CE08 - Matériaux métalliques et inorganiques et procédés associés

Self-organized multifunctional multi-material structures for LED lighting – SMARtLEDS

SMARtLEDs : Auto-organized multi-functional multi-materials structures for LED lighting.

In LED devices, many photons emitted by the source are not part of the final produced light due to internal reflective effects which trap them inside the luminescent coating associate to the device. These effects can be reduced through the surface patterning of coatings. The classical patterning methods offer great performances but are expensive and not suited for a large-scale use. This is why this project aims to develop ZnO/phosphor composite structures as a low cost and green alternative.

Improvement of the spectral and spatial extraction of the light produced by a LED device through the development of multi-materials ZnO/phosphor structures.

The stakes of this project are the enhancement of the spectral and spatial light extraction in a LED device, by combining the currently used phosphor YAG :Ce with a network of ZnO nanowires of controlled pattern and period. Adding ZnO nanowires to the phosphor will improve the spectral profile of the final emission through the enhancement of the too low red component of YAG :Ce in particular. Indeed, in the current devices, this low red contribution is often compensated by an expensive rare-earth doped red phosphor, oftenhard to elaborate and moisture sensitive. In our innovative structure, this latter would be then replaced by ZnO, a cheaper and rare-earth free phosphor.<br />Furthermore, the patterning brought by the ZnO nanowires network will improve the spatial extraction of the emitted light thanks to the opto-geometric properties of the final structure. An enhancement of at least 20% of the emission intensity compared to the current devices is expected, upon prior modellings.<br />The challenge is twofold since the project aims to improve the performances of LED devices while reducing the rare-earth consumption. Indeed, this resource’s management plays an important role in global geopolitical and environmental issues.

Two different approaches will be used to elaborate the composite architectures developed in the frame of this project. One top-down consisting in the impregnation of ZnO nanowires networks with YAG :Ce nanoparticles, and one bottom-up consisting in growing ZnO nanowires networks directly onto a crystallized YAG:Ce layer. Both those approaches involve low-energy and semi-industrial to industrial scale processes. To this extent, the synthesis of the ZnO nanowires will be achieved through a selective growth process involving a photosensitive resin spin-coated on a substrate followed by an hydrothermal growth of the nanowires at 90 °C. The crystallization of the ZnO seed-layer is made using a 540 °C heating-treatment.
According to the chosen approach, the phosphor will be either commercial or synthesized via sol-gel or solvothermal process. The synthesis way will determine the the particles morphology and size. The crystallized YAG :Ce coatings linked to the bottom-up approach will be sol-gel derived.
The structural characteristics of the elaborated composite architectures will be optimized thanks to numerical modellings allowing in particular to define the optimal nanowires density. It will also allow to define the optimal period and pattern of the network leading to the most appropriate spatial and spectral light extraction for the target applications.
The developed composites will be characterized in terms of structural, morphological and optical properties. The sustainability of those properties will also be studied in realistic condition of use of a LED device.
At the end of the project, all those studies will lead to the development of a LEDs demonstrator and the assessment of its photometric parameters.

In this study, ceramic matrix (YAG: Ce) was combined with zinc oxide (ZnO) nanowires (NWs), a stable and low-cost material. Synthesized under environmentally friendly conditions, these materials were combined into luminescent architectures as coatings. The objectives are to increase the intensity of the emitted light thanks to the surface structuring provided by the NWs, organized or not in the form of micrometric periodic arrays, and to provide the red component via ZnO. Part of this work was devoted to the optimization of the elaboration process of the ZnO NW arrays in order to ensure its robustness. The NW arrays were obtained by photo-etching of a ZnO-based photosensitive sol-gel resist followed by the growth of NWs by hydrothermal way. The process, optimized on a silicon substrate, was successfully transposed on a quartz substrate and then on a photoluminescent YAG:Ce layer, the latter having been synthesized via a sol-gel process. Different grating configurations were studied by varying their period, width and height.
The characteristics of the developed architectures have been optimized through numerical simulations to define the optimal NF density, periodicity and pattern of the NF array leading to the most appropriate spectral and spatial light extraction for the targeted application. These samples combining YAG:Ce with ZnO NWs are characterized by an improvement in luminescence intensity of up to 50% depending on the architecture configuration.

The results obtained in the SMARtLEDs project have been put to good use in Aubry Martin's thesis, defended on January 31, 2023 («Elaboration and characterization of self-organized multifunctional multi-material structures for LED lighting«), as well as in publications and oral and poster presentations at national and international conferences.
This work has been the subject of 3 published papers, and 2 others are in the process of being written. The first two publications have made it possible to target the architecture combining NWs (without network) and YAG matrix the most suitable for the rest of the work. The third one showed how the organization of NWs in periodic arrays can enhance light extraction via coupled resonance effects. The work developed within the framework of SMARtLEDs has also been presented at 6 international conference and 2 national ones.

Amara, N.; Martin, A.; Potdevin, A.; Riassetto, D.; Messaoud, M.; Réveret, F.; Chadeyron, G.; Bouaziz, J.; Langlet, M.
J. Alloys Compd. 2020, 842, 155708, doi.org/10.1016/j.jallcom.2020.155708.
Amara, N.; Martin, A.; Potdevin, A.; F. Reveret, Riassetto, D. ; Chadeyron, G.; Langlet, M. Nanomaterials 2022, 12(15), 2568; doi.org/10.3390/nano12152568
Martin, A., Potdevin, A., Réveret, F., Centeno, E., Smaali, R., Omeis, F., Riassetto, D., Kachan, E., Jourlin, Y., Chadeyron, G., Langlet, M., Adv. Optical Mater. 2023, 2300695. doi.org/10.1002/adom.202300695

1 popularization article in the« minute recherche« : « Elaboration d’architectures complexes luminescentes destinées à l’éclairage à LEDs », paru en octobre 2021 (https://www.uca.fr/recherche/sciences-et-societe/la-minute-recherche/elaboration-darchitectures-complexes-luminescentes-destinees-a-leclairage-a-leds)-
6 Lectures at international conferences:
A. MARTIN et al.
“Luminescence properties of multiscale nanostructured coatings combining ZnO nanowires and Y3Al5O12:Ce3+”
International Conference on Luminescence 2023 (ICL2023), 27 août-1er septembre 2023, Paris.
E. CENTENO et al.
“Enhanced photoluminescence of ZnO nanowire coatings and gratings”
invitée, The 13th International Conference on Metamaterials, Photonic Crystals and Plasmonics (META 2023), 18-21 juillet 2023, Paris.
F. OMEIS, et al.
“Enhancement of Light Emission Using ZnO Grating Nanowires: An Experimental and Numerical Study”
8th International Conference on Antennas and Electromagnetic Systems (AES), 24-27 mai 2022, Marrakesh (Maroc).
A. MARTINet al.
“Development of sol-gel derived ZnO nanowires (NWs) arrays for optical applications”
Sol-Gel 2022, 24-29 juillet 2022, Lyon.

“Enhancement of the light extraction efficiency of sol-gel derived YAG:Ce coatings using ZnO nanowires (NWs) array”
EUROMAT 2021 (conférence virtuelle), 13-17 septembre 2021.
“Nanostructuration of YAG:Ce coatings by ZnO nanowires: a smart way to enhance light extraction efficiency”
EMRS Fall meeting 2021 (conférence virtuelle), 20-23 septembre 2021.

1 Presentation at a national conference
“Zinc oxide nanowires gratings by soft chemistry for optical applications”
JSGAuRA14, 14 avril 2022, Saint-Etienne.

1 POSTER at a National Conference
“Elaboration of multiscale nanostructured luminescent coatings by combining ZnO nanowires and Y3Al5O12:Ce3+ luminescent coatings”
C’Nano 2023, 15-17 mars 2023, Poitiers.

The SMARtLEDs project involves three laboratories in the Auvergne-Rhône-Alpes region (ICCF, LabHC, LMGP) and addresses the LED lighting field. Indeed, this project will allow obtaining a white light emission with stable photometric parameters in agreement with the requirements of the interior lighting in terms of colorimetry (Color Rendering Index (CRI) > 90, color temperature ~ 4000K).
Our work will focus on the structuring of Y3Al5O12: Ce3+ (YAG: Ce3+) phosphors (Ps) coatings via their association with ZnO nanowires (NWs) organized in the shape of periodic networks. These will both improve the light extraction and provide the red component to achieve CRI in line with the lighting specifications. It will thus be possible to overcome the use of current red phosphors, expensive and not very stable over time, formulated from rare earth elements doped nitrides or sulfides as well as Mn4+ activated fluorides. YAG: Ce3+ and ZnO compounds will be developed by low cost techniques (sol-gel, hydrothermal and solvothermal synthesis processes). Their combination should lead to a white light emission meeting the lighting requirements. An enhancement of the emission intensity of at least 20% compared to the current devices is being targeted based on first modellings.
Different phenomena resulting from these structures will favor the light extraction. Thanks to their nanometric diameter and their high aspect ratio, the NWs will act as nano-antenna guiding the light emitted by the LEDs and converted by the Ps towards the outside. The 3D architectures consisting of NWs will also lead to an increase in the number of the photon’s leaky cones or their angular broadening, thus multiplying the light paths by diffusion within the NWs networks. The dimensionality and periodicity of the NW networks will also be optimized through numerical simulations to reduce internal reflection losses and to control the light’s extraction angles. In addition, ZnO NWs will favor the emission of a light characterized by an optimized CRI, thanks to their spectral contribution in the red region under blue LED excitation. That is absolutely crucial because as aforementioned there are few rare-earth-free phosphors, efficient and stable over time, offering a red emission under excitation by a blue LED. Thus, the use of ZnO NWs to provide this component is a key point of this project.
For the self-organized structures, two complementary strategies will be studied in a comparative way: the use of a YAG:Ce coating under the ZnO seed layer or the impregnation of the NWs networks with a YAG:Ce nanoparticles suspension. The originality of the project lies in the proposed association including 1- a low cost lithographic (exposure / development) method based on ZnO sol-gel resists specifically developed for this purpose and compatible with photoetching processes, 2- the localized growth of ZnO NWs on photoetched ZnO networks, 3- YAG:Ce Ps with optimized optical performances and whose selected synthesis processes allow adapted shaping 4- a multifunctional, multi-materials and multi-scale architecture leading to an optimized extraction of the light emitted by the LED and to improved optical and colorimetric performances.
A new LED devices concept with increased performances will be proposed at the end of this project and will result in the implementation of a prototype.

Project coordination


The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


UJM/LabHC Laboratoire Hubert Curien
LMGP Laboratoire des Matériaux et du Génie Physique

Help of the ANR 335,340 euros
Beginning and duration of the scientific project: September 2019 - 42 Months

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