Session V29: Focus Session: Carbon Nanotubes and Related Materials XIII: Synthesis

High-Resolution Nanofabrication Using a Highly-Focused Electron Beam

Carbon nanotubes and metallic nanowires have unusual and potentially useful electrical transport properties. Local control of the parameters of such nano-objects would open even wider possibilities for their applications. We have used the highly focused, high-energy electron beam of a transmission electron microscope to locally modify such nano-objects. In particular, it was possible to drill 2.5 nm diameter nanoholes in multiwall carbon nanotubes (MWNT's). Similarly sized holes were etched through metallic nanowires. We have also fabricated larger nanoholes, as large as 11 nm wide in a 26 nm diameter MWNT, as well as constrictions in MWNT's. Transport measurements of such nanodevices is our future goal.   

Optical Patterning of Three-Dimensional Carbon Nanotube Microstructures

We present an optical, non-contact method for patterning three-dimensional carbon nanotube microstructures. In this method, a 1$\mu $m diameter focused laser spot is used to burn patterns in dense arrays of vertically grown multiwalled carbon nanotubes. The threshold for laser burnout and the depth of burnout are determined by Raman spectroscopy and scanning electron microscopy. Using a high precision translation stage to control the position of the laser spot on the sample, we create several 3D patterns to illustrate this method's potential use for the rapid prototyping of carbon nanotube microstructures [1]. After laser surface treatment, we observe undercut profiles, changes in nanotube density, and nanoparticle formation, which provide insight into the unique evolution of the nanotube microstructures during the burnout process. This non-lithographic method provides new opportunities for chemically sensitive~applications of nanotubes and expands their possible applications into new areas. \newline [1] Hung, Wei Hsuan, Kumar, Rajay, Bushmaker, Adam, Cronin, Stephen B., and Bronikowski, Michael J. Rapid prototyping of three-dimensional microstructures from multiwalled carbon nanotubes. \textit{Applied Physics Letters} \textbf{91}, 093121 (2007).   

A Real Time Detection System for Dielectrophoretic Deposition of Carbon Nanotubes

Single-walled carbon nanotubes (SWCNTs) have showed considerable potential as building blocks for electronics and sensors but are very difficult to integrate and assemble into larger systems. Dielectrophoretic deposition allows the large-scale positioning and alignment of SWCNTs but requires precise control to reproducibly generate single-tube devices. We investigate dielectrophoretic deposition of SWCNTs using an \textit{in situ} detection system. This apparatus locks into both a small AC signal and the large, mixed-down dielectrophoretic signal, generated by the nonlinearities of the device, making it possible to halt deposition once a nanotube has made electrical contact. This results in a higher yield of single SWCNTs deposited between two electrodes.   

Irradiation-induced phenomena in carbon nanomaterials

The irradiation of solids with energetic particles such as electrons or ions is associated with disorder, normally an undesirable phenomenon. However, recent experiments [for an overview, see A.V Krasheninnikov, F. Banhart, Nature Materials, 6 (2007) 723] on bombardment of carbon nanostructures with energetic particles demonstrate that irradiation can have beneficial effects and that electron or ion beams may serve as tools to change the morphology and tailor mechanical, electronic and even magnetic properties of nanostructured carbon systems. We systematically study irradiation effects in carbon nanotubes and other forms of nano-structured carbon experimentally and theoretically by employing various atomistic models ranging from empirical potentials to time-dependent density functional theory. In my presentation, I will briefly review the recent progress in our understanding of ion-irradiation-induced phenomena in nano-structured carbon and present our recent theoretical [A.V Krasheninnikov, et al., Phys. Rev. Lett., 99 (2007) 016104, A. Tolvanen et al, Appl. Phys. Lett. 91 (2007) 173109.] and experimental [O. Lehtinen et al., to be published] results. I dwell on the ``beneficial'' role of defects and impurities in nanotubes and related systems. Finally, I will present the results of simulations of irradiation-induced pressure build-up inside nanotubes encapsulated with metals [L. Sun, et al., Science 312 (2006) 1199]. Electron irradiation of such composite systems in the transmission electron microscope gives rise to contraction of nanotube shells and thus to high pressure. The irradiation-stimulated pressure can be as high as 40 GPa, which makes it possible to study phase transformations at the nanoscale with high spatial resolution. I will also address the mechanisms of plastic deformation of small metal particles inside carbon shells at high temperatures, which may be important for understanding catalytic growth of carbon nanotubes.   

Syntheses and characterization of SWCNT assemblies prepared on silicon substrates with different methods of patterning catalyst particles

Techniques for controlled way of preparation of single wall carbon nanotubes (SWCTN) on substrates continue to be of interest, including for potential applications in integrated circuits and nanodevices. We report our work, undertaken to elucidate a number of favorable conditions for controlled patterning and growth of high quality SWCNT. Synthesis is carried out under ambient pressure with methane used as feed gas. A key factor for successful SWCNT growth is known to be the catalytic precursor. We compare several methods of controlled deposition of the catalyst precursors on oxidized silicon substrates using methods such as e-beam lithography, photolithography and scanning probe writing methods that allow for maximum flexibility and high efficiency for incorporating SWCNTs into devices or circuit architectures. The advantages and some limitations of these methods of selective patterning will also be addressed. Analysis and characterization of the as-grown SWCNTs was performed by Raman Spectroscopy, AFM and SEM.   

\textit{In-situ} calorimetric studies of SWCNT growth

Single-walled carbon nanotubes (SWCNTs) were grown inside of a differential scanning calorimetry (DSC) apparatus with an attached mass spectrometer (MS), using different hydrocarbons (CH$_{4}$ and C$_{2}$H$_{4})$ and alumina supported (Fe, Fe/Mo, and Ni) catalysts. This set-up allowed to \textit{in situ} follow the evolution of calorimetric, thermogravimetric and MS data during the synthesis. A Raman spectrometer (with laser excitations wavelengths 532 and 785 nm) was used for verification of the growth of SWCNTs. DSC studies at temperatures $\sim $650-900 \r{ }C of the interaction between the hydrocarbons and the preliminary reduced alumina supported catalysts showed a release (C$_{2}$H$_{4})$ or absorption (CH$_{4})$ of heat depending on the type of hydrocarbon used. The effect of this energy on the growth of SWCNTs was studied. We found that the incubation time for nanotube nucleation depends on the hydrocarbon type and flow rate, as well as on the synthesis temperature. The origin of the initial endothermic peak observed during nanotube growth with both hydrocarbon sources will be discussed. Furthermore, the kinetics and thermodynamic of hydrocarbon decomposition, carbon atoms diffusion and solid carbon structure formation dependence on the catalyst and synthesis parameters will also be presented.   

The role of carbon solubility in Fe-C nano-clusters on the growth of small single-walled carbon nanotubes

Various diameters of alumina-supported Fe catalysts are used to grow single-walled carbon nanotubes (SWCNTs) with chemical vapor decomposition. We find that the reduction of the catalyst size requires an increase of the minimum temperature necessary for the growth. We address this phenomenon in terms of solubility of C in Fe nanoclusters and, by using first principles calculations, we devise a simple model to predict the behavior of the phases competing for stability in Fe-C nanoclusters at low temperature. We show that, as a function particles size, there are three scenarios compatible with steady state-, limited- and no-growth of SWCNTs, corresponding to unaffected, reduced and no solubility of C in the particles. The result raises previously unknown concerns about the growth feasibility of small and very-long SWCNTs within the current Fe CVD technology, and suggests new strategies in the search of better catalysts. Research supported by Honda R.I. and NSF.   

Air assisted growth of long aligned carbon nanotube films

We report on air assisted growth of ultra long aligned bundles of multiwall carbon nanotubes. We found that the growth rate of carbon nanotubes is highly enhanced by introducing a small mount of oxygen during the catalytic decomposition of ferrocene-xylene mixture at 790$^{\circ}$C. Millimeter long aligned carbon nanotube films were easily synthesized on silicon dioxide as well as metal substrates by controlling the air flow. Electron microscopy investigations reveal that the films are composed of dense aligned multi-wall CNTs with the diameters ranging from about 30-100 nm. We will also present our preliminary results on the electrical transport measurement performed on these long nanotube bundles.   

Boron-Doped Carbon Nanotube Films

Here we report room temperature optical and resistivity studies on transparent thin films of bundled single-walled carbon nanotubes exposed to B$_{2}$O$_{3}$ at 1000$^{o}$C. This reaction is proposed to B-dope the films. They are stable in air. At 300K the four-probe sheet resistance and the optical transmission in the NIR-UV range are used to evaluate the effects of this chemical exposure. Our preliminary results show that for films with a visible optical transmittance around 80{\%} (550nm), the sheet resistance in the pristine film is lowered from $\sim $2K$\Omega $ to $\sim $300$\Omega $ via B$_{2}$O$_{3}$ exposure, a factor of five decrease. We find that the magnitude of the decrease in the sheet resistance increases in samples with higher transmission. Our results suggest that boron-doped SWNT may provide a better approach to touch-screen technology, as well as for transparent contacts in solar cells.   

\textit{In situ} Transient Growth Kinetics of Vertically-Aligned Carbon Nanotube Arrays

Here, transient growth kinetics are induced during individual VANTA synthesis experiments in order to understand how changes in total pressure and hydrocarbon partial pressure affect subsequent growth kinetics and wall number of nanotubes within the same array under start/stop and pulsed growth delivery. Transient interruptions or changes in hydrocarbon flow are revealed by rapid changes in slope and frequency of the oscillating, exponentially-decaying TRR signal. The associated regions of the nanotube array reveal kinked, band-like patterns along the width of the array as observed in cross-sectional scanning electron microscope (SEM) images. These bands serve as `growth-markers' to measure length intervals and calibrate growth rates before, during, and after transient perturbations to continuous growth. In addition, extended growth interruptions are explored to understand catalyst poisoning mechanisms. Finally, growth of size-selected, multilayered VANTAs was performed to investigate the interfaces between different growth regions by HRTEM   

Chirality-resolved kinetic analysis of single-walled carbon nanotube growth by \textit{in-situ} Raman spectroscopy

We investigate the chirality-resolved growth kinetics of single-walled carbon nanotubes (SWCNTs) by \textit{in-situ} Raman spectroscopy. The SWCNTs are synthesized by ethanol CVD from Co nanoparticle catalysts with pre-defined size before the CVD process. The chirality-sensitive radial breathing mode (RBM) signals in Raman spectra are observed during the CVD process at 80-120 Pa. We have reasonably assigned the chiral indices of the RBM signals observed at higher temperature during the CVD process by taking into account the temperature dependence of the resonance condition of SWCNTs. The growth kinetics analyzed from the time evolution of each RBM signal in \textit{in-situ} spectra reveals that the nanotube nucleation occurs just after the supply of the carbon source gas and does not significantly depend on the growth pressure and chirality. In addition, we have found that the growth duration depends on the growth pressure and chirality and that the graphitic encapsulation of catalyst particles terminates SWCNT growth. These findings make it possible to clarify the chirality-sensitive growth behavior of SWCNTs.   

DC Anhydrous Electrodeposition of Carbon Nanotubes

Electrodeposition is a versatile technique to fabricate carbon nanotube films on conducting substrates. Due to different responses of metallic and semiconducting nanotubes against electric field in solution, it can be used to discriminate nanotubes based on these types. Depending on whether the applied field is ac or dc, nanotubes are moved across electrodes by either dielectrophoresis or electrophoresis. In ordinary dc electrodeposition, water has been used as a solvent. We have found that, using organic solvents from which water is carefully removed, low dc fields electrodeposit nanotubes quite efficiently [1]. A millimeter thick film can be obtained within a minute with 15V/cm. Furthermore, the process is highly selective; the films are adhered so strongly that it requires scratching the substrate to remove, all nanotubes form thin straight bundles that lie parallel to the substrate, the films show no metallic Raman peaks and have 4 order of magnitude higher resistivity than the original sample [2]. A recent study shows that the electric field is not responsible for adhesion. Other than the first layer that is directly on the substrate surface, the van der Waals force dominates adhesion of nanotubes. [1] Y. Abe, R. Tomuro, M. Sano, Adv. Mater. 17, 2192 (2005). [2] R. Tomuro, T. Matsumoto, M. Sano, Jpn. J. Appl. Phys. 45, L578 (2006).   

Understanding the Growth of Carbon Nanotubes by Catalyst-Assisted Chemical Vapor Deposition

In catalyst-assisted chemical vapor deposition, carbon nanotubes are formed when a curved graphene island lifts off the surface of the catalyst particle on which it is growing. While this growth technique offers effective control over patterning and alignment, control over nanotube radius and chirality is ultimately tied to understanding the point at which lift-off occurs. We use atomistic approaches to model the lift-off process via the interplay between the excess energy required to grow a curved (and thus, necessarily defected) graphene island and the interaction energy between the growing island and the underlying catalyst. The atomistic approach combines Monte Carlo methods with \textit{ab initio} total energy electronic structure methods to explore island formation, growth, and lift-off on a catalyst surface. Using this approach, we are able to systematically study the effect of incident atomic flux rate, growth temperature, and catalyst curvature. The different defect topologies in the growing graphene cap that result from different growth conditions are a key parameter in determining the chirality of the nanotubes.