Multilevel Approach for Luminescent Materials Modeling
Goals and Requirements
In recent years, interest in luminescent materials with visible light emission has significantly increased. As the wave length and emission intensity are determined by the interaction of the activator ion with the host matrix, it is necessary to screen an infinite number of ion–matrix systems for finding highly effective luminescent materials with the desired properties. To understand how the nature of the material affects its emission properties, a theoretical description of the system is required; it will predict emission properties and considerably reduce the number of expensive experiments.
Application
- Plasma display panels
- Mercury-free lamps
- Emission detectors (scintillators)
- Glowing paints and additions
The development of new luminescent materials for VUV excitations requires the knowledge of optical transitions of rare-earth ions used as activators.
Kintech Solution
Kintech Lab, by request of GE Global Research, has developed a multilevel theoretical approach for the analysis of excited states. This approach can be used far any rare-earth ions in a wide-gap inorganic host matrix. The main idea of this approach was to combine advantages of first-principles calculations in determining the local geometry of rare-earth sites with the efficiency of model calculations for excited 4fN–15d states of rare-earth ions in host matrices. The work on this project was done by a team of experts from RRC Kurchatov Institute and Photochemistry Center RAS. Let us illustrate the application of this approach by the example of a LaPO4:Pr3+ system. This phosphor has attracted considerable interest as a potential quantum splitting phosphor and exhibits an anomalous emission behavior.
Multilevel approach for predicting the optical properties of phosphor
- Upon 4f–5d excitation in Ce3+ one of oxygen atoms leaves the nearest coordination sphere of Ce3+. As a result, the coordination number of cerium decreases from 9 to 8.
Step 1. Local geometry of the Re3+ site in ground and excited states was calculated using the first-principles DFT approach. Calculations were done for cerium and praseodymium ions.
- Crystal filed potential for d electrons can be represented by a superposition of separate contributions v(k) from individual metal–ligand pairs involved in the nearest coordination sphere of the cerium ion.
Step 2. The obtained geometries of the Ce3+ site were used in the calculations of the crystal field potential VCF of 5d electrons in terms of the angular overlap model. Because the Ce3+ and Pr3+ sites in LaPO4 are close to each other in geometry parameters and the characteristics of 5d orbitals, the same angular overlap model parameters were used for both ions.
Step 3. The 5d crystal field potential obtained for LaPO4:Ce3+ was then used to calculate the energy spectrum of 4f15d1 states and the Stokes shift in LaPO4:Pr3+. In the excited state, the lower edge of the Pr3+ 4f15d1 band lies below the 1S0 level; this prevents 4f–4f optical transitions, and only intense 4f15d1 -> 4f2 interconfigurational optical transitions are observed in the emission spectrum at room temperature.
The proposed combined approach can predict Stockes shifts and emission spectra within the accuracy of the experimental measurements (~500 cm-1). This opens up wide possibilities for the screening of “rare-earth ion / inorganic host matrix” systems without limitations on the local symmetry of metal sites.
