Core–Shell Engineering to Enhance the Spectral Stability of Heterogeneous Luminescent Nanofluids

Lucía Labrador-Páez, Marco Pedroni, Krisjanis Smits, Adolfo Speghini, Francisco Jaque, José García-Solé, Daniel Jaque,* and Patricia Haro-González

The tendency to the miniaturization of devices and the peculiar properties of the nanoparticles have raised the interest of the scientific community in nanoscience. In particular, those systems consisting of nanoparticles dispersed in fluids, known as nanofluids, have made it possible to overcome many technological and scientific challenges, as they show extraordinary properties. In this work, the loss of the spectral stability in heterogeneous luminescent nanofluids is studied revealing the critical role played by the exchange of ions between different nanoparticles. Such ion exchange is favored by changes in the molecular properties of the solvent, making heterogeneous luminescent nanofluids highly unstable against temperature changes. This work demonstrates how both temporal and thermal stabilities of heterogeneous luminescent nanofluids can be substantially improved by core–shell engineering. This simultaneously avoids the leakage of luminescent ions and the effects of the solvent molecular changes.

core shell

Published in Particle & Particle Systems Characterization.


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Unveiling Molecular Changes in Water by Small Luminescent Nanoparticles

Lucía Labrador-Páez, Dragana J. Jovanovic´, Manuel I. Marqués, Krisjanis Smits,
Slobodan D. Dolic´, Francisco Jaque, Harry Eugene Stanley, Miroslav D. Dramic´anin,
José García-Solé, Patricia Haro-González, and Daniel Jaque*

Nowadays a large variety of applications are based on solid nanoparticles dispersed in
liquids—so called nanofluids. The interaction between the fluid and the nanoparticles
plays a decisive role in the physical properties of the nanofluid. A novel approach
based on the nonradiative energy transfer between two small luminescent nanocrystals
(GdVO4:Nd3+ and GdVO4:Yb3+) dispersed in water is used in this work to investigate
how temperature affects both the processes of interaction between nanoparticles and
the effect of the fluid on the nanoparticles. From a systematic analysis of the effect
of temperature on the GdVO4:Nd3+ → GdVO4:Yb3+ interparticle energy transfer, it
can be concluded that a dramatic increase in the energy transfer efficiency occurs for
temperatures above 45 °C. This change is properly explained by taking into account
a crossover existing in diverse water properties that occurs at about this temperature.
The obtained results allow elucidation on the molecular arrangement of water
molecules below and above this crossover temperature. In addition, it is observed
that an energy transfer process is produced as a result of interparticle collisions that
induce irreversible ion exchange between the interacting nanoparticles.

smits small

DOI: 10.1002/smll.201700968

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Luminescence and Raman Detection of Molecular Cl2 and ClClO Molecules in Amorphous SiO2 Matrix

Abstract Image

Chlorine is a common undesirable impurity in synthetic SiO2 glass for ultraviolet optics and optical fibers. It is usually incorporated into glass as bound Si–Cl groups or interstitial Cl2molecules. We report a high-sensitivity detection of Cl2 in amorphous SiO2 (a-SiO2) by photoluminescence (PL) and also by Raman spectroscopy. The Cl2 PL emission band at 1.22 eV (1016 nm) appears at T < 160 K and shows a characteristic vibronic progression with separations ≈(520–540) cm–1 and an average lifetime of ≈5 ms at 13 K. Its excitation spectrum coincides with the shape of the 3.78 eV (328 nm) optical absorption band of Cl2 in a-SiO2, corresponding to the X → A 1Πu transition to repulsive excited state. Direct X → a singlet-to-triplet excitation was also observed at 2.33 eV (532 nm). Cl2 PL may serve as a sensitive and selective tool for monitoring Cl impurities and their reactions in a-SiO2. A Raman band of Cl2 is found at 546 cm–1. Cl2 photodissociation at energies up to 4.66 eV (266 nm) was not detected, pointing to a strong cage effect in a-SiO2 matrix. However, 7.9 eV (157 nm) photolysis of interstitial O2molecules gives rise to a Raman band at 954 cm–1, indicating a formation of dichlorine monoxide isomer, ClClO molecule by reaction of O atoms with interstitial Cl2.

J. Phys. Chem. C, 2017, 121 (9), pp 5261–5266

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Radioluminescence, thermoluminescence and dosimetric properties of ZnO ceramics

Larisa Grigorjeva, Aleksejs Zolotarjovs, Sergej Yu Sokovnin, Donats Millers, Krisjanis Smits, Vladislav G. Il`ves

Two types of ZnO ceramics were fabricated and characterized by XRD, SEM methods. The radioluminescence spectra were measured within the 300–550 K range. The defect luminescence band peaking at ~2.35 eV is the dominant one in radioluminescence spectra in both of the fabricated ceramics. The thermostimulated luminescence (TSL) glow-curves were measured after X-ray irradiation at 300 K. It was concluded that the complex overlapping peak within the 320–450 K temperature range consists of two components (~360–375 K and 400–420 K). The ratio of component intensities differs in both ceramics. The positions of high temperature TSL components (480–520 K) also differ in both samples; therefore not only sintering conditions but also the properties of the initial powder are very important for characteristics of TSL. A linear dependence of peak intensity on irradiation dose was observed up to ~3 kGy for ceramic 1 and up to 9 kGy for ceramic 2.


Ceramics International

DOI: 10.1016/j.ceramint.2017.02.016

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Effect of Mg2+ ions co-doping on luminescence and defects formation processes in Gd3(Ga,Al)5O12:Ce single crystals

V. Babin , P. Bohacek , L. Grigorjeva , M. Kucera , M. Nikl , S. Zazubovich , A. Zolotarjovs
Photo- and radioluminescence and thermally stimulated luminescence characteristics of Ce3+ – doped
and Ce3+, Mg2+ co-doped Gd3(Ga,Al)5O12 (GAGG) single crystals of similar composition are investigated in the 9-500 K temperature range. The Ce3+ – related luminescence spectra and the photoluminescence decay kinetics in these crystals are found to be similar. Under photoexcitation in the Ce3+ – and Gd3+ – related absorption bands, no prominent rise of the photoluminescence intensity in time is observed neither in GAGG:Ce,Mg nor in GAGG:Ce crystals. The afterglow is strongly reduced in GAGG:Ce,Mg as compared to GAGG:Ce, and the afterglow decay kinetics is much faster. Co-doping with Mg2+ results in a drastic decrease of the thermally stimulated luminescence (TSL) intensity in the whole investigated temperature range and in the appearance of a new complex Mg2+ – related TSL glow curve peak around 285 K. After irradiation in the Ce3+ – related 3.6 eV absorption band, the TSL intensity in GAGG:Ce,Mg is found to be comparable with that in the GAGG:Ce epitaxial film of similar composition. The Mg2+ – induced changes in the concentration, origin and structure of the crystal lattice defects and their influence on the scintillation characteristics of GAGG:Ce,Mg are discussed.

 2017 gagg paper

Optical Materials 66 (2017) 48-58

DOI: 10.1016/j.optmat.2017.01.039

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Colorimetric gas detection by the varying thickness of a thin film of ultrasmall PTSA-coated TiO2 nanoparticles on a Si substrate

Urmas Joost, Andris Šutka, Meeri Visnapuu, Aile Tamm, Meeri Lembinen, Mikk Antsov, Kathriin Utt, Krisjanis Smits, Ergo Nõmmiste and Vambola Kisand

Colorimetric gas sensing is demonstrated by thin films based on ultrasmall TiO2 nanoparticles (NPs) on Si substrates. The NPs are bound into the film by p-toluenesulfonic acid (PTSA) smits 2017 imageand the film is
made to absorb volatile organic compounds (VOCs). Since the color of the sensing element depends on the interference of reflected light from the surface of the film and from the film/silicon substrate interface, colorimetric detection is possible by the varying thickness of the NP-based film. Indeed, VOC absorption causes significant swelling of the film. Thus, the optical path length is increased, interference wavelengths are shifted and the refractive index of the film is decreased. This causes a change of color of the sensor element visible by the naked eye. The color response is rapid and changes reversibly within seconds of exposure. The sensing element is extremely simple and cheap, and can be fabricated by common coating processes.


Beilstein Journal of Nanotechnology

Beilstein J. Nanotechnol. 2017, 8, 229–236.

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Tunneling luminescence in long lasting afterglow of SrAl2O4:Eu,Dy

Virginija Liepina, Donats Millers, Krisjanis Smits

A long lasting (afterglowing) luminescent material SrAl2O4: Eu2+, Dy3+ was prepared by high temperature solid-state reaction method. The processes responsible for long lasting luminescence were studied by means of luminescence spectra, thermally stimulated luminescence and afterglow kinetics.Two processes are found to contribute in excited Eu2+ creation – the thermally released electrons recombination and electron tunneling from trap to Eu3+. The possible scheme of long lasting luminescence mechanism is proposed.

virginija raksts image.

Journal of Luminescence

DOI: 10.1016/j.jlumin.2017.01.011

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