Upconverting nanoparticles | EPFL Graph Search

Upconverting nanoparticles (UCNPs) are nanoscale particles (diameter 1–100 nm) that exhibit photon upconversion. In photon upconversion, two or more incident photons of relatively low energy are absorbed and converted into one emitted photon with higher energy. Generally, absorption occurs in the infrared, while emission occurs in the visible or ultraviolet regions of the electromagnetic spectrum. UCNPs are usually composed of rare-earth based lanthanide- or actinide-doped transition metals and are of particular interest for their applications in in vivo bio-imaging, bio-sensing, and nanomedicine because of their highly efficient cellular uptake and high optical penetrating power with little background noise in the deep tissue level. They also have potential applications in photovoltaics and security, such as infrared detection of hazardous materials. Before 1959, the anti-Stokes shift was believed to describe all situations in which emitted photons have higher energies than the

Functionally graded polymer nanocomposites using magnetic fields

Tommaso Nardi

Functionally Graded Materials (FGM) are a class of materials in which the composition and the properties gradually change over the volume. The present PhD project aims to generate a full understanding of a novel synthetic strategy for polymeric FGMs based on the magnetophoretic motion of magnetic nanofillers in a polymer matrix under the application of an external magnetic field gradient. Due to its high rate, costâeffectiveness and lowâtoxicity, UV polymerization was considered as the first curing option for the magnetic nanocomposites. The curability problem deriving from the presence of opaque particles was tackled by studying two different systems based on the same hyperbranched acrylated polymer matrix (HBP): one containing bare Fe3O4 nanoparticles whereas the other loaded with Fe3O4@SiO2 coreâshell nanoparticles. The employment of Fe3O4@SiO2 NPs for the synthesis of magnetic polymeric films showed to be effective towards the inclusion of 10âtimes higher particle volume fractions compared to formulations including bare Fe3O4. By adding Fe3O4@SiO2 nanoparticles to the UVâcurable matrix, the motion of the magnetic responsive filler was induced upon the application of magnetic field gradients generated by permanent magnets. The process was speeded up through the employment of a simple setâup composed of two block magnets in repulsion configuration and through the functionalization of the particle surfaces with an acrylated silane. From the experimental analysis of the photocuring process, the residual stresses arising during the photopolymerization of functionally graded composite coatings based on the UVâcurable HBP matrix and Fe3O4@SiO2 nanoparticles were evaluated with a Finite Element Method approach. These coatings were able to consistently decrease the residual stresses at the coatingâsubstrate interface compared to those encountered either in composites with homogeneous compositions or in the pure polymer, provided that reinforcing fillers were concentrated at the coatingâsubstrate interface. If instead nanoparticles were concentrated at the free surface, graded nanocomposite coatings could reduce by as much as 40% the interfacial stress arising from thermal variations, as demonstrated through numerical simulations. Nanoindentation tests were carried out on the surface of polymer nanocomposites exhibiting either graded or homogeneous distributions of Fe3O4@SiO2 nanoparticles in the UVâcurable matrix. It was experimentally shown how only at relatively small indentation depths, large increases in modulus and hardnesswere obtained for graded composites with respect to their homogeneous counterparts. Through a Material Point Method approach, experimental nanoindentation tests were successfully simulated. Finally, nanocomposite systems based on epoxy matrices were considered. In particular, composites with gradients in permittivity were synthesized through the application of a magnetic field to suspensions of highâpermittivity Fe3O4@TiO2 particles in an epoxy resin. The application of the magnetic force not only induced the movement of the particles, but also caused their alignment in high aspect ratio structures, resulting in rather steep permittivity gradients. Numerical simulations clarified how the asâsynthesized materials, employed as insulators, have the potential to efficiently reduce the electric field stress at the electrodeâinsulator interface and have intrinsic durability advantages over their homogeneous counterparts.

EPFL2015

Ultrafast Studies of Hot-Hole Dynamics in Au/p-GaN Heterostructures

Giulia Tagliabue

Harvesting non-equilibrium hot carriers from photo-excited metal nanoparticles has enabled plasmon-driven photochemical transformations and tunable photodetection with resonant nanoantennas. Despite numerous studies on the ultrafast dynamics of hot electrons, to date, the temporal evolution of hot holes in metal-semiconductor heterostructures remains unknown. An improved understanding of the carrier dynamics in hot-hole-driven systems is needed to help expand the scope of hot-carrier optoelectronics beyond hot-electron-based devices. Here, using ultrafast transient absorption spectroscopy, we show that plasmon-induced hot-hole injection from gold (Au) nanoparticles into the valence band of p-type gallium nitride (p-GaN) occurs within 200 fs, placing hot-hole transfer on a similar timescale as hot-electron transfer. We further observed that the removal of hot holes from below the Au Fermi level exerts a discernible influence on the thermalization of hot electrons above it, reducing the peak electronic temperature and decreasing the electron-phonon coupling time relative to Au samples without a pathway for hot-hole collection. First principles calculations corroborate these experimental observations, suggesting that hot-hole injection modifies the relaxation dynamics of hot electrons in Au nanoparticles through ultrafast modulation of the d-band electronic structure. Taken together, these ultrafast studies substantially advance our understanding of the temporal evolution of hot holes in metal-semiconductor heterostructures and suggest new strategies for manipulating and controlling the energy distributions of hot carriers on ultrafast timescales.

2018

Lanthanide light for biology and medical diagnosis

Jean-Claude Bünzli

Optical imaging emerges as a vital component of the various techniques needed to meet the stringent requirements of modern bioanalysis and bioimaging. Lanthanide luminescent bioprobes (LLBs) have greatly contributed to this field during the past 35 years because they have definite advantages such as little or no photobleaching and, thanks to time-gated detection, high sensitivity. The review summarizes the numerous tools offered by LLBs under their various forms, coordination compounds, nanoparticles, upconverting nanoparticles and their bioconjugates. It then focuses on biosensing, including point-of-care analysis, and then on both in vitro and in vivo bioimaging with visible and NIR light. The last section compares the performances of LLBs versus those of other commonly used bioprobes (organic dyes, quantum dots, and transition metal complexes). It is concluded that although LLBs will not replace all of existing bioprobes, they add invaluable new specific technologies to the biologist and medical doctor toolboxes. A good deal of improvements are achieved through nanotechnologies, which demonstrates that progresses in biosciences depend on the intersection of different disciplines, photophysics, chemistry, biochemistry, nanotechnology, and materials science. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier Science Bv2016

Functionally graded polymer nanocomposites using magnetic fields

Tommaso Nardi

EPFL2015