Synthesis and photoluminescent properties of oxide-based phosphors for light emitting diodes

Abstract

Phosphor converted light emitting diodes (pc-LEDs) are an attractive alternative to traditional incandescent and fluorescent lights because of their superior efficiency, long lifetime, environmental friendliness, energy conservation, small size, and so on. Phosphor is one of the key materials that affecting luminescent efficiency, service lifetime, color rending index and color temperature of LEDs. The purpose of this dissertation is to develop new single component oxide-based phosphors for ultraviolet (UV), near UV or blue LEDs applications. Molybdates have shown promise as hosts for phosphors because of their chemical and physical stabilities. It has been reported that the excitation band of molybdates with MoOx group depends on the coordination number x, and generally，it will shifts to longer wavelength region with the increasing x. Near UV light with specific wavelength can be absorbed by Sm3+ due to its f-f transitions. Sm3+ doped Sr2CaMoO6 phosphor was synthesized using a high temperature solid state reaction. Dual-mode luminescence were observed in the samples: warm white light can be obtained from Sr1.995Sm0.005CaMoO6 phosphor pumped by 380 or 410 nm excitation energy; it is also competitive as yellow-emitting phosphor for blue chip pumped white LEDs and gives three emission bands at 567, 603 and 650 nm, presenting yellow luminescence upon 466 nm radiation. Full visible-light spectral emission of the phosphor comes from Sm3+ as well as intrinsic luminescence of MoO6 group. Band gap of Sr2CaMoO6 estimated from its diffuse reflection spectra and calculated by CASTEP mode shows its semi-conducting character. All the results show that the Sr2-xSmxCaMoO6 phosphors have considerable potential for applications in near UV LED or pumped by blue LED chip. Then a novel red emitting phosphor Sr2CaWO6:Sm3+ synthesized though a solid state reaction was developed for LEDs applications. Its crystal structure was analyzed and refined via Rietveld full-pattern fitting method based on XRD patterns. The CASTEP module of Materials Studio was used to investigate its band structure and density of states. Optical band gap was calculated through UV-vis diffuse reflectance spectrum and compared with the value calculated through density functional theory (DFT). Raman spectra were recorded to confirm the substitution of cations by Sm3+ ions. Broad W-O charge transfer band and narrow excitation band at 406 nm from Sm3+ ions have comparable intensity in Sr2CaMoO6:Sm3+ sample. However, after the introduction of charge compensators Li+, Na+ or K+, the intensity of near UV excitation band at 406 nm strength up to twice as much as before. The emission spectra under excitation into the charge transfer absorption and f-f transition of Sm3+ indicate low-symmetry Sm3+ centres are preferentially excited via f-f transitions. Different intensity ratios of electronic and magnetic dipole allowed transitions under different radiations show that charge compensators Li+, Na+ or K+ can influence the chemical environment around Sm3+. Compared with Sr2CaMoO6:Sm3+, energy transfer from WO6 to Sm3+ in Sr2CaWO6:Sm3+ is much more efficient due to its charge transfer band lies in shorter wavelength region. Quantum efficiency of Sr2CaMoO6:Sm3+ is not high because energy transfer from host lattice to Sm3+ luminescence centers is not efficient, which usually results from energy migration among MoO6 groups. In this situation, WO6 groups are introduced into molybdate host lattice to block energy loss. Therefore, in order to make full use of high efficiency of Sr2CaWO6:Sm3+ and broad near UV absorption band of Sr2CaMoO6:Sm3+, a series of Sm3+ doped Sr2CaMo1-xWxO6 phosphors were synthesized through solid state reaction process. It was found there is a big difference between the calculated optical band gap of Sr2CaMo0.02W0.98O6: 2%Sm3+ and that of Sr2CaMo0.01W0.99O6: 2%Sm3+. Band structure and density of state of Sr2CaMo0.5W0.5O6 were examined to explore the origination of charge transfer transition. Site occupation of Sm3+ in host lattice was discussed based on Raman spectra and group theory analysis. When 98% Mo were substituted by W, the excitation band at 350 nm by monitoring 608 nm Sm3+ red emission is about 8.5 times stronger than that in Sr2CaMoO6: 2%Sm3+. Thus in Sr2CaMo0.02W0.98O6: 2%Sm3+ not only concentration quenching among MoO6 groups can be prevented but also efficient excitation band in near UV region can be realized. It can find potential applications as red emitting phosphor for near UV LED. In addition to making use of energy transfer from host lattice to rare earth ions to enhance their absorption in UV and near UV region, energy transfer from Bi3+ to Eu3+ is quite common. As a result of our previous work, energy level rules of Bi3+ in host compounds have been established. Based on relationship between transition energy values of Bi3+ and host environment, as well as crystal structure and refractive index of CdWO4, environmental factor he of Cd site was calculated and 1S0→3P1 transition of Bi3+ was predicted to be 3.281 eV(378 nm). It means that CdWO4:Bi3+ can absorb near UV light and therefore Bi3+, Eu3+ doubly doped CdWO4 phosphors were synthesized. The excitation at 350 nm in Bi3+ doped CdWO4 was assigned to be Bi3+ 1S0→3P1 transition according to the energy level rules. The origination of O-W charge transfer transition has been analyzed using the calculated band structure and density of states of CdWO4 based upon DFT. The approach to charge compensation that two impurity ions substituting for three Cd2+ sites was investigated. The mechanism of energy transfer from Bi3+ to Eu3+ is determined to be quadrupole–quadrupole interaction and the critical distance of energy transfer from Bi3+ to Eu3+ is calculated to be 15.31 Å. The quantum efficiency, CIE chromaticity and thermal quenching properties have also been investigated. Similarly, Bi3+, Eu3+ doped CaY4(SiO4)3O phosphors were synthesized by a high temperature solid state reaction. The structure of CaY4(SiO4)3O is characterized by three non-equivalent cationic sites with different coordination and cation-oxygen distances. By means of dielectric theory of the chemical bond for complex crystals, several chemical bond parameters of CaY4(SiO4)3O were calculated and integrated to yield environmental factor he. According to quantitative equations between transition energy values of Bi3+ and environmental factor he, excitation bands at 308 and 226 nm were assigned to 1S0→3P1 transition of Bi3+ in Y(6h) and Y(4f) site, respectively. Another excitation band centered at 210 nm should be the overlap of Bi3+ A-band in Ca site and C-band in Y(6h) site. Optical band gap of pure CaY4(SiO4)3O was calculated using Kubelka–Munk method from diffuse reflectance spectra. Red emission can be realized in CaY4(SiO4)3O:Bi3+, Eu3+ under UV light excitation because of efficient energy transfer from Bi3+ to Eu3+ and decay behaviors of Bi3+ and Eu3+ emission were investigated. Excitation and emission spectra of CaY4(SiO4)3O:Bi3+, Eu3+ and commercial Y2O3: Eu3+ were measured to compare their efficiency. Quantum efficiency of CaY4(SiO4)3O:2%Bi3+, 7%Eu3+ at 310 and 393 nm excitations were also given. At last, Ce3+/Mn2+/Tb3+ triply activated CaY4(SiO4)3O phosphor was prepared through a high temperature solid state reaction. Dual energy transfer from Ce3+ to Mn2+ and Tb3+ occurred and color tunable emission can be realized by modulating their relative PL intensity. Energy transfer efficiency was investigated and the process was demonstrated to be a resonant type via a dipole-quadrupole mechanism. The critical distance RC for energy transfer calculated by quenching concentration method and spectral overlap route is 7.23 and 7.55 Å, respectively. The as-obtained samples show color tunable luminescence and the corresponding CIE chromaticity coordinates were given. All results show CaY4(SiO4)3O: Ce3+, Mn2+, Tb3+ phosphor could be a promising candidate for UV pumped LEDs. Our work presented herein focuses on the investigation of different kinds of oxide-based phosphors for UV, near UV or blue LEDs. Sm3+ singly doped Sr2CaMoO6 has been proved to be a promising white phosphor under 380 or 411 nm excitations or yellow phosphor pumped by 466 nm blue light. Sr2CaWO6:Sm3+ can emit red luminescence under UV or 406 nm light radiations and compensators Li+, Na+, K+ can enhance absorption in near UV region. The emission intensity at 608 nm of Sr2CaMo0.02W0.98O6: 2%Sm3+ was found to be 8.5 times stronger than that of Sr2CaMoO6:Sm3+. Making use of energy level rules of Bi3+ in host compounds, Bi3+, Eu3+ doubly doped CdWO4 and CaY4(SiO4)3O phosphors were designed and developed. Ce3+/Mn2+/Tb3+ triply activated CaY4(SiO4)3O phosphor were also prepared and proved to be promising white phosphor for UV LEDs. Continuation of this work will hopefully further increase the efficiency and fabricate the phosphors with LED chips.