Thermoluminescence properties of the Eu2+-, R3+-doped calcium aluminate materials, CaAl2O4:Eu2+,R3+, were studied above room temperature. The trap depths were estimated with the aid of the preheating and initial rise methods. The seemingly simple glow curve of CaAl2O4:Eu2+ peaking at ca. 80 degrees C was found to correspond to several traps. The Nd3+ and Tm3+ ions, which enhance most the intensity of the high-temperature TL peaks, form the most suitable traps for intense and long-lasting persistent luminescence, too. The location of the 4f and 5d ground levels of the R3+ and R2+ ions were deduced in relation to the band structure of CaAl2O4. No clear correlation was found between the trap depths and the R3+ or R2+ level locations. The traps may thus involve more complex mechanisms than the simple charge transfer to (or from) the R3+ ions. A new persistent luminescence mechanism presented is based on the photoionization of the electrons from Eu2+ to the conduction band followed by the electron trapping to an oxygen vacancy, which is aggregated with a calcium vacancy and a R3+ ion. The migration of the electron from one trap to another and also to the aggregated R3+ ion forming R2+ (or R3+-e-) is then occurring. The reverse process of a release of the electron from traps to Eu2+ will produce the persistent luminescence. The ability of the R3+ ions to trap electrons is probably based on the different reduction potentials and size of the R3+ ions. Hole trapping to a calcium vacancy and/or the R3+ ion may also occur. The mechanism presented can also explain why Na+, Sm3+, and Yb3+ suppress the persistent luminescence.
The disintegration
of hexagonal NaYF4:Yb3+,Er3+ upconverting
nanoparticles (UCNP) was studied by
incubating various nanoparticle concentrations in aqueous suspensions
over time while monitoring the upconversion emission intensity and
measuring the dissolved particle-constituting ion concentrations.
The results revealed that the ions dissolved into water resulting
apparently in anisotropic structural disintegration of the UCNPs as
observed with transmission electron microscopy. The UCNP disintegration
caused partial loss of active ions Yb3+ and Er3+ from the host matrix and therefore decrease in the upconversion
luminescence intensity. The decrease, however, was strongly dependent
on the UCNP concentration, and dramatic drop in the intensity was
observed especially at diluted nanoparticle suspensions, where the
nanoparticles disintegrated almost completely until the solubility
equilibrium was achieved. At the concentrated suspensions the equilibrium
was achieved already with minimal disintegration, and the change in
the luminescence intensity was negligible. Further, due to the high
impact of fluoride ions on the solubility equilibrium the disintegration
of the UCNPs could be prevented by adding fluoride to the suspension.
The reported disintegration of NaYF4:Yb3+,Er3+ nanoparticles in diluted aqueous suspensions should be taken
into consideration when the UCNPs are used at low concentrations in
analytical applications and in guiding the design of improved shell-stabilized
UCNPs.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.