How do nanoparticles grow in optical fibers?

Using atom probe tomography, physicists have demonstrated the change in composition of nanoparticles as they grow in an optical fiber doped with rare earth ions.

Optical fibers containing oxide nanoparticles doped with rare earth ions are of great interest because they would allow the luminescence properties to be tailor-made by adapting the composition of the nanoparticles. A thorough characterization of these amorphous nanoparticles becomes therefore essential if we want to control all of the properties of these fibers. For the first time, an international research team led by a researcher from the Nice Institute of Physics (INPHYNI, CNRS / Univ. Côte d'Azur) has unveiled a direct link between the composition of nanoparticles of magnesium silicates (between 2 and 20 nm), formed by phase separation in optical fibers, and their size. For example, a nanoparticle with a radius of 2 nm contains around 2.5% at. of magnesium, compared to 5% at. if the radius is 6 nm. These results were discovered using atom probe tomography, in collaboration with the company CAMECA Instruments. Molecular dynamics simulations show that these changes in composition cause variations in the local chemical environment of rare earth ions.

These findings have several important consequences for the optical properties of doped fibers. Until now, it was considered that the nanoparticles should be as small as possible in order to limit losses by light scattering. These results however challenge this doxa by demonstrating that the smallest nanoparticles have compositions close to that of the matrix from which the fiber is derived, and are therefore not as important as initially thought for modifying the luminescence properties of rare earth ions. Furthermore, a change in composition should be accompanied by a change in the refractive index of the particles as a function of their sizes. However, until now, light scattering models such as Rayleigh scattering only consider a single refractive index for nanoparticles, irrespective of their size. Finally, a crucial question concerns the growth of nanoparticles and the formation of nanocrystals. The question is whether or not the first germs are still amorphous or already crystalline. Considering these results, we can state that the first amorphous germs can grow while changing composition, until this latter evolves to the level necessary for crystallization.


Figure

(a) Nanoparticles observed by TEM. They correspond to location close to the central region of the fiber core. Particle size varies between a few nanometers and 200 nm. (b) Diffraction pattern (SAED) on nanoparticles. The particles are amorphous. (c) XY orientation map of 3D distribution. The Mg is in pink and the O is in blue. (d) APT reconstruction of Mg-based nanoparticles surrounded by silica matrix.


Reference

Compositional Changes at the Early Stages of Nanoparticles Growth in Glasses.

Wilfried Blanc, Isabelle Martin, Hugues François-Saint-Cyr, Xavier Bidault, Stephane Chaussedent, Chrystel Hombourger, Sabrina Lacomme, Philippe Le Coustumer, Daniel R Neuville, David Larson, Ty J Prosa, Christelle Guillermier.

The Journal of Physical Chemistry 123 (2019) 29008
DOI: 10.1021/acs.jpcc.9b08577


Actualité prepared by the communication service of INP-CNRS.

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