Tuning of emission bands of phosphors and quantum dots, and their applications to white light sources, and understanding of electronic structure via quantum calculation

Abstract

In this thesis, emission control of phosphors and quantum dots (QDs) is reported to achieve excellent white light sources and new yellow-emitting phosphors for white light-emitting diodes (LEDs) are introduced. And the combination of phosphor with semiconductor QDs is firstly tried for the generation of white light. In addition, this thesis shows that quantum calculation using density functional theory (DFT) is useful for the analyses of electronic structure of phosphors and the design of new phosphors. And it is shown that yellow-emitting phosphors in this study can be applied to white light sources using various primary sources [e.g., carbon nanotubes (CNTs)] as well as white light sources using LEDs. In chap. 3, widely used yellow-emitting $Y_3Al_5O_{12}:Ce^{3+}$ (YAG:Ce)-based phosphors are discussed. Yellow-emitting $Y_{2.94-x}Al_5O_{12}:Ce^{3+}_{0.06}$, $Pr^{3+}_x$ (YAG:Ce,Pr) phosphors where x = 0, 0.006, 0.015, 0.03, and 0.06 were synthesized to enhance red spectral intensity and YAG:Ce,Pr is reported in section 3.1. Sharp red emission peaking at about 610 nm was observed from $Y_3Al_5O_{12}:Ce^{3+}$, $Pr^{3+}$ (YAG:Ce,Pr) and it is ascribed to the transition of $^1D_2$ → $^3H_4$ of $Pr^{3+}$. Although red emission peak was observed in YAG:Ce,Pr, yellow emission intensity decreased as the amount of $Pr^{3+}$ was increased. To investigate the origin of the decrease of yellow emission via the electronic transition of $5d^1$ → 4f of $Ce^{3+}$, crystal structure, particle size, and energy transfer were considered. It is suggested that the decrease of yellow emission is attributed to the radiative or non-radiative energy transfer from $Ce^{3+}$ to $Pr^{3+}$. In the case of non-radiative energy transfer, energy transfer occurred from $Ce^{3+}$ to $Pr^{3+}$ via dipole-dipole interaction and calculated critical distance for the energy transfer was 12$\textrm{\AA}$. In Section 3.2, $Tb^{3+}$ -substituted YAG:Ce [(Y, Tb)AG:Ce] is dealt with. Yellow-emit...