The optical properties of five different nanocrystalline Y2O3:Er3+, Yb3+ samples are presented and discussed. Green and red emission was observed following excitation with 488 nm and attributed to 2H11/2, 4S3/2→4I15/2, and 4F9/2→4I15/2 transitions, respectively. Striking red enhancement was observed in the upconversion spectra when exciting the Y2O3:Er3+, Yb3+ samples with 978 nm, and it became more pronounced with an increase in Yb3+ concentration. A cross relaxation mechanism (4F7/2→4F9/2 and 4F9/2←4I11/2) was responsible for directly populating the 4F9/2 state but did not explain the difference in the magnitude of red enhancement between identically doped bulk and nanocrystalline Y2O3:Er3+, Yb3+ samples. The 4F9/2 level was populated via a nonresonant mechanism that involved the 4F9/2←4I13/2 transition that is more prevalent in the nanocrystals, which is due to the high energy phonons inherent in this type of material. In nanocrystalline Y2O3:Er3+, Yb3+, we observe a change in the upconversion mechanism responsible for populating the 4S3/2 state, from a two-photon to a three-photon process with an increase in Yb3+ concentration. An explanation to account for this behavior is presented.

Highly dispersible Tm3+/Yb3+‐doped LiYF4 nanocrystals were synthesized using a thermal decomposition method. Upon excitation with a NIR diode laser (980 nm), the dilute dispersion of the nanocrystals exhibits several strong emissions in regions spanning the deep‐UV to NIR, all originating from a single dopant/sensitizer (Tm3+/Yb3+) combination. The material is envisioned to have potential interests in anti‐counterfeiting, biomedicine and solution‐based scintillation applications.

In this study, we report on the remarkable two-photon excited fluorescence efficiency in the "biological window" of CaF(2):Tm(3+),Yb(3+) nanoparticles. On the basis of the strong Tm(3+) ion emission (at around 800 nm), tissue penetration depths as large as 2 mm have been demonstrated, which are more than 4 times those achievable based on the visible emissions in comparable CaF(2):Er(3+),Yb(3+) nanoparticles. The outstanding penetration depth, together with the fluorescence thermal sensitivity demonstrated here, makes CaF(2):Tm(3+),Yb(3+) nanoparticles ideal candidates as multifunctional nanoprobes for high contrast and highly penetrating in vivo fluorescence imaging applications.

The luminescent properties of 1 mol % Eu3+-doped cubic Lu2O3 nanocrystals prepared by a combustion synthesis route were investigated. The visible emission spectrum of the europium doped Lu2O3 nanocrystals indicate that the structural environment surrounding the dopant Eu3+ ion is distorted when compared to a bulk sample with a micrometer particle size. From the resulting emission spectra, the Ω2 and Ω4 Judd-Ofelt intensity parameters were calculated. The lifetimes of the 5D0 excited state for both the C2 and C3i sites were found to be nearly double that found for a similarly doped sample with larger particle size (bulk sample). This behavior is attributed to a change in the refractive index of the nanocrystalline material that in turn modifies the oscillator strength of the 4f ↔ 4f transitions.