# Phosphine-Free-Synthesized ZnSe/ZnS Core/Shell Quantum Dots for White Light-Emitting Diodes
## 3 Results and Discussion
Three obvious diffraction peaks located at 27.2\({}^{\circ}\), 45.3\({}^{\circ}\), and 53.5\({}^{\circ}\) corresponded to the diffraction peaks of the (111), (220), and (311) planes of zinc blende ZnSe, respectively. After the shelling procedure (Figure 4a-ii), the zinc blende structure remained and XRD peaks were slightly shifted to higher angles (27.6\({}^{\circ}\), 46.1\({}^{\circ}\), and 54.1\({}^{\circ}\)) because ZnS has smaller lattice parameters than ZnSe [36]. The high degree of crystallinity was also confirmed by high-resolution TEM (HR-TEM) images, shown in Figure 4b. The HR-TEM images and fast Fourier transformation (FFT) patterns of the ZnSe C and ZnSe/ZnS C/S QDs are shown in Figure 4b. The lattice fringes were separated by d = 0.32 nm for the ZnSe C and the ZnSe/ZnS C/S QDs, which was consistent with the lattice spacings between the (111)  
Figure 3: **(a)** Absorption and **(b)** PL spectra of the ZnSe/ZnS C/S QDs synthesized at (i) 310 \({}^{\circ}\)C, (ii) 320 \({}^{\circ}\)C, and (iii) 330 \({}^{\circ}\)C, respectively. Figure 2: **(a)** Absorption and **(b)** PL spectra of (i) ZnSe C QDs and (ii) ZnSe/ZnS C/S QDs synthesized at 310 \({}^{\circ}\)C. The inset in **(b)** shows photographs of the ZnSe/ZnS C/S QD solution under indoor light (left) and UV light (right, \(\lambda\) = 365 nm). planes of zinc blende ZnSe. The synthesized ZnSe/ZnS C/S QDs exhibited a narrow size distribution of 5.1 \(\pm\) 0.5 nm (average size \(\pm\) standard deviation). nUV LED-pumped blue LEDs were fabricated with the ZnSe/ZnS C/S QDs. Figure 5 shows the optical properties of the fabricated blue LEDs, which were evaluated at 60 mA. In the EL spectrum, two peaks were observed. One peak in the nUV region and the other peak in the blue region were attributed to the nUV LED and ZnSe/ZnS C/S QDs, respectively. Blue light was emitted from the fabricated LED due to the strong EL peak in the blue  
Figure 4: **(a) XRD patterns and (b) TEM images of (i) ZnSe C QDs and (ii) ZnSe/ZnS C/S QDs. Lower insets in (b) show HR-TEM images of (i) ZnSe C QDs and (ii) ZnSe/ZnS C/S QDs and the upper insets in (b) show FFT patterns for the corresponding HR-TEM images. **  
spectral region (Figure 5a inset). Moreover, the CIE chromaticity diagram shows that the color coordinates of the emission light from the ZnSe/ZnS C/S QD-converted LED were located in the blue region (Figure 5b). In addition to fabricating the ZnSe/ZnS C/S QD-converted blue LED, ZnSe/ZnS C/S QD-converted white LEDs were also fabricated. White light can be generated by combining blue-emitting ZnSe/ZnS C/S QDs with yellow-emitting QDs. We synthesized yellow-emitting ZCIS/ZnS C/S QDs by referring to the previous method with a slight modification [32]. As shown in Figure 6, the ZCIS/ZnS C/S QDs exhibited an absorption band in the blue spectral region and a broad PL band under 450 nm excitation. The ZCIS/ZnS C/S QDs showed a broad PL band peaking at 565 nm with a wide FWHM of 115.5 nm, resulting in yellow luminescence. The PL QY of the yellow-light-emitting ZCIS/ZnS C/S QDs was measured at 83%. The absorption band of the ZCIS/ZnS C/S QDs was well matched with the emission band of the ZnSe/ZnS C/S QDs. Thus, a combination of ZnSe/ZnS C/S QDs and ZCIS/ZnS C/S QDs can create white light. In this study, to investigate the applicability of ZnSe/ZnS C/S QDs for warm white LEDs, which can be used in the lighting field, we fabricated warm white LEDs by combining an nUV LED chip, ZnSe/ZnS C/S QDs, and ZCIS/ZnS C/S QDs. Figure 7 shows the optical properties of the warm white LEDs, which were evaluated at 60 mA. In the EL spectrum shown in Figure 7a, three peaks were observed. The peak in the nUV region is attributed to the nUV LED chip, and other peaks in the blue and yellow spectral regions are attributed to the ZnSe/ZnS C/S QDs and ZCIS/ZnS C/S QDs. Warm white light was emitted from the fabricated white LEDs due to a combination of characteristic emission peaks of the ZnSe/ZnS C/S QDs and ZCIS/ZnS C/S QDs (Figure 7a inset).