Using DFT Methods to Study Activators in Optical Materials

Abstract

Density functional theory (DFT) calculations of various activators (ranging from transition metal ions, rare-earth ions, ns 2 ions, to self-trapped and dopant-bound excitons) in phosphors and scintillators are reviewed. As a single-particle ground-state theory, DFT calculations cannot reproduce the experimentally observed optical spectra, which involve transitions between multi-electronic states. However, DFT calculations can generally provide sufficiently accurate structural relaxation and distinguish different hybridization strengths between an activator and its ligands in different host compounds. This is important because the activator-ligand interaction often governs the trends in luminescence properties in phosphors and scintillators, which can be used to search for new materials. DFT calculations of the electronic structure of the host compound and the positions of the activator levels relative to the host band edges in scintillators are also important for finding optimal host-activator combinations for high light yields and fast scintillation response. Mn 4+ Luminescence of materials when excited by photons or ionizing radiation is the foundation for numerous technologies, such as energy efficient lighting (fluorescent lamps and white LEDs), laser, medical imaging, and nuclear materials detection.1-3 Efficient luminescence in semiconductors and insulators usually relies on the localization of excited electrons and holes at certain impurities, which act as luminescence centers (or activators). An activator can trap electrons and holes for efficient radiative recombination and is the essential component of a phosphor or scintillator material. The commonly used activators are typically multivalent ions, which can insert multiple electronic states inside the bandgap of the host material. 45 Good examples of the multivalent ions that can act as luminescent centers are rare-earth ions (e.g., Ce 3+ , Eu 2+ ), 6-11 transition metal ions (e.g., Cr 3+ , Mn 4+ ), [12][13][14][15][16][17] and ns 2 ions (ions with outer electron configuration of ns 2 , such as Tl + , Sn 2+ ). 18-20Energy efficiency is critically important for phosphors used for lighting. To suppress the energy loss through nonradiative recombination, a phosphor is excited by directly exciting activators, not the host (see Fig. 1a). The excitation energy is less than the bandgap energy of the host but is large enough to excite the activator. Since the excitation occurs locally at the activator, the chance for the excited activator to interact with nonradiative recombination centers, such as deep defects, is small. This is important for achieving high quantum efficiencies for phosphors. In scintillators used for detecting ionizing radiation, the radiation creates charge carriers in the valence and conduction bands of the host, which are subsequently trapped by activators, leading to radiative recombination (see Fig. 1b). In scintillators, a portion of the charge carriers need to travel a certain distance before being trapped by the activators, which ...

Bulk assembly of organic metal halide nanotubes

“…Along the axis of the tube, the band dispersion is still small, reflecting the weakened Pb–Br hybridization due to the distortion of the PbBr 6 octahedral structure. The narrow bands near the band edges and the soft lattice of (HTMA) 3 Pb 2 Br 7 should favor charge localization and the formation of self-trapped excitons, 37 – 39 which is consistent with the absence of free exciton emission at room temperature. The top of the valence band is mostly made up of Br-4p states but has significant Pb-6s character, while the bottom of the conduction band is dominated by Pb-6p states as shown by the projected density of states (DOS) in Fig.…”

Negative Thermal Quenching of Efficient White‐Light Emission in a 1D Ladder‐Like Organic/Inorganic Hybrid Material

“…As shown in Figure a, the dispersion curves are flat for most of the directions in the first Brillouin zone, which reflects the 1D character of the crystal structure. The narrow VBs and CBs favor the localization of the charge carriers and the formation of self‐trapped excitons (STEs) …”

Needs, Trends, and Advances in Inorganic Scintillators

“…Definitely, much more reliable investigations were made for different first-principles calculations of specific crystals (see [209] as an example). Fairly good progress was achieved in first-principles calculation of activators, both rare earth ions (see [210] and [211]) and heavy ions [212], [213]. First-principle calculations make a step toward obtaining not only positions of energy levels, but some dynamical parameters, such as multiphonon capture and recombination rates [214]- [216], the parameters that are very important for reliable application of above-mentioned calculations using rate equations or kinetic Monte Carlo schemes.…”