Picosecond Absorption Spectroscopy of Excited States in BaBrCl with and without Eu Dopant and Au

Abstract

We measure picosecond absorption spectroscopy on BaBrCl samples that are undoped, doped with europium to make activated scintillators, and codoped with gold (as AuBr3 in the melt) to increase light yield. The time-resolved measurements provide insights on effects of interband excitation in BaBrCl including identification of self-trapped exciton (STE) absorption bands, the mechanism of energy transport from the host to the Eu activator, and effects of Au co-doping. First principles electronic structure calculations of STE relaxations in BaBrCl are presented. BaBrCl:Eu:Au is a beneficiary of two material modifications for improvement of performance that are intensely investigated recently-a mixed-halide solid solution that is further modified by aliovalent co-doping.

Effect of AuBr3 additive on the scintillation properties of BaBr2:Eu and Cs2LiLaBr6:Ce

“…All the grown crystals appear transparent, although with cracks. Similarly to what detected in the case of BaBrCl single crystals grown with AuBr 3 as an additive, 18 the BaBr 2 and the Cs 2 LiLaBr 6 samples show the presence of molecular bromine on the top part of the ampoules prior to their opening. The orange coloration of…”

“…17 This additive strongly improves the scintillation characteristics of BaBrCl:Eu with light yield increases up to 3 times and remarkable reductions in long scintillation tails; these improvements are accompanied by a clear reduction in thermally and optically stimulated features related to halide vacancies. In a later paper 18 we reported picosecond transient absorption measurements which indicate that the addition of AuBr 3 reduces the concentration of native halide vacancies that can capture electrons to form F centers. Moreover, the reduction of halide vacancies and their effect in breaking up electron-hole pairs created by ionizing radiation suppresses the slow self-trapped holes migration and tunneling process involved in the BaBrCl:Eu scintillation to the benefit of prompt formation of self-trapped excitons followed by the picosecond-scale dipole-dipole energy transfer toward Eu 2+ ions.…”

“…AuBr 3 mechanism in the reduction of halide vacancies is not fully clear, however no Au could be detected in the crystals and it clearly segregates as pure gold rejected on top of the boules at the end of the growth process. 18 Therefore, there are clear indications that its effect is related to the low thermal stability of this compound leading to its dissociation in metallic gold and bromine gas. 19,20 The presence of Br 2 gas in the sealed ampoules in which the crystals are grown reduces the probability of having Br evaporation from the melt and thus minimizes the concentration of halide vacancies.…”

Self-trapped excitons in soft semiconductors

“…38,83,130,132,133 Further development in the doped materials indicates that the properties of STEs can be further tuned by following the co-doping approach. 60,[134][135][136][137] Due to the different radii and electronic structure of the dopants, co-doping may induce a relatively larger lattice distortion or lead to the formation of a sub-band. 50,60,[138][139][140] Jin et al fabricated a family of lanthanide (Ln 3+ ) ions doped Bi: Cs 2 AgInCl 6 double perovskite structures, which exhibited broadband PL spectra from the visible to the near-infrared range and high PLQY.…”

Compositionally Disordered Crystalline Compounds for Next Generation of Radiation Detectors

“…However, when mixing light and heavy halides in the lattice, no significant change in the scintillation yield was found [ 158 ]. The positive role of codoping with Au ions has been established [ 159 , 160 ] as well.…”