# Phosphine-Free-Synthesized ZnSe/ZnS Core/Shell Quantum Dots for White Light-Emitting Diodes
## 1 Introduction  
Nanometer-scaled semiconductor materials called quantum dots (QDs) exhibit size-dependent electronic and optical properties due to the quantum confinement effect that is generated from spatially driven charge carriers, including holes and electrons in the ground and excited states [1; 2; 3]. Colloidal QDs are widely used in solar cells, biolabeling, light-emitting diodes (LEDs), CO\({}_{2}\) reduction in water, and radioluminescent nuclear batteries [4; 5; 6; 7; 8]. In the last three decades, cadmium-based QDs have been popular, especially for light-emitting diode and display applications, due to their superior properties, such as size-controlled emission wavelengths over the entire visible light range, narrow and symmetric emission spectra, high photoluminescence (PL) quantum yields (QYs), and high photostability [9; 10]. However, there is now growing demand to develop cadmium-free QDs to reduce environmental harm and human health issues [11]. Related to these issues, cadmium-free green- or red-light-emitting materials, such as InP, CuInS\({}_{2}\), Si-based QDs, and carbon dots, have been studied as alternatives to CdSe QDs [12; 13; 14; 15]. Additionally, blue-light-emitting, ultra-small ZnSe-based QDs are one of the outstanding candidates for cadmium-free QDs and combining them with red- and green-emitting QDs can produce nonpoisonous whiteLEDs. ZnSe QDs were first synthesized by arrested precipitation, and more recently, using reverse micelles, precipitation in the presence of stabilizers, and heating-up and hot-injection methods [16; 17]. In the hot-injection method, a reagent at room temperature is injected into the hot-reaction solution, producing a burst and homogenous nucleation, followed by slow growth, and annealing of the nanoparticles [18]. In early works, ZnSe QDs were synthesized using trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), and tributylphosphine (TBP) as the solvent and stabilizing ligands [19; 20]. Highly luminescent ZnSeTe-based QDs have been recently reported and applied to electroluminescent devices [21; 22]. In those studies, TOP was also used to synthesize ZnSeTe-based QDs. In recent years, researchers have avoided using these toxic and expensive chemical reagents [23]. For example, in a previously reported method, zinc precursor was injected into paraffin oil containing Se-ODE at high reaction temperature [23]. In this study, we focused on synthesizing eco-friendly ZnSe-based QDs using phosphine-free precursors. The ZnSe C QDs were synthesized without phosphine precursors and the ZnSe/ZnS C/S QDs were also synthesized with phosphine-free precursors via the one-pot synthesis method to improve their PL properties. There are many kinds of zinc source materials that can be used as zinc precursors, including zinc acetate, zinc scaterate, diethyl zinc, and zinc oxide, depending on the synthesis environment [23; 24; 25; 26]. In this study, we used ZnO, which is soluble in acids and alkalis and can form complexes with ligands containing O-, N-, or S-donor atoms [27].\"Zn-oleate precursors were prepared by reacting ZnO and OA in paraffin oil. To synthesize high-quality and Cd-free, blue-emitting ZnSe QDs, we investigated the effects of reaction time and temperature on the PL properties of the ZnSe QDs. In addition, we investigated the applicability of the ZnSe/ZnS C/S QDs as color-converting materials in optoelectronic devices such as LEDs. The phosphine-free synthesized blue-light-emitting ZnSe/ZnS C/S QDs were used as color-converting materials for near ultraviolet (nUV) LED-pumped blue LEDs. We also investigated the applicability of the ZnSe/ZnS C/S QDs to white LEDs. White LEDs are in high demand in the lighting and display fields because of their notable properties, including non-toxicity (no mercury), low power consumption, and long operating time [28; 29]. For the first time, blue-light-emitting ZnSe/ZnS C/S QDs and yellow-light-emitting, Zn-doped CuInS\({}_{2}\)/ZnS C/S QDs were combined to fabricate white LEDs.