Determination of the Energy Levels of Eu2+ for Next-Generation LED Phosphors

The structural-property relationship of any novel materials, whether nanoscaled or in the bulk form must be understood for their successful applications. Soft X-ray spectroscopy is used to probe the electronic structures of materials. Furthermore, density functional theory calculations are used to interpret the experimental observations, which allows linking the electronic properties to the underlying atomic arrangements and luminescence parameters. In this thesis, InN and phosphor-converted light emitting diodes (pc-LEDs) are studied using these experimental techniques and theoretical methods. InN is a key material for technological applications in opto-electronic devices because of its relatively small band gap; however, there is a persistent and active debate about its varying band gap values and their origin. The electronic structure and the band gap of ammonothermal InN powder samples are studied as a first part of this thesis. Further the origin of the measured band gap is discussed in terms of the presence of oxygen impurities and the impurity phases of InN. pc-LEDs can significantly reduce global energy consumption and are expected to dominate the lighting market. In pc-LEDs, the energetic position of activator ions with respect to the conduction band (CB) and valence band (VB) of the host lattice determines the electronic and luminescence properties including multicolor emission, adjustable bandwidth, and thermal quenching under application conditions. The second part of my study involves the direct measurements of the energy levels of activator Eu2+ ions for three highly efficient color-sensitive Eu2+-doped phosphors such as ultranarrow band blue-emitting oxoberyllates AELi2[Be4O6]:Eu2+ (AE = Ba, Sr), red-luminescence SrLi2Al2O2N2:Eu2+, and blue-luminescence nitridoberyllates MBe20N14:Eu2+ (M = Ba, Sr). New characterization methods with existence techniques such as soft X-ray resonant inelastic X-ray scattering (RIXS) and X-ray excited optical luminescence (XEOL) measurements are used for directly determining the energy level of Eu2+ ions. Modern phosphors use the 4f65d1 to 4f7 transition of Eu2+, where 4f65d1 is an excited state. We directly determine the energetic separation between the Eu2+ 4f65d1 state and the CB of host lattice using RIXS measurements. Thus, the location of the Eu2+} 4f65d1 states is below the CB of the corresponding energy separation. The 4f7 energy level is determined from the 4f65d1 4f7 transition of Eu2+ ions with respect to the 4f65d1 states using XEOL techniques. Therefore, we determine all participating energy levels of Eu2+ ions (4f65d1 and 4f7 states) of the studied phosphors. Thus, we identify all radiative processes involved in the design of next generation pc-LEDs. These experimental techniques allow direct access to the intragap states of the Eu2+-dopants, which are the source of the luminescence of Eu2+-doped phosphors