Local Structure And Luminescent Properties Of Rare Earth Ions In Four Kinds Of Inorganic Luminescent Materials

This dissertation reports the work about the local structures of rare earth ions and the luminescent properties of four kinds of inorganic luminescent materials. The dissertation consists of five chapters, including three parts of contents according to the research subjucts. The first part focuses on the site surroundings studies of two kinds of host materials using Eu3+ ions as fluorescent probe, presented in Chapter 2 and 3 respectively. The second part concerns the luminescent properties of K2GdF5:1%Er3+ crystal, foucus in the energy transfer from Gd3+ to Er3+ and the downconversion and upconversion luminescent processes, seen in Chapter 4. The last part is about ScVO4: Bi3+, Eu3+ in view of its application as red-emitting phosphor for WLED (White Light Emitting Diode), referred to Chapter 5.Chapter 1 is the introduction. Firstly, we introduce briefly the basic knowledge of rare earth ions, mainly about the electron configuration, energy-level structure, transition selection rules and crystal-field effect on luminescence of rare earth ions. Secondly, we discribe the luminescent mechanism and enumerate several rare-earth luminescent materials. Finally, we discuss the use of Eu3+ions as fluorescent probe for site surroundings study.Chapter 2 is about the luminescent properties of MIn2O4:Eu3+(M=Ca, Sr). The samples were prepared by sol-gel process. The XRD confirm the success of synthesis, and the introduction of rare earth ions do not induce the occurrence of impure phases. The room-temperature emission spectrum of CaIn2O4:1%Eu3+ under 395 nm excitation consists of radiation transitions from Eu3+5DJ (J= 0,1,2,3) to 7FJ (J= 0,1, 2,3,4). The 20 K site-selective spectra of CaIn2ruO4:1%Eu3+ suggest five types of Eu3+ site occupation in the host lattice, and three of them with similar spectral shape are considered to substitute the Ca2+ sites, while other two with very much different spectra are to replace the In3+ sites. Moreover, energy transfer could be found between different types of Eu3+ sites. The site-selective spectra of SrIn2O4:1%Eu3+ reveal similar informations of site surrounding as those of CaIn2O4:1%Eu3+.Chapter 3 is about the Eu3+-ion local environment study of KGd2F7:5%Eu3+ by site-selective spectra measurements at 20 K. The results show three types of Eu3+site exist (labeled with S1, S2 and S3). Their characteristic excitation wavelengths are 466.1,526.0 and 525.2 nm, and the corresponding characteristic emission peaks in the 5 D0â†'7F2 transition are 620.8,612.8 and 610.0 nm with the 5D0 decay lifetimes of 6.9,7.7 and 9.8 ms, respectively. The results of the 5D0 decay lifetimes and the emission intensity ratio of 5D0â†'7F2 and 5D0 â†' 7F1 transitions support that the degree of deviation from centrosymmetry of Eu3+ site decreases from S1 to S3. The emission spectra of 5D0.1 â†' 7F2 transitions of Eu3+in S2 sites are relatively narrower due to higher coordination number.Chapter 4 shows the luminescent properties of K2GdF5:1%Er3+ crystal in a wide wavelength range from NUV to NIR. Based on the experimental results, the assignments of the recorded radiative transitions have been achieved. The energy transfer from Gd3+ ions to Er3+ions is confirmed to be very efficient. When excited with shorter wavelengths (≤ 312 nm), the emission of Er3+ion is dominated by the transition of 2P3/2 â†'4I13/2 (403 nm), while the emission of 4S3/2 â†' 4I15/2 transition (544 nm) is the strongest when excited to the levels below P3/2. Moreover, when excited with shorter wavelengths (≤ 312 nm), the 4S3/2 levels are fed through the relaxation from the 2P3/2 levels. The obtained radiative lifetime of 2P3/2 is 460 μs, much longer than 248 μs of 4S3/2. The measured NIR spectrum consists of six well-distinguished emission bands dominated by the emission around 1540 nm. The up-conversion luminescence under 980 nm excitation of the sample mainly consists of bands at 557 nm and 674 nm which are confirmed to be two-photon process. The up-conversion luminescence under various temperature are also measured and temperature sensing based on the emission intensity of the two thermal coupling levels 4S3/2 and 2H11/2 is discussed.Chapter 5 shows the luminescent properties of SCVO4:Bi3+, Eu3+. XRDs confirm samples were successfully synthesized. The samples exhibit very efficient excitation in NUV range and can be excited even at 430 nm. Under NUV light excitation, all samples show the characteristic red emission of Eu3+ at 616 nm corresponding to 5D0 â†' 7F2 transition, and the intensity reach maximum when the doings concentration of Bi3+ and Eu3+ are 2% and 4%, respectively. The energy transfer from Bi3+ to Eu3+ is very efficient when the concentration of Bi3+is≤2%, while excitation mainly quenches in Bi3+ions when Bi3+exceeds 2%. The optimized sample produces an emission intensity 5.4 times of the commercial phosphor Y2O2S:5%Eu3+ under 385 nm excitation. Moreover, it also has a better CIE chromaticity coordinates than the later, closer to the National Television System Committee standard value for red-emitting phosphor. These results suggest that SCVO4:Bi3+, Eu3+ is promising for NUV excited white LEDs in illumination applications.