Layer-by-layer assembly of freestanding thin films with homogeneously distributed upconversion nanocrystals

We report a facile and highly-controlled approach to fabricating freestanding upconversion multilayer thin films containing homogeneously distributed lanthanide-doped nanocrystals. Citrate-coated NaYF(4):17%Yb, 3%Er nanocrystals were synthesized using a single-phase high-boiling-point-solvent method, followed by ligand exchange. These hydrophilic upconversion nanocrystals were dispersed in freestanding multilayer polyelectrolyte thin films by layer-by-layer assembly over a sacrificial layer. We found that the nanocomposite multilayer thin films possess outstanding mechanical stability and exhibit NIR-to-visible upconversion luminescence. The effect of the hydrophilic ligand exchange on the upconversion properties of these nanocrystals was explored by characterizing the time evolution of upconversion emission following pulsed NIR excitation. It is found that the ligand-exchange process modestly reduces the intrinsic upconversion efficiency of the nanocrystals relative to the as-synthesized oleic acid coated product. Thin films with NIR-to-visible upconversion properties may be suitable for a variety of optical-device and sensing applications.