Single-walled Carbon Nanotube Thin Films Research Articles

Mechanically neutral and facile monitoring of thermoset matrices with ultrathin and highly porous carbon nanotube films

We examine how the physical features of single-walled carbon nanotube (SWCNT) thin films can affect monitoring thermoset polymers during their manufacturing and application. Film thickness (23, 37 and 53 nm), electrode materials and configuration, and embedding within or surface application on a polymer were all investigated. During manufacturing monitoring, thicker films provided higher sensitivity than thinner ones (ΔR/R0 = 111% (53 nm) vs. 74% (23 nm)). Electrode material and configuration showed that sputtered gold contacts allow higher sensitivity and reliability compared with conductive silver adhesive, and that parallel electrode placement provides more distinct and discreet stage measurements compared to diagonal one. The opposite trend was noted for mechanical loading, where thinner films provide a higher piezoresistive response and embedded films outperform surface applied sensors (gauge factors between 23-86 and 4-10, respectively). All thin films accurately measured curing stages as well as mechanical changes and are shown to be self-adjusting when the force increases during mechanical loading. We show that manufacturing monitoring should rely on uniformly deposited electrodes and thicker films for identification of polymerization stages, whereas embedding thinner films in the matrix is recommended for highly sensitive deformation monitoring. This work outlines how simple, overlooked parameters can easily be varied to optimize the dual-stage monitoring performance of CNT-based sensors and helps to identify what parameters should be chosen for specific applications.

(Invited) Electrochemical Tuning of Thermoelectric Performance of Carbon Nanotubes

One-dimensional materials have potential for high performance thermoelectric energy conversion. To experimentally understand how one-dimensional electronic structures can enhance thermoelectric performance, we have investigated the thermoelectric properties of single walled carbon nanotubes (SWCNTs) such as the relationships among electronic structures, the location of Fermi-level, Seebeck coefficients, and Power factor by combining thermoelectric measurements and electrolyte gating techniques. Through the studies, we have revealed unique properties such as violation of thermoelectric trade-off, isotropic Seebeck coefficient, and gigantic power factor [1]. However, in addition to that, it is important to understand how the carrier density also affects the thermal conductivity of the carbon nanotube thin films for correct evaluation of ZT value. Therefore, we have developed a measurement system which can evaluate thermal conductivity by tuning the location of Fermi-level through electrochemical gating techniques. Time-domain thermoreflectance (TDTR) measurement is one of methods which can evaluate the thin film thermal conductivity. In conventional TDTR system, Al is used as metal transducer, but Al is electrochemical reactive, and thus it is very difficult to combine the TDTR measurements with electrochemical techniques. To solve this problem, we developed a TDTR system using Au as a metal transducer [2]. By using Au as a metal transducer, we can evaluate both the electrical and thermal properties at the same time. By using system, we can evaluate thermal properties of thin films during electrolyte gating, and have evaluated how the change of charrier density affects the thermal conductivity of carbon nanotube thin films [3]. By preparing the aligned single-walled carbon nanotube thin film, we evaluated the ZT value in the perpendicular direction to the nanotube axis as a function of carrier density. The ZT value in this direction can be maximized to be 0.1 by tuning the Femi-level around 1st van hove singularity [4].

Structural, Electrical, and Optical Properties of Single-Walled Carbon Nanotubes Synthesized through Floating Catalyst Chemical Vapor Deposition.

Single-walled carbon nanotube (SWCNT) thin films were synthesized by using a floating catalyst chemical vapor deposition (FCCVD) method with a low flow rate (200 sccm) of mixed gases (Ar and H2). SWCNT thin films with different thicknesses can be prepared by controlling the collection time of the SWCNTs on membrane filters. Transmission electron microscopy (TEM) showed that the SWCNTs formed bundles and that they had an average diameter of 1.46 nm. The Raman spectra of the SWCNT films suggested that the synthesized SWCNTs were very well crystallized. Although the electrical properties of SWCNTs have been widely studied so far, the Hall effect of SWCNTs has not been fully studied to explore the electrical characteristics of SWCNT thin films. In this research, Hall effect measurements have been performed to investigate the important electrical characteristics of SWCNTs, such as their carrier mobility, carrier density, Hall coefficient, conductivity, and sheet resistance. The samples with transmittance between 95 and 43% showed a high carrier density of 1021-1023 cm-3. The SWCNTs were also treated using Brønsted acids (HCl, HNO3, H2SO4) to enhance their electrical properties. After the acid treatments, the samples maintained their p-type nature. The carrier mobility and conductivity increased, and the sheet resistance decreased for all treated samples. The highest mobility of 1.5 cm2/Vs was obtained with the sulfuric acid treatment at 80 °C, while the highest conductivity (30,720 S/m) and lowest sheet resistance (43 ohm/square) were achieved with the nitric acid treatment at room temperature. Different functional groups were identified in our synthesized SWCNTs before and after the acid treatments using Fourier-Transform Infrared Spectroscopy (FTIR).

Conjugated Polymers with Self-Immolative Sidechain Linkers for Carbon Nanotube Dispersion.

Single-walled carbon nanotubes (SWNTs) are promising materials for generating high-performance electronic devices. However, these applications are greatly restricted by their lack of purity and solubility. Commercially available SWNTs are a mixture of semi-conducting (sc-) and metallic (m-) SWNTs and are insoluble in common solvents. Conjugated polymers can selectively disperse either sc- or m-SWNTs and increase their solubility; however, the conductivity of conjugated polymer-wrapped SWNTs is largely affected by the polymer side chains. Here, a poly(fluorene-co-phenylene) polymer that contains a self-immolative linker as part of its sidechains is reported. The self-immolative linker is stabilized with a tert-butyldimethylsilyl ether group that, upon treatment with tetra-n-butylammonium fluoride (TBAF), undergoes a 1,6-elimination reaction to release the sidechain. Sonication of this polymer with SWNTs in tetrahydrofuran (THF) results in concentrated dispersions that are used to prepare polymer-SWNT thin films. Treatment with TBAF caused side-chain cleavage into carbon dioxide and the corresponding diol, which can be easily removed by washing with solvent. This process is characterized by a combination of absorption and Raman spectroscopy, as well as four-point probe measurements. The conductance of the SWNT thin films increased ≈60-fold upon simple TBAF treatment, opening new possibilities for producing high-conductivity SWNT materials for numerous applications.

A comprehensive study of spray coating technique parameters and chemical postdeposition treatments for transparent conducting SWCNT thin films

The demands for transparent conducting films (TCFs) are continuously increasing and the future of this industry is going toward large scale, cheap and mechanically stable electronics. This study shows a simple way to fabricate low-cost transparent conductive single-walled carbon nanotube (SWCNT) thin films by spray coating. The work employed here demonstrates that it is possible to fabricate highly efficient SWCNT TCFs without the use of expensive ultrasonic spray coating equipment. Highly conductive TCFs were prepared by dispersing SWCNTs in an N-Methyl-2-pyrrolidone (NMP) solution. The spray coated TCF properties were studied by the effect of spray gun carrier gas pressure, concentration, and volume sprayed onto the substrates. The prepared thin films were subjected to post treatments with nitric acid (HNO3) using different treatment methods and times in order to improve the electrical conductivity. To further investigate the effect of p-doping in SWCNT thin films, HNO3 treatments were combined with thionyl chloride (SOCl2). The experimental results indicated that spray coating is an effective method to obtain highly conductive films without any chemical treatment, resulting in 273 Ω/sq sheet resistance, at 84% in the visible range. Additionally, the optimized films treated with both HNO3 and HNO3/SOCl2 showed great improvements at 83%–85% in the visible range and in sheet resistances followed by 98 and 103 Ω/sq with very good FOM values (1.6 × 10−3 and 2 × 10−3, respectively).

Reconfigurable nonlinear losses of nanomaterial covered waveguides

Abstract Optical waveguides covered with thin films, which transmittance can be controlled by external action, are widely used in various applications from optical modulators to saturable absorbers. It is natural to suggest that the losses through such a waveguide will be proportional to the absorption coefficient of the covering material. In this letter, we demonstrate that under certain conditions, this simple assumption fails. Instead, we observe that the reduction of the material loss of the film can lead to an increase in the propagation losses through the waveguide. For this, we use a side polished fiber covered with a single-walled carbon nanotube thin film whose absorption can be attenuated either by a short pulse illumination (due to absorption saturation) or with electrochemical gating. For the films thicker than 50 nm, we observe saturable absorption to turn into optical limiting with nonmonotonic dependence on the incident power. With a numerical simulation, we identify that this nontrivial behavior comes from mode reshaping due to changes in the absorption coefficient of the covering film. We demonstrate the applicability of the observed effect by fabricating the device which nonlinear optical response can be controllably switched between saturable absorbing and optical limiting. Finally, we utilize an analytical approach to predict the required parameters and corresponding nontrivial shapes of the nonlinear absorbance curves. These results provide new perspectives for engineering complex reconfigurable nonlinear optical responses and transmittance dependences of nanomaterial covered waveguides.

Single-Walled Carbon Nanotube (SWCNT) thin films via automatic spray coating and nitric acid vapor treatment

In the realm of optoelectronics to produce thin film, SWCNT (Single-Walled Carbon Nanotube) is a promising alternative to Indium Tin Oxide (ITO). However, it is still challenging to manufacture SWCNT thin film that is low-cost, homogenous and has minimal sheet resistance. In this work, SWCNT thin films were deposited by using the automated spray coating method. To enhance the film quality and optoelectronic characteristics, post-deposition treatment through nitric acid vapor was carried out. The optimized SWCNT thin films exhibited optical transparency in the range of 60.4% at 800 nm and sheet resistance of 6.6 Ω/square. The sheet resistance of our fabricated SWCNT thin films was lower than the value of ITO films (10 Ω/square), putting forth evidence which suggests the potential of SWCNTs as an alternative to ITO.

Fully solution-processed carbon nanotubes thin film transistors and PMOS inverters on glass substrate

We report fully solution-processed thin film transistors and PMOS inverters fabricated on glass substrates using single-walled carbon nanotubes (SWCNTs) as active semiconducting material. All the electrodes (gate, source, and drain) were inkjet-printed using silver (Ag) as conductive ink. Spin coated poly-4-vinylphenol dielectric was optimized in terms of thickness and heating conditions for solution-processed SWCNTs thin film transistors to achieve a mobility equal to 0.81 cm2 V−1s−1. We will show that, hole traps at the dielectric-semiconductor interface are responsible for the hysteresis in the transfer curve, and controlled by the different sweep rate of the gate field. Drain-current transients under different bias conditions were studied and the increase in current occurs due to slow polarizations of residual dipolar groups in the dielectric. The adopted technology has been exploited to fabricate a PMOS inverter and studied for high gain and noise margin values at the supply voltage, V DD = −40 V.

Single-walled carbon nanotube (SWCNT) thin film for optoelectronics device prepared by spray coating technique.

CNT is a novel material with exceptional properties. SWCNT thin films have been used in many different applications. To fabricate the SWCNT film, various methodologies have been introduced, however, the problem of fabricating uniform SWCNT thin film remains unsolved. Spray coating technique (SCT) is the most effective method for producing a uniform SWCNT thin film. The CNT thin films were deposited on glass substrates via SCT with different nozzle distances (from 2.5 cm to 12.5 cm). The morphological, topological, electrical and optical properties of CNT thin films were characterized using appropriate techniques. The electrical analysis depicted that the sheet resistance of thin films reduced from 40220 Ω/sq. to 1330 Ω/sq. as the spray nozzle distance was increased from 2.5 cm to 10 cm sheet resistance reached 2133 Ω/sq. for 12.5 cm which confirmed the concentration of CNT was highest at 10 cm nozzle distance. The optical analysis of the CNT thin films gave transmittance values in percentage of 64%, 60%, 52%, 43%, and 46%, respectively for the nozzle distance of 2.5 cm, 5 cm, 7.5 cm, 10 cm, and 12.5 cm. Transmittance percentage decreased from 2.5 cm to 10 cm nozzle distance, indicative of an uniform film coverage of CNT layer. The findings of this study could be a platform for producing an optimized CNT thin film using a SCT for optoelectronic devices.

Tunable THz flat zone plate based on stretchable single-walled carbon nanotube thin film

Tunable optoelectronics have attracted a lot of attention in recent years because of their variety of applications in next-generation devices. Among the potential uses for tuning optical elements, those allowing consistent parameter control stand out. We present an approach for the creation of mechanically tunable zone plate lenses in the THz range. Our devices comprise single-walled carbon nanotube (SWCNT) thin films of predetermined design integrated with stretchable polymer films. These offer high-performance and in situ tunability of focal length up to 50%. We studied the focusing properties of our lenses using the backward-wave oscillator THz imaging technique, supported by numerical simulations based on the finite element frequency domain method. Our approach may further enable the integration of SWCNT films into photonic and optoelectronic applications and could be of use for the creation of a variety of flexible and stretchable THz optical elements.

Ultrafast Optomechanical Terahertz Modulators Based on Stretchable Carbon Nanotube Thin Films

For terahertz wave applications, tunable and rapid modulation is highly required. When studied by means of optical pump–terahertz probe spectroscopy, single-walled carbon nanotube (SWCNT) thin films demonstrated ultrafast carrier recombination lifetimes with a high relative change in the signal under optical excitation, making them promising candidates for high-speed modulators. Here, combination of SWCNT thin films and stretchable substrates facilitated studies of the SWCNT mechanical properties under strain and enabled the development of a new type of an optomechanical modulator. By applying a certain strain to the SWCNT films, the effective sheet conductance and therefore modulation depth can be fine-tuned to optimize the designed modulator. Modulators exhibited a photoconductivity change of approximately 2 times of magnitude under the strain because of the structural modification in the SWCNT network. Stretching was used to control the terahertz signal with a modulation depth of around 100% without strain and 65% at a high strain operation of 40%. The sensitivity of modulators to beam polarization is also shown, which might also come in handy for the design of a stretchable polarizer. Our results give a fundamental grounding for the design of high-sensitivity stretchable devices based on SWCNT films.

(Invited) One-Dimensional Features of Electron Transport in Single-Walled Carbon Nanotube Thin Films

Single-walled carbon nanotubes (SWCNTs) are excellent conductors with high flexibility, high thermal conductivity, and high transparency. Flexible and transparent SWCNT thin films and SWCNT yarns with high mechanical strength have been produced, and in particular, the improvement of electrical conduction characteristics of these SWCNT aggregates has been studied intensively. The electrical conductivity of SWCNT aggregates have improved year by year, but it is worse than those of a single SWCNT, so far. SWCNT aggregates contain complex structures such as SWCNTs with different chirality structures, many junctions, and bundle structures, which could be the cause of deterioration of transport characteristics, however, it has not been clarified.In this study, we focused on low-density SWCNT thin films and investigated their electron transport characteristics. The low-density SWCNT thin film has a simple network structure since the network consists of the bundles of a small number of long SWCNTs. Here, the magnetic field dependence, temperature dependence, and gate voltage dependence of the electrical conductivity were measured and analyzed.The SWCNT thin films were synthesized by a floating catalyst CVD method, and they were transparent (90%T), and electrically conductive [1]. The thin film consisted of a network structure of bundles, which consisted of about 10 SWCNTs. The films were mixtures with metallic and semiconducting SWCNTs. The electrical resistivity of the SWCNT thin film was measured by the four-point probes method, and the gate voltage (Vg) was applied using an ionic liquid. The direction of the magnetic field was perpendicular to the thin film surface. The areal density of SWCNT length was estimated from the light absorption cross-section of the SWCNT thin films, and then the electrical resistivity was calculated from the actual volume of SWCNTs excluding the void space.SWCNTs generally show p-type characteristics in the atmosphere in the back-gate configuration due to the dope effects of oxygen and/or water molecules. However, the Vg dependence of the electrical resistivity of the thin film showed a bipolar behavior under the ionic liquid gate. The change ratio of the resistivity in the Vg range (-3 to +3 V) was more than 10, which suggested that not only did the non-conductive semiconducting SWCNT become conductive, but the number of percolation paths increased accordingly. Raman scattering spectroscopy also exhibited that semi-conducting SWCNTs were heavily doped by the ionic liquid gating. The electrical resistivity decreased down to approximately 20 μΩcm at room temperature with Vg=-3 V. The temperature dependence in the temperature range (30-110 K) with Vg=-3 V was explained by the variable range hopping (VRH) model with the dimensionality of one, although the SWCNT thin films are two-dimensional structures. Hall resistivity of the thin film slightly changed at different Vg, however, it was almost negligible. It is probably because most of the SWCNT thin films were linear bundle parts, and the bundle diameter was a few nm, which is much smaller than the cyclotron radius [2]. Additionally, the thermoelectric measurement was performed. The Vg dependence of thermoelectrical conductivity showed one-dimensional characteristics [3]. These measurements showed one-dimensional features of electron transport properties of SWCNT thin films. A single SWCNT can be regarded as a one-dimensional system, but in general, one-dimensional features do not appear in the electron transport of SWCNTs in the form of thin films or yarns. In this study, probably because the thin film has a very low density (light transmittance 90% T) and forms a network consisting of a thin bundle structure, the one-dimensional features clearly appeared. The one-dimensionality is one of the origins of the excellent properties of SWCNTs and it is could be a useful index for improving electrical conductivity.AcknowledgmentsI would like to thank H. Date (Univ. of Tokyo) for the experiments and measurements, Prof. T. Fujii (Univ. of Tokyo) for the measurements, and Prof. E. I. Kauppinen for the sample supply.[1] A.G. Nasibulin, et al., ACS Nano, 5 (2011) 3214.[2] M. L. Roukes, et al., Phys. Rev. Lett., 59 (1987) 3011.[3] Y. Ichinose, et al., Phys. Rev. Mater., 5 (2021) 025404.

(Invited) Understanding Chemical Doping to Improve the Thermoelectric Performance of Carbon Nanotube Networks

The thermoelectric performance of semiconductor-enriched carbon nanotube networks is intimately connected to the charge-carrier density injected into the network. This process is typically carried out by charge transfer to or from a chemical dopant, resulting in chemical counterions that reside within the network. While it is commonly accepted that the chemical structure of the dopant controls the redox properties, and therefore the ability of the dopant to modify the charge-carrier density in the carbon nanotube network, less attention is placed on the role that the residual counterion plays in the carrier transport within the doped network. We will show that the chemical structure of some novel p-type dopants based on dodecaborane clusters appended with various tunable organic moieties enables effective spatial separation of the negative counter-charge on the center of the dodecaborane cluster from the hole density injected into the carbon nanotube. This increased spatial separation, compared to more conventional oxidants like triethyloxonium hexachloroantimonate (OA) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), leads to a reduction in the coulombic potential barrier and improved hole transport. The enhanced transport can be attributed to reduced carrier localization, resulting in an increase in the thermoelectric power factor that surpasses the previous best-in-class for enriched semiconductor single-walled carbon nanotube thin film networks.

(Digital Presentation) Strategy to Enhance the Power Factor in Carbon Nanotubes

Flexible thermoelectrics, which can convert waste heat into electricity at surfaces with various shapes and moving parts, is one of important techniques for efficient use of our limited energy resources. Carbon nanotubes are one of possible candidates for flexible thermoelectrics, and then we have investigated the relationships between electronic structure, location of Fermi-energy level, morphology, and thermoelectric performance of the carbon nanotubes. Particularly, we have clarified the one-dimensional characteristics in thermoelectric properties of single walled carbon nanotubes (SWCNTs). In the case of metallic SWCNTs, the thermoelectric trade-off is violated when the Fermi-energy level is tuned around van hove-singularity [1]. Thus, simultaneous enhancement of Seebeck coefficient and electrical conductivity is possible in the metallic type. In the case of semiconducting SWCNTs, we theoretically clarified that it is very difficult to identify the 1D characters in the properties of their Seebeck coefficient, power factor, and electrical conductivity. We found it is necessary to clarify the relationships between thermoelectrical conductivity (L12 term) and electrical conductivity (σ) to discuss the dimensionality in thermoelectric properties of semiconducting nanomaterials. We observed clear peak structure in the L12-σ plot in high-purity (6,5) SWCNT thin films, reflecting the 1D traces in semiconducting SWCNTs[2]. Morphology control is also important to enhance power factor of carbon nanotubes. The Seebeck coefficient does not depend on the directions of heat flow and nanotube axis, but the electrical conductivity can be enhanced when the current direction is parallel to the nanotube axis [1, 3]. On the basis of above characteristics, we determined the following strategy to enhance the power factor (PF) of SWCNTs. Fine tuning of Fermi-energy level in the metallic type of nanotube can enhance PFAligned nanotubes with good electrical conductivity can enhance PF. Thus, we started to study the thermoelectric properties of carbon nanotube fibers with extremely good electrical conductivity, which are produced by Prof. Pasquali Group in Rice Univ [4]. As a result, we found the gigantic power factor of the nanotube fiber, which is 14 mWm-1K-2, with proper Fermi-level tuning [5]. Flexible thermoelectrics with such large PF will be useful for active cooling application.

(Invited, Digital Presentation) Optoelectronic Processes in Single-Chirality Carbon Nanotube Thin Films

Much understanding exists regarding chirality-dependent properties of single-wall carbon nanotubes (SWCNTs), primarily obtained through single-tube studies. However, macroscopic manifestations of chirality dependence have been limited, especially in electronic transport. Here, recent progress in our optical and electronic transport studies of single-chirality SWCNT thin films will be reviewed. We observed pronounced chirality-dependent electronic localization in temperature and magnetic field dependent conductivity measurements on macroscopic films of single-chirality SWCNTs [1]. We also performed optical absorption measurements in aligned single-chirality (6,5) SWCNT films, and through comparison with detailed theoretical calculations based on the Boltzmann scattering equation, we demonstrated that the background absorption is due to phonon-assisted transitions from the semiconductor vacuum to finite-momentum continuum states of excitons [2]. Furthermore, we conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting SWCNTs and found that excitons autoionize, i.e., spontaneously dissociate into electrons and holes [3]. Some of these studies were enabled by our recent improvement of the controlled vacuum filtration technique [4,5], which allows us to fabricate wafer-scale aligned SWCNTs with extremely anisotropic optical properties [6-9]. W. Gao et al., “Band Structure Dependent Electronic Localization in Macroscopic Films of Single-Chirality Single-Wall Carbon Nanotubes,” Carbon 183, 774 (2021).S. Dal Forno et al., “Origin of the Background Absorption in Carbon Nanotubes: Phonon-Assisted Excitonic Continuum,” Carbon 186, 465 (2021).F. R. G. Bagsican et al., “Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication,” Nano Letters 20, 3098 (2020).X. He et al., “Wafer-Scale Monodomain Films of Spontaneously Aligned Single-Walled Carbon Nanotubes,” Nature Nanotechnology 11, 633 (2016).W. Gao and J. Kono, “Science and Applications of Wafer-Scale Crystalline Carbon Nanotube Films Prepared through Controlled Vacuum Filtration,” Royal Society Open Science 6, 181605 (2019).F. Katsutani et al., “Direct Observation of Cross-Polarized Excitons in Aligned Single-Chirality Single-Wall Carbon Nanotubes,” Physical Review B 99, 035426 (2019).W. Gao et al., “Macroscopically Aligned Carbon Nanotubes as a Refractory Platform for Hyperbolic Thermal Emitters,” ACS Photonics 6, 1602 (2019).N. Komatsu et al., “Groove-Assisted Global Spontaneous Alignment of Carbon Nanotubes in Vacuum Filtration,” Nano Letters 20, 2332 (2020).A. Baydin et al., “Giant Terahertz Polarization Rotation in Ultrathin Films of Aligned Carbon Nanotubes,” Optica 8, 760 (2021).

Morphology and Electrical Characteristics of Single-Walled Carbon Nanotubes Film Prepared by Air Brush Technique

This study is to investigate the morphology and electrical characteristics of single-wall carbon nanotubes (SWCNTs) thin film deposition using air brush technique. A deposition setup consisting of a conventional artist air brush was developed and used to deposit SWCNT thin films and therefore the resulting film’s characteristics need to be investigated to gauge its suitability in producing uniform monolayer. The SWCNTs used were synthesized via Direct Injection Pyrolytic Synthesis (DIPS) method, with diameters ranging from 0.8 to 3 nm. The substrate was deposited using an airbrush with varying nozzle to study its effect to the resulting SWCNTs thin films’ characteristics. Subsequently, scanning electron microscope (SEM) were used to inspect the morphology and surface topography, and followed by preliminary electrical measurements. The result shows that good dispersion promotes uniform distribution of SWCNTs over large area of glass substrate. Moreover, the electrical measurement revealed that at 1 V, best morphology produced highest current at a nozzle distanceof 10 mm (15.3 μA) and the lowest current at a nozzle height of 4 mm (3.24 μA). From the results presented, it is demonstrated that conventional artist airbrush setup can be effectively used to deposit monolayer thin film of SWCNT with high degree of uniformity. This research is necessary for the process of depositing-controlled CNT thin film network, which can influence the material characteristics and performance of the variety of CNT-based device applications.

Marangoni-flow-assisted assembly of single-walled carbon nanotube films for human motion sensing

Single-walled carbon nanotubes (SWCNTs) present excellent electronic and mechanical properties desired in wearable and flexible devices. The preparation of SWCNT films is the first step for fabricating various devices. This work developed a scalable and feasible method to assemble SWCNT thin films on water surfaces based on Marangoni flow induced by surface tension gradient. The films possess a large area of 40 cm × 30 cm (extensible), a tunable thickness of 15∼150 nm, a high transparency of up to 96%, and a decent conductivity. They are ready to be directly transferred to various substrates, including flexible ones. Flexible strain sensors were fabricated with the films on flexible substrates. These sensors worked with high sensitivity and repeatability. By realizing multi-functional human motion sensing, including responding to voices, monitoring artery pulses, and detecting knuckle and muscle actions, the assembled SWCNT films demonstrated the potential for application in smart devices.

Dry-transferred single-walled carbon nanotube thin films for flexible and transparent heaters

Single-walled carbon nanotubes (SWCNTs) are considered to be ideal candidates for flexible and transparent heaters (THs). However, the conventional wet-fabrication of SWCNT thin films makes the performance of SWCNT THs fail to meet the expectation. Herein, we synthesized high-quality SWCNT thin films by floating catalyst chemical vapor deposition (FCCVD), allowing the dry and direct process of transferring films on polyethylene terephthalate (PET) as THs. A pristine SWCNT TH with a transmittance of 73.5% displayed a reasonable heating performance, reaching the saturated temperatures of ∼53 and 85 °C at the driven voltages of 7 V and 12 V, respectively. Nevertheless, after doping the identical SWCNT film by AuCl3, it rises to ∼70 and 139 °C at 7 V and 12 V, respectively. We held the point that the improved heating performance was mainly attributed to the reduced sheet resistance and film densification by AuCl3 doping. We also found our SWCNT TH showed excellent thermal stability and mechanical flexibility. Due to the continuous synthesis and facile transfer of FCCVD, our SWCNT THs demonstrate potential applications in modern technologies, such as smart windows and defoggers.

Controlled synthesis of single-walled carbon nanotubes by floating catalyst CVD for transparent conducting films: A critical role of loops

Single-walled carbon nanotubes (SWCNTs) are ideal candidates for transparent conductive films (TCFs) due to their excellent optical transparency and electrical conductivity. The geometry of SWCNTs, including the tube diameter, bundle length and bundle diameter, is vital to high-performance TCFs. Herein, we synthesized SWCNTs by floating catalyst chemical vapor deposition (FCCVD). The SWCNT geometries were tuned by hydrogen (H2), and we found that the tube diameter, bundle length and bundle diameter increase with the H2 concentration. Besides, we observed the formation of SWCNT loop at the tube ends. Both the number and circumference of loops increased with the increment of bundle length. Further, the loops were also found to affect the conductivity of SWCNT thin film. Excessive number of loops with large size could reduce the conductivity of SWCNT thin film. At the optimized H2 concentration, we obtained the SWCNT TCF with sheet resistances of 290 and 95 Ω/sq. for the pristine and AuCl3 doped SWCNT films, respectively, at 90% transmittance. Our work demonstrates the importance of H2 for SWCNT synthesis and the critical role of loops on film conductivity, blazing new ideas for future research to obtain SWCNT TCFs with improved performance.

Arresting Photodegradation in Semiconducting Single-Walled Carbon Nanotube Thin Films

Solid-state dispersions of isolated single-walled carbon nanotubes (SWCNTs) in polymer matrices or networks of electronically coupled SWCNTs are gaining interest for a broad variety of optoelectronic applications. However, little is known about either the stability or degradation mechanisms of these systems. We show dramatic sp2-to-sp3 defect transformations of the SWCNT sidewall when either the S11 or S22 exciton transitions are optically pumped in ambient conditions, leading to the rapid decay of absorption and emission properties in less than 24 h for conditions similar to exposure to solar illumination. Importantly, we demonstrate that either (i) encapsulation to block reactive O2 from SWCNT excited states or (ii) exciton quenching via donor-to-acceptor electron transfer is an effective route for “kinetic stabilization” against photodegradation, with <8% loss in absorbance after 1200 h of illumination. We find that SWCNT:polymer loading does not impact degradation. Our study suggests that the sp3 defects are associated with the formation of oxygenic groups on the SWCNT sidewall. While such defect populations can detrimentally evolve over time in films where SWCNTs are environmentally exposed in the presence of light, we offer multiple pathways to arrest this degradation and enable their robust application as advanced optical materials in optical and electronic devices.