Eduardo D. MartÃnez
Universidade Estadual de Campinas (UNICAMP), Brazil
Title: Electrothermal control of upconversion emissions in rare-earth doped NaY(Gd)F4 nanoparticles by coupling with silver nanowires networks: A versatile platform for in situ thermal experiments
Biography
Biography: Eduardo D. MartÃnez
Abstract
Photon upconversion (UC) is a non-linear optical anti-Stokes process by which low energy photons stimulate the emission of higher energy photons. The hexagonal phase (P63/m) b-NaYF4 doped with rare-earths elements stands as one of the most efficient UC materials, finding applications in bioimaging, solar-cells and displays. However, many of these applications operate in fluctuating temperature conditions affecting each emission line of the UC spectra in a different manner. In this work, we develop a functional device to study in situ the thermal effects on UC nanoparticles (UCNPs) of different size and composition by using a percolating network of silver nanowires (AgNWs) as a highly transparent heating element. The electrical power dissipated by Joule effect allows for the electrothermal control. This device was successfully applied to characterize the thermal dependence of UC in large (>100 nm) b-NaYF4:Yb:Er(Tm,Ce-Ho) and small (<20 nm) core-shell b-NaGd4:Yb:Er(Tm,Ce-Ho)@NaYF4 UCNPs in the 20 °C-140 °C interval. Just the presence of AgNWs was enough to produce an enhancement of 20-30% in the intensity of UC emissions. We find that an increment in temperature can enhance or partially quench the emission lines selectively on each UCNPs system. The most temperature-sensitive case was that of Er doped UCNPs, in which the optical transitions 2H11/2→4H15/2 (H transition) and 4S3/2→4H15/2 (S transition), were found to reversible change in a different manner. For the case of bigger NaYF4 UCNPs, the S transition is quenched while the H transition was barely constant. For the small-sized UCNPs, the S transition remained unaffected while the H transition was sturdily enhanced. Time-resolved spectroscopy at different temperatures revealed further insights on the mechanisms involved. A rate-equation model was proposed to unravel the underlying mechanism. Finally, we take advantage of the electrothermal device to analyze in-situ other relevant chemical processes.