Photoluminescence and Scintillation Properties of Pb 2+ Based Quantum Dots in CsCI Host Crystal
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ABSTRACT The spectral and kinetic properties of photoluminescence of Pb 2+ aggregated phase in CsC1 host, together with scintillation characteristics of CsCI:Pb crystals, are reported in the 10 - 300 K temperature range. Absorption, steady-state excitation and emission spectra of Pb2+ phase in CsCI host are similar to those of CsPbCl 3 bulk crystal (emission peak at 419 nm at 10 K). The decay of the 421 nm luminescence of the Pb2 + phase in CsCl shows single exponential behavior with extremely short decay time of 40 ps at 421 nm and 10 K,which is considerably shorter than the decay times found in the decay of CsPbCl3 bulk emission (0.45, 2.8 and 12 ns at 418 nm and 10 K). In the scintillation decay of CsCI:Pb, two components with 0.95-1 ns and 2-3 ns decay times were found and no slower component is present at room temperature.
INTRODUCTION Increased interest in finding new materials for fast scintillators has appeared in recent years in connection with the demands of high energy and nuclear physics, and industrial and nuclear medicine applications'. When the scintillation is based on the radiative transition in the outer electron shell of an emission centre, created by doped or intrinsic ion in the crystal or glass host, the radiative decay rate is limited by the excited - ground state transition probability of the emission centre. For the fastest electrical dipole allowed transitions it is typically a few ns. However, as the characteristics of the excited state levels of most luminescent ions are influenced by admixture of forbidden transitions as well, the radiative lifetimes observed are e.g. 10 - 30 ns for YAP:Ce 3 1 or CeF 3 scintillators or several hundreds ns for T1V in NaI(CsI):TI or Bi 3+ in BGO. Nonradiative quenching can obviously shorten substantially the observed decay time of luminescence and scintillation (mean scintillation decay time of about 10 ns is observed in PbWO 4 at 295 K), but the intensity of luminescence and the light yield of such systems are becoming weaker, which is a serious disadvantage for most of the applications. There is still another possibility to obtain very fast emission centres in the host crystal. Picosecond decay time components (10 - 1000 ps) are observed in the emission of very small semiconductor crystallites with typical dimensions of several units or a few tens of nanometers (called quantum dots QD) embedded in glass or crystal matrix, e. g. CdS or CdSe studied mostly in glass or CuCI in NaCI (KCI) crystal matrixes. The observed fast decay component is explained by microscopic excitonic superradiance effect in QD and is the direct consequence of spatial confinement of coherent exciton motion in QD2 . It is the aim of this contribution to present a new system showing exciton superradiance effect, 155 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
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