Toward a multiscale theoretical framework for organic memristive materials

Neuromorphic computing and engineering has been the focus of intense research efforts that have been intensified recently by the mutation of Information and Communication Technologies. In fact, new computing solutions and new hardware platforms are expected to emerge to answer to the new needs and challenges of our societies. In this revolution, lots of candidates’ technologies are explored and will require leveraging of the pros and cons. In this perspective paper belonging to the special issue on neuromorphic engineering of Journal of Applied Physics, we focus on the current achievements in the field of organic electronics and the potentialities and specificities of this research field. We highlight how unique material features available through organic materials can be used to engineer useful and promising bio-inspired devices and circuits. We also discuss the opportunities that organic electronics offer for future research directions in the neuromorphic engineering field.

Neuromorphic computing holds promise for building next-generation intelligent systems in a more energy-efficient way than the conventional von Neumann computing architecture. Memristive hardware, which mimics biological neurons and synapses, offers high-speed operation and low power consumption, enabling energy- and area-efficient, brain-inspired computing. Here, recent advances in memristive materials and strategies that emulate synaptic functions for neuromorphic computing are highlighted. The working principles and characteristics of biological neurons and synapses, which can be mimicked by memristive devices, are presented. Besides device structures and operation with different external stimuli such as electric, magnetic, and optical fields, how memristive materials with a rich variety of underlying physical mechanisms can allow fast, reliable, and low-power neuromorphic applications is also discussed. Finally, device requirements are examined and a perspective on challenges in developing memristive materials for device engineering and computing science is given.

In semiconductor-based optoelectronics devices made with organic materials have the potential advantages of low cost, structural flexibility, and simple fabrication processes. They have been shown to possess both electrical and optical properties that are associated with metals and semiconductors. Increasing attention is being paid to the electro-excitation or optical-excitation process between the metal electrodes and the active organic layers, where metal / organic interfaces are present. They are now thought to be one of the device parameters that significantly influence the device performance, for instance in organic light-emitting diodes (OLED) [1], organic photovoltaic cells [2], organic field-effect transistors [3] or new organic biosensors [4-7]. This chapter is intended to elucidate the effects of coupled active surface plasmon polaritons (SPPs) on a metallic lamellar grating nanostructure with organic material on the surface. It demonstrated how an energy gap for SPPs propagating on such a grating nanostructure can be used to modify the emission properties of an adjacent thin layer of organic semiconductor material (Alq3, tris-(8-hydroxyquinoline)-aluminum). We have fabricated several grating devices with differences in pitch size and coupled organic / metal nanostructure with symmetric and asymmetric dielectric SP band gap materials. It is found that emission is significantly inhibited in the vicinity of the band gap and that the modified emission spectrum is determined by the wavelength dependence of the density of SPP states. We present recent experimental results and discuss potential applications of such an active plasmonics for biosensor with enhanced resonance energy and highly directional emission due to local interactions on the organic / metal nano-grating.

Confronted by the difficulties of the von Neumann bottleneck and memory wall, traditional computing systems are gradually inadequate for satisfying the demands of future data‐intensive computing applications. Recently, memristors have emerged as promising candidates for advanced in‐memory and neuromorphic computing, which pave one way for breaking through the dilemma of current computing architecture. Till now, varieties of functional materials have been developed for constructing high‐performance memristors. Herein, the review focuses on the emerging 2D MXene materials‐based memristors. First, the mainstream synthetic strategies and characterization methods of MXenes are introduced. Second, the different types of MXene‐based memristive materials and their widely adopted switching mechanisms are overviewed. Third, the recent progress of MXene‐based memristors for data storage, artificial synapses, neuromorphic computing, and logic circuits is comprehensively summarized. Finally, the challenges, development trends, and perspectives are discussed, aiming to provide guidelines for the preparation of novel MXene‐based memristors and more engaging information technology applications.

The search and investigation of resistive switching materials, the most consolidated form of solid-state memristors, has become one of the fastest growing areas in the field of electronics. This is not only due to the huge commercial interest in developing the so-called Resistive Random-Access Memories (ReRAMs) but also because resistive switching materials are gathering way to new forms of analog computation. Unlike in the field of traditional electronics technologies, where Silicon has monopolized most of the applications, the area of solid-state memristors is opened to a broad set of candidates that may contribute to unprecedented applications. In particular, the use of organic-based resistive switching materials can provide additional functionalities as structural flexibility for conformal integration or introduce new and cost-effective fabrication technologies. Following this new wave of organic memristive materials, this work aims at reviewing the existing models explaining the origins of resistive switching in Graphene Oxide, one of the most promising contenders on the battlefield of emerging memristive materials due to its low cost and easy processing methods. Within this manuscript, we will revisit the different theories supporting the phenomenology of resistive switching in this material nourishing the discussion with experimental results supporting the three main existing theories.

Circular dichroism (CD) spectroscopy reflects sensitively the various chemistries involved in chiral molecules and molecular systems. The CD spectra are very sensitive to the conformational changes of molecules: the rotation around the single bond including a chiral atom. It is also sensitive to the changes in the stacking interactions in the chiral DNA and RNA. Since these changes are low-energy processes, we expect that the CD spectra include a lot of information of the chiral molecular systems. On the other hand, the SAC-CI method is a highly reliable excited-state theory and gives very reliable theoretical CD spectra. Therefore, by comparing the experimental CD spectra with the theoretical SAC-CI spectra calculated for various chemical situations, one can study various chemistries such as the nature of the weak interactions involved in chiral molecular systems and biology. Based on these facts, we are developing a new molecular technology called ChiraSac, a term combining chirality and SAC-CI, to study chiral molecular systems and the chemistry involved thereof. We utilize highly reliable SAC-CI method together with many useful quantum chemical methods involved in Gaussian suite of programs. In this chapter, we review our ChiraSac studies carried out to clarify the chemistries of some chiral molecules and molecular systems: large dependences of the CD spectra on the conformations of several chiral molecules in solutions and the effects of the stacking interactions of the nuclear-acid bases in DNA and RNA on the shapes of their CD spectra. The results of our several studies show that the ChiraSac is a useful tool for studying the detailed chemistry involved in chiral molecular and biological systems.

Alkali-ion batteries, including potassium-ion batteries, lithium-ion batteries, and sodium-ion batteries are important energy storage devices; however, with the cation size increased, there exists ...

Memristive Materials and Devices for Neuromorphic Computing: state of the art and perspectives

physiological genomics is currently entering its eighth year of publication. The formation of this new journal was considered a high risk venture when first discussed in 1998. The American Physiological Society was convinced that this journal would advance and consolidate the information emerging

Grain crushing underpins key mechanical behaviours of granular materials. A variety of factors, including grading, particle shapes and loading conditions, have been recognised to affect the crushability of grains and the overall behaviour of a granular material. Among them, the role of intermediate principal stress in a general stress condition on the shear behaviour of crushable granular sand remains less understood, owing to the scarcity of experimental data and analytical tools available. In this paper, a multi-scale computational approach is employed to investigate the shear behaviour of crushable granular sand under general stress conditions with varying intermediate principal stresses and confining pressures. The computational approach features multi-scale coupling between non-smooth contact dynamics and peridynamics, and offers a rigorous way to consider the intertwined evolution of particle size and shape during the process of grain crushing. The numerical study helps to quantify comprehensively and analyse the grain crushing-induced changes of macro- and micro-scale material behaviours including strength, deformability, particle size and shape evolution, particle-scale forces and contact conditions, and the development of anisotropy. The competition between a void-filling mechanism due to grain size change and enhanced friction and interlocking due to grain shape change in dictating the deformation of crushable sand is further discussed. The findings offer insights into the complex behaviours of crushable granular materials under general stress conditions and facilitate future development of physics-based constitutive theories on crushable sand.

The origin of homochirality in life remains a mystery that some believe is essential for life, and which may result from chiral symmetry breaking interactions with galactic organic material.

Arrhythmia models: in vivo, in vitro and in silico

Conjugated polymers (CPs) were first reported to exhibit high electrical conductivity upon heavy doping in 1977. For this discovery, Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa were awarded the Nobel Prize in Chemistry in 2000. Since the initial publications, there has been tremendous research and development to make conjugated polymers useful and to expand the scope of available materials and properties. Professor Olle Inganäs has long been a tall figure in the development of conjugated polymers and the expansion of organic polymer electronics to include many diverse areas such as bioelectronics, electromechanical machines, solar cells, light-emitting diodes, energy storage, and printed electronics. Here, we assemble a collection of articles in honor of Professor Inganäs to celebrate his 35 years of pioneering work, where he has contributed over 525 papers, focused on studies of conducting polymers throughout the areas of polymer physics, electrochemistry, electronics, and optics. His current interests include energy conversion and energy storage with organic photovoltaic devices and organic supercabatteries, as well as the use of biopolymers as organizers of electronic polymers. Olle initiated his research career with the ambition to do biophysics and found inspiration in photosynthesis and novel organic electronic materials after the initial discovery of CPs. To emphasize his interdisciplinary ambition, he changed the name of his research division to Biomolecular and Organic Electronics in the year 2000, which reflected the combination of two fields: bioelectronics and organic electronics. In the field of organic electronics, he carried out fundamental studies on thermochromism, charge transport, and electrochromic properties of conducting polymers (such as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), polypyrrole, and polythiophene). An area that further developed, based on these studies, was organic light-emitting diodes (OLEDs), where Olle made seminal work including new materials for OLEDs, covering the full visible spectrum and extending into the infrared and ultraviolet, and the first tunable white OLEDs, as well as the first nano-OLEDs in the 1990s. His most productive field throughout the years has, however, been organic photovoltaics (OPVs), covering optimization of active materials, device engineering, modeling, and device physics. Olle was the first to employ a low-bandgap alternating polyfluorene to enhance the photocurrent of OPVs (2003) and extended the spectral coverage to long wavelengths. In the last three decades, he made many innovations in device engineering significantly contributing to the increase in photoconversion efficiency. Olle was also the first to employ PEDOT:PSS as a hole-transporting layer into polythiophene/C-60 photodiodes (1998) and to use aqueous solution-processed poly(ethylene oxide) as an electron-transporting layer to replace vacuum-deposited lithium fluoride (2007). PEDOT:PSS was introduced as an anode and a cathode, respectively, to replace ITO for printing large-area flexible OPVs (2002, 2006). Double efficiency was achieved in lateral reflective tandem OPVs via folded devices (2007 and 2008). Vacuum-free inverted semitransparent OPVs were realized (2009, 2012), which not only pave the way to large-scale printing of OPV modules, but also extend the installation of OPV panels to windows and facades of buildings. Olle was the first to develop optical models of the OPV using the transfer matrix model (TMM) of multilayer OPV devices (1999). Furthermore, based on classical drift–diffusion models for electrons and holes, he also developed a model combining optical and electrical processes in OPVs in 2005. Another important contribution was his first identification of the existence of charge-transfer states (CTS) and the origin of the open-circuit voltage, which was done by studying the photoluminescence and electroluminescence of polymer/fullerene bulk heterojunctions (BHJs) in OPVs (2009). Olle was the first to quantify the internal quantum efficiencies of charge generation in OPVs by investigating the device performance combined with optical modeling (2012). These studies have resulted in the emergence of nonfullerene acceptors, which minimize the exciton dissociation leading to increased power conversion efficiencies. In addition to his heavy contributions in the field of organic energy conversion, Olle recently, made noteworthy contribution to energy storage, by illustrating that lignins, a green byproduct from the pulp industry, can form a structural composite with conducting polymers to store electric charge. In the field of bioelectronics, his initial steps toward bioinspired electronic applications were taken in the 1990s, when he contributed to utilizing the effect of ionic intercalation into polypyrrole to achieve electromechanical actuation. This phenomenon was later combined with microfabrication to develop the first organic electronic micromuscle. In parallel to this work, Olle explored the conducting polymer PEDOT and its ion/electron transduction capabilities to demonstrate supercapacitors, electrochemical transistors, and displays. This work eventually enabled a true bioelectronic interfaces in the form of neural and enzymatic contacts. Olle extended his activities to biosensing by developing water-soluble conjugated poly- and oligoelectrolytes, enabling optical probes for protein folding in vitro and protein diseases in vivo, as well as DNA chips. In a recent contribution, he demonstrated electronic interfacing of the lipid membranes and ion channels of cells using PEDOT in an electrochemical transistor. This special issue contains a collection of reviews and original research papers from 13 groups, with a diverse range of research topics in which Olle made important initial contributions, ranging from organic solar cells to bioelectronics. Andersson and co-workers (article number 1807275) review all-organic polymer solar cells. Zou and co-workers (article number 1806616) present a Communication studying flexible nonfullerene solar cells. Zhang, and co-workers (article number 1900690) present a Progress Report on the limitations and perspectives of triplet-based materials for organic solar cells. Gao, and co-workers (article number 1900326) present a Progress Report on the structural and functional diversity in lead-free perovskite materials. Berggren, Crispin, and co-workers (article number 1805813) present a Progress Report on ion–electron coupled functionality in conjugated polymers. Scheblykin and co-workers (article number 1805671) present a review of emerging polarization-microscopy methods for assessing organization and excitation energy transfer in single molecules, and beyond. Müller, Hamedi, and co-workers (article number 1807286) briefly review fiber-based organic electronics ranging from the molecular scale to the mega-scale. Malliaras, Herland and co-workers (article number 1806712) present an overview of biologic functions addressed using organic electronic devices. Bazan, Nilsson, and co-workers (article number 1806701) present a Progress Report on conjugated oligoelectrolytes for biosensing and therapeutics. Jaeger, and co-workers (article number 1808210) review conjugated-polymer actuators and devices. Kemerink and co-workers (article number 1806004) present a Progress Report on photogenerated charge transport in organic electronic materials. Wang and co-workers (article number 1807019) review donor–acceptor terpolymers for high-efficiency solar cells. Lastly, Pei and co-workers (article number 1807516) present a Communication studying stretchable organometal-halide-perovskite quantum-dot light-emitting diodes. Beside academic ambitions, Olle has contributed to several startup companies including “Micromuscle”, developing tools for single-cell manipulation using polymer micromuscles; “BioChromix”, developing luminescence biodetection; “Epishine”, developing large-scale semitransparent OPVs; and “Ligna Energy” developing wood-based energy-storage devices. As a concluding remark, we quote Prof. Inganäs on the prospective outlook for the exciting field of organic electronics: “The most important aspect of organic electronic materials is found in energy conversion and storage… The energy payback time for OPV is weeks, rather than the years necessary for silicon. This is the only affordable route to avoid filling the atmosphere with even more carbon dioxide from fossil fuels.” Mahiar Max Hamedi is an Associate Professor in Fiber and Polymer Technology at KTH Royal Institute of Technology in Sweden. His specializes in combining organic and nanoelectronics materials with bulk materials, and especially biopolymer-based materials, to achieve new devices. His current research focuses on paper microfluidic devices for point-of-care diagnostics, organic bioelectronics, and nanostructured materials for energy storage. He received his Ph.D. from Linköping University under Olle Inganäs and carried out postdoctoral research under Lars Wågberg at KTH, and under George M. Whitesides at Harvard. Besides research activities, he is also an entrepreneur and private investor mainly focusing on the software industry. Anna Herland is an Associate Professor in Biohybrid Systems at KTH Royal Institute of Technology, and at Karolinska Institutet (Sweden). She received her Ph.D. in organic bioelectronics from Linköping Universitet (Sweden) under Olle Inganäs and did postdoc fellowships at Karolinska Institutet in stem cell engineering under Ana Texieira and at Harvard University (USA) in tissue engineering under Donald Ingber. Her current research focuses on creating microphysiological models of tissue, especially the central nervous system. She develops human primary and stem-cell-derived systems combined with microfluidics, and uses organic electronics or bioelectronics stimuli and read-outs for real-time assessment of biological functions. Fengling Zhang is a Professor in the Department of Physics, Chemistry and Biology at Linköping University, Sweden. She specializes in device engineering and physics of organic photovoltaic devices. Her current research activities include polymer-based electrochromics, supercapacitors, thermoelectrics and integrated photocapacitors. She received her Ph.D. degree in solid-state physics from Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, and carried out postdoctoral research under Prof. Yasuhiko Shirota at Osaka University and Prof. Olle Inganäs at Linköping University. Qibing Pei is Professor of Materials Science and Engineering and Professor of Mechanical Engineering at the University of California, Los Angeles. He specializes in synthetic polymers and composites for electronic, electromechanical, and photonic applications. His current research activities focus on stretchable electronics, nanostructured polymer composites, dielectric elastomers and bistable electroactive polymers for muscle-like actuation, and electrocaloric polymer cooling. He received his Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences, carried out postdoctoral research under Olle Inganäs at Linköping University, and worked at UNIAX Corporation (now DuPont Display) and SRI International, Menlo Park, California.

With the current research impetus on neuromorphic computing hardware, realizing efficient drift and diffusive memristors are considered critical milestones for the implementation of readout layers, selectors, and frameworks in deep learning and reservoir computing networks. Current demonstrations are predominantly limited to oxide insulators with a soft breakdown behavior. While organic ionotronic electrochemical materials offer an attractive alternative, their implementations thus far have been limited to features exploiting ionic drift a.k.a. drift memristor technology. Development of diffusive memristors with organic electrochemical materials is still at an early stage, and modulation of their switching dynamics remains unexplored. Here, halide perovskite (HP) memristive barristors (diodes with variable Schottky barriers) portraying tunable diffusive dynamics and ionic drift are proposed and experimentally demonstrated. An ion permissive poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate interface that promotes diffusive kinetics and an ion source nickel oxide (NiOx) interface that supports drift kinetics are identified to design diffusive and drift memristors, respectively, with methylammonuim lead bromide (CH3NH3PbBr3) as the switching matrix. In line with the recent interest on developing artificial afferent nerves as information channels bridging sensors and artificial neural networks, these HP memristive barristors are fashioned as nociceptive and synaptic emulators for neuromorphic sensory signal computing.

Resistive random-access memory (RRAM), also known as memristors, having a very simple device structure with two terminals, fulfill almost all of the fundamental requirements of volatile memory, nonvolatile memory, and neuromorphic characteristics. Its memory and neuromorphic behaviors are currently being explored in relation to a range of materials, such as biological materials, perovskites, 2D materials, and transition metal oxides. In this review, we discuss the different electrical behaviors exhibited by RRAM devices based on these materials by briefly explaining their corresponding switching mechanisms. We then discuss emergent memory technologies using memristors, together with its potential neuromorphic applications, by elucidating the different material engineering techniques used during device fabrication to improve the memory and neuromorphic performance of devices, in areas such as I ON/I OFF ratio, endurance, spike time-dependent plasticity (STDP), and paired-pulse facilitation (PPF), among others. The emulation of essential biological synaptic functions realized in various switching materials, including inorganic metal oxides and new organic materials, as well as diverse device structures such as single-layer and multilayer hetero-structured devices, and crossbar arrays, is analyzed in detail. Finally, we discuss current challenges and future prospects for the development of inorganic and new materials-based memristors.