Session A28: Focus Session: Carbon Nanotubes and Related Materials: Fundamentals and Applications

Carbon nanotube -- catalyst composites: from nano-complexes to aerogel functionalization

Here we present three different strategies to achieve attachment of catalytic nanoparticles to SWNTs and discuss their physical properties. In nano-complex scheme, DNA that solubilizes SWNTs is used as an anchor for Pt nanoparticle growth. Attached platinum strongly influences nanotube phonon and charge carrier distribution. For macroscopic electrodes, no special chemistry is needed. Simple solubilization of both nanoparticles (Pt) and nanotubes in polar surfactants and joint deposition on a porous membrane will result in charge coupled SWNT/Pt electrode. A particularly difficult problem in SWNT research is a task of electrically connecting nanotubes and at the same time keeping the surface available. We present an innovative solution to this problem in which SWNTs are connected through point contacts that leave the majority of the surface free. This method creates self-assembled carbon nanotube aerogel of a record low density that is both luminescent and conductive. Additional value of this material is that it is suitable for subsequent functionalizations. Platinum and titanium dioxide deposition on aerogel suggests that carbon aerogel can be used as a framework for complex structures.   

Reinforced Epoxy Nanocomposite Sheets Utilizing Large Interfacial Area from a High Surface Area Single-Walled Carbon Nanotube Scaffold

We employed single-walled carbon nanotubes (SWNTs) with the available highest specific surface area (more than 1000 m2/g) that provided very large interfacial area for the matrix to fabricate epoxy composite sheets. Through mechanical redirection of the SWNT alignment to horizontal to create a laterally aligned scaffold sheet, into which epoxy resin was impregnated. The SWNT scaffold was engineered in structure to meet the these two nearly mutually exclusive demands, i.e. to have nanometer meso-pores (2-50 nm) to facilitate homogeneous impregnation of the epoxy resin and to have mechanical strength to tolerate the compaction forces generated during impregnation. Through this approach, a SWNT/epoxy composite sheet with a nearly ideal morphology was realized where long and aligned SWNTs were loaded at high weight fraction (33 percent) with an intertube distance approaching the radius of gyration for polymers. The resultant composite showed a Young's modulus of 15.0 GPa and a tensile strength of 104 MPa, thus achieving 5.4 and 2.1 times reinforcement as compared to the neat epoxy resin.   

Single-walled carbon nanotube buckypaper and mesophase pitch carbon/carbon composites

Carbon/carbon composites consisting of single-walled carbon nanotube (SWCNT) buckypaper (BP) and mesophase pitch resin have been produced through impregnation of BP with pitch using toluene as a solvent. Drying, stabilization and carbonization processes were performed sequentially, and repeated to increase the pitch content. Voids in the carbon/carbon composite samples decreased with increasing impregnation process cycles. Electrical conductivity and density of the composites increased with carbonization by two to three times that of pristine BP. These results indicate that discontinuity and intertube contact barriers of SWCNTs in the BP are partially overcome by the carbonization process of pitch. The temperature dependence of the Raman shift shows that mechanical strain is increased since carbonized pitch matrix surrounds the nanotubes.   

Biscrolling nanotube sheets and functional guests into yarns

Multifunctional applications of textiles have been limited by the inability to spin important materials into yarns. Generically applicable methods are demonstrated for producing weavable yarns comprising up to 95 wt {\%} of otherwise unspinnable particulate or nanofiber powders that remain highly functional. Scrolled 50 nm thick carbon nanotube sheets confine these powders in the galleries of irregular scroll sacks, whose observed complex structures are related to twist-dependent extension of Archimedean spirals, Fermat spirals, or spiral pairs into scrolls. The strength and electronic connectivity of a small weight fraction of scrolled carbon nanotube sheet enables yarn weaving, sewing, knotting, braiding, and charge collection. This technology is used to make yarns of superconductors, Li-ion battery materials, graphene ribbons, catalytic nanofibers for fuel cells, and TiO$_{2}$ for photocatalysis. \\[4pt] Work done in collaboration with Shaoli Fang, Xavier Lepro-Chavez, Chihye Lewis, Raquel Ovalle-Robles, Javier Carratero-Gonzalez, Elisabet Castillo-Martinez, Mikhail Kozlov, Jiyoung Oh, Neema Rawat, Carter Haines, Mohammed Haque, Vaishnavi Aare, Stephanie Stoughton, Anvar Zakhidov, and Ray Baughman, The University of Texas at Dallas / Alan G. MacDiarmid NanoTech Institute.   

Load transfer mechanisms in cross-linked DWNT fibers

The application of carbon nanotubes (CNT) to macroscopic composite fibers has been limited by weak shear interfaces between adjacent CNT shells and composite matrix elements. A fundamental understanding of load transfer at multiple length-scales is needed to identify how the exceptional mechanical properties of CNTs can be scaled to produce high-performance fibers. Through in-situ electron microscopy tensile testing we have elucidated load transfer mechanisms across multiple scales of cross-linked double-walled nanotube (DWNT) fibers. A low density of polymer cross-links is found to increase the total energy dissipated at failure and ductility of fibers by 5 and 10X, respectively, without reducing strength. This mutiscale approach has identified a need to enhance shear interactions between individual DWNTs within the hierarchical DWNT fiber structures. Through in-situ TEM electron irradiation studies we have shown that load can be effectively transferred to inner DWNTs within bundles by covalently cross-linking the interfaces of adjacent DWNTs and shells. We have observed order of magnitude increases in strength and modulus and identified their dependence on irradiation dose. In future a combined approach of irradiation induced covalent and polymer cross-linking may lead to high-performance DWNT-based fibers and composites with tunable mechanical properties.   

Aligned Carbon Nanotubes Embedded in Elastic Polymer as Stretchable Conductors

Stretchable electronics enable new applications in a wide range of fields. Carbon nanotube (CNT) ribbons, composed of bundles of aligned millimeter-long CNTs, represent a unique opportunity for high performance stretchable conductors. In this work, we embedded CNT ribbons in elastic poly(dimethylsiloxane) (PDMS) film (or CNT/PDMS films) and systematically investigated the dependence of film resistance on the tensile strains. The CNT/PDMS films fabricated by this approach are flexible, transparent, and show constant resistance under strains in the range of 0{\%}-100{\%}. We believe that the unique stretchability of CNT ribbons reported here will open new potential applications of CNTs in the next generation intelligent electronics.   

The Anisotropic Physical Properties of Polyethylene Oxide/Magnetic Carbon Nanotubes Composite Films

Magnetic carbon nanotubes (m-CNTs) were synthesized by the tethering of $\gamma $-Fe$_{2}$O$_{3}$ nanoparticles. Subsequently, the m-CNTs were dispersed and aligned in a PEO matrix under a low externally-applied magnetic field ($<$0.3 T). The degree of crystallinity, crystal size, and crystal structure of the composite films were investigated using DSC and XRD. The electrical conductivity of the composite films showed anisotropic characteristics that were correlated to the parallel and perpendicular direction of the applied magnetic field. Young's modulus and tensile strength of the composite films increased with the increasing weight fraction of m-CNT up to 170 {\%} and 157 {\%}, respectively. The elongation at break of the composites improved as well compared to that of the pure-PEO film, due to the lowering of the glass transition temperature (T$_{g})$ and was also correlated to m-CNT content and the alignment directions.   

Changing Carbon Nanostructures by Irradiation

Changes in the force field of carbon nanostructures immediately following irradiation by light and electrons may cause important structural changes. Exposure to light may modify the morphology at the apex of carbon nanohorns during Raman spectroscopy observations [1], or exfoliate graphite layer-by-layer upon exposure to specifically shaped femtosecond laser pulses [2]. Irradiation by electrons may significantly improve the structural integrity and mechanical properties of low-quality multi-wall carbon nanotubes grown by Chemical Vapor Deposition [3]. {\em Ab initio} molecular dynamics calculations in the electronic ground and excited state help to analyze the microscopic mechanisms underlying these structural changes including photo-activated Stone-Wales transformations, cross-linking of nanotube walls at extended defect sites, and charge redistribution causing detachment of graphene monolayers.\\[4pt] [1] T. Fujimori {\em et al.} (in preparation). \newline [2] Y. Miyamoto {\em et al.}, Phys. Rev. Lett. {\bf 104}, 208302 (2010). \newline [3] M. Duchamp {\em et al.}, J. Appl. Phys. {\bf 108}, 084314 (2010).   

N-type Doping of Single-walled Carbon Nanotubes: Fundamental Properties, Spectroscopic Signatures, and Transparent Conducting Electrodes

Controllable p- and n-type doping of single-walled carbon nanotube (SWNT) films enables technologies such as FETs, LEDs, and solar cells. Because many p-type dopants for SWNTs are environmentally stable, they have been studied in greater detail and used in far more applications than their less stable n-type counterparts. As a result, further studies on n-type SWNTs are needed. We report on the effectiveness of small molecule and polymer amines as n-type dopants on thin film nanotube networks. We find significant doping-induced changes in NMR, XPS, and Raman spectra that can be used in future studies to characterize n-type SWNTs. Moreover, we find that the best amines can produce n-type transparent conducting films with nearly the same sheet resistance (at a given transparency) as p-doped HNO$_{3}$ treated films. These results serve both to increase the knowledge base in the community regarding the fundamental properties and spectroscopic signatures of n-type doped SWNTs and to expand the versatility of functional SWNT network electrodes that are typically resigned to p-type SWNTs.   

From $^3$He to Xe: adsorption isotherms on the same batch of BuckyPearls$^{TM}$ carbon nanotube bundles

We report a study of the adsorption of $^3$He, $^4$He, H$_2$, HD, D$_2$, Ne, Ar, N$_2$, Kr and Xe adsorbed on samples of BuckyPearls$^{TM}$, a form of HiPCo-type$^{TM}$ carbon nanotube bundles, from the same batch used for neutron diffraction studies of the structure of $^4$He and Ne at low temperatures. For each gas, except $^3$He and $^4$He, we have measured three or more isotherms in a range of temperatures where we can observe the completion of both the three-line phase and the first layer. We can correlate the helium and hydrogen isotopes data and the Ne data with previous neutron and/or heat capacity measurements on BuckyPearls and HiPCo bundles. By taking ratios of monolayer completion coverage for the various gases to the N$_2$ monolayer completion coverage we can compare nanotube adsorption to adsorption on exfoliated graphite. Quantum effects on adsorption can be seen by comparing areas per atom or molecule to Lennard-Jones hard core radii.   

Comparative study of small alkane and alkene molecules adsorbed on purified HiPco Single-walled carbon nanotubes

We have measured adsorption isotherms for ethylene on purified HiPco SWNTs at 11 different temperatures (between 110 and 220K). Our findings for ethylene will be compared to the results of ethane adsorption on the same substrate. Consistent with what we had found for ethane, two groups of distinct binding energy sites are observed for ethylene molecules adsorbed on the nanotube substrate. However, unlike in the case of ethane, no feature suggesting the existence of a phase transition was observed for the ethylene films. In addition, we have determined the coverage dependence of the isosteric heat of adsorption for ethylene on the same substrate. The values of the isosteric heats that we had previously determined for ethane are slightly higher than the ones obtained for ethylene, for the same fractional coverage. Our experimental isosteric heat results will also be compared with simulation results that indicate a similar trend.   

Studying of kinetics of rear earth ion (REI) nanoscale complex formation by resonant energy transfer

We observed formation of nanoscale complexes between multivalent REIs (Tb and Eu) and negatively charged DNA wrapped SWNTs, ionized in the water solution. Foerster Resonance Energy Transfer (FRET) was found to be an ideal method to confirm the complex formation. Because of its high sensitivity and non-destructive characterization approach FRET can be used to trace the kinetics of the complex formation. Strong dependence of SWNT photoluminescence (PL) on the REI concentration was detected and interpreted as a competition between the REI absorption on the SWNTs and subsequent FRET enhanced PL and the SWNT agglomeration followed by PL quenching. We measured the distance between REI and SWNT which appears to be much shorter than the one from their relative concentration in solution. We speculate that Manning condensation of the REIs on the SWNT/DNA surface happens thereby significantly reducing their spacing and making FRET possible.   

Attachment of a Genetically Engineered Antibody to a Carbon Nanotube Transistor for Detection of Prostate Cancer Biomarkers

We have developed a novel detection method for osteopontin (OPN) by attaching an engineered single chain variable fragment (scFv) protein with high binding affinity for OPN to a carbon nanotube transistor. Osteopontin is a potential new biomarker for prostate cancer; its presence in humans is already associated with several forms of cancer, arthritis, osteoporosis and stress. Prostate cancer is the most commonly diagnosed cancer and second leading cause of cancer deaths among American men and as such represents a major public health issue. Detection of early-stage cancer often results in successful treatment, with long term disease-free survival in 60-90\% of patients. Electronic transport measurements are used to detect the presence of OPN in solution at clinically relevant concentrations.