Optimization of Coating Strategies and Evaluating Performance of NaGdF₄:Yb,Er Upconversion Films on Silicon Nitride Membrane for Electron Beam Excitation-Assisted Optical Microscope toward High-Resolution Bioimaging Applications

Electron Beam Excitation-Assisted (EXA) optical microscope is a revolutionary microscopy technique that has been recently developed for achieving high-resolution biological imaging. In the EXA optical microscope, the EXA cathodoluminescence, facilitated by the nanoparticle layer, acts as a bridge film, enabling interaction between the vacuum and the atmosphere. Many costly techniques and tough materials were employed to fabricate a cathodoluminescent Si3N4 membrane film. Rare-earth ion (RE3+)-doped fluoride upconversion nanoparticles (UCNPs) coated Si3N4 membrane was fabricated for achieving high spatial resolution with a tunable particle average size of 11.5 nm. One-pot synthesis of fluorite UCNPs (NaGdF4) doped with RE3+ (Yb3+:Er3+) was achieved via a facile thermal decomposition approach. The crystal structure and morphology of the synthesized UCNPs were ascertained using XRD and TEM, respectively. The XRD confirmed the formation of a cubic-phase NaGdF4 crystal structure, while TEM analysis infers a predominantly spherical morphology with a smoother surface. XPS analysis was performed on the synthesized UCNP nanoparticles to confirm the presence of elements and dopant ions. The UCNPs were fabricated on Si3N4 substrates to foam a film using spin-coating, drop-coating, and a robotic hydrographic dip coating (HDC) technique. Fabricated UCNP/Si3N4 films were analyzed with SEM and cathodoluminescence (CL) analysis to understand the thin-film morphology and luminescence. Among all these films, which were generated using different techniques, HDC-developed films showed relatively isolated nanostructures compared to other methods. The CL emission of HDC-developed films exhibits broader emission with strong optical responses from isolated nanostructures, which promises better utility of these films for developing EXA applications for hydrated biological sample imaging.