Aligned Single-walled Carbon Nanotube Films Research Articles

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.

(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%.

Covalent Atomic Bridges Enable Unidirectional Enhancement of Electronic Transport in Aligned Carbon Nanotubes.

Interconnecting the surfaces of nanomaterials without compromising their outstanding mechanical, thermal, and electronic properties is critical in the design of advanced bulk structures that still preserve the novel properties of their nanoscale constituents. As such, bridging the π-conjugated carbon surfaces of single-walled carbon nanotubes (SWNTs) has special implications in next-generation electronics. This study presents a rational path toward the improvement of the electrical transport in aligned semiconducting SWNT films by deposition of metal atoms. The formation of conducting Cr-mediated pathways between the parallel SWNTs increases the transverse (intertube) conductance while having a negligible effect on the parallel (intratube) transport. In contrast, doping with Li has a predominant effect on the intratube electrical transport of aligned SWNT films. Large-scale first-principles calculations of electrical transport on aligned SWNTs show good agreement with the experimental electrical measurements and provide insight into the changes that different metal atoms exert on the density of states near the Fermi level of the SWNTs and the formation of transport channels.

Effect of constructive rehybridization on transverse conductivity of aligned single-walled carbon nanotube films

Alignment of densely packed single-walled carbon nanotubes (SWNTs) largely preserves the extraordinary electronic properties of individual SWNTs in the alignment direction, while in transverse direction the films are very resistive due to large energy barriers for tunneling between adjacent SWNTs. We demonstrate that chromium atoms inserted between the sidewalls of parallel SWNTs effectively coordinate to the benzene rings of the nanotubes via hexahapto bonds that preserve the nanotube-conjugated electronic structure and serve as a conduit for electron transfer. The atomically interconnected aligned SWNTs exhibit enhanced transverse conductivity, which increases by ∼2100% as a result of the photoactivated organometallic functionalization with Cr. The hexahapto mode of bonding the graphitic surfaces of carbon nanotubes with transition metal atoms offers an attractive route to the reversible chemical engineering of the transport properties of aligned carbon nanotube thin films. We demonstrate that a device fabricated with aligned SWNTs can be reversibly switched between a state of high electrical conductivity (ON) by light and low electrical conductivity (OFF) by applied potential. This study provides a route to the design of novel nanomaterials for applications in electrical atomic switches, optoelectronic and spintronic devices.

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Reliably routing heat to and from conversion materials is a daunting challenge for a variety of innovative energy technologies--from thermal solar to automotive waste heat recovery systems--whose efficiencies degrade due to massive thermomechanical stresses at interfaces. This problem may soon be addressed by adhesives based on vertically aligned carbon nanotubes, which promise the revolutionary combination of high through-plane thermal conductivity and vanishing in-plane mechanical stiffness. Here, we report the data for the in-plane modulus of aligned single-walled carbon nanotube films using a microfabricated resonator method. Molecular simulations and electron microscopy identify the nanoscale mechanisms responsible for this property. The zipping and unzipping of adjacent nanotubes and the degree of alignment and entanglement are shown to govern the spatially varying local modulus, thereby providing the route to engineered materials with outstanding combinations of mechanical and thermal properties.

Inkjet printing of aligned single-walled carbon-nanotube thin films

We report a method for the inkjet printing of aligned single-walled carbon-nanotube (SWCNT) films by combining inkjet technology with the strong wettability contrast between hydrophobic and hydrophilic areas based on the patterning of self-assembled monolayers. Both the drying process control using the strong wettability boundary and the coffee-stain effect strongly promote the aggregation of SWCNTs along the contact line of a SWCNT ink droplet, thereby demonstrating our achievement of inkjet-printed aligned SWCNT films. This method could open routes for developing high-performance and environmentally friendly SWCNT printed electronics.

Ultrahigh Magnetic Field Optical Study of Single-walled Carbon Nanotubes Film

Excitons in Single-Walled Carbon Nanotubes (SWNTs) have emerged as an ideal candidate for exploring one-dimensional (1-D) exciton physics. Exciton states which dominate optical properties of SWNTs even at room temperature, are not clarify yet. The optical absorption spectra of aligned SWNTs films under ultra high magnetic fields up to 190 T are examined to investigate this issue. Shifting and splitting of the absorption peaks due to Aharonov-Bohm effect was observed clearly above 80 T in the configuration where the magnetic fields were applied in parallel to the alignment of SWNTs. The lowest singlet exciton state has been determined through the analysis of energy splitting of excitons by the application of magnetic fields. Keywords: blue shift, optically active, optically inactive, red shift, single-turn coil system

Development of Magnetically Aligned Single-Walled Carbon Nanotubes-Gelatin Composite Films

Magnetically aligned and highly dispersed single-walled carbon nanotubes (SWNTs)-gelatin composite films were fabricated on a quartz substrate of newly designed disassemblable sample cells. After an SWNTs-gelatin composite sol in each sample cell was exposed to vertical DC magnetic fields to 28 T during the sol-gel transition, a side plate with a polytetrafluoroethylene film attached to the inner surface was removed from the sample cell carefully, and the composite gel left on the other side plate made of quartz was dehydrated in a nonmagnetic circumstance. We have found that the alignment of SWNTs is retained in uniform composite films obtained after the nonmagnetic drying process. Both polarized absorption measurements and polarized Raman scattering measurements were performed in order to confirm the alignment of SWNTs in the composite films consistently. The optical anisotropy of the composite films increased somewhat superlinearly as a function of the magnetic field applied during the sol-gel transition and reached to 0.13 at 28 T. The nonmagnetic drying process would make it easy to apply higher magnetic fields to the fabrication of aligned SWNTs films as it consumes resources to keep operating high field magnets such as hybrid magnets during the drying process.

Vertical Alignment of Single-Walled Carbon Nanotube Films Formed by Electrophoretic Deposition

Films of chemically shortened and functionalized single-walled carbon nanotubes (SWNTs) have been formed on a gold electrode by electrophoretic deposition. Applying ultrasonic energy resulted in dramatic changes of the film morphology; the deposited SWNT bundles reassembled and oriented normal to the electrode. Oriented SWNT bundles with high density (more than 250 bundles/microm (2)) not only presented narrow size distributions, but uniformly spread on the electrode. We discuss the mechanism of SWNT orientation by analyzing the variation in the film morphology with ultrasonication time. In addition, we suggest that the 3D displays of AFM images can lead to misjudgment of nanotube alignment. The method for aligning SWNTs normal to the electrode may be competitive with chemical vapor deposition or screen printing, the predominant methods by which vertically aligned SWNT films have been fabricated to date.

Area-selective growth of aligned single-walled carbon nanotube films using microwave plasma-enhanced CVD

The synthesis of dense, aligned carbon nanotubes (CNTs) on an Si substrate was achieved using microwave plasma-enhanced chemical vapor deposition. Catalytic nanoparticles were prepared on the Si substrate without a buffer layer, using pulsed arc plasma deposition, with a sintered Co and Ti composite electrode. Mixing Ti nanoparticles with Co nanoparticles prevents the formation of Co-silicides during the substrate heating process, thereby enabling fabrication of single-walled carbon nanotubes (SWNTs). In addition, catalytic nanoparticles were patterned using a lift-off method, and area-selective growth of vertical SWNTs to form organized SWNT microstructures was demonstrated.

Near-edge absorption fine structure and UV photoemission spectroscopy studies of aligned single-walled carbon nanotubes on Si(100) substrates

We report near-edge absorption fine structure (NEXAFS) and UV photoemission spectroscopy (UPS) studies of aligned single-walled carbon nanotube films on Si(100) substrates. Orientation of the films was detected in the NEXAFS spectra, with the intensity of the π* core exciton at 284.4 eV showing a strong dependence on nanotube alignment with respect to the polarization of the incident radiation. At lower angles of incidence, the intensity of the π* peak was higher for all orientations, which we attribute to the greater accessibility of the π* orbitals. UPS spectra of the films showed little angular dependence and included features consistent with the total density of states of graphite. As a result of the nanotube curvature and the distribution of nanotube chiralities, the UPS spectra are similar to angle-integrated graphite spectra.

Optical and electrical properties of preferentially anisotropic single-walled carbon-nanotube films in terahertz region

The absorption and dispersion of aligned single-walled carbon-nanotube films were measured from 0.2 to 2.0 THz using a source of freely propagating subpicosecond pulses of THz electromagnetic radiation. The real conductivity increased rapidly with increasing frequency up to 0.45 THz and decreased at a high-frequency range. The Maxwell–Garnett model, where the nanotubes were embedded in a dielectric host, fit the results of this study with the Drude–Lorentz model for nanotube network. We have observed the transverse phonon mode of 2.4 THz propagating along the c direction. This suggested that the carbon nanotube network is composed of metallic and semiconducting nanotubes embedded in an air dielectric host.

Characterization of singlewalled carbon nanotube films by terahertz spectroscopy

Characterizations of singlewalled carbon nanotube (SWCNT) films were measured using an optoelectronic terahertz (THz) time-domain spectroscopy system. The conductivities of SWCNT were found to increase as frequency increases. The purified SWCNT film was likewise revealed to have larger conductivity than unpurified SWCNT film. The anisotropically aligned SWCNT films were found to have higher conductivities when film polarization is parallel to THz pulse polarization.