TC16: Nanostructures


The main goal of TC16 at present is to perform collaborative research focused around the development of a nanostructured functional coating on glass, simultaneously with solar control and self-cleaning properties, based on TiO2 or on ZnO:Al (AZO). In particular, the goal is to use sol-gel processing for the purpose of combining solar control and photocatalytic activity on a single nanostructured glass coating, by using Bragg mirrors (BMs), which are photonic bandgap-type coatings with high reflectivity in the near-UV and NIR, further coated with mesoporous TiO2 with photocatalytic properties. Another strategy could be the realization of an AZO coating starting from already formed nanoparticles to eventually combine with a TiO2 self-cleaning coating.


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Activities and plans

PLANS

In 2017, TC16 plans to continue the development of nanostructured, transparent oxy-fluoride glass-ceramics by melt-quenching, doped with Nd and other lanthanides, for optical communications, lasers and displays, as well as nanostructured sol-gel coatings doped with sensitizer/acceptor lanthanide pairs for frequency conversion in photovoltaic solar cells, in order to increase their efficiency. Solar-control coatings which can reflect the sun light will be investigated, based on ZnO:Al or ZnO:Ga. A Round Robin exercise, together with short stays of TC members in selected laboratories, will be organized in order to promote collaborative activities.

TC activity

Transparent oxyfluoride nano-glass-ceramics have been prepared by melting-quenching and doped with five different Nd3+ concentrations (0.1-2 mol %) to obtain the most efficient 4F3/2®4I11/2,13/2 emission. It was observed by differential thermal analysis (DTA) that the addition of Nd3+ does not affect the crystallization mechanism which corresponds to a diffusion-controlled volumetric process that starts from a constant number of nuclei. Nevertheless, the presence of the dopant species affects the kinetics due to a progressive increase in Tg with increasing Nd3+ content.  LaF3 crystals with a size between 9-12 nm are obtained after proper heat treatments at Tg + 20-80 °C, as confirmed by XRD and high resolution TEM. Furthermore, energy dispersive X-ray (EDX) analysis shows the incorporation of Nd3+ ions into the LaF3 nano-crystals. Judd-Ofelt analysis from the absorption spectra further demonstrate the incorporation of Nd3+ ions into the fluoride phase in glass-ceramic samples and the most relevant parameters, radiative lifetime and stimulated emission cross section, were calculated. A detailed optical characterization clearly shows that Nd3+ ions in glass-ceramics are incorporated in both the crystalline and amorphous phases. The low temperature site-selective emission and excitation spectra, together with the different lifetime values of the 4F3/2 state, depending on the excitation and emission wavelengths, allow to isolate unambiguously the emission from Nd3+ ions in the LaF3 nanocrystals and to correlate the spectroscopic properties with the structural properties. As the Nd3+ concentration increases beyond 0.1 mol%, a stronger quenching of lifetime is observed for Nd3+ ions inside LaF3 crystals than for those dispersed in the glass matrix. This strong concentration quenching is explained by the much higher concentration of Nd3+ ions in the crystalline phase compared to that in the glass matrix, calculated from XAS-Synchrotron measurements.

The SAXS results shown in Fig. 1 (below) were roughly consistent with the TEM results. The slightly larger average size (by about 2 nm) derived from the SAXS analysis compared to the TEM results could be due to the difference between particle volume distribution (SAXS) and particle number distribution (TEM), with the former having more weight on the larger particles. A paper has been published as a result of this work, co-authored by three TC 16 members: G. Gorni, J.J. Velázquez, G.C. Mather, A. Durán, G. Chen, M. Sundararajan, R. Balda, J. Fernández and M.J. Pascual , “Selective excitation in transparent oxyfluoride glass-ceramics doped with Nd3+”,  The Royal Soc. Chem.,  DOI: 10.1039/x0xx00000x.

Sol-gel derived ceramic phosphor films were also prepared by spin-coating, doped with lanthanide sensitizer/acceptor pairs like Er/Yb and Tb/Yb, for down-conversion processes in photovoltaic Si solar cells. Fig. 2 (below) shows the XRD patterns of multilayer samples with 30 layers, consisting of a TiO2 matrix doped with different concentrations of Tb and Yb. These films are crystalline and both Anatase and Rutile phases are present. A third phase, whose XRD patterns are close to those of Tb2Ti2O7, is also present.

Committee Members: back to top

Almeida, Rui Committee Position: Chair
Castro, Yolanda
Chen, Gang
Duran, Alicia (Prof)
Martucci, Alex Committee Position: Vice Chair
Pascual, M.J.
Ribiero, Sidney
Santos, Luis
Tadanaga, Kiyoharu
Xu, Jian
Yang, Hui

Committee Contact Details: back to top

TC16: Nanostructures
Prof Rui Almeida
Instituto Superior Tecnico
Lisboa
Portugal


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