Farklı Plazmon Geometrilerinin Nadir Toprak Iyonu Takviyeli Metalik Nanopartiküllerin Üst Enerji Dönüşüm Verim Artışına Etkisinin Incelenmesi

Abstract

Rare-earth activated upconverting nanoparticles (RE-UCNPs) have recently come into light for their potential applications such as bioimaging and photovoltaics due to their excellent chemical and spectral properties. However, RE-UCNPs usually suffer from low upconversion emission efficiency owing to the small absorption cross sections induced by the forbidden transitions between 4f orbitals of the REs. Therefore, the upconverting materials synthesized till today remain too inefficient for viable implementation. For instance, the quantum yield of the extensively studied ß-NaYF4 nano crystals codoped with Yb3+ and Er3+ is usually below 1%. Researchers have been diligently working to develop new techniques and methodologies to increase the upconversion efficiency. Although significant progress has been made, the research is still ongoing to develop better strategies. Plasmon resonances can be used to achieve high upconversion efficiency by enhancing the incident electromagnetic field intensity and the radiative emission rates. Accordingly, upconversion efficiency can be enhanced by the effects and changes that occur when plasmonic materials are brought within the close proximity of the UCNPs. The intensity and peak location of the absorption and scattering spectra which also assists excitation and emission spectra are highly dependent on the composition, size and geometry of plasmonic nanoparticles. Until today, luminescence properties of various RE-UCNPs have been studied, however their luminescence efficiency did not meet the expectations due to the low absorption and excitation cross section of the rare-earth ions and non-radiative decay. Therefore, in several studies effects of plasmon enhancement on luminescence efficiency of RE-UCNPS have been investigated. Different plasmonic structures change the absorption spectrum and the radioactive decay rate affecting the emission intensity of RE-UCNPs. However, to the best of our knowledge, the effect of plasmonic structure geometry on upconversion efficiency is uncertain and hence need to be clarified. Therefore, in this project, an experimental analysis of plasmon enhanced upconversion was realized by investigating the effect of different geometrical gold plasmonic structures with different sizes and mutual separations on upconversion efficiency of RE-UCNPs for high luminescence intensity upconversion materials. In this project, the co-doped ß-NaYF4:17%Yb3+/3%Er3+ RE-UCNPs were synthesized using hydro(solvo)-thermal route. Au nanoparticles were formed via electron beam lithography in four different geometries (rectangular, elliptical, triangular and spherical disks) with different sizes (1000-50nm) and mutual separations (250-50nm). ß-NaYF4:17%Yb3+/3%Er3+ solutions were then spin-coated on the substrate with prefabricated gold plasmonic structures having different geometries to obtain plasmon enhanced RE-UCNPs. Structural characterizations of plasmon enhanced RE-UCNPs were achieved by FTIR spectroscopy and Raman spectroscopy. Size and geometry of plasmon enhanced RE-UCNPs were characterized using SEM. Spectroscopic investigations of plasmon enhanced RE-UCNPs with emphasis on their optical absorption, excitation and emission were realized. The UC emission spectra, effect of the addition and the geometry of plasmonic structures on the luminesence efficiency and color quality parameters were evaluated. The overall luminesence efficiency enhancement rates were analyzed. These findings can benefit not only the use of upconversion luminescence for renewable energy and biological applications, but can also have important implications for improving other fluorescence and excitonic systems like organic and other excitonic solar cells.