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

Abstract

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 CC, CO, –COOH, and graphitic N. Both of CO and graphitic N cause the PL redshift, whereas the CC 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 yellow-emission 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.