15 results on '"Nafezarefi, F."'
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2. Influence of Crystal Structure, Encapsulation, and Annealing on Photochromism in Nd Oxyhydride Thin Films
- Author
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Chaykina, D. (author), Nafezarefi, F. (author), Colombi, G. (author), Cornelius, S. (author), Bannenberg, L.J. (author), Schreuders, H. (author), Dam, B. (author), Chaykina, D. (author), Nafezarefi, F. (author), Colombi, G. (author), Cornelius, S. (author), Bannenberg, L.J. (author), Schreuders, H. (author), and Dam, B. (author)
- Abstract
Thin films of rare earth metal oxyhydrides show a photochromic effect, the precise mechanism of which is yet unknown. Here, we made thin films of NdH3-2xOx and show that we can change the band gap, crystal structure, and photochromic contrast by tuning the composition (O2-:H-) via the sputtering deposition pressure. To protect these films from rapid oxidation, we add a thin ALD coating of Al2O3, which increases the lifetime of the films from 1 day to several months. Encapsulation of the films also influences photochromic bleaching, changing the time dependency from first-order kinetics. As well, the partial annealing which occurs during the ALD process results in a dramatically slower bleaching speed, revealing the importance of defects for the reversibility (bleaching speed) of photochromism., ChemE/Materials for Energy Conversion & Storage, Instrumenten groep, ChemE/O&O groep, ChemE/Chemical Engineering
- Published
- 2021
- Full Text
- View/download PDF
3. Photochromic Properties of Rare-Earth Oxyhydrides
- Author
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Nafezarefi, F., Dam, B., and Delft University of Technology
- Abstract
The experiments presented in this thesis have provided new insights regarding the photochromic properties of rare earth oxyhydrides (REOxHy). We discovered that thin films of rare-earth (Y, Dy, Er, Gd, Nd) oxyhydrides show unique photochromic properties under ambient conditions. We showed that by direct current reactive magnetron sputtering of rare earth metal targets in an Ar and H2 atmosphere a metallic dihydride thin films can be made. However, above a certain critical deposition pressure, a semiconductor REOxHy thin film is formed upon exposing the films to air. These films show photochromic properties. Compared to YOxHy, the optical bandgaps of the lanthanide-based oxyhydrides are smaller, while photochromic contrast and kinetics show large variation among different cations. The photon energy required to obtain a photochromic effect is given by the optical band gap of the material, as shown in the energy threshold measurements. Photon energies larger than the band gap are required to photo-darken the rare-earth oxyhydrides. The photochromic process is reversible and bleaching occurs through thermal bleaching and possibly through interaction with the light of longer wavelengths (optical bleaching). However, the latter needs to be confirmed with further experiments. We have shown for the first time also that semiconducting YH3 does not exhibit photochromic properties, revealing the importance of the presence of both oxide and hydride ions for the photochromic effect in rare-earth oxyhydrides.
- Published
- 2020
- Full Text
- View/download PDF
4. Photochromic Properties of Rare-Earth Oxyhydrides
- Author
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Nafezarefi, F. (author) and Nafezarefi, F. (author)
- Abstract
The experiments presented in this thesis have provided new insights regarding the photochromic properties of rare earth oxyhydrides (REOxHy). We discovered that thin films of rare-earth (Y, Dy, Er, Gd, Nd) oxyhydrides show unique photochromic properties under ambient conditions. We showed that by direct current reactive magnetron sputtering of rare earth metal targets in an Ar and H2 atmosphere a metallic dihydride thin films can be made. However, above a certain critical deposition pressure, a semiconductor REOxHy thin film is formed upon exposing the films to air. These films show photochromic properties. Compared to YOxHy, the optical bandgaps of the lanthanide-based oxyhydrides are smaller, while photochromic contrast and kinetics show large variation among different cations. The photon energy required to obtain a photochromic effect is given by the optical band gap of the material, as shown in the energy threshold measurements. Photon energies larger than the band gap are required to photo-darken the rare-earth oxyhydrides. The photochromic process is reversible and bleaching occurs through thermal bleaching and possibly through interaction with the light of longer wavelengths (optical bleaching). However, the latter needs to be confirmed with further experiments. We have shown for the first time also that semiconducting YH3 does not exhibit photochromic properties, revealing the importance of the presence of both oxide and hydride ions for the photochromic effect in rare-earth oxyhydrides., ChemE/Materials for Energy Conversion & Storage
- Published
- 2020
5. Effect of the addition of zirconium on the photochromic properties of yttrium oxy-hydride
- Author
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Nafezarefi, F. (author), Cornelius, S. (author), Nijskens, J. (author), Schreuders, H. (author), Dam, B. (author), Nafezarefi, F. (author), Cornelius, S. (author), Nijskens, J. (author), Schreuders, H. (author), and Dam, B. (author)
- Abstract
Thin films of yttrium oxy-hydride have interesting, reversible photochromic properties, the origin of which is poorly understood. To investigate the role of point defect mobility, we probed the effect of lattice contraction as induced by the addition of zirconium. Interestingly, we find no loss of photochromic contrast for Y 1-z Zr z O x H y films with a small Zr cationic fractions (z < 0.15). At larger fractions the photochromic contrast is reduced. Zirconium is found to slow down the bleaching process, which suggests that the mobility of point defects may play a role in the thermal bleaching process. However, we cannot rule out substitution of zirconium in the YO x H y lattice which may also affect the photochromic properties., ChemE/Materials for Energy Conversion & Storage, ChemE/Afdelingsbureau
- Published
- 2019
- Full Text
- View/download PDF
6. Oxyhydride Nature of Rare-Earth-Based Photochromic Thin Films
- Author
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Cornelius, S. (author), Colombi, G. (author), Nafezarefi, F. (author), Schreuders, H. (author), Heller, R. (author), Munnik, Frans (author), Dam, B. (author), Cornelius, S. (author), Colombi, G. (author), Nafezarefi, F. (author), Schreuders, H. (author), Heller, R. (author), Munnik, Frans (author), and Dam, B. (author)
- Abstract
Thin films of rare-earth (RE)-oxygen-hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE-O-H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REO x H 3-2x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O 2- and H - anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications., ChemE/Materials for Energy Conversion & Storage, ChemE/Afdelingsbureau
- Published
- 2019
- Full Text
- View/download PDF
7. Oxyhydride Nature of Rare-Earth-Based Photochromic Thin Films
- Author
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Cornelius, S., Colombi, G., Nafezarefi, F., Schreuders, H., Heller, R., Munnik, F., Dam, B., Cornelius, S., Colombi, G., Nafezarefi, F., Schreuders, H., Heller, R., Munnik, F., and Dam, B.
- Abstract
Thin films of rare-earth (RE)−oxygen−hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE−O−H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REOxH3−2x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O2− and H− anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications.
- Published
- 2019
8. Effect of the addition of zirconium on the photochromic properties of yttrium oxy-hydride
- Author
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Nafezarefi, F., primary, Cornelius, S., additional, Nijskens, J., additional, Schreuders, H., additional, and Dam, B., additional
- Published
- 2019
- Full Text
- View/download PDF
9. Electronic structure and vacancy formation in photochromic yttrium oxy-hydride thin films studied by positron annihilation
- Author
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Plokker, M.P. (author), Eijt, S.W.H. (author), Naziris, F. (author), Schut, H. (author), Nafezarefi, F. (author), Schreuders, H. (author), Cornelius, S. (author), Dam, B. (author), Plokker, M.P. (author), Eijt, S.W.H. (author), Naziris, F. (author), Schut, H. (author), Nafezarefi, F. (author), Schreuders, H. (author), Cornelius, S. (author), and Dam, B. (author)
- Abstract
In order to investigate the mechanism of the photochromic effect in yttrium oxy-hydride (YOxHy) thin films, Doppler broadening positron annihilation spectroscopy (PAS) was applied to probe the electronic structure and the presence of vacancies in YOxHy and related materials as a function of composition, UV illumination and thermal annealing. The Doppler S and W parameter depth profiles of a series of Y, yttrium di-hydride YH1.9+δ and Y2O3 thin films show strong systematic changes caused by the distinct differences in electronic structure of the metals Y, YH1.9+δ and the wide band gap insulator Y2O3. The Doppler broadening parameters of photochromic YOxHy (a semiconductor with a band gap of ~2.6eV) are intermediate to those of YH1.9+δ and Y2O3. In order to probe the nanostructural changes related to the photochromic effect, the S parameter of YOxHy was monitored during in-situ UV illumination. A small but systematic increase of the S parameter was observed, possibly induced by generation of cation mono-vacancies or small vacancy clusters involving generated anion vacancies. The changes did not relax during bleaching under dark conditions, showing that the structural changes are not directly responsible for the photochromic mechanism. For temperatures above around 90°C, thermal annealing leads to a substantial increase in the Doppler S parameter, pointing to the formation of vacancies by local removal of hydrogen. Simultaneously, the optical band gap increases, consistent with an increase in O:H ratios., RST/Fundamental Aspects of Materials and Energy, RST/Neutron and Positron Methods in Materials, ChemE/Materials for Energy Conversion & Storage, ChemE/Afdelingsbureau
- Published
- 2018
- Full Text
- View/download PDF
10. Electronic structure and vacancy formation in photochromic yttrium oxy-hydride thin films studied by positron annihilation
- Author
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Plokker, M.P., primary, Eijt, S.W.H., additional, Naziris, F., additional, Schut, H., additional, Nafezarefi, F., additional, Schreuders, H., additional, Cornelius, S., additional, and Dam, B., additional
- Published
- 2018
- Full Text
- View/download PDF
11. Photochromism of rare-earth metal-oxy-hydrides
- Author
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Nafezarefi, F. (author), Schreuders, H. (author), Dam, B. (author), Cornelius, S. (author), Nafezarefi, F. (author), Schreuders, H. (author), Dam, B. (author), and Cornelius, S. (author)
- Abstract
Recently, thin films of yttrium oxy-hydride (YOxHy) were reported to show an unusual color-neutral photochromic effect promising for application in smart windows. Our present work demonstrates that also oxy-hydrides based on Gd, Dy, and Er have photochromic properties and crystal structures similar to YOxHy. Compared to YOxHy, the optical bandgaps of the lanthanide based oxy-hydrides are smaller while photochromic contrast and kinetics show large variation among different cations. Based on these findings, we propose that cation alloying is a viable pathway to tailor the photochromic properties of oxy-hydride materials. Furthermore, we predict that the oxy-hydrides of the other lanthanides are also potentially photochromic., ChemE/Materials for Energy Conversion & Storage, ChemE/Afdelingsbureau
- Published
- 2017
- Full Text
- View/download PDF
12. Photochromism of rare-earth metal-oxy-hydrides
- Author
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Nafezarefi, F., primary, Schreuders, H., additional, Dam, B., additional, and Cornelius, S., additional
- Published
- 2017
- Full Text
- View/download PDF
13. Size and synergy effects of nanofiller hybrids including graphene nanoplatelets and carbon nanotubes in mechanical properties of epoxy composites
- Author
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Chatterjee, S., Nafezarefi, F., Tai, N. H., Schlagenhauf, L., Nueesch, F. A., and Chu, B. T. T.
- Abstract
We study mechanical reinforcement in a widely used epoxy matrix with the addition of graphene nanoplatelets (GnPs) and various mixture ratios of carbon nanotubes (CNTs) with GnPs. Two different dimensions of GnPs were used with flake sizes of 5 mu m and 25 mu m to investigate the influence of nanofiller size on composite properties. In GnP reinforced composites, bigger flakes showed greater reinforcement at all GnP concentrations as they actively control the failure mechanisms in the composite. In the mixture samples, highest CNT content (9:1) showed marked improvement in fracture toughness of 76%. The CNT:GnP ratio is an interesting factor significantly influencing the properties of the epoxy based nanocomposites. The combination of high aspect ratio of CNTs and larger surface area of GnPs contribute to the synergistic effect of the hybrid samples. Thermal conductivity consistently increases with incorporation of GnPs in the matrix. Transmission electron microscopy (TEM) images confirm the uniform nanofiller dispersion achieved in the composites. For the hybrid samples CNTs are seen to align themselves on the GnP flakes creating an inter-connected strong nanofiller network in the matrix. The homogeneous nanofiller dispersions have been achieved by high shear calendaring which is a method capable of being industrially scaled up. (C) 2012 Elsevier Ltd. All rights reserved.
14. Influence of Crystal Structure, Encapsulation, and Annealing on Photochromism in Nd Oxyhydride Thin Films.
- Author
-
Chaykina D, Nafezarefi F, Colombi G, Cornelius S, Bannenberg LJ, Schreuders H, and Dam B
- Abstract
Thin films of rare earth metal oxyhydrides show a photochromic effect, the precise mechanism of which is yet unknown. Here, we made thin films of NdH
3-2 x Ox and show that we can change the band gap, crystal structure, and photochromic contrast by tuning the composition (O2- :H- ) via the sputtering deposition pressure. To protect these films from rapid oxidation, we add a thin ALD coating of Al2 O3 , which increases the lifetime of the films from 1 day to several months. Encapsulation of the films also influences photochromic bleaching, changing the time dependency from first-order kinetics. As well, the partial annealing which occurs during the ALD process results in a dramatically slower bleaching speed, revealing the importance of defects for the reversibility (bleaching speed) of photochromism., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)- Published
- 2022
- Full Text
- View/download PDF
15. Oxyhydride Nature of Rare-Earth-Based Photochromic Thin Films.
- Author
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Cornelius S, Colombi G, Nafezarefi F, Schreuders H, Heller R, Munnik F, and Dam B
- Abstract
Thin films of rare-earth (RE)-oxygen-hydrogen compounds prepared by reactive magnetron sputtering show a unique color-neutral photochromic effect at ambient conditions. While their optical properties have been studied extensively, the understanding of the relationship between photochromism, chemical composition, and structure is limited. Here we establish a ternary RE-O-H composition-phase diagram based on chemical composition analysis by a combination of Rutherford backscattering and elastic recoil detection. The photochromic films are identified as oxyhydrides with a wide composition range described by the formula REO
x H3-2 x where 0.5 ≤ x ≤ 1.5. We propose an anion-disordered structure model based on the face-centered cubic unit cell where the O2- and H- anions occupy tetrahedral and octahedral interstices. The optical band gap varies continuously with the anion ratio, demonstrating the potential of band gap tuning for reversible optical switching applications.- Published
- 2019
- Full Text
- View/download PDF
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