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Journal of Physical Studies 23(3), Article 3705 [5 pages] (2019)
DOI: https://doi.org/10.30970/jps.23.3705 LUMINESCENCE OF SrF2-Ce NANOPARTICLES UNDER OPTICAL AND X-RAY EXCITATIONT. Demkiv, O. Halyatkin, M. Chylii, T. Malyi, V. Vistovsky, L. Bulyk, L. Demkiv, A. Voloshinovskii
Ivan Franko National University of Lviv, |
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In order to identify the mechanisms of the scintillation process under the size confinement, the study of luminescence parameters under the optical and X-ray excitation of SrF$_2$:Ce nanoparticles obtained by the chemical precipitation method has been performed.
The main features of the X-ray-excited and photoluminescence spectra of SrF$_2$-Ce nanoparticles of 82 nm and 65 nm sizes well match with the luminescence parameters of bulk materials and reveal the characteristic doublet emission band of Ce$^{3+}$ ions with maximums at 310 and 329 nm, which correspond to the electronic transitions from $5d$-levels to the $4f$ ground state of cerium ion ( $^2F_{5/2}$ and $^2F_{7/2}$) split by spin-orbital interaction. In the luminescence excitation spectra in the matrix transparency range between 3.9 and 7.1 eV, the excitation bands corresponding to intracenter $4f$-$5d$-absorption transitions of cerium ions are observed. The observed drop at 10.7 eV in the range of exciton reflection maximum is associated with the near-surface radiation-free loss of excitation energy. The structure of luminescence excitation spectra in the band-to-band region (12-20 eV) corresponds to the combined density of states of valence band and conduction band. The excitation threshold at 21.9 eV is related to the multiplication of electronic excitations in particular with the creation of secondary electrons.
The intensity and time parameters of the luminescence, the features of the luminescence excitation spectra essentially depend on the nanoparticle sizes. The decrease of the luminescence intensity of cerium ions in the case of intracenter excitation is due to the resonant transfer of excitation energy to surface defects. The decrease of the luminescence decay time constant becomes significant for nanoparticles of sizes smaller than 20 nm, which indicates the dominant role of surface defects in this process. In the case of recombination luminescence (\emph{h}$\nu > E_g$), an additional quenching channel appears caused by the escape of electrons from the nanoparticle volume when the thermalization length exceeds the nanoparticle size
PACS number(s): 71.15.Mb, 71.20.-b, 78.55.Hx, 78.67.Bf, 78.70.Ps