Biophotonic Crystals With Tunable Bandgaps And Their Applications In Upconversion Luminescent Enhancement

Lanthanide-doped upconversion fluorescent materials have been widely used in light-emitting devices,fluorescence sensors and optical encryption for their unique properties,such as anti-Stokes effect,stable chemical properties,and narrow luminous bandwidth.However,the extremely low luminescent efficiency?less than 1%?of the upconversion nanoparticles?UCNPs?limits their direct application in industrial products.Therefore,how to improve the upconversion luminescent efficiency of UCNPs and enhance their fluorescence intensities is an important research topic of nanomaterials and optoelectronic displays,which has important scientific research values and industrial application prospects.Biophotonic crystals refer to periodical dielectric nanostructures with photonic bandgaps,widely existing in natural organisms,such as butterfly wings,chameleon skins,and peacock feathers.The unique properties of biophotonic crystals,e.g.photonic bandgaps,locally enhanced electromagnetic fields and photon density of states,make them potential applications in biosensors,biomimetic materials,and luminescence enhancement.In particular,in comparison to artificial photonic crystals,biophotonic crystals have the merits of easy access of large-area photonic crystal structures,lower prices,and various types of nanostructures,and hence have wider applications and more important research significance.Therefore,this paper mainly focuses on employing biophotonic crystal structures cut from butterfly-wings to selectively control the luminescent channels of UCNPs.Here,we first tuned the photon band gap of butterfly wings by controlling the amounts of the pigments of butterfly wings with the cleaning method.A photonic-crystal/UCNP hybrid nanostructures is then fabricated with assembling the UCNPs(NaYF₄:Yb³⁺,Er³⁺)onto the biophotonic crystals.We investigated the relationship between the upcoversion luminescence and the tunable bandgap of biophotonic crystals.The results show that the green and red emission luminescent channels have been respectively enhanced when the bandgap of biophotonic crystal overlaps with the corresponding emission channels,resulting in different luminescent colors.Importantly,the upconversion luminescent intensity is maximally increased upto 73 times with respect to the same density of UCNPs on glass substrate.Next,we established a simulation model to investigate the interactions between the biophotonic crystals and UCNPs.We explored the effects of photonic bandgap,near-field electromagnetic-field,and photonic state density on the radiative decay rate of UCNPs.Finally,we measured the time-resolved fluorescence spectra to prove that the photonic bandgap and the enhanced near-field electromagnetic field are the underlying physics for the enhanced upconversion luminescence.Overall,we theoretically and experimentally prove that the enhanced local density of photonic states and the perfect absorption effect of pigments are responsible for the improved upconversion efficiency and the different luminescent colors.This thesis demonstrates the interaction mechanism between biophotonic crystals and upconversion luminescent nanoparticles,provides new ideas for realizing dynamic light-emitting devices,and are beneficial to biological nanophotonics.