Highly conductive and transparent thin films fabricated with predominantly semiconducting single-walled carbon nanotubes

A predominantly semiconducting single-walled carbon nanotube-based thin conductive film was fabricated on a flexible poly(ethylene terephthalate) substrate. The structural features of the nanotubes and careful experimental scrutinization consistently yielded the films with very low surface resistance (59 Ω sq⁻¹) and high transparency (80%). The morphological studies of these films before and after acid treatment revealed the self orientation of nanotubes clustered at favorable centers.     

A scalable fabrication of highly transparent and conductive thin films using fluorosurfactant-assisted single-walled carbon nanotube dispersions

A single-walled carbon nanotube (SWCNT)/sodium dodecylbenzenesulfonate (NaDDBS) dispersion containing a fluorosurfactant was bar-coated in order to produce a highly transparent and conductive thin film (TCF) for large-area application. The addition of a small amount of fluorosurfactant greatly reduced the surface tension of the CNT-dispersed solution, which produced a uniform film of CNTs by preventing agglomeration of CNTs during the drying process, and, furthermore, rendered bar-coating as the most practical large-area coating technique for a CNT solution. This particular fluorosurfactant addition maintained a CNT dispersion in the solution, which led to a dramatic improvement in the wettability of the CNT dispersion on the substrate towards high-performance TCF films. The thickness of the CNT films was controlled simply by adjusting the amount of CNTs in the solution. Moreover, the addition of a waterborne polymethyl methacrylate (PMMA) binder to the CNT dispersion improved the adhesion of the CNT films on a glass substrate.     

Contamination-free and damage-free patterning of single-walled carbon nanotube transparent conductive films on flexible substrates

High quality patterning of single-walled carbon nanotube (SWCNT) transparent conductive films is achieved by a lift-off aluminum interlayer method, which has the advantage of resulting in contamination-free and damage-free SWCNTs. The obtained patterns preserve the electrical properties of the SWCNT films and show promising applications in flexible high frequency electronic and display devices.     

Work-function engineering of carbon nanotube transparent conductive films

We developed a method that allows control of the work function of carbon nanotube (CNT) transparent conductive films on a flexible sheet of plastic. The approach involves the deposition of small amount of metals, such as aluminum, on CNT transparent conductive films followed by a measure of the work function of the films using in situ ultraviolet photoelectron spectroscopy. Core-level spectra of the films were collected in order to investigate the chemical reaction when a small amount of aluminum was deposited on their surface in a stepwise manner. The measurements revealed that deposition of less than 0.5 nm of aluminum was enough to control the work function of CNT transparent conductive films.     

Reinforcing randomly oriented transparent freestanding single-walled carbon nanotube films

We report an improvement of the mechanical properties of transparent randomly oriented freestanding single-walled carbon nanotube (SWCNT) films by deposition of polymers using a drop casting method and aluminum oxide utilizing an atomic layer deposition (ALD) technique. Due to the thickness increase, the polymer coating resulted in an increase in toughness, however, simultaneously decreasing the ultimate tensile strength. The 100 nm thick SWCNT films ALD-coated with Al₂O₃ layer revealed significant increase in the ultimate tensile strength from 46 ± 5 to 213 ± 17 and 80 ± 4 to 318 ± 16 MPa depending on the network density, preserving the high level of porosity of the structure.     

Anisotropic electrical conduction of vertically-aligned single-walled carbon nanotube films

Anisotropic electrical conduction measurements have been carried out for thin films of vertically-aligned single-walled carbon nanotubes (VA-SWCNTs) grown by an alcohol catalytic CVD process. Combined with controlled synthesis and structure characterization by optical spectroscopy, the influence of the aligned structure on the electrical conduction has been identified. The out-of-plane conductivity of the films was measured to be about 0.56 S/mm, independently of the film thickness. On the other hand, the in-plane conductivity was found to be more than an order of magnitude smaller, which gives rise to highly anisotropic electrical conduction, reflecting the high degree of alignment in the VA-SWCNT films. The in-plane conductivity decreases with increasing film thickness, in contrast to the film of random SWCNT networks, which exhibit thickness-independent in-plane resistance. The thickness-dependent in-plane conductivity can be expounded by a growth model of vertically aligned SWCNT films in which a thin layer of nanotube networks form on top of films at the initial stage of the growth. Such electrical anisotropy of VA-SWCNT films can be useful in miniaturized sensing devices.     

Preparation and characterization of transparent and conductive thin films of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs), synthesized using the arc-discharge method and the direct-injection-pyrolytic synthesis (DIPS) method, were dispersed in a tetrahydrofuran solution containing propylamine and used to prepare transparent and conductive thin films on PET films using an airbrush technique. The SWNTs were analyzed using vis-near infrared absorption spectroscopy, Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. The surface resistivity of the SWNT films on the substrates was measured using a four-point probe conductivity measurement. The results revealed that the purity, length, and proportion of the metallic SWNTs are important factors in decreasing the sheet resistance.     

Optimisation of carbon nanotube ink for large-area transparent conducting films fabricated by controllable rod-coating method

Single-walled carbon nanotubes (SWCNTs) were dispersed in water with the help of surfactants to achieve high concentration SWCNT ink. SWCNT transparent conducting films (TCFs) were fabricated by rod coating using the SWCNT ink. A combination of two surfactants provided optimal rheological behaviour, which produced uniform films by preventing dewetting and rupture of SWCNTs during drying. The combination led to a dramatic increase of shear viscosities but no change of their wettability. The viscosity of SWCNT ink was controlled by the ratio of two surfactants. The thickness of SWCNT films could be easily varied by controlling both the concentration of SWCNT ink and the size of the wire-wound rod. The produced uniform SWCNT-TCFs treated by nitric acid have a relatively low sheet resistance of ∼80 Ω sq⁻¹ at 80% transmittance. The performance has a wide range of applied interest for touch screen and flexible electronics.     

Preparation and characterization of highly conductive transparent films with single-walled carbon nanotubes for flexible display applications

Dense, aligned single-walled carbon nanotubes (SWCNTs) were obtained by nitric acid treatment and the subsequent removal of metal impurities by HCl. The highly purified SWCNTs were dispersed with sodium dodecyl sulfate in order to obtain a stabilized suspension for spray coating on flexible polyethylene terephthalate (PET) substrate. The low sheet resistance of the resulting thin conductive film on the PET substrate was due to the interconnecting networks of highly purified SWCNT bundles. These bundles formed strong crisscross networks of nanotubes clustered together with well defined channels, thus improving the electrical and optical properties of the film. Its sheet resistance varied from 956 to 472 Ω/square with 85% optical transmittance at a wavelength of 550 nm. The films may be potential candidates for flexible display applications.     

Modulation of electrical properties in single-walled carbon nanotube/conducting polymer composites

The preparation and electrical characterization of a new class of composite layers formed by dispersing single-walled carbon nanotubes (SWNT) in 1,8-diaminonaphthalene polymer, the poly(1,8-DAN), are described.The material was grown on the surface of Pt plates by electropolymerization of 1,8-diaminonaphthalene (1,8-DAN) monomer in the presence of nanotubes. This synthesis method allows the simultaneous deposition of both the host polymer matrix and the filler nanotubes. A series of composite films were prepared using untreated nanotubes as well as nanotubes treated with KOH, HNO₃ and HNO₃/H₂SO₄ solutions. The structural features of the nanotubes and of the films produced have been investigated using Raman spectroscopy. Insight into the nature of nanotube dispersion and nanotube-polymer association was gained by AFM and STM analysis and by FE-SEM inspection after removing the outermost portion of composite films.The charge transport in composite films is found to be strongly enhanced by the nanotube insertion. Depending on the SWNTs processing, currents up to 30 mA, higher by a factor of about 140 than those of the pure poly(1,8-DAN) films, were measured with an applied voltage of 250 mV.     

Fabrication of transparent and conductive carbon nanotube/polyvinyl butyral films by a facile solution surface dip coating method

We present a simple solution surface dip coating method for fabricating transparent and conductive carbon nanotube/polyvinyl butyral (CNT/PVB) composite films. This fabrication process is simple to scale production and requires only ethanol and water as solvents, which is green and environment friendly.     

The electrical and optical properties of oriented Langmuir-Blodgett films of single-walled carbon nanotubes

Thin, transparent, electrically-conductive films containing single-walled carbon nanotubes have been fabricated using the Langmuir-Blodgett method. No additional surfactant was used and films up to 99 layers in thickness (300 nm) could readily be built-up. Both optical and electrical measurements revealed anisotropy in the film plane, suggesting alignment of the tubes during the deposition process. For the majority of films, DC conductivity measurements over the range 77–300 K showed an increase in the conductivity with increasing temperature, i.e. semi-conducting characteristics. However, some samples exhibited the opposite effect. Such metallic films reverted to semi-conducting behaviour following an electrical annealing process.     

Influence of lengths of millimeter-scale single-walled carbon nanotube on electrical and mechanical properties of buckypaper

The electrical conductivity and mechanical strength of carbon nanotube (CNT) buckypaper comprised of millimeter-scale long single-walled CNT (SWCNT) was markedly improved by the use of longer SWCNTs. A series of buckypapers, fabricated from SWCNT forests of varying heights (350, 700, 1,500 μm), showed that both the electrical conductivity (19 to 45 S/cm) and tensile strength (27 to 52 MPa) doubled. These improvements were due to improved transfer of electron and load through a reduced number of junctions for longer SWCNTs. Interestingly, no effects of forest height on the thermal diffusivity of SWCNT buckypapers were observed. Further, these findings provide evidence that the actual SWCNT length in forests is similar to the height.     

Growth of novel carbon phases by methane infiltration of free-standing single-walled carbon nanotube films

High-temperature methane infiltration of thin, free-standing films of acid-treated single-walled carbon nanotubes (SWCNT) has been studied by means of scanning electron microscopy, transmission electron microscopy and Raman spectroscopy. In the early stages of infiltration, carbon nuclei form predominantly at SWCNT bundle intersections. Further growth proceeds via the formation of graphite nanosheets - without further influence of the nanotube support. Both sheet edges and their structural imperfections act as reaction centers for subsequent deposition, likely giving rise to autocatalytic deposition kinetics. In contrast, infiltration with a H₂:CH₄ (24:1) mixture leads to the reductive activation of residual Ni/Co impurities embedded in the precursor SWCNT-felt. This is associated with a different predominant carbon deposition mode in which multiwalled carbon nanotubes grow out from the substrate.     

Field emission properties of Cu/multiwalled carbon nanotube composite films fabricated by an electrodeposition technique

Composite films of Cu and multiwalled carbon nanotubes (MWCNTs) were fabricated by an electrodeposition technique, and their field emission properties were examined. Commercially available MWCNTs with various diameters (60–150 nm) were used. The microstructure of the composite films was analyzed by scanning electron microscopy and the field emission properties were measured using a diode-type system. Cu/MWCNT composite films with homogeneous dispersion of MWCNTs were fabricated using each type of MWCNT. Bare MWCNTs were present on the surface of the composite films and the ends of the protruding tips were fixed by the deposited copper matrix. The composite films produced clear emission currents and the corresponding Fowler–Nordheim (F–N) plots showed that these were field emission currents. The turn-on electric field tended to decrease with decreasing MWCNT diameter. A light-emitting device incorporating the Cu/MWCNT composite film as a field emitter was fabricated, and its light-emitting properties were investigated. Light emission with a brightness of around 100 cd m^?2 was observed for approximately 100 h.     

Structural,electrical, and optical properties of Sb-doped SnO2 transparent conductive oxides fabricated using an electrospray technique

Sb-doped SnO₂ (ATO) thin films, for use as transparent conductive oxides (TCOs), were synthesized using an electrospray technique, and their structural, electrical, and optical properties were investigated. To elucidate the optimum fabrication conditions for the best electrical and optical properties, ATO thin films were calcined using four different temperatures, 450 °C, 550 °C, 650 °C, and 750 °C. When calcined at 650 °C, ATO thin films exhibit excellent resistivity (~8.14×10⁻³ Ω cm), superior transmittance (~91.4% at 550 nm), and good figure of merit (~11.4×10⁻⁴ Ω⁻¹) compared to the other samples. The enhanced properties of ATO thin films are attributed to high densification without formation of cracks, and the increased grain size of ATO nanoparticles.     

Transparent and flexible electrodes and supercapacitors using polyaniline/single-walled carbon nanotube composite thin films

Large-scale transparent and flexible electronic devices have been pursued for potential applications such as those in touch sensors and display technologies. These applications require that the power source of these devices must also comply with transparent and flexible features. Here we present transparent and flexible supercapacitors assembled from polyaniline (PANI)/single-walled carbon nanotube (SWNT) composite thin film electrodes. The ultrathin, optically homogeneous and transparent, electrically conducting films of the PANI/SWNT composite show a large specific capacitance due to combined double-layer capacitance and pseudo-capacitance mechanisms. A supercapacitor assembled using electrodes with a SWNT density of 10.0 μg cm(-2) and 59 wt% PANI gives a specific capacitance of 55.0 F g(-1) at a current density of 2.6 A g(-1), showing its possibility for transparent and flexible energy storage.     

Improvement of the mechanical properties of single-walled carbon nanotube networks by carbon plasma coatings

Carbon vacuum arc was used to deposit 5–25 nm thick carbon coatings on single-walled carbon nanotube (SWCNT) networks. The SWCNT bundles thus embedded in conformal coatings maintained their optical transparency and electrical conductivity. Sheet resistances of the networks were measured during the vacuum arc deposition, revealing initially a 100-fold increase, followed by significant recovery after exposing the samples to an ambient atmosphere. Nanoindentation measurements revealed improved elasticity of the network after applying the carbon coating. Pristine SWCNT networks were easily deformed permanently, but a 20 nm carbon coating strengthened the nanostructure, resulting in a fully elastic recovery from a 20 μN load applied with a Berkovich tip. In nano-wear tests on selected areas, the coated SWCNT maintained its networking integrity after two passes raster scan at loads up to 25 μN. On the other hand, the pristine networks were badly damaged under a 10 μN scan load and completely displaced under 25 μN. Raman and electron energy loss spectroscopies indicated the carbon coating on bundles to be mainly sp² bonded. Finite element modeling suggests that the low content of sp³ bonds may be due to heating by the intense ion flux during the plasma pulse.     

Reinforcement of rubber using radial single-walled carbon nanotube soot and its shock dampening properties

A soot composed of radial single-walled carbon nanotubes (r-SWCNTs), in which 70 nm length nanotubes are grown radially around the core metal particles, and nanohorn-like carbons (NHCs) was used as reinforcement for a styrene-butadiene rubber (SBR). The fracture stress of r-SWCNT soot (38 phr)/SBR was 6.3 MPa at 60% strain. Furthermore, the hardness value of r-SWCNT soot (38 phr)/SBR was 94, which is 1.38 times larger, and larger than carbon black/SBR. Additionally, the resilience of r-SWCNT soot/SBR with 38 phr filler content was markedly lower than 20% in comparison with standard carbon black filler. These results indicate that r-SWCNT soot/SBR possesses excellent kinetic energy absorbing properties.     

Transparent conductive oxide films mixed with gallium oxide nanoparticle/single-walled carbon nanotube layer for deep ultraviolet light-emitting diodes

We propose a transparent conductive oxide electrode scheme of gallium oxide nanoparticle mixed with a single-walled carbon nanotube (Ga₂O₃ NP/SWNT) layer for deep ultraviolet light-emitting diodes using spin and dipping methods. We investigated the electrical, optical and morphological properties of the Ga₂O₃ NP/SWNT layers by increasing the thickness of SWNTs via multiple dipping processes. Compared with the undoped Ga₂O₃ films (current level 9.9 × 10^-9 A @ 1 V, transmittance 68% @ 280 nm), the current level flowing in the Ga₂O₃ NP/SWNT increased by approximately 4 × 10⁵ times and the transmittance improved by 9% after 15 times dip-coating (current level 4 × 10^-4 A at 1 V; transmittance 77.0% at 280 nm). These improvements result from both native high transparency of Ga₂O₃ NPs and high conductivity and effective current spreading of SWNTs.