You see, the structure of lanthanide ions shouldn't get changed in contrast to gap energy of main phase. The latter doped by lanthanide ions excite light of certain frequency caused by different dopant. The structure of host ion lattice plays the key role in the process of upconversion. Optical transitions (not all of them, but certain ones) of lanthanides are very sensitive to symmetry & coordination of host lattice. They are described by Tanabe-Sugano diagrams, for instance. Nobody doubts that character of any of transition strongly depends on symmetry of coordination surrounding.

As for size-depence, the energy inner distribution inside lathanides lattices doesn't change unlike their surface concentration.

The explanation has briefly been reviewed. To draw to a close I'd like to attach an article where your subject is discussed from different points. I hope, you've already read before posting this question :)

Please, give me your suggestions. This subject is of great interest for me as well.

Kind regards,

MN 

Thanks MN for the paper ..I have not read it yet. I'll read it for sure.

I am talking about the quantum confinement effect. Why is it not applicable here?? 

I read the paper but did not find any reasons as such.

Lanthanide's 4f-states are almost fully localized states in the lanthanide ion, unlike for example perovskites quantum dots, Silicon quantum dots, or Au quantum dots. Thus, they are not subject to confinement effects of the host.