EXPRESS: Dye-Dependent Fluorescence Enhancement in ZnO Nanorod-Polymer Films: Excitation and Emission Contributions...

The interaction of light with nanostructured materials enables control over emission processes and photonic environments at the nanoscale. In this work, we investigate the dye-dependent fluorescence modulation induced by dielectric nanostructures using thin films based on ZnO nanorods (NRs). Two representative fluorophores with emission spanning the visible spectral range, Nile Blue and Coumarin 6, were embedded in polyvinylpyrrolidone (PVP) spacer layers of varying thicknesses and spin-coated onto ZnO NRs films prepared by a drop-coating protocol. Steady-state and time-resolved fluorescence spectroscopy were employed to quantify modifications in emission intensity and excited state dynamics arising from the interaction between the fluorophores and the nanostructured substrate. Both dyes exhibited pronounced fluorescence enhancement in the presence of the ZnO NRs, reaching up to ∼ 15× for Nile Blue and ∼ 9× for Coumarin 6 relative to glass substrates, dependent on the PVP layer thickness. Time-correlated single photon counting measurements revealed modest lifetime reductions upon deposition on ZnO, indicating that fluorescence enhancement is not dominated by strong radiative-rate modification or interfacial charge transfer. The results suggest that the observed emission amplification arises from a combination of excitation-field enhancement and improved emission extraction mediated by wavelength-dependent scattering from the ZnO NRs. The excitation-wavelength dependence of the enhancement is consistent with scattering-assisted redistribution of the excitation light, which may increase the effective excitation experienced by the fluorophores. Additionally, numerical aperture-dependent fluorescence measurements and polarization-resolved anisotropy investigations are consistent with scattering-assisted redistribution and extraction of emitter photons that would otherwise remain partially trapped within the PVP film by total internal reflection.