An international consortium (University of Strasbourg, University of Luxembourg, NIMS Japan, Ecole Centrale Lyon, C2N Saclay) led by IPCMS (team of Jean-François Dayen, DMONS), established the concept for a new generation of reconfigurable ferroelectronic logic circuits.
These devices, referred as Re-FeFET, allow for encoding and manipulating the information in a single operating unit, circumventing the famous ”Memory Wall” limitation of modern CMOS technology. By making use of the switchable polarization state of two ferroelectric gates, the electrical potential landscape within a semiconductor channel can be permanently and reconfigurably modified. Depending on the ferroelectric state encoded, the ferroelectric logic circuits can function as six alternative logic gates, while CMOS circuit are limited to a single function. Last but not least, the device can operate as a photodiode and generate photovoltaic energy. These findings rethink circuit topology and memory-logic interaction, opening up new research directions in the area of frugal computational enhancement.
Reference : Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits Ankita Ram, Krishna Maity, Cédric Marchand, Aymen Mahmoudi, Aseem Rajan Kshirsagar, Mohamed Soliman, Takashi Taniguchi, Kenji Watanabe, Bernard Doudin, Abdelkarim Ouerghi, Sven Reichardt, Ian O’Connor and Jean-Francois Dayen. ACS Nano 2023, 10.1021/acsnano.3c07952. Link.
Reconfigurable Multifunctional van der Waals Ferroelectric Devices and Logic Circuits, Ankita Ram, Krishna Maity, Cédric Marchand, Aymen Mahmoudi, Aseem Rajan Kshirsagar, Mohamed Soliman, Takashi Taniguchi, Kenji Watanabe, Bernard Doudin, Abdelkarim Ouerghi, Sven Reichardt, Ian O’Connor et Jean-Francois Dayen, ACS Nano, publié le 21 octobre 2023. Doi : 10.1021/acsnano.3c07952 Archives ouvertes : HAL
La fluorescence de nanorubans de graphène (GNR) synthétisés sur une surface métallique est étudiée avec une résolution spatiale sub-nanométrique en utilisant un microscope à effet tunnel (STM). L’émission de lumière observée implique des états topologiques qui se comportent comme des centres fluorescents localisés aux extrémités des GNR.
Depuis leur première synthèse, les nanorubans de graphène (GNR) ont suscité un intérêt considérable dans les communautés des nanosciences et nanotechnologies en raison de propriétés physiques uniques liées à leur topologie. En effet, la conformation spécifique de leurs bords est à l’origine d’états électroniques singuliers qui, à leur tour, conduisent à des propriétés de transport ou magnétiques particulières. Plusieurs études théoriques traitent en détail de la façon dont les propriétés optiques des GNR peuvent être avantageusement contrôlées par des variations à l’échelle atomique de leur largeur, de leur longueur et de la forme de leurs bords. Toutefois, les expériences portant sur les propriétés de fluorescence des GNR sont rares et limitées à des mesures d’ensemble dominées par l’émission de défauts difficilement contrôlables. De fait, les propriétés d’émission des GNR restent un territoire essentiellement inexploré.
Dans un article publié dans Science, une équipe de l’Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS – CNRS – Unistra) en collaboration avec un collègue de l’Institut des Sciences Moléculaires d’Orsay (ISMO – CNRS – Université Paris-Saclay) a développé une nouvelle méthode expérimentale permettant d’étudier les propriétés de fluorescence de GNR uniques à l’aide d’un microscope à effet tunnel (STM). Leur approche consiste à déplacer un unique ruban à l’aide de la pointe du STM depuis une surface métallique – indispensable à la synthèse du ruban – jusque sur une fine couche isolante qui permet de protéger les propriétés optiques des rubans. Les chercheurs se servent ensuite de la pointe STM pour passer un courant électrique très faible à travers le GNR. En réaction au passage du courant, les chercheurs ont observé une émission de lumière, qui s’est révélée particulièrement intense quand la pointe est positionnée aux extrémités du ruban. L’étude du spectre d’émission révèle une raie fine à une énergie bien plus faible que celle attendue pour un ruban infiniment long. Sur la base de ces résultats expérimentaux et d’une étude théorique complète, les chercheurs ont pu attribuer cette émission de lumière à des excitons localisés au niveau du bord du ruban du fait sa topologie particulière (dite non-triviale). Les bords du ruban se comportent ainsi comme des centres fluorescents, à l’image de ce qui est observé dans de nombreux matériaux isolants et semi-conducteurs. Un avantage des structures en ruban présentées dans cette étude est que l’on peut adapter le nombre et la position des centres fluorescents par ingénierie chimique, ce qui constitue un moyen efficace d’ajuster le couplage entre centres fluorescents et de contrôler leurs propriétés d’émission classiques ou quantiques. Ainsi, à moyen terme, ces structures devraient trouver leur place au sein de dispositifs optoélectroniques basés sur des briques élémentaires atomiquement plates et robustes ou comme capteurs quantiques accordables de dimension réduite.
Our modern, highly technological societies have developed within the paradigm of cheap, unlimited energy thanks to fossil fuel extraction. The resulting energy-driven human activity within the closed ecosystem that is Earth has caused multiple planetary boundaries to be exceeded. The wide-ranging impacts on our daily lives of the war-fueled and systemic, long-term energy crises have underscored our societal addiction to energy. Solving this conundrum will almost certainly require a systemic change in our society. This can occur through a combination of temperance, an astute use of existing energy solutions within planetary boundaries, and the development of radically new energy sources — if used wisely.
A multi-lab team of scientists, led by CNRS Senior Researcher Martin Bowen of the Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), has experimentally demonstrated a new, industrially promising approach to harvest the most basic form of energy available: ambient heat. Heat is the energy wasted in any process that transforms energy from one type to another while generating work. Typical processes involve the cycles of an engine, e.g in cars, refrigerators, etc… The team’s nanoscale devices output around 10nW of dc electrical power at room temperature. The resulting power density is much greater than that using conventional thermoelectric approaches that convert heat gradients into electricity. And in this case, no heat gradient was applied nominally. To achieve this remarkable, counterintuitive result, which was published in the top-tier journal Advanced Materials, the team combined two heretofore non-interacting branches of physics. Quantum thermodynamics is a rapidly developing field that has recently begun reexamining how an engine operates when its components are reduced to excited electrons on single atoms. “At this scale, an engine’s efficiency can be increased by the quantum mechanical interactions between the engine parts. One boost can arise upon measuring the state of a quantum object that acts as the engine’s working substance”, explains Andrew Jordan, a leading quantum thermodynamics theoretician from Chapman University, CA USA.
Typically, these engine cycles are studied using well-controlled, model experiments in which an ensemble of atoms is excited using optical and microwave light. Achieving an electronic counterpart would open promising possibilities for industrialization. However, so far, semiconductor physics approaches have failed to generate a quantum advantage. “This is likely because these engines typically utilize electrons that mostly behave like classical, charged particles without strong quantum interactions,” explains Heiner Linke, a leading quantum engine experimentalist at Lund University in Sweden.
The Bowen team’s solution is to implement the electronic engine using spintronics, a green nanotechnology that exploits the electron’s quantum spin property in addition to its charge. The electron is a Fermion particle, meaning that its spin can exist in one of two possible states… when measured! Between measurements, it can exist in a superposition of these two states. In atomic magnets called paramagnets, this superposition arises from the energy of ambient heat. To harvest this heat, in the team’s design, engine-fueling quantum measurements on the atom-borne spin are performed by electronically connecting the atom to the outside world using electrodes that only allow electrons of one spin type to flow within a narrow energy window. “We believe that quantum measurements, together with other quantum forms of engine fuel, can account for our experimental results”, explains Martin Bowen.
Another quantum engine fuel has been predicted to arise when the spins forming the working substance experience quantum coupling between one another. “Using electron paramagnetic resonance, we discovered that the temperature at which this transition occurs in our chains of atomic magnets matches a change in the temperature dependence of the device’s output power”, adds teammate Bertrand Vileno of the University of Strasbourg.
To build these concept features into a nanoelectronic device, the Bowen team used molecules. Indeed, phthalocyanine molecules are not only quite commonplace, but can host magnetic atoms so as to form spin chains. Also, molecules can more easily be positioned within a vertical nanopillar device than by working with single atoms directly. In addition, over the past 10 years, the Bowen-led IPCMS team has extensively researched — and patented, the spin selective properties of the interface between molecules and simple ferromagnetic metals like iron (which is plentiful). “These latest results, with a 89% spin polarization of the current, showcase how competitive this so-called ‘spinterface’ is relative to other solutions for a spin-selecting device electrode”, comments IPCMS teammate Wolfgang Weber.
Spintronic technology is normally used to encode and transmit information at a low energy cost. “When in contact with a molecular spin chain, the spinterface can help to spintronically encode information onto the chain’s quantum excited state. To promote the thermal fluctuations on the 3-member chain that are to be harvested, it was necessary to weaken the magnetic coupling between them by intercalating monolayers of non-magnetic fullerenes,” explains IPCMS teammate Samy Boukari.
This highlights the need, when designing quantum spintronic engines, to exquisitely control the quality of the heterostructure stack used to make the device. “Spintronics is already present in hard disk drives and magnetic memories. This makes it a natural vector to industrialize quantum physics… if you can controllably insert quantum spin states within a device on an industrial scale”, adds Martin Bowen. At present, crafting vertical nanopillars that contain molecules remain an academic tour-de-force that only two labs worldwide master the technology for. Both are run by the Centre National de la Recherche Scientifique (CNRS), a French research agency. To transform entire, pristine heterostructures into nanoscale vertical devices without exposing the delicate molecular layer to degrading solvents, the IPCMS nanotechnological process utilizes several steps of macroscale and nanoscale masking. For co-author Armel Bahouka of the laser-cutting firm IREPA LASER near Strasbourg who provided the high-precision shadow-masks, “these results illustrate the potential that technological solutions have to help solve fundamental problems.”
A challenging but industrially promising alternative to molecules is to directly work with individual atoms. “Magnetic tunnel junctions comprising a magnesium oxide (MgO) barrier are found in hard drive read heads and magnetic memories. They constitute a prime industrialization vector for spintronic concepts”, reminds Shinji Yuasa, director of the Research Center for Emerging Computing Technologies at the Japanese National Institute of Advanced Industrial Science and Technology (AIST). The major hurdle is now to control the insertion of magnetic atoms into these devices. “Our 10-year collaboration with the IPCMS’s Bowen team has generated precious knowledge into how oxygen vacancies in the MgO barrier drive the device performance. Switching from an information technology to an energy technology use will require mastering how to fill these oxygen vacancies with appropriately placed magnetic atoms”, explains teammate Daniel Lacour of the Institut Jean Lamour in Nancy, France. The Bowen-led team had already reported first results into a more industrializable spintronic quantum engine using MgO magnetic tunnel junctions containing magnetic carbon atoms. What plagued those fortuitous results from 2019 was that energy output was observed on only one device.
The present study bolsters the case for other academic and industrial entities to further explore this new implementation of quantum energy technologies using spintronics. This could require accessing the properties of the spintronic quantum engine’s magnetic atoms while the engine is in operation. “This very challenging experiment is precisely what the Bowen team unsuccessfully set out to undertake, using their unique prior expertise on device in-operando characterization techniques at our synchrotron facility”, remarks co-author Philippe Ohresser of Synchrotron SOLEIL near Paris, France.
Looking ahead, the Bowen-led team plans to study both the molecular and the MgO approaches, thanks to research consortiums built using fresh funding from French agencies. “This second set of results presents us with the challenge of clarifying which resources are powering these quantum spintronic engines”, remarks project consortium member Robert Whitney, who develops quantum thermodynamical theory at the LPMMC at Grenoble. “Our plans to study the engine’s magnetic atoms while in operation using microwaves could be very insightful”, adds project consortium member Joris Van Slageren, who studies quantum magnetism at the Institute of Physical Chemistry in Stuttgart, Germany. For Martin Bowen, the results are the natural progression of his 20+ year career focus on how spin-polarized currents that run across spintronic devices interact with quantum states. “What’s new is that I now get to interact with a whole new scientific community, and research technological solutions to society’s energy-driven crises. But you’ll be seeing me at Scientist Rebellion protests too, because technology alone won’t avert the major planetary catastrophe in progress.”
“Quantum advantage in a molecular spintronic engine that harvests thermal fluctuation energy” Advanced Materials
Fast photon-induced switching phenomena are the basis of nanodevices that can be operated by laser pulses. Highly promising materials for these applications are spin-crossover (SCO) compounds where a spin transition, e.g., from a diamagnetic s = 0 state to a paramagnetic s = 2 state, changes the bond length between a central metal atom and organic ligands. This transition can be induced by light or heat pulses and changes the size of the molecules. In SCO nanoparticles, reversible size changes can therefore be induced by laser pulses. While macroscopic techniques such as X-ray diffraction or magnetic measurements have already given time-resolved information on bulk ensembles of SCO nanoparticles, expansion effects at the level of individual SCO nanoparticles has so far remained difficult to study.
In a collaboration with researchers at the University of Bordeaux, ultrafast transmission electron microscopy (UTEM) at the IPCMS was applied to reveal the expansion mechanisms of individual SCO particles under nanosecond laser pulses. To facilitate the heat absorption in SCO, gold nanorods were embedded in SCO nanocrystals. Plasmonic heating of the gold rods under laser pulses leads to a controllable transfer of heat to the SCO nanoparticles which then expand rapidly due to the thermal spin transition and subsequently shrink upon cooling. With the ultrafast TEM, we are now able to measure the size of individual particles with high spatial precision and nanosecond time resolution. In this study, it is seen that elliptical SCO crystals with lengths of 200 – 500 nm expand by up to 5% within 10 – 20 ns. It is shown that the presence of plasmonic gold rods speeds up and enhances the expansion of SCO which is governed by collective effects within the nanoparticles.
Reference : Y. Hu, M. Picher, N. M. Tran, M. Palluel, L. Stoleriu, N. Daro, S. Mornet, C. Enachescu, E. Freysz, F. Banhart, G. Chastanet: Photo-Thermal Switching of Individual Plasmonically Activated Spin Crossover Nanoparticle Imaged by Ultrafast Transmission Electron Microscopy, Advanced Materials,https://doi.org/10.1002/adma.202105586
This work was carried out by a team from Institut de physique et chimie des matériaux de Strasbourg (IPCMS, CNRS/Univ. de Strasbourg) associated with a team from Institut Franche-Comté électronique mécanique thermique et optique – sciences et technologies (FEMTO-ST, CNRS/COMUE UBFC) and have been published in Nature Chemistry
Energy funnelling within multichromophore architectures monitored with subnanometre resolution. S. Cao, A. Rosławska, B. Doppagne, M. Romeo, M. Féron, F. Chérioux, H. Bulou, F. Scheurer, G. Schull, Nature Chemistry. Publié le 24 mai 2021. DOI : 10.1038/s41557-021-00697-z
In a nanocrystal array, transport occurs though hopping mechanism, leading to limited carrier diffusion length, in the 10 to 100 nm range. In the case of HgTe nanocrystals, Lan et al.  recently proposed that transport actually occurs between island of strongly coupled nanocrystals. In other word, partly delocalize states are formed within these islands and the bottleneck for conduction becomes the link between these islands. Based on this idea, it becomes of utmost interest to probe transport at the scale of a single island where transport can efficiently occur.
IPCMS recently collaborated with Emmanuel Lhuillier team of INSP (Paris), together with ONERA (Gregory Vincent, Palaiseau) and IEMN (Christophe Delerue, Lille) to address this technological lock. 
The authors exploited a smart concept of “nanotrench” electrodes, previously introduced by Dayen et al.  and developed at StNano plateform, to build high aspect ratio electrodes spaced by few nanometers only and expanding over several tenths of µm. Device size resolution is no longer set by the wavelength of the light used for the lithography but rather by device geometrical factor, down to few nanometers resolution.
We observe that for such small devices the light responsivity of a thin film of HgTe nanocrystals exposed to infrared light is strongly enhanced compared to the same film deposited on µm-spaced electrodes. Because the device is small, gain is generated and quantum efficiency above 105 are achieved (corresponding to 105 electrons flowing per absorbed photons). This large gain enables large signal to noise ratio reaching 2×1012 Jones at 200 K for a device with 2.5 µm cut-off wavelength, see figure 1b. This result over-performs the best photodiodes operating at the same wavelength. 
Carefull optoelectronics study and theoretical modeling demonstrates that transport is only driven by the part of the nanocrystal film located within the nanogap, and asset the top-notched performances of our device to the ultimate length scale of the photoactive channel. This result is important since it contradicts a common thought in the field that only photodiodes enable highest performances. The next challenge will be to achieve a matrix of such electrodes.
These result, published in Nature Communications, are important since it contradicts a common thought in the field that only photodiodes enable highest performances. The next challenge will be to achieve a matrix of such electrodes, and to address new generation of photovoltaic cells.