Cellulose nanofiber paper as an ultra flexible nonvolatile memory

Hydrogel-Based Artificial Synapses for Sustainable Neuromorphic Electronics

Hydrogels find widespread applications in biomedicine because of their outstanding biocompatibility, biodegradability, and tunable material properties. Hydrogels can be chemically functionalized or reinforced to respond to physical or chemical stimulation, which opens up new possibilities in the emerging field of intelligent bioelectronics. Here, the state-of-the-art in functional hydrogel-based transistors and memristors is reviewed as potential artificial synapses. Within these systems, hydrogels can serve as semisolid dielectric electrolytes in transistors and as switching layers in memristors. These synaptic devices with volatile and non-volatile resistive switching show good adaptability to external stimuli for short-term and long-term synaptic memory effects, some of which are integrated into synaptic arrays as artificial neurons; although, there are discrepancies in switching performance and efficacy. By comparing different hydrogels and their respective properties, an outlook is provided on a new range of biocompatible, environment-friendly, and sustainable neuromorphic hardware. How potential energy-efficient information storage and processing can be achieved using artificial neural networks with brain-inspired architecture for neuromorphic computing is described. The development of hydrogel-based artificial synapses can significantly impact the fields of neuromorphic bionics, biometrics, and biosensing.

Addition of fibers derived from paper mill sludge in paper coatings: impact on microstructure, surface and optical properties

Traditionally, cellulose nanofiber (CNF) production has primarily relied on virgin cellulose sources. Yet, the shift to using paper mill sludge (PMS) as a source for CNF underscores the significance of reusing and recycling industrial byproducts. PMS contains significant amounts of cellulose that can be extracted as a raw material. The purpose of present study is to provide a sustainable approach to PMS utilization as a paper coating additive in the cellulose nanofibrils (CNFPMS) form via simply scalable wire-wound rod coating method. The effect of CNFPMS additive amounts at two coating layers on microstructure and surface properties of coatings such as porosity, air permeability surface roughness and optical properties such as brightness, gloss and CIE L*a*b* is studied, which they can also provide insight for the eventual print performance. Results indicated that the obtained CNFPMS in paper coating shows 52% decrease in porosity, presenting significant improvement in the coating microstructure. The marginal increase in permeability coefficient and surface roughness, 54% and 10%, respectively, suggests improving color reproduction and preventing color density losses. Optical analysis showed slight decrease in brightness and gloss, as was expected. Notably, the lightness was improved, which also indicates increasing color gamut volume in printing applications. As a result, the current work offers a sustainable approach to manage PMS for use in paper coatings as a high-value-added material.

Ion-Incorporative, Degradable Nanocellulose Crystal Substrate for Sustainable Carbon-Based Electronics

Electronic wastes from transient electronics accumulate biologically harmful materials with global concern. Recycling these wastes could prevent the deposition of hazardous chemicals and toxic materials to the environment while saving scarce natural compounds and valuable resources. Here, we report a sustainable electronic device, taking advantage of carbon resources and a biodegradable cellulose composite. The device consists of an ambient-stable carbon nanotube as a semiconductor, graphene as electrodes, and a free-standing cellulose filter paper/nanocellulose composite as a dielectric layer. The dual-functional cellulose composite acting simultaneously as a robust substrate and a dielectric is demonstrated, which is compatible with solution device fabrication processes. An optimized channel dimension of 5 mm × 3 mm with the addition of ions that facilitates a charge transport realized a device with an on-current per width of 9.6 μA mm^-1, an on/off ratio >10², a field-effect mobility of 2.03 cm² V^-1 s^-1, and long-term stability over 30 days under ambient conditions. Successful separation of the carbonaceous components via an eco-friendly solution sorting protocol allowed the recycled device to display excellent electronic performance, with a recapture efficiency of 90%. This effort demonstrates a processable, low-cost, and sustainable electronic system that can be applied in the current realm of the semiconducting and sensing industry.

Biopolymer based artificial synapses enable linear conductance tuning and low-power for neuromorphic computing

Neuromorphic computing is considered a promising method for resolving the traditional von Neumann bottleneck. Natural biomaterial-based artificial synapses are popular units for constructing neuromorphic computing systems while suffering from poor linearity and limited conduction states. In this work, a AgNO₃ doped iota-carrageenan (ι-car) based memristor is proposed to resolve the non-linear limitation. The memristor presents linear conductance tuning with a higher endurance (∼10⁴), more enriched conduction states (>2000), and much lower power consumption (∼3.6 μW) than previously reported biomaterial-based analog memristors. AgNO₃ is doped to ι-car to suppress the formation of Ag filaments, thereby eliminating uneven Joule heating. Using deep learning of hand-written digits as an application, a doping-enhanced recognition accuracy (93.8%) is achieved, close to that of an ideal synaptic device (95.7%). This work verifies the feasibility of using biopolymers for future high-performance computational and wearable/implantable electronic applications.

Applications of biomemristors in next generation wearable electronics

With the rapid development of mobile internet and artificial intelligence, wearable electronic devices have a great market prospect. In particular, information storage and processing of real-time collected data are an indispensable part of wearable electronic devices. Biomaterial-based memristive systems are suitable for storage and processing of the obtained information in wearable electronics due to the accompanying merits, i.e. sustainability, lightweight, degradability, low power consumption, flexibility and biocompatibility. So far, many biomaterial-based flexible and wearable memristive devices were prepared by spin coating or other technologies on a flexible substrate at room temperature. However, mechanical deformation caused by mechanical mismatch between devices and soft tissues leads to the instability of device performance. From the current research and practical application, the device will face great challenges when adapting to different working environments. In fact, some interesting studies have been performed to address the above issues while they were not intensively highlighted and overviewed. Herein, the progress in wearable biomemristive devices is reviewed, and the outlook and perspectives are provided in consideration of the existing challenges during the development of wearable biomemristive systems.

Freestanding Translucent ZnO-Cellulose Nanocomposite Films for Ultraviolet Sensor Applications

The rapidly advancing technology of wearable and miniaturized electronics has increased the demand for low-cost high-performance flexible sensors. Herein, the preparation of translucent freestanding films consisting of cellulose nanofibers (CNFs) and ZnO nanoparticles (NPs) via a simple spray coating method is presented. The obtained nanocomposite films were thin (~10 µm) and flexible. The scanning electron microscopy and atomic force microscopy analysis revealed that the nanocomposite film was composed of regions of ZnO NP-modified CNFs and regions of aggregation of ZnO NPs with each other. The electrical conductance of the films was rapidly increased beyond 40 wt.% ZnO and reached up to >50 nA at 60 wt.% ZnO. This was attributed to the increased number of conductive paths formed by the ZnO NPs in the nanocomposite film when a certain threshold was crossed. The ZnO−CNF nanocomposite film exhibited a stable response over on/off cycles of UV light exposure. The responsivity and sensitivity of the nanocomposite film with 60 wt.% ZnO were 36.5 mA/W and 247, respectively. Even when the device was curved (radius of curvature: 3 mm), the response and sensitivity remained high. The developed nanocomposite films are expected to be applied as environmentally friendly flexible UV sensors.

Enhanced Hydrophobicity in Nanocellulose-Based Materials: Toward Green Wearable Devices

Nanocellulose is a promising material for fabricating green, biocompatible, flexible, and foldable devices. One of the main issues of using nanocellulose as a fundamental component for wearable electronics is the influence of environmental conditions on it. The water adsorption promotes the swelling of nanopaper substrates, which directly affects the devices' electrical properties prepared on/with it. Here, plant-based nanocellulose substrates, and ink composites deposited on them, are chemically modified using hexamethyldisilazane to enhance the system's hydrophobicity. After the treatment, the electrical properties of the devices exhibit stable operation under humidity levels around 95%. Such stability demonstrates that the hexamethyldisilazane modification substantially suppresses the water adsorption on fundamental device structures, namely, substrate plus conducting ink. These results attest to the robustness necessary to use nanocellulose as a key material in wearable devices such as electronic skins and tattoos and contribute to the worldwide efforts to create biodegradable devices engineered in a more deterministic fashion.

High-Speed Fabrication of Clear Transparent Cellulose Nanopaper by Applying Humidity-Controlled Multi-Stage Drying Method

As a renewable nanomaterial, transparent nanopaper is one of the promising materials for electronic devices. Although conventional evaporation drying method endows nanopaper with superior optical properties, the long fabrication time limits its widely use. In this work, we propose a multi-stage drying method to achieve high-speed fabrication of clear transparent nanopaper. Drying experiments reveal that nanopaper's drying process can be separated into two periods. For the conventional single-stage evaporation drying, the drying condition is kept the same. In our newly proposed multi-stage drying, the relative humidity (RH), which is the key parameter for both drying time and haze, is set differently during these two periods. Applying this method in a humidity-controllable environmental chamber, the drying time can be shortened by 35% (from 11.7 h to 7.6 h) while maintaining the same haze level as that from single-stage drying. For a conventional humidity-uncontrollable oven, a special air flow system is added. The air flow system enables decrease of RH by removing water vapor at the water/air interface during the earlier period, thus fabricating clear transparent nanopaper in a relatively short time. Therefore, this humidity-controlled multi-stage drying method will help reduce the manufacturing time and encourage the widespread use of future nanopaper-based flexible electronics.

Resistive switching of silicon-silver thin film devices in flexible substrates

Novel applications for memory devices demand nanoscale flexible structures. In particular, resistive switching (RS) devices are promising candidates for wearable and implantable technologies. Here, the Pt/Si/Ag/TiW metal-insulator-metal structure was fabricated and characterized on top of flexible substrates using a straightforward microfabrication process. We also showed that these substrates are compatible with sputtering deposition. RS was successfully achieved using both commercial cellulose cleanroom paper and bacterial cellulose, and polymer (PET) substrates. The bipolar switching behavior was observed for both flat and bent (under a radius of 3.5 mm) configurations. The observed phenomenon was explained by the formation/rupture of metallic Ag filaments in the otherwise insulating Si host layer.

Building memory devices from biocomposite electronic materials

Natural biomaterials are potential candidates for the next generation of green electronics due to their biocompatibility and biodegradability. On the other hand, the application of biocomposite systems in information storage, photoelectrochemical sensing, and biomedicine has further promoted the progress of environmentally benign bioelectronics. Here, we mainly review recent progress in the development of biocomposites in data storage, focusing on the application of biocomposites in resistive random-access memory (RRAM) and field effect transistors (FET) with their device structure, working mechanism, flexibility, transient characteristics. Specifically, we discuss the application of biocomposite-based non-volatile memories for simulating biological synapse. Finally, the application prospect and development potential of biocomposites are presented.

Paper-Based Disposable Molecular Sensor Constructed from Oxide Nanowires, Cellulose Nanofibers, and Pencil-Drawn Electrodes

Progress toward the concept of "a trillion sensor universe" requires sensor devices to become more abundant, ubiquitous, and be potentially disposable. Here, we report a paper-based disposable molecular sensor device constructed from a nanowire sensor based on common zinc oxide (ZnO), a wood-derived biodegradable cellulose nanofiber paper substrate, and a low-cost graphite electrode. The ZnO nanowire/cellulose nanofiber composite structure is embedded in the surface of the cellulose nanofiber paper substrate via a two-step papermaking process. This structure provides a mechanically robust and efficiently bridged network for the nanowire sensor, while ensuring efficient access to target molecules and allowing reliable electrical contact with electrodes. The as-fabricated paper sensor device with pencil-drawn graphite electrodes exhibits efficient resistance change-based molecular sensing of NO₂ as a model gas. The performance of our device is comparable to that of noble metal electrodes. Furthermore, we demonstrate cut-and-paste usability and easy disposal of the sensor device with its uniform in-plane sensing properties. Our strategy offers a disposable molecular sensing platform for use in future sensor network technologies.

Micropatterning Silver Nanowire Networks on Cellulose Nanopaper for Transparent Paper Electronics

Transparent microelectrodes with high bendability are necessary to develop lightweight, small electronic devices that are highly portable. Here, we report a reliable fabrication method for transparent and highly bendable microelectrodes based on conductive silver nanowires (AgNWs) and 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO)-oxidized cellulose nanofibers (CNFs). The AgNW-based micropatterns were simply fabricated on glass via poly(ethylene glycol) photolithography and then completely transferred to transparent TEMPO-CNF nanopaper with high bendability via vacuum-assisted microcontact printing (μCP). The AgNW micropatterns were embedded in the surface layer of TEMPO-CNF nanopaper, enabling strong adhesion to the nanopaper substrate. The resulting AgNW micropatterns on the TEMPO-CNF nanopaper showed an optical transparency of 82% at 550 nm and a sheet resistance of 54 Ω/sq when the surface density of AgNWs was as low as 12.9 μg/cm². They exhibited good adhesion stability and excellent bending durability. After 12 peeling test cycles and 60 s sonication time, the sheet resistance of the AgNW networks embedded on TEMPO-CNF nanopaper increased by only ∼0.12 and ∼0.07 times, respectively. Furthermore, no significant change in electrical resistance was observed even after 3 bending cycles to nearly 90° and 500 cycles of 80% bending strain. Moreover, the AgNW patterns on TEMPO-CNF paper were successfully applied for constructing a transparent electric circuit as well as a solid-state electrochromic device. Overall, we proposed an effective way to fabricate AgNW micropatterns on transparent nanopaper, which can be expanded to various conductive materials for high-performance paper-based electronics.

Ultralow Power Consumption Flexible Biomemristors

Low power consumption is the important requirement in memory devices for saving energy. In particular, improved energy efficiency is essential in implantable electronic devices for operation under a limited power supply. Here, we demonstrate the use of κ-carrageenan (κ-car) as the resistive switching layer to achieve memory that has low power consumption. A carboxymethyl (CM) group is introduced to the κ-car to increase its ionic conductivity. Ag was doped in CM:κ-car to improve the resistive switching properties of the devices. Memory devices based on Ag-doped CM:κ-car showed electroforming-free resistive switching. This device exhibited low reset voltage (∼0.05 V), fast switching speed (50 ns), and high on/off ratio (>10³) under low compliance current (10^-5 A). Its power consumption (∼0.35 μW) is much lower than those of the previously reported biomemristors. The resistive switching may be a result of an electrochemical redox process and Ag filament formation in the CM:κ-car under an electric field. This biopolymer memory can also be fabricated on flexible substrate. This study verifies the feasibility of using biopolymers for applications to future implantable and biocompatible nanoelectronics.

Clearly Transparent Nanopaper from Highly Concentrated Cellulose Nanofiber Dispersion Using Dilution and Sonication

Nanopaper prepared from holocellulose pulp is one of the best substrates for flexible electronics because of its high thermal resistance and high clear transparency. However, the clearness of nanopaper decreases with increasing concentration of the starting cellulose nanofiber dispersion-with the use of a 2.2 wt % dispersion, for example-resulting in translucent nanopaper with a high haze of 44%. To overcome this problem, we show that the dilution of this high-concentration dispersion with water followed by sonication for 10 s reduces the haze to less than 10% while maintaining the high thermal resistance of the nanopaper. Furthermore, the combination of water dilution and a short sonication treatment improves the clearness of the nanopaper, which would translate into cost savings for the transportation and storage of this highly concentrated cellulose nanofiber dispersion. Finally, we demonstrate the improvement of the electrical conductivity of clear transparent nanopaper prepared from an initially high-concentration dispersion by dropping and heating silver nanowire ink on the nanopaper. These achievements will pave the way toward the realization of the mass production of nanofiber-based flexible devices.

Wearable Intrinsically Soft, Stretchable, Flexible Devices for Memories and Computing

A recent trend in the development of high mass consumption electron devices is towards electronic textiles (e-textiles), smart wearable devices, smart clothes, and flexible or printable electronics. Intrinsically soft, stretchable, flexible, Wearable Memories and Computing devices (WMCs) bring us closer to sci-fi scenarios, where future electronic systems are totally integrated in our everyday outfits and help us in achieving a higher comfort level, interacting for us with other digital devices such as smartphones and domotics, or with analog devices, such as our brain/peripheral nervous system. WMC will enable each of us to contribute to open and big data systems as individual nodes, providing real-time information about physical and environmental parameters (including air pollution monitoring, sound and light pollution, chemical or radioactive fallout alert, network availability, and so on). Furthermore, WMC could be directly connected to human brain and enable extremely fast operation and unprecedented interface complexity, directly mapping the continuous states available to biological systems. This review focuses on recent advances in nanotechnology and materials science and pays particular attention to any result and promising technology to enable intrinsically soft, stretchable, flexible WMC.

Paper in Electronic and Optoelectronic Devices

Paper, one of the oldest materials for storage and exchange of human's information, has been reinvented as a building component of electronic and optoelectronic devices over the past decades with successful demonstration of paper-based or paper-using devices. These recent achievements can meet the demand for lightweight, cost-effective, and/or flexible electronic and optoelectronic devices with advanced functionality and reduced manufacturing costs. This article provides a review of electronic and optoelectronic devices relying on or making use of the unique properties achievable with paper-based materials. Basic scientific/technical principles, quantitative comparisons of material, electronic and/or optical properties, and benefits for each paper-based application are given. Application-specific research challenges, future design considerations, and development directions are also discussed.

Thermal conductivity analysis and applications of nanocellulose materials

In this review, we summarize the recent progress in thermal conductivity analysis of nanocellulose materials called cellulose nanopapers, and compare them with polymeric materials, including neat polymers, composites, and traditional paper. It is important to individually measure the in-plane and through-plane heat-conducting properties of two-dimensional planar materials, so steady-state and non-equilibrium methods, in particular the laser spot periodic heating radiation thermometry method, are reviewed. The structural dependency of cellulose nanopaper on thermal conduction is described in terms of the crystallite size effect, fibre orientation, and interfacial thermal resistance between fibres and small pores. The novel applications of cellulose as thermally conductive transparent materials and thermal-guiding materials are also discussed.

Organic bistable memory devices based on MoO3 nanoparticle embedded Alq3 structures

Organic bistable memory devices were fabricated by embedding a thin layer of molybdenum trioxide (MoO₃) between two tris-(8-hydroxyquinoline)aluminum (Alq₃) layers. The device exhibited excellent switching characteristics with an ON/OFF current ratio of 1.15 × 10³ at a read voltage of 1 V. The device showed repeatable write-erase capability and good stability in both the conductance states. These conductance states are non-volatile in nature and can be obtained by applying appropriate voltage pulses. The effect of MoO₃ layer thickness and its location in the Alq₃ matrix on characteristics of the memory device was investigated. The field emission scanning electron microscopy (FE-SEM) images of the MoO₃ layer revealed the presence of isolated nanoparticles. Based on the experimental results, a mechanism has been proposed for explaining the conductance switching of fabricated devices.

High performance transient organic solar cells on biodegradable polyvinyl alcohol composite substrates

Physically transient organic solar cells on PVA composite substrates have been successfully demonstrated for the first time.

Development and applications of transparent conductive nanocellulose paper

Increasing attention has been paid to the next generation of 'green' electronic devices based on renewable nanocellulose, owing to its low roughness, good thermal stability and excellent optical properties. Various proof-of-concept transparent nanopaper-based electronic devices have been fabricated; these devices exhibit excellent flexibility, bendability and even foldability. In this review, we summarize the recent progress of transparent nanopaper that uses different types of nanocellulose, including pure nanocellulose paper and composite nanocellulose paper. The latest development of transparent and flexible nanopaper electronic devices are illustrated, such as electrochromic devices, touch sensors, solar cells and transistors. Finally, we discuss the advantages of transparent nanopaper compared to conventional flexible plastic substrate and the existing challenges to be tackled in order to realize this promising potential.

Foldable and Disposable Memory on Paper

Foldable organic memory on cellulose nanofibril paper with bendable and rollable characteristics is demonstrated by employing initiated chemical vapor deposition (iCVD) for polymerization of the resistive switching layer and inkjet printing of the electrode, where iCVD based on all-dry and room temperature process is very suitable for paper electronics. This memory exhibits a low operation voltage of 1.5 V enabling battery operation compared to previous reports and wide memory window. The memory performance is maintained after folding tests, showing high endurance. Furthermore, the quick and complete disposable nature demonstrated here is attractive for security applications. This work provides an effective platform for green, foldable and disposable electronics based on low cost and versatile materials.

Biodegradable resistive switching memory based on magnesium difluoride

This study presents a new type of resistive switching memory device that can be used in biodegradable electronic applications. The biodegradable device features magnesium difluoride as the active layer and iron and magnesium as the corresponding electrodes. This is the first report on magnesium difluoride as a resistive switching layer. With on-off ratios larger than one hundred, the device on silicon switches at voltages less than one volt and requires only sub-mA programming current. AC endurance of 10(3) cycles is demonstrated with ±1 V voltage pulses. The switching mechanism is attributed to the formation and rupture of conductive filaments comprising fluoride vacancies in magnesium difluoride. Devices fabricated on a flexible polyethylene terephthalate substrate are tested for functionality, and degradation is subsequently demonstrated in de-ionized water. An additional layer of magnesium difluoride is used to hinder the degradation and extend the lifetime of the device.

Use of nanocellulose in printed electronics: a review

Since the last decade, interest in cellulose nanomaterials known as nanocellulose has been growing. Nanocellulose has various applications ranging from composite reinforcement to rheological modifiers. Recently, nanocellulose has been shown to have great potential in flexible printed electronics applications. The property of nanocellulose to form self-standing thermally stable films has been exploited for producing transparent and smooth substrates for printed electronics. However, other than substrates, the field of printed electronics involves the use of inks, various processing methods and the production of flexible electronic devices. This review aims at providing an overview of the use and potential of nanocellulose throughout the printed electronics field.

Surface effects in metal oxide-based nanodevices

As devices shrink to the nanoscale, surface-to-volume ratio increases and the surface-environment interaction becomes a major factor for affecting device performance. The variation of electronic properties, including the surface band bending, gas chemisorption or photodesorption, native surface defects, and surface roughness, is called "surface effects". Such effects are ambiguous because they can be either negative or beneficial effects, depending on the environmental conditions and device application. This review provides an introduction to the surface effects on different types of nanodevices, offering the solutions to respond to their benefits and negative effects and provides an outlook on further applications regarding the surface effect. This review is beneficial for designing nano-enabled photodetectors, harsh electronics, memories, sensors and transistors via surface engineering.

Conductivity of PEDOT:PSS on Spin-Coated and Drop Cast Nanofibrillar Cellulose Thin Films

Aqueous dispersion of conductive polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (

PEDOT

PSS) was deposited on spin-coated and drop cast nanofibrillar cellulose (NFC)-glycerol (G) matrix on a glass substrate. A thin glycerol film was utilized on plasma-treated glass substrate to provide adequate adhesion for the NFC-glycerol (NFC-G) film. The effects of annealing temperature, the coating method of NFC-G, and the coating time intervals on the electrical performance of the

PEDOT

PSS were characterized.

PEDOT

PSS on drop cast NFC-G resulted in 3 orders of magnitude increase in the electrical conductivity compared to reference

PEDOT

PSS film on a reference glass substrate, whereas the optical transmission was only slightly decreased. The results point out the importance of the interaction between the

PEDOT

PSS and the NFC-G for the electrical and barrier properties for thin film electronics applications.

Transparent Conductive Nanofiber Paper for Foldable Solar Cells

Optically transparent nanofiber paper containing silver nanowires showed high electrical conductivity and maintained the high transparency, and low weight of the original transparent nanofiber paper. We demonstrated some procedures of optically transparent and electrically conductive cellulose nanofiber paper for lightweight and portable electronic devices. The nanofiber paper enhanced high conductivity without any post treatments such as heating or mechanical pressing, when cellulose nanofiber dispersions were dropped on a silver nanowire thin layer. The transparent conductive nanofiber paper showed high electrical durability in repeated folding tests, due to dual advantages of the hydrophilic affinity between cellulose and silver nanowires, and the entanglement between cellulose nanofibers and silver nanowires. Their optical transparency and electrical conductivity were as high as those of ITO glass. Therefore, using this conductive transparent paper, organic solar cells were produced that achieved a power conversion of 3.2%, which was as high as that of ITO-based solar cells.

Cost-Effective and Highly Photoresponsive Nanophosphor-P3HT Photoconductive Nanocomposite for Near-Infrared Detection

The advent of flexible optoelectronic devices has accelerated the development of semiconducting polymeric materials. We seek to replace conventional expensive semiconducting photodetector materials with our cost-effective composite system. We demonstrate in this work the successful fabrication of a photoconductive composite film of poly(3-hexylthiophene-2,5-diyl) (P3HT) mixed with NaYF4:Yb,Er nanophosphors that exhibited a ultrahigh photoresponse to infrared radiation. The high photocurrent measured was enabled by the unique upconversion properties of NaYF4:Yb,Er nanophosphors, where low photon energy infrared excitations are converted to high photon energy visible emissions that are later absorbed by P3HT. Here we report, a significant 1.10 × 10(5) times increment of photocurrent from our photoconductive composite film upon infrared light exposure, which indicates high optical-to-electrical conversion efficiency. Our reported work lays the groundwork for the future development of printable, portable flexible and functional photonic composites for light sensing and harvesting, photonic memory devices, and phototransistors.

Chemical Modification of Cellulose Nanofibers for the Production of Highly Thermal Resistant and Optically Transparent Nanopaper for Paper Devices

Optically transparent cellulose nanopaper is one of the best candidate substrates for flexible electronics. Some types of cellulose nanopaper are made of mechanically or chemically modified cellulose nanofibers. Among these, nanopapers produced from chemically modified cellulose nanofibers are the most promising substrate because of their lower power consumption during fabrication and higher optical transparency (lower haze). However, because their thermal durability is as low as plastics, paper devices using chemically modified nanopaper often do not have sufficiently high performance. In this study, by decreasing the carboxylate content in the cellulose nanofibers, the thermal durability of chemically modified nanopaper was drastically improved while maintaining high optical transparency, low coefficient of thermal expansion, and low power consumption during fabrication. As a result, light-emitting diode lights illuminated on the chemically modified nanopaper via highly conductive lines, which were obtained by printing silver nanoparticle inks and high-temperature heating.

Chiral nematic porous germania and germanium/carbon films

We report our extensive attempts and, ultimately, success to produce crack-free, chiral nematic GeO2/cellulose nanocrystal (CNC) composite films with tunable photonic properties from the controlled assembly of germanium(iv) alkoxides with the lyotropic liquid-crystalline CNCs in a mixed solvent of water/DMF. With different pyrolysis conditions, the photonic GeO2/CNC composites can be converted into freestanding chiral nematic films of amorphous GeO2, and semiconducting mesoporous GeO2/C and Ge/C replicas. These new materials are promising for chiral separation, enantioselective adsorption, catalysis, sensing, optoelectronics, and lithium ion batteries. Furthermore, the new, reproducible synthesis strategies developed may be applicable for constructing other composites and porous materials with chiral nematic ordering.

High-performance green flexible electronics based on biodegradable cellulose nanofibril paper

Today's consumer electronics, such as cell phones, tablets and other portable electronic devices, are typically made of non-renewable, non-biodegradable, and sometimes potentially toxic (for example, gallium arsenide) materials. These consumer electronics are frequently upgraded or discarded, leading to serious environmental contamination. Thus, electronic systems consisting of renewable and biodegradable materials and minimal amount of potentially toxic materials are desirable. Here we report high-performance flexible microwave and digital electronics that consume the smallest amount of potentially toxic materials on biobased, biodegradable and flexible cellulose nanofibril papers. Furthermore, we demonstrate gallium arsenide microwave devices, the consumer wireless workhorse, in a transferrable thin-film form. Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using ecofriendly materials.

Water-resistant, transparent hybrid nanopaper by physical cross-linking with chitosan

One of the major, but often overlooked, challenges toward high end applications of nanocelluloses is to maintain their high mechanical properties under hydrated or even fully wet conditions. As such, permanent covalent cross-linking or surface hydrophobization are viable approaches, however, the former may hamper processability and the latter may have adverse effect on interfibrillar bonding and resulting material strength. Here we show a concept based on physical cross-linking of cellulose nanofibers (CNF, also denoted as microfibrillated cellulose, MFC, and, nanofibrillated cellulose, NFC) with chitosan for the aqueous preparation of films showing high mechanical strength in the wet state. Also, transparency (∼70-90% in the range 400-800 nm) is achieved by suppressing aggregation and carefully controlling the mixing conditions: Chitosan dissolves in aqueous medium at low pH and under these conditions the CNF/chitosan mixtures form easily processable hydrogels. A simple change in the environmental conditions (i.e., an increase of pH) reduces hydration of chitosan promoting multivalent physical interactions between CNF and chitosan over those with water, resulting effectively in cross-linking. Wet water-soaked films of CNF/chitosan 80/20 w/w show excellent mechanical properties, with an ultimate wet strength of 100 MPa (with corresponding maximum strain of 28%) and a tensile modulus of 4 and 14 GPa at low (0.5%) and large (16%) strains, respectively. More dry films of similar composition display strength of 200 MPa with maximum strain of 8% at 50% air relative humidity. We expect that the proposed, simple concept opens new pathways toward CNF-based material utilization in wet or humid conditions, which has still remained a challenge.

A miniaturized flexible antenna printed on a high dielectric constant nanopaper composite

A high-dielectric-constant and flexible cellulose nanopaper composite is prepared by mixing a small amount of silver nanowires with cellulose nanofibers. The nanopaper antenna is downsized by about a half when using the nanopaper substrate. The nanopaper antenna has potential in wearable wireless communication devices.

Resistive switching memory based on bioinspired natural solid polymer electrolytes

A solution-processed, chitosan-based resistive-switching memory device is demonstrated with Pt/Ag-doped chitosan/Ag structure. The memory device shows reproducible and reliable bipolar resistive switching characteristics. A memory device based on natural organic material is a promising device toward the next generation of nonvolatile nanoelectronics. The memory device based on chitosan as a natural solid polymer electrolyte can be switched reproducibly between high and low resistance states. In addition, the data retention measurement confirmed the reliability of the chitosan-based nonvolatile memory device. The transparent Ag-embedded chitosan film showed an acceptable and comparable resistive switching behavior on the flexible plastic substrate as well. A cost-effective, environmentally benign memory device using chitosan satisfies the functional requirements of nonvolatile memory operations.

3 nm Thick Lignocellulose Nanofibers Obtained from Esterified Wood with Maleic Anhydride

Esterification with maleic anhydride before mechanical treatments enabled wood to fibrillate into thin and uniform thick lignocellulose nanofibers. The esterification did not affect the crystal structure of the cellulose, and carboxyl groups introduced by the esterification facilitated the fibrillation of the wood. Moisture in the reaction system caused hydrolysis of some of the lignin and hemicellulose, thereby assisting the fibrillation. The esterification significantly reduced the number of passes through the disk mill required for the production of lignocellulose nanofibers with large specific surface areas. By using a high-pressure homogenizer, 97 wt % of the esterified wood was fibrillated into 3 nm thick lignocellulose nanofibers.

Chemically-modified cellulose paper as a microstructured catalytic reactor

We discuss the successful use of chemically-modified cellulose paper as a microstructured catalytic reactor for the production of useful chemicals. The chemical modification of cellulose paper was achieved using a silane-coupling technique. Amine-modified paper was directly used as a base catalyst for the Knoevenagel condensation reaction. Methacrylate-modified paper was used for the immobilization of lipase and then in nonaqueous transesterification processes. These catalytic paper materials offer high reaction efficiencies and have excellent practical properties. We suggest that the paper-specific interconnected microstructure with pulp fiber networks provides fast mixing of the reactants and efficient transport of the reactants to the catalytically-active sites. This concept is expected to be a promising route to green and sustainable chemistry.

Effects of drying temperature and ethanol concentration on bipolar switching characteristics of natural Aloe vera-based memory devices

Natural Aloe vera provides a biodegradable, biocompatible, and renewable avenue for the sustainable development of electronics.