Tuning the Energy Transfer Efficiency between Ce 3+ and Ln 3+ Ions (Ln=Tm, Sm, Tb, Dy) by Controlling the Crystal Phase of NaYF 4 Nanocrystals

NaYF 4 is a superior host matrix to study the luminescence properties of lanthanide (Ln 3+ ) ions. Ln 3+ ions in hexagonal‐phase NaYF 4 (β‐phase) nanocrystals (NCs) exhibit strong luminescence via an upconversion process compared to cubic NaYF 4 (α‐phase) NCs. However, in Ce 3+ /Ln 3+ ‐doped NaYF 4 NCs (Ln=Tm, Tb, Sm, Dy) the α‐phase NaYF 4 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 3+ ions emission band with excited energy levels of those Ln 3+ ions in α‐phase compared to β‐phase NCs. This difference is attributed to different crystal‐field splitting of Ce 3+ 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 4 NCs with respect to their barycenter. The enhanced luminescence from α‐phase NaYF 4 NCs is advantageous as they are prepared at a relatively lower temperature and shorter reaction times compared to β‐NaYF 4 NCs.