Session W6: Focus Session: Carbon Nanotube Synthesis, Structure and Defects

Chirality selective growth of carbon nanotubes from one-dimensional fusion of aromatic compounds

We have investigated the formation of carbon nanotubes (CNTs) from one-dimensional coalescence of various polyaromatic compounds with different edge structures within an outer tube template. Transformation of the filled precursors into an inner tube was confirmed upon high-temperature thermal annealing. These newly formed inner tubes were then extracted through ultrasonication, as reported in our previous study [1]. High resolution transmission electron microscope observations of the samples together with the photoluminescence analyses of the extracted dispersions reveal that the chirality of the inner tubes generated were greatly affected not only by the edge structures but also by the intermediates formed. In particular, graphene nanoribbon-like intermediates obtained from perylene-derivative can be preferentially converted into near-zigzag CNTs with (8,1) chirality. The present method may provide the controlled growth of chiral-specific CNTs, which has been difficult to achieve using ordinary synthesis approaches.\\[4pt] [1] Y. Miyata, et al., ACS Nano 2010, 4, 5807.   

Hierarchical 3D microstructures from pyrolysis of epoxy resin

Nature is replete with examples of microscale dendrites connected to tree-like backbones ranging from the overall structures of trees to vascular networks. These branched structures have emerged as a result of an optimization between the maximization of a surface area and the minimization of transport losses. Elucidating these sophisticated designs proposed by nature is of paramount importance for the creation of higher-efficiency materials. The fabrication of such structures is however particularly challenging at small scale. In this paper, we focus on amorphous carbon microstructures, which provide a wide electrochemical stability window, excellent bio-compatibility, and cost-effective fabrication. However, relatively few methods have been developed for the fabrication of hierarchical amorphous carbon microstructures.Here we show that novel anisotropic microarchitectures comprising vertically aligned amorphous carbon nanowires CNWs can be made by oxygen plasma treatment of epoxy resins, followed by pyrolysis. Interestingly, these structures can also be shaped into deterministic three-dimensional (3D) hierarchical structures where nanowires are anchored to a microsized solid carbon core. These structures could play a key role in the development of new electrodes for microsensors, bioprobes, batteries, and fuel cells.   

Semiconducting nanotube dominant chemical vapor deposition synthesis of isopropanol carbon feedstock

The development of guided chemical vapor deposition (CVD) growth of single wall carbon nanotubes provides great platform for wafer-scale integration of aligned nanotube into circuits and systems. However, the co-existence of the metallic and semiconducting nanotubes is still a major problem for the development of carbon nanotube based nanoelectronics. To address this limitation, we developed a method to get semiconducting dominant nanotube by using isopropanol carbon feedstock. We achieved a purity of 87{\%} of semiconducting nanotube growth, which was verified by measuring single nanotube transistors fabricated from aligned nanotube arrays. Besides, Raman spectrum was characterized to confirm the enhanced fraction of semiconducting nanotube as well. To further understand chemical mechanism of synthesis at atomic level, we performed the mass spectrum study and compared the measurement results from different carbon source. Furthermore, to discuss the future application of this synthesis method, we fabricated thin-film transistor from as-grown nanotube network. Transistor with on/off ratio over 104 and mobility up to 116 cm2/v\textbullet s was achieved, which shows great potential for thin-film transistor applications.   

Synthesis and Characterization of Carbon Nanotubes Produced From Thermal Decomposition of Nickellocene

We have employed a direct thermal deposition technique, which used Nickellocene both as the catalyst as well as the carbon source, to grow films of carbon nanotubes (CNT). The CNT films obtained using this procedure were characterized using Transmission Electron Microscopy which indicated the presence of thin diameter carbon nanotubes as well as single walled CNT ropes. Volumetric adsorption measurements were performed to determine the porosity and specific surface areas of these samples. Electrical transport measurements performed on long ropes of CNTs extracted from these bulk films will be presented and will be discussed in the framework of transport theories of quasi-one dimensional systems.   

Covalently Functionalized Carbon Nanotubes for Electronics

Covalent chemistry on carbon nanotubes generates useful and stable functionalities, but it also generally damages their electronic properties, which is a critical drawback for device applications. Here we present two approaches to achieve covalent functionalization of carbon nanotubes without compromising on their electronic properties. For each case, we demonstrate the fabrication of functional carbon nanotube devices. First, double-walled carbon nanotubes (DWNTs) are functionalized using a monovalent reaction with aryldiazonium salts. Absorption and Raman spectroscopy along with electrical measurements show that the functionalization occurs strictly on the outer wall and preserves the optical and transport properties of the inner wall. Functionalized-DWNT devices are operated with similar characteristics as pristine single-walled carbon nanotube (SWNT) devices [1]. Second, SWNTs are functionalized with different addends using a divalent carbene reaction. For both metallic and semiconducting species, electrical measurements of numerous functionalized and unfunctionalized SWNT devices show identical characteristics. Ref: [1] Bouilly D. et al. ACS Nano, 5 (6), 4927 (2011)   

Mechanocapillary forming of carbon nanotube microstructures

The hierarchical structure and organization of filaments within both natural and synthetic materials can determine a wide variety of collective chemical and physical functionalities. Carbon nanotubes (CNTs) are known for their record properties, and densely packed CNTs are therefore expected to enable new materials having outstanding multifunctional performance. However, it remains a significant challenge to build highly ordered assembles of CNTs, and this challenge has largely limited the design and properties of macroscale CNT yarns and sheets, and CNT-based surfaces and interfaces. We have created a versatile technique called \textit{capillary forming} to manipulate patterned vertically aligned (VA-) CNTs into diverse 3D microarchitectures, and to enable their integration in applications ranging from microsystems to macroscale functional films. Capillary forming relies on shape-directed capillary rise during solvent condensation, followed by evaporation-induced shrinkage. Three-dimensional transformations result from shrinkage of the vapor-liquid-solid interface and the resulting heterogeneous strain distribution in the microstructures. A portfolio of microscale CNT assemblies with highly ordered internal structure and freeform geometries including straight, bent, folded and helical profiles, are fabricated using capillary forming. The mechanical stiffness and electrical conductivity of capillary formed CNT micropillars are 5 GPa and 10$^4$ S/m respectively. These values are at least hundred-fold higher than as-grown CNT forests and exceed the properties of typical microfabrication polymers. Finally, the potential applications of these structures are demonstrates as vertical microsprings with geometrically tunable compliance, and hydrogel-driven microtransducers.   

{\em Ab initio} studies of defects in carbon nanofoam structures

We combine {\em ab initio} density functional and GW calculations to study the stability, electronic and elastic properties of hypothetical cellular foam-like carbon nanostructures. These systems with a mixed sp$^2$/sp$^3$ bonding character are structurally related to bundles of carbon nanotubes. The cross-sectional honeycomb structure may accommodate the same type of structural defects as the honeycomb lattice of graphene. The infinite 3D foam structure is a narrow-gap semiconductor, with the binding energy of atoms nearly 0.5 eV lower than graphene. Quasi-2D layered foam structures of finite thickness may be stabilized by terminating caps. When exposed to hydrostatic pressure, the unit cells of the foam deform significantly from their initially hexagonal shape.   

Energetical Stability of Near-Armchair Carbon Nanotubes: A Systematic First-Principles Study

We perform the systematic first-principles study of 57 kinds of carbon nanotubes (CNTs) and investigate the energetical stability of so-called ``near-armchair'' CNTs. The density functional theory computational code which utilizes the helical symmetry and has been developed in our group is extensively used in the present work. Because the computational effort can be drastically reduced by using the helical symmetry of CNTs, the systematic study of fully-optimized CNTs including the experimentally abundant CNTs was finally achieved in this work. As a result, it is found that ``near-armchair'' CNTs including (6,5) and (7,5) CNTs are energetically more stable than other CNTs. This result corresponds well with the high abundance of these near-armchair CNTs experimentally reported so far. In addition, by performing the systematic analysis of CNT bond-lengths and angles, the presence of the geometrical family pattern after their structural optimizations has been revealed for the first time. It is also confirmed that the geometrical optimization plays a very important role in predicting electronic structures of chiral CNTs as well as achiral CNTs. The fundamental gap corrections associated with the geometrical optimizations are sizable even in one nm diameter CNTs.   

Diameter and Chiral Selective Purification of SWNT and DWNT Using CO$_2$

Oxidation of carbon nanotubes in air is a well known method to eliminate carboneous impurities from raw material. Recently, it has also been used to selectively remove single-wall carbon nanotubes (SWNT) present in double-walled carbon nanotubes (DWNT) soot[1]. Here, we propose a more efficient purification process for both SWNT and DWNT based on a high temperature oxidation in a pure CO$_2$ gas flow. This treatment, combined with a standard reflux in nitric acid, provides fast oxidation of amorphous carbon and removal of other impurities without affecting the structure of the carbon nanotubes. Parameterization of the treatment allowed us to observe both diameter and chirality dependence of the nanotubes reaction rate with CO$_2$. This selective character was applied to produce thin films of clean and highly enriched DWNT and of semiconducting SWNT. Microscopy and spectroscopy analyses will be shown and reveal that those films are composed of very high quality carbon nanotubes (micrometers long, very low impurity and catalyst concentration, low Raman $I_D/I_G$ ratios). Also, no significant nanotube shortening is observed following the different diameter or chirality enrichment treatments.\\*[1em] [1] K. Iakoubovskii et al., J. Phys. Chem. C, 112, 30 (2008)   

Radial Elasticity Measurement of Single-walled Carbon Nanotubes by Atomic Force Microscopy

By applying well-calibrated tapping mode and contact mode AFM upon horizontally aligned SWCNTs grown directly on quartz substrates, we have obtained effective radial modulus (E$_{radial})$ of SWCNTs with diameters less than 2 nm. The measured E$_{radial}$ decreases from 57 to 9 GPa with the increase of the SWCNT diameter from 0.92 to 1.91 nm. Our experimental result is consistent with the computational data obtained using the modified molecular structure mechanics model. We have also compared our measurements with the reported experimental results obtained on SWCNTs with diameters from 2 to 3 nm. Our measurements of large diameter SWCNTs (diameter close to 2nm) are in agreement with the reported data and modeling. However, our measurements of SWCNTs with smaller diameters deviate from this previous study. This has been explained by the compressibility of the substrate and the AFM tip. By employing Hertzian theory, we specifically exploit the components of deformation of the AFM tip-SWCNT-substrate system. Further calculation using our measured E$_{radial}$ indicates that under the same normal force the deformation of our quartz-SWCNT-silicon tip system can be as much as 96{\%} more than the deformation of the SWCNT compressed between a rigid substrate and an AFM tip.   

Synthesis and high temperature stability of amorphous Si(B)CN-MWCNT composite nanowires

We demonstrate synthesis of a hybrid nanowire structure consisting of an amorphous polymer-derived silicon boron-carbonitride (Si-B-C-N) shell with a multiwalled carbon nanotube core. This was achieved through a novel process involving preparation of a boron-modified liquid polymeric precursor through a reaction of trimethyl borate and polyureasilazane under atmospheric conditions; followed by conversion of polymer to glass-ceramic on carbon nanotube surfaces through controlled heating. Chemical structure of the polymer was studied by liquid-NMR while evolution of various ceramic phases was studied by Raman spectroscopy, solid-NMR, Fourier transform infrared and X-ray photoelectron spectroscopy. Electron microscopy and X-ray diffraction confirms presence of amorphous Si(B)CN coating on individual nanotubes for all specimen processed below 1400 degree C. Thermogravimetric analysis, followed by TEM revealed high temperature stability of the carbon nanotube core in flowing air up to 1300 degree C.