Tuning the Energy Transfer Efficiency between Ce and Ln Ions (Ln=Tm, Sm, Tb, Dy) by Controlling the Crystal Phase of NaYF Nanocrystals.

NaYF is a superior host matrix to study the luminescence properties of lanthanide (Ln ) ions. Ln ions in hexagonal-phase NaYF (β-phase) nanocrystals (NCs) exhibit strong luminescence via an upconversion process compared to cubic NaYF (α-phase) NCs. However, in Ce /Ln -doped NaYF NCs (Ln=Tm, Tb, Sm, Dy) the α-phase NaYF NCs shows strong luminescence compared to their counterpart β-phase NCs despite the latter being much larger in size. This is attributed to comparatively large overlap between Ce ions emission band with excited energy levels of those Ln ions in α-phase compared to β-phase NCs. This difference is attributed to different crystal-field splitting of Ce ions 4f-5d band in different crystal environments of the α-phase (cubic crystal field environment) and β-phase (trigonal prismatic with equatorials crystal field environment) NaYF NCs with respect to their barycenter. The enhanced luminescence from α-phase NaYF NCs is advantageous as they are prepared at a relatively lower temperature and shorter reaction times compared to β-NaYF NCs.