News Article | May 3, 2017
A novel fabrication procedure is used to produce flexible devices that include inorganic semiconductor nanowires and that can compete with organic devices in terms of brightness. Nitride LEDs are coming to replace other light sources in almost all general lighting, as well as in displays and life-science applications. Inorganic semiconductor devices, however, are naturally mechanically rigid and cannot be used in applications that require mechanical flexibility. Flexible LEDs are therefore currently a topic of intense research, as they are desirable for use in many applications, including rollable displays, wearable intelligent optoelectronics, bendable or implantable light sources, and biomedical devices. At present, flexible devices are mainly fabricated from organic materials. For example, organic LEDs (OLEDs) are already being used commercially in curved TV and smartphone screens. However, OLEDs have worse temporal stability and lower luminescence (especially in the blue spectral range) than nitride semiconductor LEDs. Substantial research efforts are thus being made to fabricate flexible inorganic LEDs.1 The conventional approach for flexible inorganic LED fabrication consists of number of steps, i.e., layer lift-off, microstructuring, and transfer to plastic supports. To avoid the microstructuring step and facilitate the lift-off, it is advantageous to shrink the active element dimensions and to use bottom-up nanostructures (such as nanowires, NWs) rather than 2D films. These NWs—i.e., elongated nanocrystals with a submicrometer diameter—have remarkable mechanical and optoelectronic properties that stem from their anisotropic geometry, high surface-to-volume ratio, and perfect crystallinity. In addition, such NWs are mechanically flexible and can withstand high levels of deformation without suffering plastic relaxation. Efficient LEDs that include nitride NWs have previously been demonstrated, and in our work,2 we make use of nitride NWs as the active material for flexible LEDs. Our polymer-embedded NWs offer an elegant solution to create flexible optoelectronic devices in which we combine the high efficiency and long lifetimes of inorganic semiconductor materials with the high flexibility of polymers. In our devices, the NW arrays—which are embedded in a flexible film and can be lifted-off from their native substrate—can sustain large deformations because of the high flexibility of the individual NWs. Furthermore, the footprints of individual NWs are much smaller than the typical curvature radius of LEDs (i.e., on the order of a few millimeters or more). In our approach, we used catalyst-free metal-organic chemical vapor deposition (MOCVD) to grow self-assembled gallium nitride (GaN) NWs on c-plane sapphire substrates.3 These NWs (with lengths of about 20μm and radii of about 0.5–1.5μm) have core/shell n–p junctions into which we incorporate multiple radial indium gallium nitride (InGaN)/GaN quantum wells. We control the emission color by changing the indium concentration of the InGaN emitting layer. In our actual device fabrication process4—see Figure 1(a)—the NW array is embedded into the polydimethylsiloxane (PDMS), peeled-off from the sapphire host substrate, and we then flip the composite NW/polymer membrane onto an arbitrary substrate to conduct the metal back-contacting. We subsequently flip the layer again and mount it on a flexible substrate (a metal foil or plastic), at which point we front-contact it with a flexible and transparent electrode. For the front contact we chose a silver NW mesh—see Figure 1(b)—which is characterized by mechanical flexibility, good electrical conductivity, and optical transparency. Figure 1. (a) Schematic illustration of the fabrication process for flexible LEDs that are based on a vertical nitride nanowire (NW) array. Ni: Nickel. Au: Gold. Ti: Titanium. (b) Scanning electron microscope image of the spin-coated silver (Ag) NW network on the polydimethylsiloxane (PDMS)/NW membrane. This silver NW network is used to form the transparent top-contact of the device. The protruding LED NWs are circled in red. We have used this technological procedure to fabricate blue and green flexible NW LEDs.4 We find that our devices exhibit typical behavior for nitride NW LEDs, i.e., with a light-up voltage of about 3V. Moreover, our LEDs can be bent to a curvature radius of ±3mm without any degradation of their electrical or luminescent properties. Photographs of our NW LEDs under operation in flat conditions, and during upward or inward bending are shown in Figure 2. Our flexible NW LEDs also have reasonable stability over time, unlike conventional OLEDs. Indeed, storing our devices in ambient conditions for several months does not cause their properties to degrade, whereas the lifetime of an OLED without encapsulation is limited to only several hours. Figure 2. Photographs of the blue (top), green (middle), and white (bottom) flexible LEDs at operation under different bending conditions. We have also used our composite NW/polymer membrane architecture to realize a flexible white LED (see Figure 2). To achieve this device we follow the standard approach of down-converting blue emission with yellow phosphors, i.e., to get white light from a blue–yellow mixture. To adapt this scheme for our flexible NW LEDs, we added yellow cerium-doped yttrium aluminum garnet phosphors into the PDMS layer between the NWs and covered the surface with an additional phosphorous-doped PDMS cap.5 The phosphor particles we use are smaller than 0.5μm so that they can fill the gaps between the NWs. The light that is emitted by the NWs is thus partially converted by the phosphors from blue to yellow, and we achieve a broad spectrum (covering almost the full visible range). Our NW membrane lift-off and transfer procedure allows free-standing layers of NW materials with different bandgaps to be assembled without any constraints relating to lattice-matching or compatability of growth conditions. Our approach therefore provides a large amount of design freedom and modularity, i.e., because it enables materials with very different physical and chemical properties to be combined (which cannot be achieved with monolithic growth). We made use of this modularity to demonstrate a two-color device, in which we combined two flexible LED layers that contain different active NWs: see Figure 3(a). In this device, we mounted a fully transparent flexible blue LED on top of a green LED. We were able to bias the two LEDs separately by producing either blue or green light, or by simultaneously producing a light mixture. We show the electrolumniscence spectra from the different layers of this bicolor flexible LED in Figure 3(b). Figure 3. (a) Schematic illustration of a blue-green two-color flexible NW LED, in which a fully transparent blue LED is mounted on top of a green LED. The two LEDs are biased separately (i.e., V1 and V2). (b) Electroluminescence (EL) spectra (in arbitrary units) of the two-color flexible NW LED. The blue, green, and red curves show the emissions from the top layer, bottom layer, and both layers together (biased simultaneously), respectively. In summary, we have successfully demonstrated a new procedure for the fabrication of efficient, flexible nitride NW LEDs. In our approach, we embed GaN NWs within a PDMS membrane and have realized blue, green, and white LEDs that exhibit good bending, electrical, luminescent, and temporal stability characteristics. The modularity of our technique means that we can also produce bicolor devices in which one LED is mounted upon another. Our approach thus opens up new routes to achieving efficient flexible LEDs and other optoelectronic devices, such as red-green-blue flexible LEDs or displays, flexible NW-based photodetectors6, or solar cells. In our future research we will concentrate on improving the efficiency of our flexible light-emitting devices, which is not yet comparable to that of commercialized rigid thin-film LEDs. We will also try to integrate the flexible light sources into life-science applications. This work has been financially supported through the 'PLATOFIL' project (ANR-14-CE26-0020-01), the EU H2020 ERC ‘NanoHarvest’ project (grant 639052), and by the French national Labex GaNex project (ANR-11-LABX-2014). The device fabrication was performed at the Centrale de Technologie Universitaire's Institut d'Electronique Fondamental (CTU-IEF) Minerve technological platform, which is a member of the Renatech Recherche Technologique de Base network. Center for Nanosciences and Nanotechnologies Paris-Sud University, CNRS Nan Guan is a PhD candidate in physics. He received both his master's and engineering degrees in optics from Université Paris-Saclay, France, in 2015. His current research interests include nanofabrication, characterization, and optical simulations for nitride nanowire LEDs. Xing Dai received her PhD in applied physics from Nanyang Technological University, Singapore, in 2014. During her time as a postdoctoral researcher at Paris-Sud University, she focused on flexible nanowire LEDs. She is currently a process-development engineer at Almae Technologies. Maria Tchernycheva received her PhD in physics from Paris-Sud University in 2005. She joined CNRS in 2006, where she currently leads the ‘NanoPhotoNit’ research group. Her research focuses on the fabrication and testing of novel optoelectronic devices that are based on semiconductor nanowires. Quantum Photonics, Electronics and Engineering (PHELIQS) Institute for Nanoscience and Cryogenics, French Alternative Energies and Atomic Energy Commission (CEA) Joël Eymery obtained his engineering degree, PhD, and habilitation from Université Grenoble Alpes, France, and now leads CEA's Nanostructures and Synchrotron Laboratory. His research is focused on the development of nanowire physics, including metal–organic vapor-phase epitaxy growth of nitride compounds, structural and optical characterization, and the development of nanodevice demonstrators. Christophe Durand received his PhD in physics from the Université Joseph Fourier, France, in 2004. Since 2006, he has been an associate professor at the Université Grenoble Alpes. In his research, he focuses on the synthesis of novel III-N nanostructures by metal–organic vapor-phase epitaxy to develop new optoelectronic applications. Quantum Photonics, Electronics and Engineering (PHELIQS)Institute for Nanoscience and Cryogenics, French Alternative Energies and Atomic Energy Commission (CEA) 2. N. Guan, X. Dai, J. Eymery, C. Durand, M. Tchernycheva, Nitride nanowires for new functionalities: from single wire properties to flexible light-emitting diodes. Presented at SPIE Photonics West 2016. 3. R. Koester, J.-S. Hwang, D. Salomon, X. Chen, C. Bougerol, J.-P. Barnes, D. Le Si Dang, et al., M-plane core-shell InGaN/GaN multiple-quantum-wells on GaN wires for electroluminescent devices, Nano Lett. 11, p. 4839-4845, 2011. 4. X. Dai, A. Messanvi, H. Zhang, C. Durand, J. Eymery, C. Bougerol, F. H. Julien, M. Tchernycheva, Flexible light-emitting diodes based on vertical nitride nanowires, Nano Lett. 15, p. 6958-6964, 2015. 5. N. Guan, X. Dai, A. Messanvi, H. Zhang, J. Yan, E. Gautier, C. Bougerol, et al., Flexible white light emitting diodes based on nitride nanowires and nanophosphors, ACS Photonics 3, p. 597-603, 2016.
Roy R.N.,Grenoble Alpes |
Roy R.N.,CEA Grenoble |
Bonnet S.,Grenoble Alpes |
Bonnet S.,CEA Grenoble |
And 5 more authors.
Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, EMBS | Year: 2015
Mental workload estimation is of crucial interest for user adaptive interfaces and neuroergonomics. Its estimation can be performed using event-related potentials (ERPs) extracted from electroencephalographic recordings (EEG). Several ERP spatial filtering methods have been designed to enhance relevant EEG activity for active brain-computer interfaces. However, to our knowledge, they have not yet been used and compared for mental state monitoring purposes. This paper presents a thorough comparison of three ERP spatial filtering methods: principal component analysis (PCA), canonical correlation analysis (CCA) and the xDAWN algorithm. Those methods are compared in their performance to allow for an accurate classification of mental workload when applied in an otherwise similar processing chain. The data of 20 healthy participants that performed a memory task for 10 minutes each was used for classification. Two levels of mental workload were considered depending on the number of digits participants had to memorize (2/6). The highest performances were obtained using the CCA filtering and the xDAWN algorithm respectively with 98% and 97% of correct classification. Their performances were significantly higher than that obtained using the PCA filtering (88%). © 2015 IEEE.
News Article | February 27, 2017
The mystery that is the origin of flowering plants has been partially solved thanks to a team from the Laboratoire de Physiologie Cellulaire et Végétale (CNRS/Inra/CEA/Université Grenoble Alpes), in collaboration with the Reproduction et Développement des Plantes laboratory (CNRS/ENS Lyon/Inra/Université Claude Bernard Lyon 1) and Kew Gardens (UK). Their discovery, published in the journal New Phytologist on February 24, 2017, sheds light on a question that much intrigued Darwin: the appearance of a structure as complex as the flower over the course of evolution. Terrestrial flora is today dominated by flowering plants. They provide our food and contribute color to the plant world. But they have not always existed. While plants colonized the land over 400 million years ago, flowering plants appeared only 150 million years ago. They were directly preceded by a group known as the gymnosperms, whose mode of reproduction is more rudimentary and whose modern-day representatives include conifers. Darwin long pondered the origin and rapid diversification of flowering plants, describing them as an "abominable mystery". In comparison with gymnosperms, which possess rather rudimentary male and female cones (like the pine cone), flowering plants present several innovations: the flower contains the male organs (stamens) and the female organs (pistil), surrounded by petals and sepals, while the ovules, instead of being naked, are protected within the pistil. How was nature able to invent the flower, a structure so different from that of cones? The team led by François Parcy, a CNRS senior researcher at the Cell and Plant Physiology Laboratory (CNRS/Inra/CEA/Université Grenoble Alpes), has just provided part of the answer. To do so, the researchers studied a rather original gymnosperm called Welwitschia mirabilis. This plant, which can live for more than a millennium, grows in the extreme conditions of the deserts of Namibia and Angola, and, like other gymnosperms, possesses separate male and female cones. What is exceptional is that the male cones possess a few sterile ovules and nectar, which indicates a failed attempt to invent the bisexual flower. Yet, in this plant (as well as in certain conifers), the researchers found genes similar to those responsible for the formation of flowers, and which are organized according to the same hierarchy (with the activation of one gene activating the next gene, and so on)! The fact that a similar gene cascade has been found in flowering plants and their gymnosperm cousins indicates that this is inherited from their common ancestor. This mechanism did not have to be invented at the time of the origins of the flower: it was simply inherited and reused by the plant, a process that is often at work in evolution. The study of the current biodiversity of plants thus enables us to go back in time and gradually sketch the genetic portrait of the common ancestor of a large proportion of modern-day flowers. The team is continuing to study other traits to better understand how the first flower emerged. Explore further: What 'pine' cones reveal about the evolution of flowers More information: Edwige Moyroud et al. A link between LEAFY and B-gene homologues insheds light on ancestral mechanisms prefiguring floral development, New Phytologist (2017). DOI: 10.1111/nph.14483
News Article | November 28, 2016
Home > Press > Leti and Grenoble Partners Demonstrate Worlds 1st Qubit Device Fabricated in CMOS Process: Paper by Leti, Inac and University of Grenoble Alpes Published in Nature Communications Abstract: Leti, an institute of CEA Tech, along with Inac, a fundamental research division of CEA, and the University of Grenoble Alpes have achieved the first demonstration of a quantum-dot-based spin qubit using an industry-standard fabrication process. Published in the November 24 issue of Nature Communications, and the topic of an invited paper at IEDM 2016, this proof-of-concept breakthrough uses a device fabricated on a 300-mm CMOS fab line. The device consists of a two-gate, p-type transistor with an undoped channel. At low temperature, the first gate defines a quantum dot encoding a hole spin qubit, and the second one defines a quantum dot used for the qubit readout. All electrical, two-axis control of the spin qubit is achieved by applying a phase-tunable microwave modulation to the first gate. Semiconductor spin qubits reported so far were realized in academic research facilities. Unlike those demonstrations, the present research achievement relies on the technology of FDSOI field-effect transistors. The standard single-gate transistor layout is modified in order to accommodate a second closely spaced gate, which serves for the qubit readout. Another key innovation lies in the use of a p-type transistor, meaning that the qubit is encoded by the spin of a hole and not the spin of an electron. This specificity makes the qubit electrically controllable with no additional device components required for qubit manipulation. Our one-qubit demonstrator brings CMOS technology closer to the emerging field of quantum spintronics, said Silvano De Franceschi, Inacs senior scientist. Maud Vinet, Letis advanced CMOS manager and a co-author of the paper, said, This proof-of-concept result, obtained using a CMOS fab line, is driving a lot of interest from our semiconductor industrial partners, as it represents an opportunity to extend the impact of Si CMOS technology and infrastructure beyond the end of Moores Law. The way toward the quantum computer is still long, but CEA is leveraging its background in physics and computing, from technology to system and architecture, to build a roadmap toward the quantum calculator. While superconducting circuits are already providing basic quantum processors with several qubits (up to nine), spin qubits in silicon are at a much earlier stage of development. The immediate next steps will be demonstrating a few (n>2) coupled qubits, and developing a strategy for long-range coupling of qubits. In the long run, leveraging the integration capabilities of CMOS technology will be a clear asset for large-scale qubit architectures, said De Franceschi. Within a European collaborative project (see www.mos-quito.eu), we are also developing cryogenic CMOS electronics for the future co-integration of silicon qubits and classical control hardware. The specific added value of thin-film FDSOI is having a back-gate, which can be used to tune the QD electrical state or the dot-to-dot coupling. That avoids the need for an overlapping top gate and the need to deal with the crosstalk. An additional advantage is that conventional FDSOI comes with undoped channels, which is expected to be an advantage for co-integrated control electronics. It is anticipated that the built-in parallelism in the treatment of quantum information will open new perspectives for cryptography, database searching and simulation of quantum processes. This opportunity is triggering major research initiatives all around the world and, because of that, significant progress can be expected in the coming years. Silvano De Franceschi, INACs senior scientist, will present an invited paper entitled SOI Technology for Quantum Information Processing at IEDM 2016, Tuesday, Dec. 6. About Leti As one of three advanced-research institutes within CEA Tech, Leti serves as a bridge between basic research and production of micro- and nanotechnologies that improve the lives of people around the world. It is committed to creating innovation and transferring it to industry. Backed by its portfolio of 2,800 patents, Leti partners with large industrials, SMEs and startups to tailor advanced solutions that strengthen their competitive positions. It has launched 59 startups. Its 8,500m² of new-generation cleanroom space feature 200mm and 300mm wafer processing of micro and nano solutions for applications ranging from space to smart devices. With a staff of more than 1,900, Leti is based in Grenoble, France, and has offices in Silicon Valley, Calif., and Tokyo. Follow us on www.leti.fr and @CEA_Leti. CEA Tech is the technology research branch of the French Alternative Energies and Atomic Energy Commission (CEA), a key player in innovative R&D, defence & security, nuclear energy, technological research for industry and fundamental science. CEA was identified by Thomson Reuters as the most innovative research organization in the world. CEA Tech leverages a unique innovation-driven culture and unrivalled expertise to develop and disseminate new technologies for industry, helping to create high-end products and provide a competitive edge. About Inac (France) CEA-Inac counts 500 people in 6 laboratories, constituting joint research units with University Grenoble Alpes, and some with CNRS and Grenoble Institute of Technology. Inac is a major player in basic research and its research focuses are on (i) nanoscience, namely photonics, spintronics, nanoelectronics, polymer science and nanochemistry; (ii) cryogenic technologies mainly for space and large instruments; (iii) health (DNA damages) & biosensors; and (iv) correlated electron systems (superconductivity). Inac develops strong activities in nano- and material characterization (synchrotron, neutrons, NMR and EPR, TEM, ions) through internal or shared research centres and with Inac research groups located at ESRF and ILL. Inac manages a 700 m2 clean room for upstream research and fast devices prototyping. Inac has three major commitments: (i) creating frontier science results in basic research (350 publications per year); (ii) creating value by ensuring technology transfer through patents, start-ups, and partnerships in applied research; and (iii) training of first class scientists at undergraduate, graduate, and postdoctoral level. Website: inac.cea.fr. 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News Article | April 21, 2016
The LEAFY protein assembling in small chains on DNA (white and red) thanks to its DNA binding domain (pale or dark green) and oligomerization domain (pale or dark blue). Credit: Camille Sayou et al., Nature Communications The LEAFY protein, a transcription factor responsible for flower development, is able to assemble itself in small chains made up of several proteins. This mechanism allows it to bind to and activate regions of the genome that are inaccessible to a single protein. These results were obtained by scientists in the Laboratoire de Physiologie Cellulaire Végétale (CNRS/Inra/CEA/Université Grenoble Alpes) and the Institut de Biologie Structurale (CNRS/CEA/Université Grenoble Alpes), working in collaboration with their international partners. Published on 21 April 2016 in Nature Communications, they open the way to new research opportunities regarding the regulation of gene expression. The LEAFY protein plays an essential role in the beauty of the plant kingdom: it governs the development of flower buds and their different organs (sepals, petals, stamens and pistils). This protein is a transcription factor that is necessary to decipher the genetic code and is endowed with two important domains: one that binds to DNA to activate floral genes, and the second of a hitherto unknown type. The scientists have demonstrated that the latter is a so-called "oligomerization" domain that allows the LEAFY proteins to assemble in small chains. In this form, they become capable of binding to compacted regions of the chromatin while a single LEAFY protein cannot achieve this. The experiments performed suggest that once assembled in chains, the different DNA binding domains cooperate, thus improving their binding to regions that are normally too condensed to be recognized. LEAFY can thus activate the genes at the origin of the development of flower organs. LEAFY may therefore play the role of a "pioneer factor" capable of binding to the dense chromatin structure of certain genomic regions and initiating epigenetic changes that lead to gene expression. Human beings and animals do not have the LEAFY protein but other transcription factors containing oligomerization domains. These new findings therefore suggest that these domains may also contribute to giving pioneering properties to transcription factors in other kingdoms. This work thus opens new avenues for understanding the role of these factors in the regulation of gene expression. Explore further: New insights into cooperativity in gene regulation More information: Camille Sayou et al. A SAM oligomerization domain shapes the genomic binding landscape of the LEAFY transcription factor, Nature Communications (2016). DOI: 10.1038/NCOMMS11222