Flexible Electronic Materials Research Articles

Grain boundary passivation via balancing feedback of hole barrier modulation in HfO2-x for nanoscale flexible electronics

Flexible electronics has attracted considerable attention owing to its enormous potential for practical applications in various fields. However, the massive strain produced during bending degrades the device. Especially at grain boundaries, due to the accumulation of defects, this degradation is exacerbated in flexible electronic devices. The importance of electrically inactivated grain boundaries increases as devices scale down to the nanoscale. Here, we propose an HfO2-x thin film that can be used as an excellent material for flexible electronics with versatile functionality, especially for grain boundary passivation. Various electrical phases of HfO2-x thin films with conducting to insulating behavior, which originates from oxygen deficiency, have been fabricated on flexible substrates. Furthermore, owing to the most stable charge state of oxygen vacancies, oxygen-deficient HfO2-x shows p-type conductivity. Current mapping by conductive atomic force microscopy reveals that current flow is hindered at grain boundaries due to the formation of potential barriers. This phenomenon is also observed in bent flexible thin films on convex and concave molds, leading to tensile and compressive strains, respectively. Although the defect concentration increases because of lattice deformation during bending, more holes are trapped at the grain boundaries, resulting in an increased hole barrier height. We believe that grain boundary passivation through hole barrier modulation during bending would pave the way for advances in hafnia-based nanoscale flexible electronics.

Direct Comparison of the Effect of Processing Conditions in Electrolyte-Gated and Bottom-Gated TIPS-Pentacene Transistors

Among the plethora of soluble and easy processable organic semiconductors, 6,13-Bis(triisopropylsilylethynyl)pentacene (TIPS-P5) is one of the most promising materials for next-generation flexible electronics. However, based on the information reported in the literature, it is difficult to exploit in field-effect transistors the high-performance characteristics of this material. This article correlates the HMDS functionalization of the silicon substrate with the electrical characteristics of TIPS-P5-based bottom gate organic field-effect transistors (OFETs) and electrolyte-gated organic field-effect transistors (EGOFETs) fabricated over the same platform. TIPS-P5 transistors with a double-gate architecture were fabricated by simple drop-casting on Si/SiO2 substrates, and the substrates were either functionalized with hexamethyldisilazane (HMDS) or left untreated. The same devices were characterized both as standard bottom-gate transistors and as (top-gate) electrolyte-gated transistors, and the results with and without HMDS treatment were compared. It is shown that the functionalization of the silicon substrate negatively influences EGOFETs performance, while it is beneficial for bottom-gate OFETs. Different device architectures (e.g., bottom-gate vs. top-gate) require specific evaluation of the fabrication protocols starting from the effect of the HMDS functionalization to maximize the electrical characteristics of TIPS-P5-based devices.

Mechanical properties of 2D Zr n +1C n (n = 1, 2) MXenes with and without functionalization

Transition metal carbides and nitrides (MXenes) are considered the new generation of flexible electronic materials because of their superior mechanical strength and flexibility. Based on the density functional theory, the structures, electronic properties and mechanical properties of the 2D Zr-based MXenes with and without surface functional groups (O, F and OH) are investigated systematically to explore their elastic properties and tensile fracture mechanism. The results reveal the tensile strength and critical strain under biaxial tensile direction can reach 52 GPa, 12% for Zr2C and 55 GPa, 19% for Zr3C2, more outstanding than the mechanical behavior of the pristine Ti2C (47 GPa, 9.5%). The tensile behaviors of the functionalized Zr n +1CnT2 (n = 1, 2, T = O, F, OH) strongly depend on the crystallographic orientation and the surface functional group. The phonon spectrum under the critical strain indicates the tensile fracture of the pristine Zr-based MXenes was determined by phonon instability, except along the armchair direction of Zr2C and zigzag direction of Zr3C2. During tensile strain, the collapse of Zr n +1C n F2 and Zr n +1C n (OH)2 (n = 1, 2) are mainly caused by internal Zr–C bond rupture and transfer to the surface. While the O-functionalized Zr n +1C n O2 (n = 1, 2) presented the opposite collapse trend. Additionally, according to the research results of critical strain, elastic modulus and electrical conductivity, F/OH-terminated Zr2C MXene is relatively more suitable for flexible sensors of wearable devices than Zr3C2T2.

Design and synthesis of novel phenolphthalein-based poly (aryl ether) resin containing isopropyl groups

With the rapid development of 5G technology, low dielectric constant materials present a widespread application prospect in microelectronic devices. Poly (aryl ether) resins have been widely used in this field due to their excellent thermal stability, mechanical properties and electrical insulation properties, however, reducing its dielectric constant remains a challenge. In this study, novel phenolphthalein poly (aryl ether ketone) containing isopropyl groups and fluorene groups was successfully synthesized with high molecular weight via SN2 aromatic nucleophilic polycondensation reaction. At lower frequencies ranging from 10 Hz to 1 MHz and higher frequencies from 10 GHz to 42 GHz, the films exhibit the intrinsic dielectric constant in the range of 2.4–2.7. The resin has good thermal stability, mechanical properties and insulation properties. Thus, the functional poly (aryl ether ketone) possesses attractive potential applications in the field of high-performance flexible electronic packaging materials.

Versatile Copolymer for Stretchable and Self-healable Liquid-free Ionic Conductive Elastomers.

To facilitate the practical use of ionic conductive materials for flexible electronics, the issues existing in hydrogels and ionogels, such as low thermostability and possible solvent leakage, need to be resolved but are inevitable. Liquid-free ionic elastomers (ICEs) as an alternative option are free of such concerns but have been facing the drawbacks of low conductivity and less satisfying mechanical properties. Here, a versatile copolymer with π-π stacking and cation-π interactions for high-performance ICE is proposed. The ICEs presented tunable mechanical and electrical properties by varying the feed ratio of the ternary monomers. The optimized ICE possessed high stretchability and strength, fast shape-recovery, self-healing, decent conductivity, and desirable stability against heat and under ambient conditions. The use of virgin and self-healed ICEs as the conductors for dielectric elastomer actuators (DEA) is demonstrated and exhibits comparable actuating performance to the reported DEA employing organogels and ionogels. The work provides a facile approach for fabricating ICEs with versatile properties that can be used for flexible electronics.

Silicene: an excellent material for flexible electronics

The outstanding properties of graphene have laid the foundation for exploring graphene-like 2D systems, commonly referred to as 2D-Xenes. Among them, silicene is a front-runner due to its compatibility with current silicon fabrication technologies. Recent works on silicene have unveiled its useful electronic and mechanical properties. The rapid miniaturization of silicon devices and the useful electro-mechanical properties of silicene necessitate the exploration of potential applications of silicene flexible electronics in nano electro-mechanical systems. Using a theoretical model derived from the integration of ab initio density-functional theory and quantum transport theory, we investigate the piezoresistance effect of silicene in the nanoscale regime. As with graphene, we obtain a small value of the piezoresistance gauge factor (GF) of silicene, which is sinusoidally dependent on the transport angle. The small GF of silicene is attributed to its robust Dirac cone and strain-independent valley degeneracy. Based on the obtained results, we propose to use silicene as an interconnect in flexible electronic devices and as a reference piezoresistor in strain sensors. This work will hence pave the way for exploring flexible electronics applications in other 2D-Xene materials.

High stretchability and conductive stability of flexible hybrid electronic materials for smart clothing

In this study, a flexible hybrid electronic (FHE) material with enhanced conductivity was developed and successfully applied to electromyograms (EMG) and electrocardiograms (ECG) of smart clothing. The FHE material comprises a highly stretchable patterned circuit with dry flexible electrodes. The pre-stretched circuit was obtained by mixing three-dimensional (3D) dendritic silver nanoparticles (AgNPs) with two-dimensional (2D) reduced graphene oxide (rGO) in a thermoplastic elastomer and patterning the resultant as wires using steel plate stencil printing. Based on the nano-dispersion principle, the geometrical differences of nanomaterials could be exploited to form a 3D continuous phase conductive network with improved tensile properties. A highly stretchable and bendable conductive film with a resistance of 1.4 × 10−2 ohm/sq was obtained by mixing 80 wt% of 3D dendritic AgNPs and 1 wt% of rGO to produce pre-strained nanowires with a horseshoe pattern. Furthermore, the conductive film could be coated with a polyurethane (PU) layer to achieve good stability in the washing test. In the tensile fatigue test, the resistance variation ΔR of horseshoe patterned nanowire was only 1.9 ohm under 500 stretches. Additionally, a low-resistance film (∼103 ohm.cm) was fabricated by adding 7 wt% of carbon black and 2D graphene nanoplatelets (CB/GNPs, 4:1) and styrene maleic anhydride-amide (SMA-amide, 10:1 ratio with carbon material), yielding superior fatigue resistance. Finally, a design combining the pre-stretched circuit pattern with dry sensing flexible conductive electrodes was developed and successfully applied for EMG and ECG of smart clothes.

Study and Modeling of Graphene-Boron-Nitride Heterostructures

When we talk about nano devices, the molecule and its interface with electrodes play a key role. So, one of the major objectives is to select an organic nanomaterial with extensive applications, which requires smart synthesis of appropriate materials and an understanding of their properties. Here we modeled a device, which not only adds another “protuberance” to learn about the transport properties of the molecule but also helps in grasping its use as a considerable material for future flexible electronics. Modeling of materials at the nano-level not only provides fundamental insight into the properties of crystalline defects but also gives a reasonable understanding of phase stability and learning of processes like atomic diffusion interface migration. For the development of devices at a mesoscopic and macroscopic level and with atomistic input parameters, this recognition serves as a guide. We tried to model how the layers of one type of molecule and the interaction of two different types of molecular layers control the junction charge transport characteristics.

Hierarchical Wrinkles for Tunable Strain Sensing Based on Programmable, Anisotropic, and Patterned Graphene Hybrids.

Flexible, stretchable, wearable, and stable electronic materials are widely studied, owing to their applications in wearable devices and the Internet of Things. Because of the demands for both strain-insensitive resistors and high gauge factor (GF) strain-sensitive materials, anisotropic strain sensitivity has been an important aspect of electronic materials. In addition, the materials should have adjustable strain sensitivities. In this work, such properties are demonstrated in reduced graphene oxide (RGO) with hierarchical oriented wrinkle microstructures, generated using the two-step shrinkage of a rubber substrate. The GF values range from 0.15 to 28.32 at 100% strain. For device demonstrations, macrostructure patterns are designed to prepare patterned wrinkling graphene at rubber substrate (PWG@R). Serpentiform curves can be used for the constant-value resistor, combined with the first-grade wrinkles. Strip lines can increase the strain-sensing property, along with the second-grade wrinkles. The patterned sensor exhibits improved GF values range from 0.05 to 49.5. The assembled sensor shows an excellent stability (>99% retention after 600 cycles) with a high GF (49.5). It can monitor the vital signs of the throat and wrist and sense large motions of fingers. Thus, PWG@R-based strain sensors have great potential in various health or motion monitoring fields.

Constructive adaptation of 3D-printable polymers in response to typically destructive aquatic environments.

In response to environmental stressors, biological systems exhibit extraordinary adaptive capacity by turning destructive environmental stressors into constructive factors; however, the traditional engineering materials weaken and fail. Take the response of polymers to an aquatic environment as an example: Water molecules typically compromise the mechanical properties of the polymer network in the bulk and on the interface through swelling and lubrication, respectively. Here, we report a class of 3D-printable synthetic polymers that constructively strengthen their bulk and interfacial mechanical properties in response to the aquatic environment. The mechanism relies on a water-assisted additional cross-linking reaction in the polymer matrix and on the interface. As such, the typically destructive water can constructively enhance the polymer's bulk mechanical properties such as stiffness, tensile strength, and fracture toughness by factors of 746% to 790%, and the interfacial bonding by a factor of 1,000%. We show that the invented polymers can be used for soft robotics that self-strengthen matrix and self-heal cracks after training in water and water-healable packaging materials for flexible electronics. This work opens the door for the design of synthetic materials to imitate the constructive adaptation of biological systems in response to environmental stressors, for applications such as artificial muscles, soft robotics, and flexible electronics.

Plant‐Based Substrate Materials for Flexible Green Electronics

AbstractWith the increasing use of soft and flexible electronics, there is a growing need to develop substrate materials that mitigate potential environmental risks associated with non‐degradable electronics waste from synthetic substrate materials. To address this issue, the authors develop a novel, 2D plant‐based substrate termed “sporosubstrate”, which is made of non‐allergenic natural pollen. The pollen particle has a double‐layered architecture with an ultra‐tough sporopollenin exine, and a soft cellulose intine is engineered through an eco‐friendly process. In this manner, a readily available, economical, biodegradable, and biocompatible microgel can be prepared. This microgel can be used to create a variety of flexible shapes with customized mechanical, geometrical, electronic, and functional properties and performance characteristics such as thermal, chemical, and mechanical stability and optical transparency. Moreover, the authors demonstrate here different applications of the flexible natural substrate made of pollen microgel for use in electronic devices for health monitoring and wearable wireless heating. The results of this work point to opportunities for the development of a new class of flexible green electronics based on plant‐based materials in applications such as wearable sensors, implantable devices, and soft robotics.

Integrating quasi-one-dimensional superconductors on flexible substrates

In recent years, the field of flexible electronics has become one of the cross-disciplinary research hotspots, attracting worldwide attention and making rapid advances. So far, there has been plenty of research on the use of two-dimensional (2D) materials in flexible electronics, including graphene, transition metal dichalcogenide, and so on. In this work, we successfully prepared quasi-one-dimensional (Q1D) Nb2Pd0.73S5.97 superconductors on flexible paper by mechanical friction and systematically studied their physical properties at low temperatures. Superconductivity with transition temperature (Tc) ∼ 6.05 K by Meissner effect was observed in Nb2Pd0.73S5.97 wires coated on flexible paper, and a resistance drop at 4.80 K was confirmed in electrical transport measurements. The lower critical field (Hc1) of coated paper shows anisotropy effect under parallel and perpendicular magnetic fields, exhibiting a 2D-like feature, unlike the bulk Nb2Pd0.73S5.97 fibers. Our work provides a broader platform for the application of low-dimensional materials in flexible functional devices.

Facile Surface Modification of Ultrathin Gold Nanowires Film with Hydrogen Sulfide for Improvement of Stability

Ultrathin gold nanowires (AuNWs) have an important role and promising applications in areas of wearable sensors, matter detection, and flexible transparent electronic materials. But oleylamine (OAm) ligand on the ultrathin AuNWs surface leads to high junction resistance and the inherent Rayleigh instability leads to decomposition into gold spheres within a few days. To eliminate these two disadvantages, herein, the implementation of one facile H2S gas treatment method to improve the conductivity and stability of ultrathin AuNWs is tried. The conductivity of AuNWs on the interdigital electrode can be improved by 4 × 107 times because the AuNWs became bent and cross‐linked, due to the H2S exchange of the ligand OAm. In addition, the conductivity of the three‐layer ultrathin AuNW film on polyethylene terephthalate (PET) can be improved by at least 2000 times. The ultrafine structure of AuNWs can be maintained for more than 1 week at room temperature, which attributes to H2S that can protect the surface structure of AuNWs. The findings highlight a new method of the ultrathin AuNWs for exceeding improvement in electronic conduction and stability.

Positive temperature coefficient of resistance in Na2S interacted polyaniline on cellulose substrate: A flexible electronic material

Flexible electronic materials of polyaniline (PANI) chemically interacted with Na2S have been developed with the objective of improving and tuning its electrical transport properties and optical properties. The chemical interaction of Na2S with PANI was realized by the synthesis process of physical vapor polymerization of aniline over cellulose substrate. The prepared samples of PANI interacted with Na2S at different concentrations (3%, 5%, 7%, and 10%) have been studied through X-ray diffraction (XRD) measurement, scanning electron microscope (SEM) measurement, analysis of FTIR spectra, I-V measurement, and analysis of UV–vis–NIR absorption spectra. From XRD measurement it was observed that the peaks corresponding to the emeraldine form of PANI were obtained during polymerization, which is electrically conductive. Being partially polymerized, this form of PANI provides the scope for further functionalization through chemical interaction with Na2S. As a consequence of chemical interaction with Na2S, its resistivity was found to increase systematically with the increase of concentration of Na2S. A twelve times change in resistivity was recorded with Na2S interaction. The temperature dependence of electrical conductivity shows an excellent positive temperature coefficient of resistance, which is of profound technological significance for self-controlled overheating protection of electronic devices.

CNT-assisted design of stable liquid metal droplets for flexible multifunctional composites

Gallium-based liquid metals (LMs) hold great promise as potential star materials for wearable and flexible electronics due to their superior electrical conductivity, high thermal conductivity, and good deformability. However, it remains still a huge challenge to realize the good dispersion of LM droplets. Herein, we demonstrated the design of highly stable LM droplets with the assistance of carbon nanotubes (CNTs). The resultant CNT@LM droplets showed excellent dispersion stability, without obvious precipitation even after settling for 30 days. The CNT@LM droplets could be easily sprayed onto diverse polymer substrates (thermoplastic polyurethanes, cellulose, and polyethylene terephthalate), showing an electrical conductivity of 1.0 × 106 S/m and a superior electromagnetic interference shielding effectiveness of 50.9 dB at only 5 μm thickness. Moreover, the CNT@LM droplets exhibited a homogeneous dispersion in waterborne polyurethane and it was demonstrated the as-prepared composite films excellent Joule heating performance with a high saturation temperature (63.0 °C) at a low supplied voltage (2.0 V). In addition, a superior out-plane thermal conductivity of 2.19 W/(m·K) was achieved for the composite films. This work provides a novel idea to address the agglomeration of LMs, opening a promising avenue for their practical applications in wearable and flexible electronics.

Scalable production of high-quality carbon nanotube dispersion in aqueous solution using cellulose as dispersant by a freezing/thawing process

Carbon nanotube (CNT) dispersion, which enables scalable fabrication of printed and flexible electronic materials, has brought a revolutionary improvement in CNT-based devices. To progress from the laboratory to commercial applications, it will be necessary to develop simple and scalable methods to produce high-quality CNT dispersions. Here, we propose a freezing/thawing strategy assisted by cellulose to produce high-quality CNT dispersion in an aqueous solution on a large scale. The freezing/thawing process simultaneously serves two purposes: (1) one is to cleave CNTs from their bulk via water-freezing expansion force; (2) the other is to dissolve cellulose in NaOH/urea aqueous solution, facilitating the wrapping of cellulose on the surface of CNTs to improve the stability of the dispersion. The obtained cellulose-dispersed CNT (CDCNT) dispersion is highly homogeneous and stable because of the formation of CNT/cellulose/NaOH/urea inclusion complexes. Importantly, the freezing/thawing strategy using cellulose promises scalable production, including a high CNT concentration of 11.3 mg/mL, high yield of 75%, high production capacity, and unlimited system size, much superior to common ultrasonication. Furthermore, it is demonstrated that the CDCNT dispersion can be used as a precursor solution for fabricating different CNT-based materials via various techniques, such as spinning, dyeing, screen printing, and 3D printing, indicating versatile applications.

Near-Amorphous Conjugated Polymers: An Emerging Class of Semiconductors for Flexible Electronics

The emergence of flexible electronics with broad application prospects has put forward new requirements for organic semiconductors, i.e., not only the electrical properties but also the mechanical properties. Among the strategies to achieve synergy between mechanical and electrical properties, near-amorphous polymers are competitive candidates due to their advantages of intrinsic flexibility, high mobility, and high reproducibility. Near-amorphous polymers with excellent performance are continuing to attract attention, and recently, some significant progress has been achieved in this field. In this perspective, we summarize these excellent advances on near-amorphous polymers from the point of view of their molecular design principles, their resulting charge transport and mechanical properties, and their potential applications in flexible electronic devices. Finally, we give a brief discussion on the challenges and opportunities of near-amorphous materials in flexible electronics from the perspectives of semiconductor layer materials themselves, devices, and multifunctional integration.

Conductive, self-healing, and repeatable graphene/carbon nanotube/polyurethane flexible sensor based on Diels-Alder chemothermal drive

Flexible conductive materials have excellent stretchability and sensitivity, so they have great development potential in motion detection, artificial electronic skin and other fields. Three-dimensional (3D) carbon nanomaterials are important materials for preparing flexible conductive polymers with good electrical conductivity. But its cost is too high to be widely promoted. In this paper, the modified graphene (FGO) and carbon nanotubes (FMCNTs) were prepared to three-dimensional structures to provide excellent electrical conductivity for the polymer. The reversible bond was introduced into the waterborne polyurethane by using Diels-Alder (DA) chemical crosslinking. Then the conductive and self-healing flexible electronic materials FGO/FMCNTs/WPU were prepared by penetration. The results showed that the composites had excellent thermal and mechanical properties. When the content of nanomaterial was 6 mg/ml, the stress was 9.58 MPa, the strain was 207, and the electrical conductivity was 0.0283 S/m. Nanomaterials also showed good self-healing ability because they had three-dimensional structures. The application test of the composite material showed that it could accurately monitor the human body motion. This indicated that the composite material can be used to manufacture flexible electronic devices.

Study of capacitance type flexible electronic devices based on polyacrylamide and reduced graphene oxide composite hydrogel

Flexible electronic devices can transform various external stimuli into electrical signals, and have broad prospects in wearable devices, soft machines, artificial intelligence and other fields. However, the flexible electronic materials based on hydrogel lack mechanical strength and stability. Meanwhile, the electrodes in hydrogels are also easily corroded by electrolytes. In this work, the capacitive sensor based on polyacrylamide-rGO (PAM-rGO) composite hydrogel was studied to solve these problems. Due to the effect of hydrogen bonds, the reduced graphene oxide (rGO) can greatly enhance the mechanical strength of hydrogels. At the same time, the hydrogel is encapsulated in the capacitor, and the stability of the capacitor increases due to the decrease of the water loss of the hydrogel. The PAM-rGO composite hydrogel did not contain free anion and cation, the corrosion of the electrode decreased and the service life was prolonged. The wearable device of PAM-rGO composite hydrogel sensor performed well in tensile and compression stimulation tests, and the linear relationship reached 0.998 and 0.992 in turn, showing potential applications in the field of flexible electronic devices.

Application of conductive polymer hydrogels in flexible electronics

AbstractFlexible electronic devices to obtain accurate and efficient information interactions between humans and machines have gained increasing attention in recent years. A series of soft materials for flexible electronics have been developed to improve device performance in terms of electrical and mechanical properties. Among them, conductive polymer‐based hydrogels (CPHs), which combine the tunable electronic properties of conductive polymers and the soft mechanical properties of hydrogels, are promising candidates for next‐generation wearable electronic devices. This review summarized the material design and preparation of CPHs, and presented the properties of CPHs, including tunable conductivity, outstanding mechanical performance, biocompatibility, self‐healing capability, resistance to freezing, and solution processability. In particular, their emerging applications in flexible electronics devices including flexible supercapacitors, flexible sensors, and biomedical electronics are highlighted. Furthermore, perspectives on existing challenges and opportunities in this field are discussed.