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 2H11/2,4S3/24I15/2, and 4F9/24I15/2 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 (4F7/24F9/2 and 4F9/24I11/2) 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 4F9/24I13/2 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.

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