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White light quantum dots (WQDs) such as CdSe and ZnxCd1-xS are considered source candidates in solid-state lighting (SSL) due to their unique optical properties. However, the low fluorescence quantum yield (QY, < 60 %), low luminous efficacy (~11.9 lm/W), and poor stability (~100 hr) limits their commercial application. In order to solve the above problems, a high QY of ZnxCd1-xS WQDs (x=0.2, 0.5 and 0.8, named as Zn0.2, Zn0.5 and Zn0.8, respectively) was prepared by using unsaturated fatty acid-oleic acid (OA) and non-coordinating solvent-1-octadecene (ODE) at low temperature (180 °C). Moreover, the Zn0.5 WQDs was coated with silica to improve stability, and named as Zn0.5@SiO2. The results show that the QYs of Zn0.2, Zn0.5 and Zn0.8 WQDs are 82, 86 and 51 %, and the TEM images show that the morphologies of those WQDs are spherical, and the particle size are 2.7, 2.6 and 2.5 nm, respectively. The particle size of Zn0.5 WQDs in Zn0.5@SiO2 is 2.6 nm, and the emission wavelength is red-shifted from 391/501 to 418/533 nm due to agglomeration. The CIE coordinates, color rendering index (CRI), correlated color temperature (CCT) and luminous efficacy of Zn0.5-based white light-emitting diode (WLED) are (0.36, 0.43), 73, 4784 K and 30.6 lm/W, respectively. The CIE coordinates, CRI, CCT and luminous efficacy of Zn0.5@SiO2-based WLED are (0.42, 0.46), 72, 3732 K and 19.4 lm/W, respectively. After long-term operation for 261 h, the luminous efficacy of Zn0.5- and Zn0.5@SiO2-based WLED decreased by 82 and 14 %, respectively. The above results confirm that ZnxCd1-xS WQDs with high QY can be successfully prepared at low temperature by using unsaturated fatty and non-coordinating solvent, and coating the WQDs with silica can increase WLED stability. This result can make WQD a big step towards the application of solid state lighting.
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