SYNTHESIS, MORPHOLOGY AND LUMINESCENT PROPERTIES OF RARE EARTH DOPED VISIBLE UP-CONVERSION NANOPHOSPHORS FOR BIO-IMAGING APPLICATIONS

Abstract

RE doped cubic phase KGdF4 up-conversion nanoparticles (UCNPs) have been synthesised by a wet chemical synthesis procedure. The phase confirmation was achieved via XRD analysis. The morphology and size distribution of the UCNPs was analysed through HR-TEM technique. From Debye-Scherrer and HR-TEM calculations, the size of the particles was found out to be in the range of 6-8nm. These morphological characterisations revealed a high degree of crystallinity in the host lattice. EDAX analysis was conducted to confirm the presence and corresponding weight percentages of precursor elements in the host lattice. The as prepared UCNPs exhibit the property of up-conversion as was evident after conducting UC studies on the samples. Under 980nm CW laser excitation, the samples emit strong emission in the visible region and the intensity of which increases with increase in the concentration of sensitizer (Yb3+) ion. In-depth analysis of energy transfer mechanisms in the UCNP lattice under NIR excitation was conducted. Cooperative energy transfer (CET), energy transfer up-conversion (ETU) and excited/ground state absorptions (ESA/GSA) were established as the main UC mechanisms in the KGdF4 lattice depending upon the sensitizeractivator combination. Inokuti-Hirayama (IH) model was also applied to establish the nature of energy transfer between sensitizer and activator ions as dipole-dipole in nature. Decay kinetics revealed high lifetimes for these samples for the visible emission under NIR excitation. EPR studies were conducted to study the effects of paramagnetic gadolinium ion on the lattice symmetry of KGdF4. The calculated g values from the EPR spectra correspond to the “U” spectrum of gadolinium and match well with the reported values. These UCNPs have sizes well within the cellular range. Along with the ability to exhibit upconversion and having high lifetimes, these UCNPs can be easily used as alternatives to xix conventional dyes and quantum dots for bio-imaging and other solid state lighting (SSL)/ wLEDs applications.