Thermal quenching of luminescence

The quantum efficiency of the band tail luminescence is largest at low temperature, low excitation intensity, and in samples of low defect density. Other conditions cause competing non-radiative processes which quench the luminescence intensity. Direct recombination to defect states in samples of high defect density is discussed in Section 8.4.1. The other main non-radiative mechanism is thermal 

The competition between the radiative and non-radiative rates, and Pnr, defines the luminescence quantum efficiency. 

Non-radiative quenching therefore usually reduces both the intensity and the decay time. These relations apply only when the different recombination processes compete directly. The actual processes are usually more complicated, with some competing and non-competing paths and a distribution of rates. 

The conventional theory of thermal quenching by excitation of a carrier out of a shallow trap predicts a thermally activated process. No single activation energy is observed in a-Si H, but it is found that the luminescence efficiency, follows the relation (Collins, Paesler and Paul 1980), 

18 shows the thermal quenching of a-Sij. C iH alloys for different x (Liedke et al. 1989). The parameter 7 is about 25 K in a-Si H and increases with the carbon concentration. It should not be too surprising that the temperature dependence originates from the exponential distribution of band tail states, or that the derivation of Eq. (8.44) follows the multiple trapping approach. Thermal quenching 

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A more difficult problem to address is that of the thermal quenching of luminescence. Rathei than recombining radiatively across the band gap, free excitons within a quantum dot are readily bounc to lattice phonons at temperatures exceeding lOO C. " An example of this phonon-driven effect car be found in a Sandia National Lab study on the thermal stability of CdS and CdSe quantum dots encapsulated in both epoxy and silicon as is routinely done for plication to a LED device. " In that study, the luminescence from CdS quantum dots decreased by 50% at 75C. Similar thermal quenching... 

The average of 10 s gives /23, so that the measured value of 20-25 K is of the correct magnitude for the band tail slope in a-Si H. The larger in the alloys is because they have broader band tails. The derivation of Eq. (8.46) makes several approximations, of which the most severe is the neglect of the distribution of lifetimes, so that one should not expect an accurate fit. Also it is tacitly assumed that only one type of carrier is thermally excited. The non-radiative process obviously involves both carriers. The question of what is the rate-limiting step in the process is complicated and, in fact, the thermal quenching of the luminescence depends on the defect density and on the excitation intensity. 

Thermal Quenching of the Uranate Luminescence of Ordered Perovskites A2BW06-U + ... 

Fortunately, it is not necessary to include all the excited states in the model describing the thermal quenching of the uranate luminescence. In order to explain the... 

Examples of europium complexes 30-32 that have been applied in temperature sensors or dual pressure- and temperature-sensitive paints are listed in Table 5. The respective ligand structures are shown in Scheme 7. The temperature dependency is quantified as average luminescence intensity temperature coefficient 7 [%/°C]. Usually, it is determined in a temperature range from 1 to 40 or 50°C. These examples exceed the intensity temperatiue coefficients of other established temperature sensitive probes such as ruthenium(II)-tris-(l,10-phenanthroline) [121]. Generally, the lifetime temperature coefficients are significantly lower. This indicates that thermal quenching of the triplet state of the antenna chromophore plays an important role. Due to the narrow emission band of europium complexes at 615 nm even triple sensors for temperature, oxygen, and pH are achievable [122]. 

Dorenbos P (2005) Thermal quenching of Eu " 5d-4/ luminescence in inorganic compounds. J Phys Condens Matira 17 8103... 

Fig. 14.27 Scheme for the anti-thermal-quenching of Bi luminescence in LUVO4 Bi. The white solid circle stands for hole the wine solid circle for electron and the depth of trap A is 86 °C. Paths 1 though 3 denote the nonradiative relaxation processes. Reproduced from Ref. [36] by permission of John Wiley Sons Ltd. 

Fig. 34.10. Thermal quenching of the luminescence. The relative intensity of the luminescence from a number of phosphors, obtained by excitation with 254 nm radiation, plotted as a function of absolute temperature (after Blasse and Bril, 1970).

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