Novel optical signatures of sub-3 nm rare earth sesquioxide nanocrystals.

Europium and terbium based sesquioxide nanomaterials, known for their characteristic red and green luminescence, respectively, have recently garnered much research attention due to their size-dependent optical properties. Here, we present systematic investigation of the size-dependent optical properties Eu$_{2}$O$_{3}$, Tb$_{2}$O$_{3}$, and Gd$_{2}$O$_{3}$:Eu$^{3+}$ / Tb$^{3+ }$nanocrystals (NCs) in the size range of 1-3 nm in diameter. We observe a new luminescence peak at 620 nm in Eu$_{2}$O$_{3}$ and Gd$_{2}$O$_{3}$:Eu$^{3+}$ NCs, which represents modulation of the $^{7}$F$_{2}$ transition in Eu$^{3+}$ ion. Intensity modulation with respect to the 612 nm is observed as a function of nanocrystal size. For the Tb$_{2}$O$_{3}$ NCs, a new luminescence signature at 548 nm characterizes modulation of the $^{7}$F$_{5}$ transition in Tb$^{3+}$ ion. In addition, we probe the effect of NC size on the luminescence efficiencies of the doped and pure sesquioxide NCs. The concentration quenching effect, which leads to low luminescence efficiencies in bulk, pure sesquioxides, is explored in sub-3 nm sesquioxides.