Microscopic Study of Dopant Distribution in Europium Doped SrGa₂S₄: Impact on Thermal Quenching and Phosphor Performance

Abstract

White light emitting diodes start to dominate lighting and display applications. However, the properties of the phosphors used in these devices strongly depend on synthesis conditions. A better understanding of how performance-determining mechanisms such as thermal quenching are influenced by synthesis conditions and sample composition is necessary to achieve the required standards in a goal-oriented strategy. In this paper, a microscopic thermal quenching study on green-emitting SrGa 2 S 4 :Eu 2+ phosphors by means of cathodoluminescence spectroscopy and energy dispersive X-ray analysis in a scanning electron microscope is used to extend our knowledge beyond averaged information obtained on bulk material. Elemental and cathodoluminescence mapping at different temperatures made it possible to determine thermal quenching profiles for sub-micrometer sized areas. These revealed a broad range of local quenching temperatures for samples with ill-distributed dopant ions. For the associated activation energy an upper limit of 0.61 eV was identified, corresponding to the intrinsic thermal quenching of isolated europium ions. Furthermore, the results confirm a previously suggested thermal quenching model which involves the presence of both isolated and clustered dopant ions. Inorganic luminescent materials or phosphors are currently applied on a large scale in various novel technologies. Most notably, white light-emitting diodes (LEDs), which are applied in lighting and display technologies, rely on high-performance phosphors. [1][2][3] Here, the emission of a blue light source, based on a Ga 1-x In x N LED chip is partly absorbed by the phosphor blend that is applied on top of this chip or in a remote fashion. [4][5][6] The phosphor re-emits the absorbed energy as green, yellow and red light, which is mixed with the remaining blue LED emission as to get an overall white emission. This requires subtle fine-tuning, both in terms of material composition, 7,8 as well as device manufacturing 5,9 to achieve white LED devices with the desired properties.From the materials point of view, phosphor properties are highly dependent on synthesis conditions, often in an unpredictable and hardto-control way. This can be attributed to an incomplete understanding of the underlying physical mechanisms for various macroscopic properties. 10 As an example, thermal quenching (TQ), i.e. the decreased internal quantum efficiency of luminescent materials at elevated temperatures, has been attributed to various causes, either intrinsic to the luminescent center itself or due to external influences such as the presence of defects or the local clustering of luminescent ions. 10,11 The undetermined nature of TQ makes it difficult to engineer synthesis routines to yield phosphors with a minimized TQ and an optimized performance.Phosphors are generally studied at the macroscopic level, i.e measurements are typically performed on milligram quantities, a scale which is of relevance for applications. Nonetheless, if materials are inhomogeneous in terms...

Blind spheres of paramagnetic dopants in solid state NMR

“…In case of luminescent materials, the doping homogeneity is of major importance to establish an optimal performance. 33,34 When luminescent ions are too close, energy can easily be transferred between them, leading to uncontrollable migration of energy in the dopant sublattice that can eventually be nonradiatively dissipated at so-called luminescence killer centres. This effect is usually referred to as concentration quenching 35,36 and has been related to the dopant distribution obtained from NMR lineshape analysis in a few cases.…”

“…the decrease of luminescence quantum yield for increasing temperatures, 37 is severely worsened for inhomogeneously doped materials due to areas with a locally more elevated doping concentration. 34 Instead of asking for the NMR properties of the visible paramagnetic signals, it may be interesting to ask for the fraction of NMR invisible signal. If a spherical regime around a paramagnetic centre is assumed, from which no NMR signals can be detected, it is possible to relate the signal loss to the size of this sphere of influence which is known under the name blind-sphere 38,39 or wipe-out radius.…”

Direct Evidence of Intervalence Charge-Transfer States of Eu-Doped Luminescent Materials

“…The IVCT states reached vertically are structurally very stressed, hence the corresponding absorption bands are very broad; their calculated peak energies are given in Table 1. The peak energies span wide intervals: 15100- Energy (10 3 cm -1 ) 12 16 20 24 28 Energy ( 20800 cm´1 (CaF 2 ); 18200-23900 cm´1 (SrF 2 ); and 22000-27700 cm´1 (BaF 2 ). The superpositions of all the IVCT transitions lead to the very broad IVCT absorption envelopes plotted in green in Fig.…”

Micro-light-emitting diodes with quantum dots in display technology

“…Further, the narrow linewidths of QDs enable them to form viable active elements of LEDs in high-resolution displays. The optical properties of typical phosphors and QDs are summarised in Table 1 [21][22][23][24][25][26][27][28][29] , showing the superiority of QD materials for next-generation display technology.…”