Influence of Covalency on the Mn 4+ 2 E g → 4 A 2g Emission Energy in Crystals

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 ...

Section: Ecs Journal Of Solid State Science and Technology 5 (1) R30mentioning

confidence: 99%

Using DFT Methods to Study Activators in Optical Materials

2015

mentioning

confidence: 99%

“…Investigation of the optical properties of the Mn 4+ dopant showed that fluoride hosts are preferred for LED phosphors over oxide hosts, since only the ionic nature of fluorides results in a sufficiently small nephelauxetic effect, maintaining the energy of the emitting 2 E g → 4 A 2g transition within the visible part of the spectrum, corresponding to red emission with a zero phonon line in the 617-624 nm range. 8,9 For instance in the oxide phosphors, GdAlO 3 :Mn 4+ , SrTiO 3 :Mn 4+ and Y 2 Sn 2 O 7 :Mn 4+ the emission is well beyond 650 nm and thus unsuitable for visible displays or lighting. [10][11][12] The interest in K 2 SiF 6 :Mn 4+ as a red phosphor started with the development of a synthesis method by Adachi and Takahashi, etching Si wafers in HF in the presence of KMnO 4 .…”

mentioning

confidence: 99%

Luminescent Behavior of the K₂SiF₆:Mn⁴⁺Red Phosphor at High Fluxes and at the Microscopic Level

Sijbom

Joos

Martin

et al. 2015

Phosphor-converted white light-emitting diodes (LEDs) are becoming increasingly popular for general lighting. The non-rare-earth phosphor K 2 SiF 6 :Mn 4+ , showing promising saturated red d-d-line emission, was investigated. To evaluate the application potential of this phosphor, the luminescence behavior was studied at high excitation intensities and on the microscopic level. The emission shows a sublinear behavior at excitation powers exceeding 40 W/cm 2 , caused by ground-state depletion due to the ms range luminescence lifetime. The thermal properties of the luminescence in K 2 SiF 6 :Mn 4+ were investigated up to 450 K, with thermal quenching only setting in above 400 K. The luminescence lifetime decreases with increasing temperature, even before thermal quenching sets in, which is favorable to counteract the sublinear response at high excitation intensity. A second, faster, decay component emerges above 295 K, which, according to crystal field calculations, is related to a fraction of the Mn 4+ ions incorporated on tetragonally deformed lattice sites. A combined investigation of structural and luminescence properties in a scanning electron microscope using energy-dispersive X-ray spectroscopy and cathodoluminescence mappings showed both phosphor degradation at high fluxes and a preferential location of the light outcoupling at irregularities in the crystal facets. The use of K 2 SiF 6 :Mn 4+ in a remote phosphor configuration is discussed. Most phosphor-converted white LEDs contain phosphors doped with rare earths such as divalent europium and trivalent cerium. These ions feature relatively broad emission bands based on the parity allowed 5d-4f transition. They are often easily excited with blue light and can show high quantum efficiency, even at elevated temperature. To improve the color rendering of white LEDs, red phosphors are added to the traditional blue LED and yellow Y 3 Al 5 O 12 :Ce (YAG:Ce) phosphor combination. These red phosphors need to be stable and have a high quantum efficiency. The emission spectrum should both be sufficiently red (>600 nm) and well within the eye sensitivity curve, to obtain a high luminous efficacy.1 Sulfide phosphors doped with Eu 2+ , such as (Ca,Sr)S:Eu 2+ are known for their efficient red emission, 2 but they lack stability in humid environments and the eye sensitivity is low for part of their broad emission band.3 Nitride phosphors doped with Eu 2+ are often chemically more stable, but their synthesis at high pressure and temperature is a drawback.4 Most europium-doped nitride phosphors show a relatively broad emission band. 5,6 Cost and supply issues of the rare-earth materials pave the way for transition-metal-doped phosphors. 7 In particular the Mn 4+ ion is a promising alternative for Eu 2+ as it shows line emission from parity and spin-forbidden d-d transitions in the red and near-infrared spectral region. Investigation of the optical properties of the Mn 4+ dopant showed that fluoride hosts are preferred for LED phosphors over oxide hosts, since only the io...

“…More details about this remarkable feature of these two energy levels can be found in recent publications. 20,21 It is obvious that if Dq = 0, we recover the energy level scheme of a free ion. With increasing Dq we see how the energy levels are split and how they behave.…”

Section: Electronic Properties Of Ions With D 3 Electron Configuratiomentioning

confidence: 99%

Critical Review—A Review of the Electronic Structure and Optical Properties of Ions with d³Electron Configuration (V²⁺, Cr³⁺, Mn⁴⁺, Fe⁵⁺) and Main Related Misconceptions

Brik

Srivastava

2017

Self Cite

Tetravalent manganese ions with the 3d 3 electronic configuration have recently began to play a major role as a red photon generator in the LED based lighting and display devices. The goal of this article is to tutorize the fundamental optical properties of Mn 4+ and to clear up some common misconceptions that we have encountered in the archival literature that pertains to the spectroscopic properties of this ion and other ions with the same electron configuration. The methods to properly interpret the optical spectra of these systems are also discussed. This is accomplished by presenting a discussion on the electronic properties of the free d 3 ions and the changes which occur when the ion is introduced in a crystalline lattice. It is hoped that such systematic presentation of spectroscopic properties of the d 3 ions and their variation from the free state to the crystalline solids will be useful for many researchers -mainly experimentalists -actively working in the field and will help them avoiding many mistakes when presenting their experimental results. pl). This paper is part of the JSS Focus Issue on Visible and Infrared Phosphor Research and Applications.The development in 1994 of the first high brightness LED by the Nichia Corporation was followed by a renaissance of interest in developing phosphors, which in combination with the blue LED radiation would produce white light with suitable efficacy and color rendering for general lighting applications. Basically, the research has targeted the development of custom-made new phosphors (such as the nitride and oxynitride family of materials) and changing the composition of existing phosphors (such as the garnet family of materials) to produce luminescent materials that satisfy the pertinent requirements of general illumination devices. Such phosphors have been based on the luminescence of Rare-earth (RE) and Transition metal (TM) ions. Among the improved phosphors that is devised to produce red photons in general illumination and display devices is the family of phosphors with the general formulation A 2 MF 6 activated with Mn 4+ , a TM ion with the 3d 3 electronic configuration. Since the recent commercialization of the K 2 SiF 6 :Mn 4+ phosphor there has been burgeoning interest among phosphor scientist and engineers to research and develop alternative red emitting Mn 4+ activated phosphors. According to the Web of Science data base, the number of research papers containing two key words "phosphor" and "Mn 4+ " increased from nearly 0 in 2007 to more than 80 in 2016, with the number of citations going up to about 1500 in 2016 alone (Fig. 1). 1 Having seen such a reviving interest in the Mn 4+ doped phosphors, on one hand, and having met several incorrect statements in some of those papers, on the other hand, we deemed it necessary to write a tutorial review of basic spectroscopic properties of such systems with a clear aim of helping scientists, mainly targeting the young and experimental researchers, to avoid simple mistakes and basic misunderstanding. ...