Effects of size and surface on luminescence properties of submicron upconversion NaYF₄:Yb,Er particles

Bulk NaYF₄:Yb,Er particles (∼1.4 μm particle size) were synthesized using a hydrothermal method. As-synthesized particles were subsequently ball milled to three average particle sizes, namely, ∼260 nm, 160 nm, and 100 nm. The x-ray diffraction pattern showed an hcp phase for as-synthesized and ball-milled particles with a predominant (100) peak. Room temperature emission spectra showed no size dependent peak shifts or peak broadening. The intensities of both green and red emissions decreased with increasing milling time. Segregation of Er ions was detected on the surfaces of milled particle that reduced the sensitizer-activator transition probability, resulting in decreased emission intensities. The green-to-red emission ratio was correlated to the surface enrichment of Er, which affected the cross-relaxation of luminescence dynamics.