices.