Session Z7: Focus Session: Carbon Nanotubes: Synthesis

Developing Single-Wall Carbon Nanotubes into an Industrial Material through the Super-Growth CVD Method

Since the discovery of the carbon nanotube (CNT) 20 years ago, extensive effort has been made to utilize their exceptional intrinsic properties toward industrial applications. However, availability has significantly thwarted these endeavors. In one section of my presentation, I will describe our efforts toward the economical mass-production of single-walled carbon nanotubes (SWCNT) based on the water-assisted chemical vapor deposition technique, from which highly efficient synthesis of vertically aligned SWCNTs grow from substrates (SWCNT forests). Further, I will discuss our work to promote the industrial use of SWCNTs as a member of the Technology Research Association for Single-Walled Carbon Nanotubes (TASC) (A consortium of five companies and AIST founded for the specific purpose of developing SWCNT industrial technology.) Specifically, I will present our progress on developing the technology for the synthetic control of SWCNTs and the development of standardized evaluation techniques for the purpose of understanding the relationship between the SWCNT forest structure, e.g. length, density, crystallinity, etc and the targeted property, e.g. conductivity, mechanical reinforcement, etc. Finally, I will present several examples of applications from composites to CNT-based devices.   

Oxygen-Assisted Synthesis of Single-Walled Carbon Nanotubes

Water-assisted chemical vapor deposition (CVD) has become a standard synthesis method for high quality single-walled carbon nanotubes (SWCNTs). Some drawbacks of the water-assisted method, however, include good control of water concentrations in the feedstock and poor control of SWCNT diameters below 2.0 nm. Here, we describe a variation of water-assisted CVD that uses dry feedstocks with a small, controlled quantity of molecular oxygen. Reactions of oxygen with hydrogen in the reaction zone provide all the benefits of water-assisted growth at the substrate while maintaining dry valves and flowmeters. In addition, the oxygen-based technique allows water concentrations in the system to be varied precisely and with short time constants. Perhaps because of the improved control, we find that the SWCNT diameter can be easily tuned by changing the oxygen concentration during the growth phase. Changing the oxygen concentration over the range of 0.5{\%} to 1{\%} varied the resulting SWCNT diameters from 1.5 to 0.5 nm, with typical diameter distributions less than $+$/- 30{\%}. Control of SWCNT growth within this diameter range is ideal for probing opto-electronic properties of individual SWCNTs and SWCNT devices.   

Chirality-controlled synthesis of single-wall carbon nanotubes using vapour phase epitaxy

Due to the superior electrical properties such as high intrinsic carrier mobility and current-carrying capacity, single wall carbon nanotubes (SWCNT) hold great promise for electronic application. Since the electronic property of a SWCNT strongly depends on its chirality, the lack of synthetic control in chirality has long been recognized as a fundamental impediment in the science and application of SWCNTs Here we demonstrate a general strategy for producing carbon nanotubes with predefined chiralities by using purified single-chirality nanotubes as seeds for subsequent metal-catalyst-free growth, resembling vapour phase epitaxy commonly used for semiconductor films. In particular, we have successfully synthesized (7, 6), (6, 5), and (7, 7) nanotubes, and used Raman spectroscopy to show unambiguously that the original chiralities of the nanotube seeds are preserved. Furthermore, we have performed electrical measurements on synthesized individual (7, 6) and (6, 5) nanotubes, confirming their semiconducting nature. The vapour phase epitaxy approach is found to be highly robust and should enable a wide range of fundamental studies and technological developments.   

Towards Single-Chirality Armchair Carbon Nanotube Ensembles using Combined Size Exclusion Chromatography and Density Gradient Ultracentrifugation

Recently, density gradient ultracentrifugation (DGU) has been shown to produce aqueous ensembles enriched in armchair carbon nanotubes (CNTs), introducing new experimental insight into the photophysics of one-dimensional metals. However, despite these successes, DGU-produced armchair CNT ensembles contain multiple armchair species, which is not ideal for extracting chirality-specific optical quantities. Sample heterogeneity is partly due to tube-to-tube variability in other CNT properties such as end-capping, CNT diameter and length, resulting in differences in the observed CNT mass density. For example, CNT sedimentation velocity increases with decreasing tube length, resulting in a given CNT species appearing in multiple separated fractions after DGU. Here, using surfactant-based, size exclusion chromatography, high-concentration, uniform length CNT fractions were produced. These fractions were subsequently used for armchair enrichment DGU with the expectation that greater uniformity of the starting CNT material will lead to more monodispersed fractions, enhancing separation towards the goal of single-chirality armchair ensembles. The resulting separated fractions were analyzed using optical absorption and resonant Raman spectroscopy to assess improvement in separation.   

Removal of surfactants and adducts from solution-processed single-walled carbon nanotubes

The use of single-walled carbon nanotubes (SWCNTs) in scalable electronics and optoelectronics requires purification of the material to remove contaminants from the growth, and enrichment of the semiconducting fraction of the material through sorting. Centrifugation of aqueous suspensions of SWCNTs allows for both purification and sorting in successive steps with the aid of surfactants, but the suspension process causes oxidative damage to the SWCNTs and the surfactants are difficult to remove from the SWCNT sidewall after deposition on the substrate. These residual surfactants and adductive defects negatively impact device performance. We present a two-step approach towards reducing this disorder post-deposition using mild oxidation to remove the surfactant followed by vacuum annealing to heal the SWCNT sidewall. Thermal gravimetric analysis and temperature programmed desorption show the optimal conditions and fundamental mechanisms. Characterization of the results using Raman spectroscopy, atomic force microscopy, and electronic transport measurements show that the quality of the material is maintained.   

Directed assembly of one-dimensional functional nanostructures

One-dimensional (1D) nanostructures have unique electronic, optical and mechanical properties that have attracted intense interest over the past two decades. Single wall carbon nanotubes (SWNTs) and semiconducting nanorods have long been recognized as potential candidates for future nanoelectronic applications. The small size and the fact that these nanostructures are synthesized either at high temperatures or in solution make it difficult to organize them in complex architectures, a key requirement for their exploitation. As a step toward this goal, we are developing approaches leading to the controlled and ordered arrangement of nanoobjects on lithographically patterned, chemically (or biochemically) functionalized surfaces. One approach consists in patterning metallic nanodots that serve as anchors by selective functionalization with single stranded DNA (ssDNA) or with other chemical moieties. End functionalized nanostructures are attached to the dots through DNA hybridization or through a covalent bond. A second approach consists in patterning hydrophilic regions on hydrophobic substrates. Ion-complexed nanostuctures selectively bind to the hydrophilic pattern.   

A novel fabrication process of hundreds of field effect transistors around one single carbon nanotube for molecular assembly

Carbon nanotube field effect transistors (CNTFETs) can be used both as stand-alone electronic devices and as basis for other devices, but high-throughput fabrication remains an important challenge. In one specifically demanding application, CNTFETs are lithographically `cut' and rejoined with single molecules in the gap, to yield circuits for studying transport properties of single molecules. Because of the extreme precision required, such devices have a fabrication yield of only a few percent, which severely limits the speed of implementing CNT-molecule devices. In addition, the diversity of CNT structures provides an additional source of heterogeneity that makes collection of meaningful statistics difficult. Here we report a novel fabrication method to produce a chip with over 600 CNTFETs fabricated on one CNT. We use long (1cm) flow-aligned CNTs grown by chemical vapor deposition. Two photolithography steps are then used to pattern contacts and define a mask to burn away extra CNTs by oxygen plasma. We present the statistics of the transport properties of these devices including threshold voltage and on-state resistance. The devices are then lithographically cut and reconnected with DNA to provide consistent measurements of DNA conductance.   

Can diamond nanowires grow inside carbon nano\-tubes?

We investigate the possibility of templated growth of diamond nanowires from functionalized diamondoid molecules enclosed in a carbon nanotube (CNT). Our {\em ab initio} density functional theory studies identify suitable candidate molecules and conditions, under which such molecules may fuse to narrow diamond nanowires with C$_8$H$_8$ or C$_7$H$_8$ unit cells inside a CNT. We find that the unique environment inside a narrow carbon nanotube, which can be suitably represented by a cylindrical potential, subjects enclosed molecules to a high pressure, caused by ``capillary'' forces, and orients them in a suitable way favoring fusion and constraining the resulting structure. Based on total energy calculations, we find that fusion of C$_{10}$H$_{16}$ adamantane molecules requires additional energy, whereas fusion of C$_{14}$H$_{18}$(COOH)$_2$ diamantane di-acid molecules in hydrogen atmosphere occurs as an exothermic reaction. Our canonical molecular dynamics calculations at elevated temperatures indicate likely intermediate products occurring during this reaction.   

Characteristics of thin graphene sheets prepared by a laser ablation method

Graphenes are innovative carbon materials having a sheet-like structure; these materials are thought to have many applications in the fields of electrochemistry, biomedicine, and so on. In this study, we showed that thin graphene sheets (TGSs) prepared by a laser ablation method had a distinctive structure: even-number layered graphenes (2-, 4-, 6- and 8-layers) were preferentially grown (ca. 90{\%}), and their population decreased as the layer number increased. These phenomena have not been observed in graphenes prepared with other methods. Our results suggest a new growth mechanism in which single-layer graphene is unstable and bends to form bi-layers, and the bi-layers then go on to stack and form thicker TGSs. The inter-layer distances estimated by transmission electron microscope images were about 15{\%} larger than that of bulk graphite in the bi-layer TGSs, and they approached the bulk value as the layer number increased. Furthermore, we showed surface-selective functionalization of TGSs by mild oxidation with H$_{2}$O$_{2}$ at room temperature, indicating the possibility of multi-modal functionalization, which will make the graphene more attractive in various applications.   

Plasmon Heat Transport Between Vertical Carbon Nanotube Forest and Different Substrates

Near-field radiative heat transfer between vertical forest of carbon nanotubes and different metallic and dielectric substrates has been studied using the formalism of the fluctuational electrodynamics. Proper matching between surface plasmons in nanotubes and surface polaritons in the substrate was demonstrated to be crucial for the efficient thermal coupling across the interface. Particularly, thermal Kapitza conductance between nanotubes and such polar dielectrics as quartz, sapphire and GaAs (with surface phonon-polariton energies $\sim$ 30-50 meV) is substantially higher than that between nanotubes and BN and SiC (with polartion energies $>$ 100 meV), or metals (with plasmon-polaritons in the visible range). Further optimization of heat transport can be achieved by tweaking nanotube length.   

Unusual thermal conduction characteristics of phase change composites with single-walled carbon nanotube inclusion

Thermal energy storage using phase transition materials is often employed in many engineering applications. However, the low thermal conductivity of such materials inhibits its use for large scale applications. Recently, Zheng et al. [Nature Comm. 2011] demonstrated an efficient technique using graphite suspensions to tune the thermal and electrical conductivity using temperature regulation. In this work, we report large contrasts in thermal conductivity enhancement of nano composites with single walled carbon nanotube (SWCNT) inclusions using first order phase transition process. SWCNTs synthesized by alcohol CVD were dispersed in n-octadecane by tip-sonication with sodium deoxycholate as the surfactant. Thermal conductivity measurements were carried out with transient hot-wire system [Mater. Express 2012]. Thermal conductivity enhancement in the liquid state was found to be nominal and is consistent with the predictions of Maxwell- Garnett type effective medium theory. However, in the frozen state nearly a 2.5 fold increase in thermal conductivity was observed. Similar temperature dependent thermal conductivity contrast was observed when exfoliated graphite nanoplatelets were used as the inclusions.   

Explosive Characteristics of Carbonaceous Nanoparticles

Explosion testing has been performed on 20 codes of carbonaceous particles. These include SWCNTs (single-walled carbon nanotubes), MWCNTs (multi-walled carbon nanotubes), CNFs (carbon nanofibers), graphene, diamond, fullerene, carbon blacks and graphites. Explosion screening was performed in a 20 L explosion chamber (ASTM E1226-10 protocol), at a (dilute) concentration of 500 g/m$^{3}$, using a 5 kJ ignition source. Time traces of overpressure were recorded. Samples exhibited overpressures of 5-7 bar, and deflagration index K$_{\mathrm{St}} =$ V$^{1/3}$ (dp/pt)$_{\mathrm{max}}$ $\sim$ 10 - 80 bar-m/s, which places these materials in European Dust Explosion Class St-1 (similar to cotton and wood dust). There was minimal variation between these different materials. The explosive characteristics of these carbonaceous powders are uncorrelated with particle size (BET specific surface area). Additional tests were performed on selected materials to identify minimum explosive concentration [MEC]. These materials exhibit MEC $\sim$ 10$^{1}$ -10$^{2}$ g/m$^{\mathrm{3}}$ (lower than the MEC for coals). The concentration scans confirm that the earlier screening was performed under fuel-rich conditions (i.e. the maximum over-pressure and deflagration index exceed the screening values); e.g. the true fullerene K$_{\mathrm{St}}$ $\sim$ 200 bar-m/s, placing it borderline St-1/St-2.