Aligned Single-walled Carbon Nanotubes Research Articles

Tailored twisted CNT bundle with improved inter-tube slipping performances

The exceptional mechanical properties of carbon nanotubes (CNTs) have encouraged the development of high-performance synthetic fibers in composite materials. To understand the effect of twisting on the mechanical and slipping performances of CNT bundles, molecular dynamics simulation is applied to investigate the tensile performances, failure modes, and pull-out responses of twisted CNT bundles. A molecular model comprising nineteen parallel aligned single-walled carbon nanotubes (SWCNTs) is twisted into bundles at angles of 0°, 10°, and 20°, and further tensile and pull-out simulations are performed. The tensile simulation indicates that compared with the non-twisted CNT bundle showing a tensile strength of about 82 GPa with obvious inter-tube slipping, the 10°-twisted bundle exhibits a tensile strength of approximately 70 GPa with SWCNTs fracture as the main failure mode, which indicates that twisting improves the inter-tube slipping performance without causing excessive strength reduction. Comparatively, when the twisting angle is 20°, no inter-tube slipping is observed and the tensile strength of the CNT bundle is measured to be 55 GPa, which decreases by approximately 32.9 %. The pull-out simulations further reveal that the pull-out forces increase as the twisting angle increases and the weakened bundle strength of twisted bundle is attributed to the repulsive van der Waals forces caused by the reduced distances between inter-tubes. Essentially, twisting is unfavorable for the overall mechanical strength while torsional densification mitigates the inter-tube slipping, which indicates that a trade-off need to be achieved. This paper provides insights into the tensile and slipping performances of twisted CNT bundles and forms a basis for enhancing the assembled CNTs bundle as the next-generation reinforcing phase in composite materials.

Study on dynamic mechanism of controllable SWCNTs arrays prepared by self-assembly

Single walled carbon nanotubes (SWCNTs) are seamless hollow tubular structures formed by a layer of graphite curled along a specific spiral vector. Individual SWCNT is restricted by the inconvenient manipulation in its nanoscale, and unable to maximize its functionality in the macro field. As a result, the two-dimensional array is the most basic structure for manufacturing SWCNTs devices in practical application. However, the fidelity of the array pattern and the distribution morphology of individual SWCNT in the large-scale array directly affect its performance, which also poses a challenge to the research on assembly methods and their underlying mechanisms. In this paper, the dynamic mechanism of controllable SWCNTs arrays prepared by self-assembly method has been studied from molecular dynamics to hydrodynamic by a combination of experiments and simulations. Some key factors limiting the fidelity of the arrays and the distribution morphology of individual SWCNT in the array, such as the self-assembly template, the concentration of the SWCNTs dispersion, are investigated and optimized by experiments. Finally, inspired by the results of the mechanism research, a blade-coating procedure is applied in self-assembly process to improve the alignment of SWCNTs in the array. The improved alignment of SWCNTs arrays are verified through morphology analysis and volt-ampere (V-A) characteristics compared with the SWCNTs randomly distributed arrays. The explicitly results are not only helpful to understand dynamic phenomena during self-assembly process and the influencing factors of the self-assembly method but also will provide meaningful guidance in the design, fabrication of SWCNTs arrays to prevent distortion in large scale and further promote their industrial application in manufacturing next-generation micro-nano devices.

Alignment of DNA Wrapped Single Walled Carbon Nanotubes Sensors

Single walled carbon nanotubes (SWNT) that are well aligned have a broad range of applications, including solar cells, sensors, transistors, and optoelectronic devices. Alignment of SWNT is typically performed either during SWNT fabrication or out of post-processed SWNT solutions through vacuum filtration, spin-coating, electromagnetic fields, surfactant-aided microfluidic alignment techniques or concentration-dependent shear alignment. Alignment of SWNT during fabrication requires a lot of time and money and precludes the ability to wrap the SWNT with DNA, which is what creates the SWNT’s sensing capabilities. Unfortunately, most of these techniques have not been widely adopted since it is labor-intensive, difficult to reproduce or hard to scale up. We developed a scalable method to align DNA wrapped SWNT through heat activation without altering the SWNT’s wrapping and sensing capabilities. This research will pave the way for the development of both research and industrial scale aligned SWNT from aqueous solutions for next-generation of SWNT based optical sensors.

Assembly and Characterization of Length-Sorted, Aligned Boron Nitride/Carbon Nanotube Thin Film Composites Via Slow Vacuum Filtration

Globally aligned single-wall carbon nanotubes (SWCNTs) are a promising, scalable platform for studying fundamental physical phenomena and an important vehicle for optoelectronic device applications. Recent work by Rust et al. [1] has identified nanotube length as an important mediating factor in the degree and scale of SWCNT alignment via slow vacuum filtration (SVF).Building on this work and that of Walker et al. [2,3], we explore the co-assembly of length-sorted SWCNTs and boron nitride nanotubes (BNNTs) atop polycarbonate track-etched membranes. BNNTs have recently gained interest as a chirality-agnostic, wide-bandgap material for encapsulation, insulation, thermal/mechanical robustness, and visible-light transparency, while also having a similar high aspect ratio morphology, making them an exceptional and alignable host matrix for individualized CNT species. To prepare such hybrid assemblies, a polymer-assisted, depletion-based method was first used to length-sort different diameter SWCNT (alkane filled electric-arc, CoMoCAT, metallic-sorted Tuball), and BNNT nanotube populations. Thereafter, we investigated the impacts of tube lengths, relative BN/C fraction, mass loading, and presence of a global director force on the multi-scale assembly behavior. A combination of birefringence microscopy and polarized Raman spectroscopy probed and identified trends in global and local ordering under these conditions.[1] Rust, C., Shapturenka, P., Spari, M., Jin, Q., Li, H., Bacher, A., Guttmann, M., Zheng, M., Adel, T., Hight Walker, A. R., Fagan, J. A., & Flavel, B. S. (2022). The Impact of Carbon Nanotube Length and Diameter on their Global Alignment by Dead-End Filtration. Small (forthcoming).[2] Walker, J. S., Fagan, J. A., Biacchi, A. J., Kuehl, V. A., Searles, T. A., Hight Walker, A. R., & Rice, W. D. (2019). Global Alignment of Solution-Based Single-Wall Carbon Nanotube Films via Machine-Vision Controlled Filtration. Nano Letters, 19(10), 7256–7264.[3] Walker, J. S., Macdermid, Z. J., Fagan, J. A., Kolmakov, A., Biacchi, A. J., Searles, T. A., Walker, A. R. H., & Rice, W. D. (2022). Dependence of Single‐Wall Carbon Nanotube Alignment on the Filter Membrane Interface in Slow Vacuum Filtration. Small, 18, 2105619.

Confinement-induced nonlocality and casimir force in transdimensional systems.

We study within the framework of the Lifshitz theory the long-range Casimir force for in-plane isotropic and anisotropic free-standing transdimensional material slabs. In the former case, we show that the confinement-induced nonlocality not only weakens the attraction of ultrathin slabs but also changes the distance dependence of the material-dependent correction to the Casimir force to go as contrary to the ∼1/l dependence of that of the local Lifshitz force. In the latter case, we use closely packed array of parallel aligned single-wall carbon nanotubes in a dielectric layer of finite thickness to demonstrate strong orientational anisotropy and crossover behavior for the inter-slab attractive force in addition to its reduction with decreasing slab thickness. We give physical insight as to why such a pair of ultrathin slabs prefers to stick together in the perpendicularly oriented manner, rather than in the parallel relative orientation as one would customarily expect.

Printable Aligned Single-Walled Carbon Nanotube Film with Outstanding Thermal Conductivity and Electromagnetic Interference Shielding Performance

HighlightsUltrathin, lightweight, and ultraflexible aligned single-walled carbon nanotube (SWCNT) films were fabricated via a facile, environmentally friendly, and scalable printing methodology.The aligned pattern and outstanding intrinsic properties of SWCNTs rendered “metal-like” thermal conductivity, excellent mechanical strength, hydrophobicity, and remarkable electromagnetic interference (EMI) shielding performance of the films.The excellent EMI shielding stability and reliability when subjected to mechanical deformation, chemical corrosion, and extreme environments demonstrated the significant potential of the films in aerospace, defense, and smart wearable electronics.

Liquid Crystalline Behaviors of Single-Walled Carbon Nanotubes in an Aqueous Sodium Cholate Dispersion.

Controlling the alignment of single-walled carbon nanotubes (SWCNTs) on the macroscopic scale is critical for practical applications because SWCNTs are extremely anisotropic materials. One efficient technique is to create an effective SWCNT dispersion, which shows a liquid crystal (LC) phase. A strong acid treatment can realize SWCNT liquid crystalline dispersions. However, strong acids pose a substantial safety risk, which renders the process unfit for mass production. Herein, an isolated SWCNT dispersion displaying an LC behavior is prepared using sodium cholate without an acid treatment, and its phase transition behaviors are systematically investigated across the isotropic to biphasic to nematic phases. As the SWCNT concentration increases, the dispersion undergoes an isotropic-to-nematic phase transition in which the spindle-shaped LC droplets, or the so-called tactoids, and the Schlieren textures can be observed in the intermediate biphasic state and the nematic phase, respectively. The arrangements of SWCNTs in the tactoids and the Schlieren structures are directly investigated by polarized optical microscopy. The clear LC behaviors of the CNT dispersion suggest that the CNT orientations can be controlled by the normal surfactant-assisted method, which is a crucial advantage for the liquid-phase processing of CNT fibers and films.

(Invited, Digital Presentation) Macroscopically Aligned Carbon Nanotubes for Photonics, Electronics, and Thermoelectrics

The remarkable flexibility, stable chemical structure, and extraordinary thermal, electrical, and optical properties of carbon nanotubes (CNTs) are promising for a variety of applications in flexible and/or high-temperature electronics, optoelectronics, and thermoelectrics, including wearables, refractory photonics, and waste heat harvesting [1]. However, the long-standing goal in the preparation of CNT ensembles is how to maintain the extraordinary properties of individual CNTs on a macroscopic scale. The polydispersity and randomness remain two main challenges.Here, we will discuss different methods for creating macroscopically aligned CNTs, including spontaneous formation of wafer-scale aligned CNT films via controlled vacuum filtration [2-4] and production of ultrahigh-conductivity CNT fibers and films through solution spinning and coating [5,6]. We will then describe the optical [2,7-11], dc and ac electrical [2,12-17], thermal [18], and thermoelectric [19-21] properties of these materials. These results are promising for device applications in various fields such as flexible CNT broadband detectors [22-26], spectrally selective thermal emitters [11], and thermoelectric devices [20,21]. W. Gao et al., “Macroscopically Aligned Carbon Nanotubes for Flexible and High-Temperature Electronics, Optoelectronics, and Thermoelectrics,” Journal of Physics D: Applied Physics 53, 063001 (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).N. Komatsu et al., “Groove-Assisted Global Spontaneous Alignment of Carbon Nanotubes in Vacuum Filtration,” Nano Letters 20, 2332 (2020).N. Behabtu et al., “Strong, Light, Multifunctional Fibers of Carbon Nanotubes with Ultrahigh Conductivity,” Science 339, 182 (2013).L. W. Taylor et al., “Improved Properties, Increased Production, and the Path to Broad Adoption of Carbon Nanotube Fibers,” Carbon 171, 689 (2021).K. Yanagi et al., “Intersubband Plasmons in the Quantum Limit in Gated and Aligned Carbon Nanotubes,” Nature Communications 9, 1121 (2018).W. Gao et al., “Continuous Transition between Weak and Ultrastrong Coupling through Exceptional Points in Carbon Nanotube Microcavity Exciton–Polaritons,” Nature Photonics 12, 362 (2018).M. E. Green et al., “Bright and Ultrafast Photoelectron Emission from Aligned Single-Wall Carbon Nanotubes through Multiphoton Exciton Resonance,” Nano Letters 19, 158 (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).X. Wang et al., “High-Ampacity Power Cables of Tightly-Packed and Aligned Carbon Nanotubes,” Advanced Functional Materials 24, 3241 (2014).A. Zubair et al., “Carbon Nanotube Fiber Terahertz Polarizer,” Applied Physics Letters 108, 141107 (2016).D. Tristant et al., “Enlightening the Ultrahigh Electrical Conductivities of Doped Double-Wall Carbon Nanotube Fibers by Raman Spectroscopy and First-Principles Calculations,” Nanoscale 18, 19668 (2016).N. Komatsu et al., “Modulation-Doped Multiple Quantum Wells of Aligned Single-Wall Carbon Nanotubes,” Advanced Functional Materials 27, 1606022 (2017).F. R. G. Bagsican et al., “Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication,” Nano Letters 20, 3098 (2020).A. Baydin et al., “Giant Terahertz Polarization Rotation in Ultrathin Films of Aligned Carbon Nanotubes,” Optica 8, 760 (2021).S. Yamaguchi et al., “One-Directional Thermal Transport in Densely Aligned Single-Wall Carbon Nanotube Films,” Applied Physics Letters 115, 223104 (2019).K. Fukuhara et al., “Isotropic Seebeck Coefficient of Aligned Single-Wall Carbon Nanotube Films,” Applied Physics Letters 113, 243105 (2018).Y. Ichinose et al., “Solving the Thermoelectric Trade-Off Problem with Metallic Carbon Nanotubes,” Nano Letters 19, 7370 (2019).N. Komatsu et al., “Macroscopic Weavable Fibers of Carbon Nanotubes with Giant Thermoelectric Power Factor,” Nature Communications 12, 4931 (2021).S. Nanot et al., “Broadband, Polarization-Sensitive Photodetector Based on Optically-Thick Films of Macroscopically Long, Dense, and Aligned Carbon Nanotubes,” Scientific Reports 3, 1335 (2013).X. He et al., “Photothermoelectric p-n Junction Photodetector with Intrinsic Broadband Polarimetry Based on Macroscopic Carbon Nanotube Films,” ACS Nano 7, 7271 (2013).X. He et al., “Carbon Nanotube Terahertz Detector,” Nano Letters 14, 3953 (2014).X. He, F. Léonard, and J. Kono, “Uncooled Carbon Nanotube Photodetectors,” Advanced Optical Materials 3, 989 (2015).A. Zubair et al., “Carbon Nanotube Woven Textile Photodetector,” Physical Review Materials 2, 015201 (2018).

(Invited, Digital Presentation) Global Alignment of Carbon Nanotubes Via High Precision Microfluidic Dead-End Filtration

Single wall carbon nanotubes (SWCNTs) dispersed by negatively charged sodium deoxycholate (DOC) or positively charged cetrimonium bromide (CTAB) are shown to assemble into aligned films (3.8 cm²) on polycarbonate membranes by slow flow dead-end filtration. Global alignment (S2D max » 0.85) is obtained on both pristine polyvinylpyrrolidone (PVP) coated membranes and those with an intentional 150 nm – 600 nm groove pattern from hot-embossing. In all cases, a custom microfluidic setup capable of precise control and measurement of the volume rate, trans-membrane pressure and the filtration resistance is used to follow SWCNT film formation. Conditions associated with the formation of SWCNT crystallites or their global alignment are identified and these are discussed in terms of membrane fouling and the interaction potential between the surface of the membrane and nanotubes. SWCNT alignment is characterised by cross polarized microscopy, AFM, SEM and Raman spectroscopy.https://doi.org/10.1002/adfm.202107411

(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).

(Invited) Optically Detected Magnetic Resonance of Triplet Excitons in Sorted (6,5) and (7,5) SWCNTs

Single-wall carbon nanotubes (SWCNTs) possess unique electronic and optical properties, strongly depending on their exact chiral structure. Their quasi one-dimensional character results in the formation of strongly bound electron-hole pairs (excitons) that can even be observed at room temperature (i.e. binding energies of the order of several hundred meV).[1] The exciton fine structure of SWCNTs is quite complex, with multiple singlet and triplet excitonic states, of which only one is optically allowed, thereby resulting in extremely low fluorescence (PL) quantum yields. While the singlet excitons have already been investigated thoroughly, little is known about the longer-living triplet excitons.In this work, we report the characterization of triplet excitons in chirality-purified samples of (7,5) and (6,5) SWCNTs by means of optically detected magnetic resonance spectroscopy (ODMR), a technique that measures the influence on the emission of CNTs when making transitions between the spin levels of the triplet excitons. In both chiralities the nanotubes are shown to sustain two types of triplet exciton states, as shown by the symmetry and magnitude of their zero-field splitting parameters in the spin Hamiltonian, which are determined by careful fitting of magnetic-field orientation-dependent ODMR spectra of in-plane aligned SWCNTs. While the triplet excitons only weakly depend on the nanotube environment, an increase in zero-field splitting is found for (6,5) with respect to (7,5) nanotubes, in good agreement with the tighter confinement expected in the narrower-diameter nanotubes.[1] M.S. Dresselhaus et al, Ann. Rev. Phys. Chem. 2007, 58, 719[2] D. Stich et al, Nature Photon. 2014 , 8,139; J. Palotás et al, ACS Nano 2020, 14, 11254

Dependence of Single-Wall Carbon Nanotube Alignment on the Filter Membrane Interface in Slow Vacuum Filtration.

The recent introduction of slow vacuum filtration (SVF) technology has shown great promise for reproducibly creating high-quality, large-area aligned films of single-wall carbon nanotubes (SWCNTs) from solution-based dispersions. Despite clear advantages over other SWCNT alignment techniques, SVF remains in the developmental stages due to a lack of an agreed-upon alignment mechanism, a hurdle which hinders SVF optimization. In this work, the filter membrane surface is modified to show how the resulting SWCNT nematic order can be significantly enhanced. It is observed that directional mechanical grooving on filter membranes does not play a significant role in SWCNT alignment, despite the tendency for nanotubes to follow the groove direction. Chemical treatments to the filter membrane are shown to increase SWCNT alignment by nearly 1/3. These findings suggest that membrane surface structure acts to create a directional flow along the filter membrane surface that can produce global SWCNT alignment during SVF, rather serving as an alignmenttemplate.

Carbon nanotubes integrated into polyamide membranes by support pre-infiltration improve the desalination performance

Carbon nanotubes (CNTs) are promising for realizing ultrafast membranes with implications to molecular separations and beyond. However, it is a big challenge to harness the potential of CNTs for designing scalable yet high-performance membranes. Here we systematically explore the role of loading and vacuum-assisted alignment of CNTs for improving the desalination performance of polyamide (PA) based thin-film composites. To rule out the dispersion instability issues, we focused on carboxylated single-walled CNTs (SWCNTs) commercially available in the market. After applying a pre-treatment for cleaning, we deposited SWCNTs on porous polysulfone supports by vacuum filtration and coated a PA layer on top via interfacial polymerization. Morphological assessments supported by polarized Raman microspectroscopy allowed the quantification of SWCNT alignment. At an optimum SWCNT loading, which we found critical for alignment, the water permeability of resulting membranes significantly improved without compromising NaCl selectivity. Also, we achieved an improved boric acid selectivity, arguably owing to the hydrophobic nature of nanotube channels. Moreover, nanotubes promoted resistance against chlorine degradation and improved mechanical strength. Vacuum deposition is instrumental for infiltrating SWCNTs into the support layer, but a mat layer forms between the support and PA layers when SWCNT loading exceeds the limit that the support pores can accommodate. Given that we use ordinary SWCNTs and a scalable methodology (vacuum-assisted infiltration), the developed membranes are promising for practical applications.

(Invited) Understanding the Process Variables to Achieve Global Alignment of Single-Wall Carbon Nanotubes

Carbon nanotubes (CNTs) are a 1D dimensional material with anisotropy in their thermal, electronic and optical properties. It is therefore a grand challenge to control their orientation in an ensemble and prepare large scale aligned thin-films. In this direction, slow flow dead-end filtration using polyvinylpyrrolidone-coated polycarbonate membranes has recently been shown to achieve global alignment of CNTs across an entire membrane. This route is highly attractive because it allows for purified dispersions to be used but it has proven to be difficult to reproduce. In this work, we present a custom dead-end filtration setup capable of precise in-situ measurement of flow, trans-membrane pressure and effective retention. This setup allows us to identify the important variables to achieve global alignment and explain why others have found it hard to reproduce in their own laboratory.

Alignment dynamics of single-wall carbon nanotubes under electric field in different surfactant solutions

The dynamics of metallic single-wall carbon nanotube (SWCNT) alignment inside various viscous media under electric field is investigated in this simulation work for the manifestation of macroscale aligned SWCNT films. An alternating current (AC) electric field was applied to the liquid solution of several surfactants (DIW, DMF, CHEX, SDS, and DOC) containing SWCNTs. The time required for the SWCNTs to get aligned to the applied AC electric field was simulated for different initial conditions for all the surfactants. An analytical model based on dielectrophoresis induced torque was employed. The model considers the viscosity and conductivity of the surrounding medium. The influence of SWCNT length, SWCNT radius, and frequency of the AC field on the assembly of SWCNTs were studied. Our analysis showed that a longer and narrower SWCNT prompts faster assembly to an aligned SWCNT aggregation. Furthermore, the effect of the concentration of SDS and the effect of electric field strength for DIW surfactant were also investigated. Viscosity plays a significant role in the alignment process. Slower SWCNT alignment is caused by a medium of higher viscosity.

Thermal properties of single-walled carbon nanotube forests with various volume fractions

Needs of a material for thermal management in reduced-sized electronic devices drive single-walled carbon nanotubes (SWCNTs) to be one of the most promising candidates due to their excellent thermal properties. Many numerical and experimental studies have reported on understanding thermal properties of the SWCNT for thermal device applications. In the present study, thermal diffusivity and conductivity of SWCNT forests in the axial direction with various volume fractions of SWCNTs were measured by laser flash analysis technique and the same properties of an individual SWCNT were derived. The volume fraction was controlled up to 25 vol% by biaxial mechanical densification which maintains SWCNT alignment. While the thermal diffusivity of SWCNT forests was almost constant, it increased when the volume fraction was higher than 17 vol%, suggesting that some SWCNTs, which did not serve as a thermal path in the case of lower volume fraction, were connected with the other SWCNTs. Through the increase of inferred thermal conductivity equivalent to an individual SWCNT after 17 vol%, we also realized that the enhancement of volume fraction of SWCNT forest diminished thermal boundary resistance in good agreement with the tendency inferred from the reported numerical data.

Principles of carbon nanotube dielectrophoresis

Dielectrophoresis (DEP) describes the motion of suspended objects when exposed to an inhomogeneous electric field. It has been successful as a method for parallel and site-selective assembling of nanotubes from a dispersion into a sophisticated device architecture. Researchers have conducted extensive works to understand the DEP of nanotubes in aqueous ionic surfactant solutions. However, only recently, DEP was applied to polymer-wrapped single-walled carbon nanotubes (SWCNTs) in organic solvents due to the availability of ultra-pure SWCNT content. In this paper, the focus is on the difference between the DEP in aqueous and organic solutions. It starts with an introduction into the DEP of carbon nanotubes (CNT-DEP) to provide a comprehensive, in-depth theoretical background before discussing in detail the experimental procedures and conditions. For academic interests, this work focuses on the CNT-DEP deposition scheme, discusses the importance of the electrical double layer, and employs finite element simulations to optimize CNT-DEP deposition condition with respect to the experimental observation. An important outcome is an understanding of why DEP in organic solvents allows for the deposition and alignment of SWCNTs in low-frequency and even static electric fields, and why the response of semiconducting SWCNTs (s-SWCNTs) is strongly enhanced in non-conducting, weakly polarizable media. Strategies to further improve CNT-DEP for s-SWCNT-relevant applications are given as well. Overall, this work should serve as a practical guideline to select the appropriate setting for effective CNT DEPs.

Theoretical modeling and experimental validation of Electrical conductivity for highly aligned single-walled carbon nanotube thin films

A new model capable of accurately predicting the electrical conductivity of highly aligned single-walled carbon nanotube thin films has been developed. The films ranged between 1 and 11 layers thick. The well-established Lamination theory failed to predict the experimentally measured values for the films of different thicknesses. However, this model, based on modified and expanded techniques applied within the realm of the lamination theory, could predict the exact values taking into consideration the inter-layer interaction that is crucial for the properties and performance of nano- films.

Oil-water separation performance of aligned single walled carbon nanotubes membrane: A reactive molecular dynamics simulation study

Carbon nanotubes (CNTs) have played pivoting role in water remediation owing to peculiar features such as high porosity and desirable chemical and mechanical properties. This research is dedicated to separation of oil–water systems by using aligned armchair SWCNTs–based membrane via reactive molecular dynamic (MD) simulation and density functional theory (DFT) calculations. It was found that no water molecules transmitted through CNT with 5.7 Å diameter, but CNTs with diameters of 7 Å and 8.5 Å were both able to transfer water molecules, although CNT with diameter of 7 Å was more efficient for water permeation. The incorporation of hexane was observed into CNTs with diameters of 7 Å and 8.5 Å and also toluene into latter one. Moreover, the determination of interaction energy, electronic structures and barrier energy with DFT-D3 calculation for hexane molecule incorporating into the nanotube cavity confirms our reactive MD simulation outcomes. It was revealed that armchair CNTs membrane with diameter of 7 Å has more efficacy for the separation of oil–water mixture compared to other counterparts though it could not avoid the permeance of small oil components such as linear hydrocarbons. Our reactive MD simulations findings provided a novel strategy for the application of aligned CNTs membranes in water treatments with voluntarily viable motivations, high efficiency and stability as well as long lifetime.

(Invited) Macroscopically Aligned Carbon Nanotubes for Efficient Thermophotovoltaic Energy Conversion

Single-walled carbon nanotubes (SWCNTs) exhibit many unique properties arising from their quantum confinement. However, the lack of a controllable large-scale alignment procedure had limited the exploration of their unique properties. Our recent work has overcome this problem by demonstrating a vacuum-filtration based technique to align SWCNTs on wafer-scale. These aligned SWCNT films exhibit many interesting optical properties such as extreme optical anisotropy or hyperbolic dispersion. Besides, aligned SWCNTs are flexible, refractory (stable up to 1900 K), inexpensive, and based on earth-abundant carbon making them an excellent refractory infrared nanophotonic material platform. Such a refractory infrared material platform is exactly what is necessary to enable efficient thermophotovoltaic energy conversion.Thermophotovoltaics is a solid-state heat-to-electricity conversion technique. It relies upon photovoltaic conversion of thermal light emanating from a hot surface to electricity. The broadband nature of thermal light makes the thermophotovoltaic conversion inefficient. Optical resonators, filters, spectrally selective thermal emitters are a few methods proposed to increase the efficiency of thermophotovoltaics. Selective thermal emitters are very promising due to their ease of design and integration. Selective thermal emitters are hot surfaces that emit thermal radiation in a narrow spectral band above the bandgap of the photovoltaic cell. The suppression of thermal emission below the bandgap and enhancement above the bandgap are the key requirements for efficient thermophotovoltaic conversion. Generally, optical properties of materials at high temperatures limit the performance of such selective emitters and the current efficiency record is about 8%. But, the hyperbolic optical property of aligned SWCNTs together with their thermal stability make aligned SWCNTs ideal for efficient thermophotovoltaics.Here in this work, we study the infrared optical properties of aligned SWCNTs at high temperatures (1000 K) and demonstrate spectrally selective thermal emission from plain and nano-patterned thin films. Using deep sub-wavelength sized optical cavities, we show that aligned SWCNTs exhibit hyperbolic dispersion in almost of all of infrared and enhance thermal photon density by at least 100 times. Such a large enhancement in thermal photon density leads to a large suppression of unwanted sub-bandgap thermal photons and a large enhancement of above-bandgap photons. Our analysis shows that aligned SWCNTs enable 30% or higher thermophotovoltaic efficiency when operating at 1300 K with no concentration. With near-field concentration, aligned SWCNTs can allow conversion efficiencies over 50%.