Three nonequivalent centers of
Cs (A, B, and C) in monoclinic phase and
C2 and
S6 centers in cubic phasewere identified in the Gd
2O
3:Eu
3+ nanocrystals with spectral techniques. Size dependence in the spectraindicated that the excitations from both host and charge-transfer band (CTB) for the
5D
0 7F
2 transition ofEu
3+ ions were nearly equal for a larger size of 135 nm of the cubic phase; however, with decreasing the sizeto or less than 23 nm, the excitations by the CTB dominated. The variation of excitation leading to thesymmetry and energy change in the
C2 and
S6 sites was also observed for larger particle sizes. The Judd-Ofelt intensity parameters
(
= 2, 4) for Gd
2O
3:Eu
3+ nanoparticles were experimentally determined. Theparameters
were found to significantly change with the sizes of Gd
2O
3:Eu
3+ from nanoparticles to bulkmaterial. With decreasing the size from 135 to 15 nm, the
quantum efficiencies for
5D
0 reduced from 23.6%to 4.6% due to the increasing ratio of surface to volume.