Relaxation of electronic excitations in CaF2 nanoparticles

Abstract

The luminescence properties of CaF2 nanoparticles with various sizes (20–140 nm) are studied upon the excitation by VUV and x-ray quanta in order to reveal the influence of ratio of mean free path and thermalization length of charge carriers and nanoparticle size on the self-trapped exciton luminescence. The luminescence intensity for exciting quantum energies corresponding to optical creation of exciton and to the range of electronic excitation multiplication is not so sensitive to nanoparticle size as for quanta with energy of Eg < hν < 2Eg. The dependences of luminescence intensity on nanoparticle size at the excitation by quanta of various energies are discussed in terms of electron-phonon and electron-electron scattering lengths and energy losses on surface defects.

Defect luminescence in CaF2 nanoparticles

“…This limiting size of nanoparticles coincides with the expected mean free path of the primary photoelectron obtained in Ref. [2] using an approximate theoretical evaluation. The size of CaF 2 nanoparticles increased to over 50 nm, leads to some increase the light output with a tendency to its subsequent saturation.…”

“…with the self-trapped triplet exciton emission. Thus, in [2] there were not observed any processes of radiation defect formation in CaF 2 nanoparticles and their potential impact on the spectral characteristics of luminescence.…”

“…[2] that for intrinsic luminescence of CaF 2 nanoparticles, excited by optical photons with hν4 E g energy, as well as for pulsed X-ray induced excitation, the luminescence light output dramatically decreases for nanoparticles smaller than 50 nm. This limiting size of nanoparticles coincides with the expected mean free path of the primary photoelectron obtained in Ref.…”

“…These phosphors are capable of combining high light output and absorption capacity of the ionizing radiation intrinsic to some inorganic materials with short luminescence fading times of organic molecules [1,2]. Relative inexpensiveness and ease of synthesis of nanophosphors compared to single crystalline fluorescent materials are their obvious advantages.…”

“…It was found that for high energy quanta of the luminescence excitation, in particular, for X-ray photons, a decisive factor is the consideration of the mean free path for the primary photoelectrons during creation of the secondary electronic excitations within the matrix, which should not exceed the size of nanoparticles [2].…”