Free-standing Carbon Nanotube Films Research Articles

Enhanced Thermal Conductivity of Free-Standing Double-Walled Carbon Nanotube Networks.

Nanomaterials are driving advances in technology due to their oftentimes superior properties over bulk materials. In particular, their thermal properties become increasingly important as efficient heat dissipation is required to realize high-performance electronic devices, reduce energy consumption, and prevent thermal damage. One application where nanomaterials can play a crucial role is extreme ultraviolet (EUV) lithography, where pellicles that protect the photomask from particle contamination have to be transparent to EUV light, mechanically strong, and thermally conductive in order to withstand the heat associated with high-power EUV radiation. Free-standing carbon nanotube (CNT) films have emerged as candidates due to their high EUV transparency and ability to withstand heat. However, the thermal transport properties of these films are not well understood beyond bulk emissivity measurements. Here, we measure the thermal conductivity of free-standing CNT films using all-optical Raman thermometry at temperatures between 300 and 700 K. We find thermal conductivities up to 50 W m-1 K-1 for films composed of double-walled CNTs, which rises to 257 W m-1 K-1 when considering the CNT network alone. These values are remarkably high for randomly oriented CNT networks, roughly seven times that of single-walled CNT films. The enhanced thermal conduction is due to the additional wall, which likely gives rise to additional heat-carrying phonon modes and provides a certain resilience to defects. Our results demonstrate that free-standing double-walled CNT films efficiently dissipate heat, enhancing our understanding of these promising films and how they are suited to applications in EUV lithography.

Free-Standing CNT Film for Interlaminar Toughening: Insight into Infiltration and Thickness Effects.

Carbon fiber reinforced polymer composites have the advantages of being lightweight, having high strength and designability, and having been extensively used. However, the interlaminar toughness and delamination resistance of these composites are relatively poor due to their laminated structure and intrinsic brittleness of resin matrix. In this paper, commercialized free-standing carbon nanotube (CNT) films, drawn from CNT forests, were used to toughen the interlaminar interfaces of the composites. The effects of resin infiltration state and thickness of CNT films on the interlaminar toughening effect were systematically investigated. The results show that the pre-infiltration treatment of CNT films with acetone diluted epoxy resin solution can effectively improve the degree of resin infiltration. Compared with the samples containing untreated CNT film, the Mode I and Mode II interlaminar fracture toughness of the treated samples were significantly improved. The GIC reached a maximum of 1412.42 J/m2 at a CNT film thickness of 5 µm, which was about 61.38% higher than that of the baseline. At a CNT film thickness of 15 µm, the GIIC reached a maximum value of 983.73 J/m2, approximately 67.58% higher than that of the baseline. The corresponding toughening mechanisms were also systematically analyzed.

Stress-Strain and Resistance-Strain Hysteresis in Single-Wall Carbon Nanotube Films for Stretchable Battery Electrodes

The need to protect the vulnerable and limited resources that are naturally available while meeting the growing energy demand of the world makes the sustainable development of renewable energy of paramount importance. To address these needs, a new type of battery electrode has been developed at Honda Research Institute (HRI), based on free-standing single-wall carbon nanotube (SWNT) films containing active battery materials as dispersed nanoparticles. The SWNT films provide both mechanical support and electrical conductivity, removing the need for metal electrodes (increasing the energy density of the battery) and binder materials (making the process environmentally friendly).Under cyclic strain, the SWNT-based electrodes show hysteretic behaviors in both stress-strain and electrical resistance-strain relations. The electrical resistance-strain behavior shows a different trend from the previously observed behavior on 2D SWNT films (on elastic substrate). We hypothesize that this change can be attributed to the differences in the SWNT network structures in the 2D and 3D films. To understand the microscopic origin of the hysteresis, we performed coarse-grained molecular statics (CGMS) simulations of the SWNT film. The simulations show that the intrinsic Van der Waals interaction between SWNTs is responsible for the observed robust mechanical properties of the electrodes. In addition, our study also reveals that the sparsity of the 3D morphology results in the current being carried across the simulation box only by a few paths. As a consequence, the resistance hysteresis is governed entirely by the change in the power-carrying capacity of the network due to the breaking and formation of contacts between the SWNTs during loading-unloading. The qualitative nature of the resistance-strain hysteresis can thus be attributed to the rapid formation of inter-nanotube contacts, as the nanotube bundles (elongated by initial loading) buckle during unloading.

Pore structure changes in free-standing single-wall carbon nanotube film on vacuum high-temperature annealing

• Highly pure and crystalline free-standing SWCNT films were fabricated. • Impurities can be removed completely by vacuum high-temperature annealing. • Vacuum annealing can improve crystallinity of SWCNT film. • Vacuum annealing eliminates nanowindows and other defects from SWCNT film. The fabrication of free-standing single-wall carbon nanotube (SWCNT) film is promising for better understanding of SWCNT and many applications. Here we fabricated highly pure and crystalline free-standing SWCNT film using vacuum high-temperature annealing. Impurities can be removed thoroughly by vacuum annealing at 2073 K and 10 −1 Pa for 3 h. Also, the vacuum annealing was quite efficient for removing nanowindows and improving crystallinity of the SWCNT film. The transmission electron microscopic observation and X-ray diffraction confirmed almost no change in the average tube diameter and bundle structure of SWCNT. The vacuum high-temperature annealing decreases the N 2 adsorption amount at 77 K by more than half, supporting closing nanowindows on graphene wall of SWCNT by the vacuum annealing. Our free-standing SWCNT film is expected to promote a fundamental understanding of electrochemical energy issues by the application these films as the well-characterized electrodes.

High-Performance Cold Cathode X-ray Tubes Using a Carbon Nanotube Field Electron Emitter.

A cold cathode X-ray tube was fabricated using a carbon nanotube (CNT) field electron emitter made by a free-standing CNT film which is composed of a highly packed CNT network. A lot of CNT bundles with a sharp tip are vertically aligned at the edge of the thin CNT film with a length of 10 mm and a thickness of 7 μm. The cold cathode X-ray tube using the CNT field emitter presents an extremely high tube current density of 152 A/cm2 (corresponding to tube current of 106.4 mA), the electron beam transmittance of 95.2% and a small focal spot size (FSS) of 0.5 mm. In addition, the cold cathode X-ray tube also shows stable lifetime during 100 000 shots. High emission current density of the cold cathode X-ray tube is mainly attributed to a lot of electron emission sites at an edge of the CNT film. The small FSS is caused by an ensemble of the CNT field electron emitter made by a free-standing thin CNT film and the optimized curve-shape elliptical focusing lens. Based on obtained results, the cold cathode X-ray tube can be widely used for various X-ray applications such as medical diagnosis systems and security check systems in the future.

Easy Fabrication of Performant SWCNT-Si Photodetector

We propose a simple method to fabricate a photodetector based on the carbon nanotube/silicon nitride/silicon (CNT/Si3N4/Si) heterojunction. The device is obtained by depositing a freestanding single-wall carbon nanotube (SWCNT) film on a silicon substrate using a dry transfer technique. The SWCNT/Si3N4/Si heterojunction is formed without the thermal stress of chemical vapor deposition used for the growth of CNTs in other approaches. The CNT film works as a transparent charge collecting electrode and guarantees a uniform photocurrent across the sensitive area of the device. The obtained photodetector shows a great photocurrent that increases linearly with the incident light intensity and grows with the increasing wavelength in the visible range. The external quantum efficiency is independent of the light intensity and increases with the wavelength, reaching 65% at 640 nm.

High-Performance Field Electron Emitters Fabricated Using a Free-Standing Carbon Nanotube Film

The carbon nanotube (CNT) field emitter was fabricated using a thin free-standing CNT film, indicating a line-shape CNT field emitter. Field emission properties of the CNT field emitter were investigated in both diode and triode configurations. The CNT field emitter showed a low turn-on electric field of 1.8 V/μm and a high emission current of 40.3 mA, corresponding to the emission current density of 96 A/cm in the diode configuration. It also exhibited a high anode current of 40 mA, corresponding to the anode current density of 95.2 A/cm in the triode configuration. In addition, the CNT field emitter showed a good electron beam transmittance of 86.4% and excellent emission stability without degradation for 15 h. The main reason for the high performance of our CNT field emitter is caused by the high density of emission sites at the edge of the CNT film.

Free-standing carbon nanotube film for high efficiency monopole antenna

A flexible radio-frequency (RF) antenna allows wearable smart devices to have various functionalities. To develop a high-performance RF antenna, an outstanding radiation efficiency should be achieved. Metallic films are commonly utilized in RF antenna electrodes owing to their high electrical conductivity, which is related to the radiation efficiency. However, their high density and low mechanical stability under deformed states, such as the folding of metallic films, hinder their use as flexible RF antenna electrodes. In this study, we fabricated a carbon-nanotube (CNT) film via the direct-spinning method for use in a flexible RF antenna electrode. The network structure of the CNT film endows it with a high electrical conductivity and allows it to be freestanding without a polymeric binder. The freestanding CNT film-based RF antenna attained a high radiation efficiency of 85.6%, commercially useable level, and its performance remained stable even under mechanical deformation, such as bending and folding. This high performance is attributed to the network structure of the CNT film composed of pure CNT bundles.

Saturable absorption and nonlinear refraction in free-standing carbon nanotube film: Theory and experiment

The nonlinear absorption and refraction of free-standing films made of single-walled carbon nanotubes (SWNTs) have been investigated experimentally and theoretically. By solving the quantum kinetic equations that take into account both the intra- and interband transitions, we obtain the analytical expression for the SWNT nonlinear conductivity. The nonlinear absorption coefficient and saturation intensity of the film comprising randomly orientated SWNTs have been calculated in a broad spectral range spanning over M11, S11, and S22 absorption bands. The effects of the laser pulse duration and dynamic Burstein−Moss shift on the saturation intensity have been revealed. We demonstrate in the experiment that, under irradiation with femtosecond laser pulses, the absorption modulation depth of SWNT film at resonance wavelength 1375 nm is as high as 30%. The observed saturation intensity minimum is red-shifted with respect to the absorption maximum due to the dynamic Burstein−Moss shift. The saturation intensity within the S22 band is 26−fold lower than that out of the band at 795 nm. The closed-aperture z-scan measurements reveal the negative nonlinear refractive index n2 = −3.1 × 10−12 cm2/W at 795 nm.

Carbon nanotube EUV pellicle tunability and performance in a scanner-like environment

Background: An extreme ultraviolet (EUV)-transparent pellicle must be used during lithography to protect the photomask from fall-on particles. A pellicle made of free-standing carbon nanotube (CNT) films stops particles despite the presence of gaps while demonstrating high EUV transmission, mechanical stability, low EUV scattering and reflectivity, and DUV transmission that enables through-pellicle mask inspection. Aim: The CNT EUV pellicle properties can be tailored based on the diversity of CNT structures and tunability of their configuration within the CNT film (density, bundle size, composition, etc.) as shown in this work. A remaining challenge is extending the CNT EUV pellicle lifetime in the scanner environment of EUV-induced hydrogen-based plasma, and the effects on different CNT films are explored here. Approach: Optical and thermal properties of different CNT pellicles with respect to the CNT material type, density, composition, and bundle size were explored. The ability of uncoated CNT EUV pellicles to withstand high EUV powers in the hydrogen-based environment was tested. Transmission, spectroscopic, and chemical mapping of the exposed CNT membranes were performed to explore the material modifications under various exposure conditions. Results: Uncoated CNT pellicles withstand 600-W source power equivalent in the EUV scanner-like gas environment but exhibit structural changes with prolonged exposure. Multiwalled CNT pellicles exhibit less EUV transmission change as compared to single-walled CNT pellicles under the same exposure conditions. The protection of CNT material from structural degradation by means of coating was shown. Conclusions: These investigations add to the understanding of CNT EUV pellicle tunability for optimal performance and lifetime limiters of CNT pellicles under the influence of EUV radiation and plasma. We anticipate the need for coating the CNT pellicle to protect the CNT material against plasma damage for the current scanner conditions. Optimization of both the CNT membrane and its coating is in progress.

Fluorination-assisted preparation of self-supporting single-atom Fe-N-doped single-wall carbon nanotube film as bifunctional oxygen electrode for rechargeable Zn-Air batteries

Freestanding membrane electrodes with a high electrocatalytic activity and stability are desirable for use in high-performance metal-air batteries. Here, we report a gas-phase fluorination-assisted method to anchor single atom Fe-Nx moieties on a freestanding single-wall carbon nanotube (SWCNT) film that involves fluorination and defluorination/ammoniation processes. It is shown that the fluorination followed by defluorination/ammoniation harvests Fe atoms from the residual Fe nanoparticles used for SWCNT growth to form high density of Fe-Nx active sites, while retaining the flexibility, integrity and high quality of the films so that they can be directly used as a bifunctional oxygen electrode for rechargeable Zn-Air batteries. The single-atom Fe-Nx loaded SWCNT film shows an even better catalytic activity than commercial Pt/C-Ir/C catalysts, and an assembled Zn-Air battery using it exhibits a low charge-discharge voltage gap of 0.84 V under 20 mA/cm2, a high peak power density of 210 mW/cm2, and excellent long-time cycling stability. In addition, a flexible all-solid-state Zn-Air battery was assembled which has a stable open circuit voltage when bent to different angles.

Lithiophilic amide-functionalized carbon nanotube skeleton for dendrite-free lithium metal anodes

Despite numerous efforts to stabilize lithium (Li) metal anodes and suppress dendrite growth, achieving stable Li plating/stripping with long cycle life remains a critical challenge because of a combination of complex degradation modes including nonuniform Li nucleation, huge volume change and uncontrollable side reaction between Li metal and electrolyte. Herein, a free-standing carbon nanotube film functionalized with lithiophilic amide-containing functional groups (Af-CNT) is designed to regulate Li nucleation uniformly and prevent dendrite growth. Owing to the strong interaction between Li/amide and low nucleation overpotential rendered by the lithiophilic functional groups, Li is preferentially deposited along the nanotubes or into the cavities, resulting in a smooth Li deposition morphology. Furthermore, the highly conductive and porous carbon framework can lower local current density for Li plating/stripping and then homogenize the Li deposition. As a result, the Af-CNT electrode enables an ultralow overpotential of ~ 14 mV accompanied with a high Coulombic efficiency (CE) of 97.8% over 300 cycles in a Li||Af-CNT half-cell and realizes a dendrite-free deposition and long-term stable cycling for 1800 h. In full cell configurations with LiFePO4 (LFP) cathode, the Af-CNT@Li anode exhibits excellent cycle stability and low polarization.

Enhanced saturable absorption in the laser-treated free-standing carbon nanotube films.

We demonstrate an increase of optical transmittance and saturable absorption of laser-treated free-standing single-walled carbon nanotube (SWNT) films. The combined acid and low-power non-destructive laser treatment ensures an enhancement of linear transmittance across the visible range and double-digit increase of the saturable absorption of femtosecond laser radiation at 795 nm. The saturable absorption coefficient and the ratio of saturable to non-saturable losses increase by 26% and 35%, correspondingly, while the saturation intensity decreases by 20% because of the treatment. Our analysis indicates that with the performed treatment one can significantly improve the nonlinear optical properties of free-standing SWNT-based ultrafast saturable absorbers.

Free-Standing N-Doped Carbon Nanotube Films with Tunable Defects as a High Capacity Anode for Potassium-Ion Batteries

Potassium-ion batteries (KIBs) have aroused enormous interest for future energy storage technology. However, the current anodes for KIBs greatly suffer from the rapid capacity fading and inferior rate capability. Herein, a free-standing flexible anode, that is, nitrogen-doped carbon nanotube paper (NCTP), which is derived from the pyrolysis of organic polypyrrole materials, is demonstrated for high-performance potassium storage. The correlations between the material structure and electrochemical properties have been investigated by a series of material analysis and characterizations, as well as electrochemical tests. The research results show that the annealing temperature dramatically affects the N-doping content, the carbon defects, and the graphitization degree. Electrochemical tests indicate that the NCTP annealed at 700 °C displays the best performances with a high reversible capacity of 250.1 mA h g-1 at 100 mA g-1 and superior rate capability retaining 133 mA h g-1 at 5 A g-1. The excellent electrochemical properties are derived from a synergic contribution from the moderate N-doping, carbon defect, and high electronic conductivity of the materials. The facile pyrolysis strategy and the appealing performances involved in this work could provide some hints to manipulate high-performance anode materials of KIBs.

Rapid, efficient, and non-destructive purification of single-walled carbon nanotube films from metallic impurities by Joule heating

We demonstrate a simple and resource-efficient approach for rapid purification of single-walled carbon nanotube (SWCNT) films from catalyst impurities by Joule heating in a vacuum. As a result, the catalyst concentration decreases by a factor of 27 (from 17 to <1 wt%) after heating at 1600°C for 2 min. Moreover, the nanotube structure remains almost intact as the purified films show a high ratio of intensities of G to D bands of Raman spectrum IG/ID = 180. The technique perfectly fits for the devices based on free-standing carbon nanotube films such as protective membranes, infrared detectors, and sound generators. For purified SWCNT films, we show the signal-to-noise ratio for the bolometers to increase by a factor of 2.5 and 24% sound pressure gain for the air-coupled thermophones. The method can be easily extended to other applications, for instance, the SWCNT-based devices of limited liquid and harsh reagent tolerance like nanotube aerogels and drug delivery systems.

Chemically Cross-Linked Carbon Nanotube Films Engineered to Control Neuronal Signaling.

In recent years, the use of free-standing carbon nanotube (CNT) films for neural tissue engineering has attracted tremendous attention. CNT films show large surface area and high electrical conductivity that combined with flexibility and biocompatibility may promote neuron growth and differentiation while stimulating neural activity. In addition, adhesion, survival, and growth of neurons can be modulated through chemical modification of CNTs. Axonal and synaptic signaling can also be positively tuned by these materials. Here we describe the ability of free-standing CNT films to influence neuronal activity. We demonstrate that the degree of cross-linking between the CNTs has a strong impact on the electrical conductivity of the substrate, which, in turn, regulates neural circuit outputs.

A Freestanding Single-Wall Carbon Nanotube Film Decorated with N-Doped Carbon-Encapsulated Ni Nanoparticles as a Bifunctional Electrocatalyst for Overall Water Splitting.

Noble‐metal free, cost‐effective, and highly stable catalysts with efficient activity for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) have attracted tremendous research interest in recent years. Here, a flexible, self‐standing hybrid film comprising a N‐doped single‐wall carbon nanotube (SWCNT) network on which are anchored Ni nanoparticles encapsulated by a monolayer of N‐doped carbon (NCNi) is reported. The films are prepared by floating catalyst chemical vapor deposition followed by NH3 treatment. The material obtained at optimum conditions shows excellent bifunctional electrocatalytic activity in alkaline media with low overpotentials of 190 and 270 mV for HER and OER, respectively, to reach a current density of 10 mA cm−2. A current density of 10 mA cm−2 at 1.57 V is achieved when this freestanding and binder‐free rod‐shaped NCNi/SWCNT assembly is used as cathode and anode in 1 m KOH solution for overall water splitting, presenting one of the best values reported to date.

Silica-Mediated Formation of Nickel Sulfide Nanosheets on CNT Films for Versatile Energy Storage.

An effective, nondestructive, and universal strategy to homogeneously modify freestanding carbon nanotube (CNT) films with various active species is essential to achieve functional electrodes for flexible electrochemical energy storage, which is challenging and has attracted considerable research interest. In this work, a generalizable concept, to utilize silicon oxide as the intermediate to uniformly decorate various metal sulfide nanostructures throughout CNT films is reported. Taking nickel sulfide nanosheet/CNT (NS/CNT) films, in which the NS nanosheets are homogeneously attached on the intact few-walled CNTs, as an example, the sheet-like NS provides sufficient active sites for redox reactions and the CNT network acts as an efficient electron highway, maintaining the structural integrity of the composite and also buffering volume changes. These merits enable NS/CNT films to meet the requirements of versatile energy storage applications. When used for supercapacitors, a high specific capacitance (2699.7 F g-1 /10 A g-1 ), outstanding rate performance at extremely high rates (1527 F g-1 /250 A g-1 ), remarkable cycling stability, and excellent flexibility can be achieved, among the best performance so far. Moreover, it also delivers excellent performance in the storage of Li and Na ions, meaning it is also potentially suitable for Li/Na ion batteries.

Free-standing carbon nanotube films for extreme ultraviolet pellicle application

To enable high volume manufacturing with extreme ultraviolet (EUV) lithography, a pellicle membrane is needed to protect the reticle from particles at EUV source powers beyond 250 W. Identifying a membrane with high EUV transmission, mechanical integrity, thermal stability, and chemical resistance to the scanner environment is extremely challenging; yet, these properties are required to realize next-generation EUV pellicle solutions. Free-standing carbon nanotube (CNT) film as an alternative next-generation core pellicle material is proposed. We demonstrate that free-standing CNT films possess very high EUV transmission (up to 99%) and good transmission uniformity (∼0.4 % half range), mechanical stability (maximum deflection ∼0.08 mm at 2 Pa), thermal stability (no transmission change under greater than 250 W equivalent EUV power in vacuum), and scalability to a full pellicle size (∼15 × 12 cm2). The capability of the CNT membrane to withstand high EUV power in the presence of H2 for a limited time is demonstrated. Other CNT membrane properties are presented that are important for the pellicle application: low EUV scattering, low EUV reflectivity, and sufficient transmission to enable through-pellicle inspection with DUV light or electrons. The ability of the CNT film to stop particles is tested. The influence of hydrogen at higher EUV powers and prolonged exposures on the lifetime of the CNT pellicle remains the current research focus. Approaches for coating the free-standing CNT films for protection are discussed.

High-Performance and Lightweight Thermal Management Devices by 3D Printing and Assembly of Continuous Carbon Nanotube Sheets.

Free-standing carbon nanotube films or buckypaper can provide a significant platform to develop practical applications of nanocarbon materials. For this research, buckypaper with high thermal conductivity (20 W/m K) and large surface area (350 m2/g) was mass produced in-house to investigate for use in lightweight thermal management devices. Floating catalyst chemical vapor deposition carbon nanotube sheets were also studied in this work. We introduced two manufacturing techniques to use the sheets for heat dissipation: (1) printing conductive composite ink on the sheets to make lightweight thermal devices, such as heat sinks and (2) assembling the sheets directly into 3D structures that were mounted on the back of heat-generating devices. These manufacturing techniques resulted in extremely lightweight, high-performance heat dissipation devices compared with other heat sink materials.