# Phosphine-Free-Synthesized ZnSe/ZnS Core/Shell Quantum Dots for White Light-Emitting Diodes
## 3 Results and Discussion  
The growth of the ZnSe C QDs was investigated by absorption spectra, as shown in Figure 1a. The first absorption peak was observed at 407 nm for the aliquot taken at 5 min and continuously redshifted to 418 nm as the reaction time increased. This shift resulted from the increase in core size, and it indicated a size-dependent, electronic-energy band structure [33]. Consequently, a redshift in the absorption spectra of the ZnSe C QDs was observed with the growth of the ZnSe due to quantum confinement effects [33]. PL spectra also shifted from 413 nm to 425 nm with increasing reaction time. The absolute PL QY of the ZnSe C QDs was measured to be 19.4%. As the reaction time increased, the bandwidth of the ZnSe C QDs became narrower. The full width at half-maximum (FWHM) of the PL band of the ZnSe C QDs was 15.5 nm for the aliquot taken at 5 min and 13.5 nm for the aliquot taken at 60 min, which indicates that size focusing occurred during the growth of the ZnSe C QDs. The synthesized ZnSe C QDs exhibited narrow size distribution (4.6 \(\pm\) 0.2 nm, average size \(\pm\) standard deviation). The photophysical properties of the synthesized ZnSe C QDs can be improved by the formation of a ZnSe/ZnS C/S structure, since the ZnS shell decreases surface defects and increases confinement of the exciton [34; 34]. The effect of ZnS shelling on the optical properties of the ZnSe QDs was investigated (Figure 2a,b). Figure 2a shows the absorption spectra of ZnSe C and ZnSe/ZnS C/S QDs. The formation of the ZnS shell on the ZnSe core decreased the surface defects of the ZnSe core [16]; thus, PL intensity increased, as shown in Figure 2b. The absolute PL QY of the ZnSe/ZnS C/S QDs was measured to be 37.4%. The ZnSe/ZnS C/S QDs exhibited their first absorption peak at different wavelengths, depending on the reaction temperature of the ZnSe/ZnS C/S QDs. The reaction temperature was increased from 310 \({}^{\circ}\)C to 330 \({}^{\circ}\)C, and absorption peaks were observed at 421 nm, 423 nm, and 427 nm for the ZnSe/ZnS C/S QDs synthesized at 310 \({}^{\circ}\)C, 320 \({}^{\circ}\)C, and 330 \({}^{\circ}\)C, respectively (Figure 3a). Since increasing the reaction temperature affected nucleation and growth of the QDs, larger QDs could be synthesized at higher reaction temperatures and a  
Figure 1: (**a**) Absorption and (**b**) PL spectra of the as-synthesized ZnSe C QDs as a function of reaction time ((i) 5 min, (ii) 10 min, (iii) 30 min, and (iv) 60 min)). The inset in (**b**) shows photographs of the ZnSe C QD solution (reaction time was 60 min) under indoor light (left) and UV light (right, \(\lambda\) = 365 nm). redshift of the absorption band was observed due to the quantum confinement effect [23]. Figure 3b shows that the PL intensity of the ZnSe/ZnS C/S QDs synthesized at 310 \({}^{\circ}\)C was higher than those of the ZnSe/ZnS C/S QDs synthesized at 320 \({}^{\circ}\)C and 330 \({}^{\circ}\)C. As the temperature was increased from 310 \({}^{\circ}\)C to 320 \({}^{\circ}\)C and 330 \({}^{\circ}\)C, PL intensity decreased. According to [PERSON]'s group, the PL QY of the QDs depends on the sizes of the QDs [35]. Thus, it is believed that, when the temperature is 320 \({}^{\circ}\)C, the size of the ZnSe/ZnS C/S QDs is larger than the optimal size of the ZnSe/ZnS C/S QDs, which shows the best PL property. When the ZnSe/ZnS C/S QDs were synthesized at 300 \({}^{\circ}\)C, blue light was hardly observed. As a consequence, the optimized reaction temperature in this study was 310 \({}^{\circ}\)C.  
In addition to spectroscopic studies, XRD patterns and TEM images were obtained to investigate the structure of the ZnSe C and ZnSe/ZnS C/S QDs in which ZnSe C was synthesized at 310 \({}^{\circ}\)C for 1 h and the ZnS shell was formed by heat treatment at 310 \({}^{\circ}\)C for 50 min. As shown in the XRD pattern, the peak positions of the ZnSe C QDs were in agreement with bulk ZnSe with a zinc blende structure (Figure 4a-i).