Effects of size and surface on optical properties of NaYF4:Yb,Er Nanoparticles

Abstract

The use of upconversion (UC) materials in bioapplications is hindered by decrease of emission intensity when the particles are reduced to nanoscale dimension. This thesis focused on the effects of size and surface on optical properties of hcp NaYF4:Yb,Er particles, which is one of the most efficient near infrared-to-visible UC materials. Room temperature emission spectra showed that for hcp NaYF4:Yb,Er nanoparticles with sizes from ~ 11 to 110 nm, the green intensity increased by 69 times and the red intensity increased by 145 times. Decrease of emission intensities with decreasing size was qualitatively attributed to the increased total nonradiative transition rates, for which the increase of ¿OH quenching rates was responsible as revealed by calculations. Via Raman spectrum identification, red shift of the effective phonon frequencies of hcp NaYF4:Yb,Er nanoparticles with increasing size were associated with the size-dependent expansion of lattice parameter. The experimental evidence of Yb3+ surface segregation was presented. Homogeneous Re3+ distribution in UC nanoparticle was found to be invalid.