The optical properties of afterglow nanoparticles were successfully improved by the addition of polyethylene glycol (PEG) to an afterglow colloidal solution. Afterglow nanoparticles— : , —were prepared by laser ablation in liquid. The quantum yields and the decay curves were measured by a fluorescence spectrophotometer. An increase in the amount of PEG added to the solution increased the quantum yield of the nanoparticles and improved the afterglow property in the initial portion of the decay curve. However, the afterglow property did not change after a substantial amount of time had passed. The afterglow nanoparticles were capped with PEG molecules, and surface defects of the nanoparticles were passivated, which decreased the optical properties. 1. Introduction In the past several years, there has been increasing interest in nanoparticles due to their unique properties and various applications in research fields, such as biotechnology and electronics. There are many methods available to create nanoparticles, such as gas-phase preparation [1, 2] and liquid-phase preparation [3, 4]. One of the liquid-phase preparation methods is laser ablation in liquid, which has recently been studied extensively [5–9]. While laser ablation in the gas phase, such as under vacuum and in argon, has been used for various research purposes [10], laser ablation, in liquid, has been investigated more recently [11, 12]. In the case of laser ablation in liquid, the target material in the liquid is irradiated with a focused, pulsed laser beam without using a chamber. Precipitation methods, such as the sol-gel method, are well known as synthesis methods for nanoparticles. However, it is difficult to prepare multielement nanoparticles with these techniques. In the case of the trioctylphosphine oxide (TOPO) method [4], the popular synthesis method used to create quantum dots, coating materials are limited to specific surfactants, such as TOPO. In contrast, in the case of laser ablation in liquid, multielement nanoparticles can be easily prepared. By using laser ablation in liquid, surfactant-free nanoparticles can be prepared, and/or a desirable coating material can be selected. We prepared functionalized multielement nanoparticles with afterglow properties by using laser ablation in liquid [13]. Afterglow is a promising optical property for various fields because of the long emitting time (e.g., several hours) after the excitation is blocked. In the case of normal fluorescence, materials emit during excitation but do not emit after the excitation is blocked. On the contrary, in
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