A highly durable, stretchable, transparent and conductive carbon nanotube-polymeric acid hybrid film

Flexible and Stretchable Light-Emitting Diodes and Photodetectors for Human-Centric Optoelectronics

Optoelectronic devices with unconventional form factors, such as flexible and stretchable light-emitting or photoresponsive devices, are core elements for the next-generation human-centric optoelectronics. For instance, these deformable devices can be utilized as closely fitted wearable sensors to acquire precise biosignals that are subsequently uploaded to the cloud for immediate examination and diagnosis, and also can be used for vision systems for human-interactive robotics. Their inception was propelled by breakthroughs in novel optoelectronic material technologies and device blueprinting methodologies, endowing flexibility and mechanical resilience to conventional rigid optoelectronic devices. This paper reviews the advancements in such soft optoelectronic device technologies, honing in on various materials, manufacturing techniques, and device design strategies. We will first highlight the general approaches for flexible and stretchable device fabrication, including the appropriate material selection for the substrate, electrodes, and insulation layers. We will then focus on the materials for flexible and stretchable light-emitting diodes, their device integration strategies, and representative application examples. Next, we will move on to the materials for flexible and stretchable photodetectors, highlighting the state-of-the-art materials and device fabrication methods, followed by their representative application examples. At the end, a brief summary will be given, and the potential challenges for further development of functional devices will be discussed as a conclusion.

Interface Engineering for High-Performance Thermoelectric Carbon Nanotube Films

Carbon nanotubes (CNTs) stand out for their exceptional electrical, thermal, and mechanical attributes, making them highly promising materials for cutting-edge, lightweight, and flexible thermoelectric applications. However, realizing the full potential of advanced thermoelectric CNTs requires precise management of their electrical and thermal characteristics. This study, through interface optimization, demonstrates the feasibility of reducing the thermal conductivity while preserving robust electrical conductivity in single-walled CNT films. Our findings reveal that blending two functionalized CNTs offers a versatile method of tailoring the structural and electronic properties of CNT films. Moreover, the modified interface exerts a substantial influence over thermal and electrical transfer, effectively suppressing heat dissipation and facilitating thermoelectric power generation within CNT films. As a result, we have successfully produced both p- and n-type thermoelectric CNTs, attaining impressive power factors of 507 and 171 μW/mK2 at room temperature, respectively. These results provide valuable insights into the fabrication of high-performance thermoelectric CNT films.

Molecular Doping Modulation and Applications of Structure-Sorted Single-Walled Carbon Nanotubes: A Review

Single-walled carbon nanotubes (SWCNTs) that have a reproducible distribution of chiralities or single chirality are among the most competitive materials for realizing post-silicon electronics. Molecular doping, with its non-destructive and fine-tunable characteristics, is emerging as the primary doping approach for the structure-controlled SWCNTs, enabling their eventual use in various functional devices. This review provides an overview of important advances in the area of molecular doping of structure-controlled SWCNTs and their applications. The first part introduces the underlying physical process of molecular doping, followed by a comprehensive survey of the commonly used dopants for SWCNTs to date. Then, it highlights how the convergence of molecular doping and structure-sorting strategies leads to significantly improved functionality of SWCNT-based field-effect transistor arrays, transparent electrodes in optoelectronics, thermoelectrics, and many emerging devices. At last, several challenges and opportunities in this field are discussed, with the hope of shedding light on promoting the practical application of SWCNTs in future electronics.

Interfacial Chemical Bridging Constructed by Multifunctional Lewis Acid for Carbon Nanotube/Silicon Heterojunction Solar Cells with an Efficiency Approaching 17.7

Single-wall carbon nanotube/silicon (SWCNT/Si) heterojunction shows appealing potential for use in photovoltaic devices. However, the relatively low conductivity of SWCNT network and interfacial recombination of carriers have limited their photovoltaic performance. Herein, a multifunctional Lewis acid (p-toluenesulfonic acid, TsOH) is used to significantly reduce the energy loss in SWCNT/Si solar cells. Owing to the charge transfer doping effect of TsOH, the conductivity and work function of SWCNT films are optimized and tuned. More importantly, a chemical bridge is constructed at the interface of SWCNT/Si heterojunction. Experimental studies indicate that the phenyl group of TsOH can interact with SWCNTs through π-π interaction, meanwhile, the oxygen in the sulfonic functional group of the TsOH molecule can graft on the dangling bonds of the Si surface. The chemical bridge structure effectively suppresses the recombination of photogenerated carriers. The TsOH coating also works as an antireflection layer, leading to a 19% increment of the photocurrent. As a result, a champion power conversion efficiency of 17.7% is achieved for the TsOH-SWCNT/Si device, and it also exhibits an excellent stability, retaining more than 96% of the initial efficiency in the ambient air after 1 month.

Transparent Conducting Films Based on Carbon Nanotubes: Rational Design toward the Theoretical Limit

Electrically conductive thin-film materials possessing high transparency are essential components for many optoelectronic devices. The advancement in the transparent conductor applications requires a replacement of indium tin oxide (ITO), one of the key materials in electronics. ITO and other transparent conductive metal oxides have several drawbacks, including poor flexibility, high refractive index and haze, limited chemical stability, and depleted raw material supply. Single-walled carbon nanotubes (SWCNTs) are a promising alternative for transparent conducting films (TCFs) because of their unique and excellent chemical and physical properties. Here, the latest achievements in the optoelectronic performance of TCFs based on SWCNTs are analyzed. Various approaches to evaluate the performance of transparent electrodes are briefly reviewed. A roadmap for further research and development of the transparent conductors using "rational design," which breaks the deadlock for obtaining the TCFs with a performance close to the theoretical limit, is also described.

Electrostatic interaction-controlled dispersion of carbon nanotubes in a ternary composite for high-performance supercapacitors

Effective dispersion of carbon nanotubes (CNTs) is of great importance to achieve their intrinsic performance. Normally, it is believed that CNT dispersion is decided by interactions between CNTs and their dispersants, while other interactions are often neglected. Herein, three ionic surfactants, sodium dodecyl sulfate (SDS), dodecyl dimethyl betaine (BS-12) and cetyltrimethylammonium bromide (CTAB), are used to disperse CNTs in a ternary composite, i.e., poly(p-phenylenediamine)-phosphomolybdic acid@reduced graphene oxide (DMoG), respectively, leading to three different DMoGC composites. It has been found that the CNT dispersion in DMoGC was mainly controlled by electrostatic interactions between the surfactants and DMoG, which further exerted vital influences on the constitution, content, morphology, porous structure and supercapacitive performance of the DMoGC composites. Among the three surfactants, cationic CTAB showed the best CNT dispersion, while amphoteric BS-12 could hardly disperse CNTs in DMoGC, leading to DMoGC-CTAB with a 2 times larger specific surface area (152.3 m² g^-1) and 1.5 times higher specific capacitance (422 F g^-1) than those of DMoGC-(BS-12). Our study can provide valuable guidelines for selecting/designing effective dispersants to prepare multi-component composites containing uniformly dispersed CNTs.

Stable Doping of Single-Walled Carbon Nanotubes for Flexible Transparent Conductive Films

Possessing excellent electronic and mechanical properties and great stability, single-walled carbon nanotubes (SWCNTs) are exceptionally attractive in fabricating flexible transparent conductive films. Doping is a key step to further enhance the conductivity of the SWCNT films and the reliable doping is highly needed. We developed a feasible strategy that uses solid acids such as phosphotungstic acid (PTA) to dope the SWCNT films stably relying on the nonvolatility of the dopants. The sheet resistance of the films was reduced to around a half of the original value meanwhile with no obvious change in transmittance. The doping effect maintained during a 700 days' observation. The excellent flexibility of the PTA-doped films was demonstrated by a bending test of 1000 cycles, during which the sheet resistance and transmittance was basically unaffected. The blue shifts of G band in the Raman spectra and the increase of work function measured by the Kelvin probe force microscopy both reveal the p-type doping of the films by PTA. The strong acidity of PTA plays a key role in the doping effect by increasing the redox potential of the ambient O₂ and thus the Fermi level of the SWCNTs is brought down. The great feasibility and robustness of our doping strategy are desirable in the practical application of SWCNT-based flexible transparent conductive films. This strategy can be extended to the p-type doping of various CNT-based assemblies (such as sponges and forests) as well as other material families, expanding the application spectrum of polyacids.

Co-Ni Basic Carbonate Nanowire/Carbon Nanotube Network With High Electrochemical Capacitive Performance via Electrochemical Conversion

In this work, the Co-Ni basic carbonate nanowires were in-situ grown on carbon nanotube (CNT) network through a facile chemical bath deposition method, which could be further converted into active hydroxide via cyclic voltammetry strategy. A series of carbonate nanowire/nanotube with different Co/Ni ratio revealed the different growth status of the nanowires on CNT network. The nanostructures of the as-synthesized samples were examined via powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) techniques. The Co/Ni ratio of the carbonate largely affected the size of the nanowires, that the low Co/Ni ratio was beneficial for thin nanowire formation and the nanowires loading on CNT network. Subsequently, the electrochemical performance of the Co-Ni basic hydroxides was studied in a three-electrode test system. The nanowires with low Co/Ni ratio 1/2 can form nanowire array on individual CNTs, which exhibited better electrochemical capacitive performance than the composite network with high Co/Ni ratio nanowires after electrochemical activation. The addition of Co enhanced the rate performance of the hydroxide/CNT, especially improved the long cycle stability largely compared to the rate performance of pure Ni converted hydroxide/CNT composite film reported by our previous research. This result is valuable for the design of inorganic electrochemical active composites based on conductive networks for energy conversion/storage applications.

Rapid nanowelding of silver nanowires by focused-light-scanning for high-performance flexible transparent electrodes

Silver nanowires (AgNWs) have been considered as one of the most promising flexible transparent electrodes (FTEs) material for next-generation optoelectronic devices. However, the large contact resistance between AgNWs could deteriorate the conductivity of FTEs. In the present work, high-performance AgNWs FTEs were obtained by means of focused-light-scanning (FLS), which could lead to the large-area, rapid and high-quality welding between AgNWs within a short time, forming the reliable and stable AgNWs network. The results of the optoelectronic tests show that after FLS, the sheet resistance of the AgNWs FTEs sharply decreased from 5113 Ω/sq to 7.7 Ω/sq, with maintaining a high transmittance (∼94%). Finally, a high-performance flexible transparent heater was fabricated by using FLS, showing reach a relatively high temperature in a short response time and rapid response at low input voltage. The findings offer an effective pathway to greatly improve the conductivity of AgNWs FTEs.

Multifunctional Additives for High-Energy-Density Lithium-Ion Batteries: Improved Conductive Additive/Binder Networks and Enhanced Electrochemical Properties

Cylindrical-type cells have been widely adopted by major battery and electric vehicle manufacturers owing to their price competitiveness, safety, and easy expandability. However, placement of electrodes at the core of cylindrical cells is currently restricted because of high electrode curvature and the lack of specialized electrodes and electrode materials. Here, we report the realization of highly flexible high-energy-density electrodes (active material loading of >98.4%) that can be used at the cores of cylindrical cells. The improved properties result from the introduction of a multifunctional poly(melamine-co-formaldehyde) (MF copolymer) additive, which yields a relatively more fluidic and well-dispersed slurry using only 0.08 wt %. MF copolymer-mediated formation of completely wrapped CNT/PVDF networks on LiCoO₂ (LCO) and accompanying contact enhancement between LCO and carbon nanotubes (CNTs) resulted in an increase of electrical and mechanical properties of the two types (composites with or without collectors) of electrodes compared with those of additive-free electrodes. Flexibility tests were carried out by rolling electrodes onto cylinder substrates (diameters of ca. 1 and 10 mm); this process resulted in relatively lower resistance changes of the MF copolymer-containing electrodes than for the reference electrodes. In addition, capacity retention after 100 cycles for cells with and without MF copolymers was approximately 10-20% better for the samples with the MF copolymer than for those without. CNT/PVDF networks with MF copolymers were proven to induce a relatively thin and stable cathode electrolyte interface layer accompanying the chemical bond formation during cycling.

Thermoresponsive and Conductive Chitosan-Polyurethane Biocompatible Thin Films with Potential Coating Application

Conductive thin films have great potential for application in the biomedical field. Herein, we designed thermoresponsive and conductive thin films with hydrophilicity, strain sensing, and biocompatibility. The crosslinked dense thin films were synthesized and prepared through a Schiff base reaction and ionic interaction from dialdehyde polyurethane, N-carboxyethyl chitosan, and double-bonded chitosan grafted polypyrrole. The thin films were air-dried under room temperature. These thin films showed hydrophilicity and conductivity (above 2.50 mS/cm) as well as responsiveness to the deformation. The tensile break strength (9.72 MPa to 15.07 MPa) and tensile elongation (5.76% to 12.77%) of conductive thin films were enhanced by heating them from 25 °C to 50 °C. In addition, neural stem cells cultured on the conductive thin films showed cell clustering, proliferation, and differentiation. The application of the materials as a conductive surface coating was verified by different coating strategies. The conductive thin films are potential candidates for surface modification and biocompatible polymer coating.

High-Performance Flexible Transparent Conductive Films Enabled by a Commonly Used Antireflection Layer

Recently, silver nanowire-based transparent conductive films (AgNW-based TCFs) with excellent comprehensive performance have aroused wide and great interest. However, it is always difficult to simultaneously improve the performances of TCFs in all aspects. In this work, by introducing silica nanoparticles (SiO₂-NPs) with a smaller particle size, several properties of AgNW-based TCFs were optimized successfully. The transmittance and conductivity were improved simultaneously, and smaller particle size was proven to be more suitable to achieve TCFs with excellent optoelectrical properties. Typically, an AgNW/SiO₂-based TCF with a sheet resistance of 250 Ω/sq and transmittance of 93.6% (including the poly (ethylene terephthalate) substrate, abbreviated as PET) could be obtained by using SiO₂-NPs with a size of ∼21 nm, and this transmittance is even higher than that of the bare PET (91.8%) substrate. We demonstrated that the layer formed through self-assembly of SiO₂-NPs can cut down the light scattering on the AgNW surface through total reflection, thus leading to a low haze of AgNW/SiO₂-based TCFs. Very interestingly, the SiO₂-NPs conducted away most of the heat generated during laser ablation, protecting the AgNWs from excessive melt and PET from empyrosis, and thus ensuring the TCFs with high transmittance and patterning accuracy. Besides, AgNW/SiO₂-based TCFs have smaller surface roughness, better flexibility, and adhesive force. To the best of our knowledge, the comprehensive performance of the AgNW/SiO₂-based TCFs reaches the highest level among recently reported novel TCFs.

An active carbon-nanotube polarizer-embedded electrode and liquid-crystal alignment

We report a method for constructing an active optical polarizer using an aligned carbon nanotube (CNT) sheet that is flexible, bendable, transparent, conductive, and also serves to anchor liquid-crystal (LC) molecules. A horizontally aligned CNT sheet was obtained by mechanical stretching from a vertically grown CNT forest, which was then transferred onto a substrate. A liquid polymer was infiltrated into the CNT sheet followed by UV curing, while a part of the CNT sheet was still exposed on the film surface without polymer coating. The polymer-embedded CNT sheet (P-ECS) film with 10 layers of CNT sheets exhibited a good polarization efficiency of 87%, a sheet resistance of 340 Ω□^-1, and excellent ability to align LC molecules. The high stability of the P-ECS film was confirmed from the very low variation of sheet resistance (2%) and transmittance (10%) observed during a bending test of 1000 cycles. In addition, a twisted nematic LC device constructed using the P-ECS films shows a good bright-dark switching performance. The P-ECS film functions simultaneously as a transparent electrode, a film-type polarizer, and a LC alignment layer, demonstrating the multi-functionality of the active CNT film. This study thus highlights a wide range of possible applications for active polarizers and flexible displays.

Nanomaterial-Enabled Flexible and Stretchable Sensing Systems: Processing, Integration, and Applications

Nanomaterial-enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial-enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real-world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial-enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.

Silsesquioxane-Polythiophene Hybrid Copolymer as an Efficient Modifier for Single-Walled Carbon Nanotubes

One silsesquioxane-polythiophene hybrid copolymer, with combined star-like structure and intramolecular heterogeneity, was synthesized and sufficiently characterized via various methods, including FTIR, NMR, and SEC measurements. According to the exploration and characterization results, it was much more efficient at modifying SWNTs than its linear analogs in aqueous solution. The hydrophobic silsesquioxane core and PEDOT chains could locally anchor to the surface of the nanotubes, while the soluble flexible copolymer chains extended into the solution and rigid conjugated chains provided some π-π stacking effect to enhance adhesive force with the conjugated structure of the carbon nanotube, imparting steric stabilization to nanotube dispersion. The noncovalent interaction with SWNTs and solubility in aqueous solution improved the electrochemical characteristics of the modified-SWNT composite and availed for the preparation of a flexible and transparent electroactive film. Accordingly, this kind of silsesquioxane-polythiophene hybrid copolymer will be forwarded to apply to the assembling of flexible optoelectronic devices.

Nonuniform functional group distribution of carbon nanotubes studied by energy dispersive X-ray spectrometry imaging in SEM

Functionalization is a key technique to improving the dispersibility of carbon nanotubes (CNTs) in solvents and polymer matrices for producing versatile CNT-based materials. Therefore, a robust and efficient characterization method is required to confirm that the functionalization on the CNT surface is spatially uniform. Although several imaging techniques for transmission electron microscopes can characterize the spatial localization of elements chemically bound to an isolated CNT surface, they are unsuitable for examinations on a practical scale because of their limited scanning area. Here, we present high spatially resolved energy dispersive X-ray spectrometry (EDS) imaging of functionalized single-walled CNTs (SWCNTs) in scanning electron microscopy (SEM). Highly sensitive EDS detection and drift-free operation enables our technique to image the light elements of SWCNTs with sufficient spatial resolution (<10 nm). We describe an experimental visualization of the spatial distribution of the functionalization on individual SWCNT bundle structures and discuss the CNT de-bundling mechanism via surface modification and the uniformity of the CNT dispersion state.

A stretchable laminated GNRs/BNNSs nanocomposite with high electrical and thermal conductivity

The rapid development of modern electronics has accelerated the demand for stretchable components with high thermal management capability because increasing the power density and miniaturization of electronic devices generate greater heat. However, stretchable electronics with enhanced heat dissipation have been rarely reported. In this study, a stretchable laminated nanocomposite-based conductor with both robust electric conductivity and enhanced thermal management capability was fabricated. With the optimized GNRs and BNNS contents, this conductor exhibited a thermal conductivity enhancement of 266%, leading to a decrease in the working temperature from 57.4 °C to 29.2 °C. Even under 100% strain, the fluctuation of the equilibrium operational temperature was within 10%. Moreover, the conductor showed outstanding electric performance under 200% strain with an R/R₀ value of 1.46. Whether stretched and tested in a Moebius-belt shape or under hard-environmental conditions such as in seawater, crude oil, and even integrated in a wireless charging circuit, the significant reliability of this conductor was recorded. Thus, our results are promising to provide a practical approach for the fabrication of stretchable electronic devices working in high temperature environments associated with extreme thermal stresses and under extreme circumstances such as sea rescue operations and marine oil pollution remediation.