Sebastian Mahlik

Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White Light-Emitting Diodes

Red phosphors AMF6:Mn(4+) (A = Na, K, Cs, Ba, Rb; M = Si, Ti, Ge) have been widely studied due to the narrow red emission bands around 630 nm. The different emission of the zero-phonon line (ZPL) may affect the color rendering index of white light-emitting diodes (WLED). The primary reason behind the emergence and intensity of ZPL, taking KNaSiF6:Mn(4+) as an example, was investigated here. The effects of pressure on crystal structure and luminescence were determined experimentally and theoretically. The increase of band gap, red shift of emission spectrum and blue shift of excitation spectrum were observed with higher applied pressure. The angles of ∠FMnF and ∠FMF(M = Si, Ti, Ge) were found clearly distorted from 180° in MF6(2-) octahedron with strong ZPL intensity. The larger distorted SiF6(2-) octahedron, the stronger ZPL intensity. This research provides a new perspective to address the ZPL intensity problem of the hexafluorosilicate phosphors caused by crystal distortion and pressure-dependence of the luminescence. The efficacy of the device featuring from Y3Al5O12:Ce(3+) (YAG) and KNaSiF6:Mn(4+) phosphor was 118 lm/W with the color temperature of 3455 K. These results reveal that KNaSiF6:Mn(4+) presents good luminescent properties and could be a potential candidate material for application in back-lighting systems.

High pressure luminescence spectra of CaMoO4:Ln3+ (Ln = Pr, Tb).

Photoluminescence spectra and luminescence kinetics of pure CaMoO(4) and CaMoO(4) doped with Ln(3+) (Ln = Pr or Tb) are presented. The spectra were obtained at high hydrostatic pressure up to 240 kbar applied in a diamond anvil cell. At ambient pressure undoped and doped samples exhibit a broad band emission extending between 380 and 700 nm with a maximum at 520 nm attributed to the MoO(4)(2-) luminescence. CaMoO(4) doped with Pr(3+) or Tb(3+) additionally yields narrow emission lines related to f-f transitions. The undoped CaMoO(4) crystal was characterized by a strong MoO(4)(2-) emission up to 240 kbar. In the cases of CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), high hydrostatic pressure caused quenching of Pr(3+) and Tb(3+) emission, and this effect was accompanied by a strong shortening of the luminescence lifetime. In doped samples, CaMoO(4):Pr(3+) and CaMoO(4):Tb(3+), quenching of the emission band attributed to MoO(4)(2-) was also observed, and at pressure above 130 kbar this luminescence was totally quenched. The effects mentioned above were related to the influence of the praseodymium (terbium) trapped exciton PTE (ITE-impurity trapped exciton) on the efficiency of the Pr(3+) (Tb(3+)) and MoO(4)(2-) emissions.

Luminescence of Gd2(WO4)3:Ln3+ at ambient and high hydrostatic pressure

This paper presents a spectroscopic characterization of Gd(2)(WO(4))(3):Ln(3+) (Ln=Eu, Pr, Tb and Dy). The luminescence and luminescence kinetics were measured under pressures up to 250 kbar. It was found that pressure quenches the luminescence of Pr(3+) and Tb(3+), whereas the emission of Eu(3+) and Dy(3+) was stable up to 250 kbar. This effect was related to a decrease in the ionization energy of Pr(3+) and Tb(3+) caused by pressure induced increase in energies of the Ln(2+) and Ln(3+) ions with respect to the band edges. Analysis of emission and excitation spectra allowed us to estimate the energies of the ground states of Ln(3+) and Ln(2+) with respect to the valence and conduction band edges of the Gd(2)(WO(4))(3) host. Differences between energies of the ground states of Ln(2+) and Ln(3+) have also been calculated.

Improvement of the Water Resistance of a Narrow-Band Red-Emitting SrLiAl3N4:Eu2+ Phosphor Synthesized under High Isostatic Pressure through Coating with an Organosilica Layer

A SrLiAl3 N4 :Eu(2+) (SLA) red phosphor prepared through a high-pressure solid-state reaction was coated with an organosilica layer with a thickness of 400-600 nm to improve its water resistance. The observed 4f(6) 5d→4f(7) transition bands are thought to result from the existence of Eu(2+) at two different Sr(2+) sites. Luminescence spectra at 10 K revealed two zero-phonon lines at 15377 (for Eu(Sr1)) and 15780 cm(-1) (for Eu(Sr2)). The phosphor exhibited stable red emission under high pressure up to 312 kbar. The configurational coordinate diagram gave a theoretical explanation for the Eu(2+/3+) result. The coated samples showed excellent moisture resistance while retaining an external quantum efficiency (EQE) of 70 % of their initial EQE after aging for 5 days under harsh conditions. White-light-emitting diodes of the SLA red phosphor and a commercial Y3 Al5 O12 :Ce(3+) yellow phosphor on a blue InGaN chip showed high color rendition (CRI=89, R9=69) and a low correlated color temperature of 2406 K.

High pressure luminescence spectra of CaMoO4:Pr3+

Steady state and time resolved luminescence measurements of CaMoO(4) doped with Pr(3+) as a function of hydrostatic pressure in the 1-175 kbar range are presented. It has been observed that with increasing pressure the spectral features shift towards lower energies, the decay times of both (3)P(0) and (1)D(2) emitting levels become shorter and the intensity of the (3)P(0) emission decreases to complete quenching at about 110 kbar, whereas that of the (1)D(2) emission increases in the 0-100 kbar range and then rapidly decreases when the pressure exceeds 127 kbar. A variation of the structure of the spectral manifolds indicates that a pressure induced phase transition of the host lattice occurs in the 80-100 kbar range. The quenching of the luminescence and the shortening of the decay times have been accounted for by means of a model that takes into account the role played by a praseodymium trapped exciton in the excited state dynamics of the investigated material.

Pressure effect on the zero-phonon line emission of Mn4+ in K2SiF6

In this work, effects of pressure and temperature on the luminescent properties of the K2SiF6:Mn(4+) system have been presented. At ambient pressure, the luminescent spectrum of Mn(4+) consists of several lines at 610-650 nm attributed to phonon repetitions of the (2)Eg → (4)A2g transition and does not contain the zero phonon line (ZPL). At pressure above 9 kbar, an additional line at about 624 nm occurs, which can be attributed to the ZPL of the (2)Eg → (4)A2g transition in the Mn(4+) ions. This change in the emission spectra is accompanied by shortening of the luminescence decay time. Further increasing pressure up to 220 kbar causes the red shift of all emission lines. Upon releasing pressure, all observed lines are going back to their previous positions. The ZPL remains visible even at ambient pressure. Taking into account XRD and Raman spectra at ambient pressure before and after compression-decompression, we have attributed these changes to pressure-induced local structure change of MnF6 (2-) octahedron.

High pressure and time-resolved luminescence spectra of Ca3Y2(SiO4)3 doped with Eu2+ and Eu3+.

Tricalcium diyttrium trisilicon oxide, Ca(3)Y(2)(SiO(4))(3) doped with Eu(2+) and Eu(3+) belongs to a very limited number of hosts able to accommodate both Eu(3+) and Eu(2+) ions, which might make it useful for white light emitting diodes (WLEDs) based on UV chip technology. In this contribution we present a detailed study of photoluminescence kinetics and high pressure spectroscopy of Eu(3+) and Eu(2+) doped Ca(3)Y(2)(SiO(4))(3). At ambient pressure and room temperature, under excitation with near-UV radiation, a broad emission band from 400 to 550 nm due to the 4f(6)5d(1)→4f(7)((8)S(7/2)) transition in Eu(2+) was observed, as well as several emission peaks in the region between 550 and 710 nm, ascribed to the (5)D(0)→ (7)F(J) (J = 0-4) transitions in Eu(3+). The bluish green luminescence related to Eu(2+) in the Ca(3)Y(2)(SiO(4))(3) exhibits a small red pressure-induced shift reaching -5.2 cm(-1)/kbar. The red shifts of the luminescence lines related to Eu(3+) ion emission vary from 0.15 to -0.54 cm(-1)/kbar. Time-resolved photoluminescence was measured at different temperatures and pressures. Luminescence decay traces were studied for the bluish green emission band of Eu(2+) and for the red emission peak due to the (5)D(0) →  (7)F(2) transition of Eu(3+). Decay times slightly decreased with increasing pressure.

Pressure-induced phase transition in LiLuF4:Pr3+ investigated by an optical technique

The luminescence and luminescence kinetics of LiLuF(4) doped with 1.5 at.% of Pr(3+) obtained at high hydrostatic pressure changing from ambient to 220 kbar applied in a diamond anvil cell are presented. It has been shown that pressure causes shift of the emission lines toward the red with rates of the order of single cm(-1) kbar(-1). The pressure-induced phase transition from tetragonal to fergusonite structure for pressure above 100 kbar was observed. The crystal field calculations performed showed that this phase transition reduces the point symmetry of the Pr(3+) site from the S(4) to the C(2) point group.

Pressure dependence of the emission in CaF2 : Yb2+

We present a detailed spectroscopic investigation of CaF2 doped with Yb(2+) performed at high hydrostatic pressure which is applied in a diamond anvil cell. At ambient pressure and at temperatures lower than 175 K, the luminescence consists of a single broad band peaked at 18 500 cm(-1), attributed to the recombination of impurity-trapped excitons. Increasing pressure causes the luminescence to be observable at higher temperature. At a pressure of 72 kbar luminescence can be observed up to 275 K. The emission lineshape does not strongly depend on pressure below 85 kbar. However, at 85 kbar it is blue shifted to 21 630 cm(-1). This is attributed to the known phase transition of the CaF2 crystal from cubic to the orthorhombic phase. The absolute energy of the ground and 4f(13)5d states of Yb(2+) as well as the energy of the impurity-trapped exciton with respect to valence and conduction bands have been estimated. The results, are discussed in comparison with the pressure dependences observed for the luminescence of BaF2 : Eu(2+) and CaF2 : Eu(2+). The difference between the spectral properties of Eu(2+) and Yb(2+) is attributable to the fact that the ground and 4f(6)5d states of Eu(2+) are placed deeper in the CaF2 bandgap than the ground and excited 4f(13)5d states of Yb(2+), whereas the energies of the impurity-trapped exciton states for Yb(2+) and Eu(2+) with respect to the conduction band are approximately the same.

Aluminate Red Phosphor in Light-Emitting Diodes: Theoretical Calculations, Charge Varieties, and High-Pressure Luminescence Analysis

Searching for a non-rare-earth-based oxide red-emitting phosphor is crucial for phosphor-converted light-emitting diodes (LEDs). In this study, we optimized a blue and UV-light excited Sr4Al14O25:Mn phosphor exhibiting red emission peaked at ∼653 nm, which was successfully synthesized by solid-state reaction. The crystal structure, micromorphology, and luminescent properties of Sr4Al14O25:Mn phosphors were characterized by X-ray Rietveld refinement, high-resolution transmission electron microscopy, and photoluminescence spectra. The band gap and electronic structure of Sr4Al14O25 were analyzed by density functional theory calculations using the hybrid exchange-correlation functional. The crystal field environment effect of Al sites from introducing activator Mn ions was investigated with the aid of Raman 27Al nuclear magnetic resonance spectra and electron spin resonance. The pressure dependent luminescent properties and decay time of this compound were presented. The tricolor display spectrum by combining blue InGaN chips, commercial β-SiAlON:Eu2+ green phosphor, and Sr4Al14O25:Mn red phosphor were evaluated for commercial applications: using the present Sr4Al14O25:Mn red phosphor converted LED as a backlighting source.