On The Validity of the Defect- Induced Negative Thermal Quenching of Eu²⁺-Doped Phosphors

Abstract

In this paper, defect-induced negative thermal quenching (NTQ) of Eu2+-doped phosphors is overviewed. NTQ denotes that the integrated emission intensity of a given phosphor increases continuously with increasing temperature up to a certain elevated temperature. The NTQ phenomenon of Eu2+ luminescence was reportedly observed in a broad variety of lattices. The NTQ of these Eu2+-doped phosphors was generally ascribed to thermally stimulated detrapping of the excitation light stored in defects (traps) and subsequent energy transfer from the defects to the Eu2+ 5d levels. Validity of defect- induced NTQ of Eu2+-doped phosphors is assessed and factors that may contribute to the measured emission intensity of a given phosphor at elevated temperatures are discussed. It is suggested that it is debatable whether NTQ could be an intrinsic property of the blue-emitting phosphor Na3Sc2(PO4)3: Eu2+, and whether the emission intensity enhancement with increasing temperature for Eu2+-doped phosphors could be related to energy transfer from defects. The temperature dependence of the measured emission intensity alone seems not to be a good measure for evaluating TQ property of a phosphor, since it is affected by not only the quantum efficiency of a phosphor but also some extrinsic factors at elevated temperatures.

Negative Thermal Quenching of Photoluminescence: An Evaluation from the Macroscopic Viewpoint

“…The key reason for this predicament, in my opinion, is that the NTQ reported in the literature is based largely on the temperature-dependent emission spectral intensity of a specific phosphor. However, the temperature-dependent emission spectral intensity is not a reliable measure for evaluating luminescence TQ [52]. The contribution of the change in geometrical configuration to the emission spectral intensity at elevated temperatures has been ignored.…”

“…However, it is a great challenge to maintain the consistency and stability of the measurement conditions, especially the surface morphology of the sample and geometrical configuration of the spectrofluorometer at different temperatures when measuring the temperature-dependent emission spectral intensity using a conventional spectrofluorometer equipped with a temperature-control accessory. The main reasons are as follows [51,52]:…”

“…The excitation saturation is generally caused by ground-state depletion and associated with the luminescence lifetime of the phosphor [64,71,72]. The luminescence lifetime of Mn 4+ -activated phosphors (on the order of milliseconds at room temperature) is longer than that of Ce 3+ -activated phosphors (in the order of nanoseconds) and Eu 2+ -activated phosphor (in the order of microseconds) [26,52,73]. The experimental results showed that Mn 4+ -activated fluorides had a considerably lower threshold for excitation saturation than that of Eu 2+ and Ce 3+ -activated phosphors in WLEDs [58,74,75].…”

“…The author of this paper also reviewed and commented on the reports about NTQ for Mn 4+ -doped fluoride phosphors and Eu 2+ -doped phosphors. Based on a comprehensive analysis of corresponding experimental data, it was suggested that the mechanisms proposed for NTQ of Na 3 Sc 2 (PO 4 ) 3 :Eu 2+ , caused by the phase transformation details [49] of Mn 4+ -activated fluoride phosphors being due to the increase in phonon numbers at high temperatures [50,51], and of Eu 2+ -activated phosphors stemming from defect-related energy transfer, were unconvincing [52]. It seems problematic for one to attribute NTQ to an intrinsic property of these phosphors.…”

“…It seems problematic for one to attribute NTQ to an intrinsic property of these phosphors. The NTQ phenomenon observed by the researchers is likely a measurement error caused by the volume change arising from the thermal expansion of the phosphor under investigation during the measurement process [49][50][51][52]. The temperature-dependent emission spectral intensity alone seems not to be a reliable measure for evaluating fluorescence TQ, especially for those phosphors having a larger volume change at elevated temperatures due to thermal expansion or phase transition, since emission spectral intensity is affected by not only the quantum efficiency (QE) of a phosphor but also extrinsic factors at elevated temperatures.…”