  Two-photon polymerization (2PP) 3D printing is a well-known technique for
fabricating passive micro/nanoscale structures, such as microlenses and
inversely designed polarization splitters. The integration of light emitting
nanoparticle (NP) dopants, such as quantum dots (QDs) and rare-earth doped
nanoparticles (RENPs), into a polymer resist would enable 3D printing of active
polymer micro-photonic devices, including sensors, lasers, and solid-state
displays. Many NPs are stabilized with oleic acid ligands to prevent
degradation, but oleate-capped NPs (oc-NPs) tend to agglomerate in nonpolar
media despite the hydrophobicity of the ligand. This results in an uneven
distribution of NPs in polymers and increased optical extinction properties. In
this work, we propose a general approach for dispersing various oc-NPs in
commercial 3D printable polymers. We achieve controlled growth of small carbon
chains around the oc-NPs by functionalizing the NPs with methyl-methacrylate
monomers. The proposed approach is validated on RENPs (~65 nm) and CdSe/ZnS
quantum dots (~12 nm) using different commercial polymer resists (IP-Dip and
IP-Visio). Dispersions of functionalized NPs (f-NPs) have improved NP density
by an order of magnitude and are shown to be stable for several weeks with
minimal impact on printing quality. Our approach is generalizable to a variety
of oc-NPs and ultimately leads to higher quality polymer-based optical and
electronic devices.
