Quantitative evaluation of various NIR-to-red upconversion mechanisms in NaYF_(4):20%Yb^(3+),2%Er^(3+)nanoparticles

In the most popular NaYF_(4):Yb/Er upconversion nanoparticles(UCNPs),the red emission is attributed to four potential excitation routes encompassing two-and threephoton excitation processes.Consequently,this red emission typically exhibits a super-quadratic dependency on near-infrared(NIR)excitation intensity,with the nonlinear order n being dependent on the individual contributions(Cis)of these four excitation routes.Notably,the Cis values are not constant but significantly impacted by the surface quenching of the UCNPs,leading to a decrease in the n value.However,a quantitative assessment of these variable Cis has not been undertaken,hindering a comprehensive understanding of the quenching effect on the UC mechanisms.In this work,we prepare four NaYF_(4):Yb/Er nanocrystal samples with varying degrees of surface quenching,achieving through the modulation of particle size and core-shell structure.We quantitatively evaluate the Cis values and identify the primary excitation route responsible for the red emission.Our results reveal that the contribution of three-photon excitation increases from 7%in the 30 nm bare core to 74%in 90 nm core with shell at an excitation intensity of 200 mW cm^(−2).This observation highlights the impact of surface quenching suppression.Furthermore,we discover that the quenching effect operates by reducing the lifetimes of the Yb^(3+)2F_(5/2)and Er^(3+)4S3/2 levels,while enhancing the NIR emission intensity ratio of the Er^(3+)4I_(13/2)→4I_(15/2)transition to the Yb^(3+)2F_(5/2)→2F_(7/2)transition.Our findings provide physical insights into the excitation mechanisms underlying the red UC emission in NaYF_(4):Yb/Er UCNPs.

Mechanisms and absolute quantum yield of upconversion luminescence of fluoride phosphors

Mechanisms of upconversion luminescence（UCL） of SrF2：Er phosphors corresponding to the 4G11∕2→4I15∕2,2H9∕2→4I15∕2,4F5∕2→4I15∕2,4F7∕2→4I15∕2,2H11∕2→4I15∕2,4S3∕2→4I15∕2,4F9∕2→4I15∕2, and 4I9∕2→4I15∕2 transitions upon excitation of the 4I11∕2 level of Er3＋ions were investigated. Energy transfer upconversion processes are responsible for the populating of the 2H9∕2,2H11∕2,4S3∕2, and 4F9∕2 levels. Cooperative process is the dominant mechanism of luminescence from 4S3∕2 and 4F9∕2 levels for SrF2：Er with high concentrations of Er3＋ions. The UCL from 4G11∕2 and 4F5∕2 is explained by excited-state absorption. Cross-relaxation processes take part in the population of 4F9∕2 and 4I9∕2 levels. For quantifying material performance, the Er3＋-concentration dependence of UCL and the absolute quantum yields of SrF2：Er were studied. The most intensive visible luminescence was obtained for SrF2：Er（14.2%） with 0.28% maximum quantum yield.

Investigation of the mechanism of upconversion luminescence in Er^(3+)/Yb^(3+) co-doped Bi_2Ti_2O_7 inverse opal

Three-dimensional-ordered Yb/Er co-doped Bi2Ti207 inverse opal, powder, and disordered reference sam- ples are prepared and their upconversion （UC） emission properties and mechanisms are investigated. Sig- nificant suppression of UC emission is detected when the photonic band-gaps overlap with Er3＋ UC green emission bands. Interestingly, green and red UC emissions follow a two-photon process in the powder sample but a three-photon one in the inverse opal.