Session U27: Focus Session: Carbon Nanotubes: Growth

Critical silicon dioxide thickness for CVD growth of single-walled carbon nanotubes

Chemical vapor deposition (CVD) has shown remarkable control over the efficient and directed assembly of single-walled carbon nanotubes, making CVD a primary growth method for device applications. Due to the high temperatures involved in CVD, the chemical compatibility between the substrate, feedstock, and catalyst must be understood. Using x-ray photoelectron spectroscopy (XPS), we have studied the evolution of the chemical state of an iron nitrate catalyst during the initial temperature ramp of a standard CVD process. Heating the catalyst on clean silicon or on silicon with a native oxide leads to the formation of a silicide at 800~$^{o}$C, inhibiting single-walled nanotube growth. By 900~$^{o}$C, a typical growth temperature, all of the iron catalyst has been incorporated into the silicide. Thicker silicon oxide layers, on the order of 10~nm, effectively prevent silicide formation, enabling high yield growth.   

Evolution of the Catalyst Nanoparticles during CVD Growth of Carbon Single-Walled Nanotubes

Despite intense studies, the growth mechanism of carbon single- walled nanotubes (SWNTs) is still debated and current synthesis methods do not allow for full control over the growth. There has been much discussion of whether the active catalytic species are in the liquid or solid phase during SWNTs formation, which is a key to understand and to control the growth of these materials. However, the actual phase of the catalyst and its evolution during carbon SWNTs growth still has to be experimentally verified. We report the observation of carbon induced solid-liquid and solid--liquid-solid phase transitions of the iron nanocatalyst during the synthesis, using differential scanning calorimetry and Raman scattering measurements. We found that as long as the nanocatalyst is in a liquid state, SWNTs growth occurs and continues until its solidification. Moreover, no growth was observed below the eutectic point, when the catalyst is always in solid phase.   

Growth of vertically aligned multiwall and uniform carbon nanotubes on self-assembled ferromagnetic Fe and Co nanowires

A novel approach to grow vertically aligned and uniformly separated carbon nanotubes on self-assembled \textit{$\alpha $}-Fe is reported. We have previously demonstrated that the growth of LaSrFeO$_{3}$ perovskite oxide by Pulsed Laser Deposition under reducing environments leads to spontaneous formation of an array of single crystalline \textit{$\alpha $}-Fe nanowires embedded in an antiferromagnetic LaSrFeO$_{4}$ matrix. The diameter and spacing of these ferromagnetic nanowires can be controlled by changing the temperature of growth. We now show that these thin films containing self-assembled $\alpha $-Fe nanowires can be used as a template to grow vertically aligned carbon nanotubes using Plasma Enhanced Chemical Vapor Deposition. Acetylene (C$_{2}$H$_{2})$ and ammonia (NH$_{3})$ were used as a carbon source and diluting gas, respectively. Self-assembled \textit{$\alpha $}-Fe nanowires serve as a nucleation sites for the growth of vertically aligned multiwall carbon nanotubes (MWCNTs). The size of carbon nanotubes can be controlled by changing the diameter of \textit{$\alpha $}-Fe nanowires. The results of Transmission Electron Microscopy, Raman spectroscopy and field emission data will be presented. By means chemical mechanical polishing we can achieve atomically smooth surfaces and improve the quality of the carbon nanotubes grown on these nanowires.   

Comparison of Efficiencies of Binary and Ternary Composite Catalysts in Arc-Discharge Synthesis of Single-Walled Carbon Nanotubes

The catalyst composition is a major factor determining the efficiency of single-walled carbon nanotubes (SWNTs) synthesis. The binary catalyst composed of transition metal (TM) and rear earth metal (REM) proved to be the most efficient combination for the arc-discharge technique. Recently we proposed a quantitative procedure to assess the relative carbonaceous purity of bulk quantities of SWNT soot on the basis of solution phase NIR spectroscopy.[1] We applied this technique to obtain direct comparison of the efficiency of variety of binary and ternary TM-TM, TM-REM, TM-TM-REM composite catalysts for the arc-discharge synthesis. We found that substituting of either component of the most popular Ni-Y composition by different transition metal (Fe, Co) and rare earth metal (Se, La) affects significantly the efficiency of the SWNT synthesis and modifies the SWNT diameter distribution. This work is supported by DOD/DARPA/DMEA under Award No.DMEA90-02-2-0216. . [1] M.E. Itkis et al., \textit{Nano Lett.} \textbf{2003}, $3$, 309-314.   

Growth of Single Wall Carbon Nanotubes from Iron Oxide Catalyst

Carbon nanotubes are cylindrical nanostructures with hexagonal networks of carbon atoms with interesting electronic and mechanical properties. CNT synthesis route involving the catalytic decomposition of hydrocarbons on metal particles have been widely popular. We have shown that single wall carbon nanotubes of diameters less than 2 nanometers can be grown directly from catalyst particles comprising of maghemite ($\gamma $-Fe$_{2}$O$_{3})$ on a Silicon substrate using the conventional CVD process. The sizes of SWNT were measured using Atomic Force Microscopy. The average tube diameter was measured to be 1.0$\pm $0.2 nm. FTIR and X-ray diffraction measurements were performed to characterize the catalyst iron oxide. Scanning Electron Microscopy measurements revealed that the catalyst oxide particles formed in clusters of 100 nm diameters. Transmission M\"{o}ssbauer measurements at room temperature showed the presence of only a superparamagnetic doublet, characteristic of nanophase iron oxides. The crystallographic, morphological and magnetic properties of the catalyst metal powders and the properties of the resulting SWNTs will be presented. .   

Current-Controlled Nanotube Growth and Zone-Refinement

We present methods by which the growth of a single carbon nanotube (CNT) can be precisely controlled by an electrical current. In one method a CNT is grown to a predetermined geometry inside another nanotube, which serves as a reaction chamber. Another method allows a preexisting marginal quality multiwall CNT to be zone-refined into a higher quality multiwall CNT by driving a catalytic bead down the length of the nanotube, which can be many microns long. In both methods the speed of nanotube formation is adjustable, and the growth can be stopped and restarted at will.   

Highly nitrogen and boron doped nanotubes: a route to synthesis and study of their properties by spatially resolved EELS

Doping C-nanotubes with B and/or N is expected to be a particular interesting way for tuning electronic and mechanical properties. BN nanotubes are predicted to behave as insulators whereas B(N) doped C-nanotubes are expected to be metallic, independent of their structure. In this framework, we have developped, both at Onera and GDPC, original routes to the synthesis of BN singlewall nanotubes (BN-SWNTs) and to highly doped nitrogen multi wall nanotubes (CN$_{x}$-MWNTs). CN$_{x}$-MWNTs were produced by a CVD method, using an aerosol injector which sprays in the reactor, heated at 950\r{ }C, a liquid mixture of organic compounds with a controlled N/C ratio and suitable metal complexes as the catalyst precursors$^{1}$. This procedure leads to dense amounts of MWNTs with controlled N/C ratios which can exceed 15-20{\%} in average. Upon doping, tubes get a characteristic compartimentalized structure with a reduced number of layers identified in transmission electron microscopy. Using spatially resolved electron energy loss spectroscopy (EELS), N is found to be preferentially localized in inner layers and in the compartiments where the concentration can exceed 40 at.{\%}. Structure of core losses in EELS reveals a high dependance of the N environment to the local concentration :chemical bonding of N can be graphitic, pyridinic or pyrrolic, this latter case being found for highest N concentrations. Relationships between these structural properties and formation mechanism will be discussed$^{2}$. BN-SWNTs are issued from the vaporization of a BN target by a continuous CO$_{2}$ laser under a N$_{2}$ atmosphere$^{3}$. We present here the first investigation on their electronic properties by two ways: first, analysis of the dielectric response of low loss EELS recorded on individual tubes provides the first identification of plasmons and of interband transitions in these tubes and the first measure of their gap found to be close to 5.8eV$^{4}$. Second, optical absorption spectra measured on macroscopic samples strongly suggest the existence of a Frenkel exciton with a binding energy in the 1eV range$^{5}$. 1-M. Glerup et al Chem. Commun 2542 (2003). 2-M. Castignolles et al submitted to Phys. Rev B (2005) 3- R. Lee et al, Phys. Rev. B Rapid Comm 64, 121405-1 (2001) 4-R. Arenal et al, submitted to Phys. Rev. Lett. (2005) 5-J.S. Lauret et al, Phys. Rev. Lett. (2005) in press. Coauthors: M. Castignolles$^{1,2}$, R. Arenal$^{1}$, O. St\'{e}phan$^{3}$, M. Glerup$^{2,4}$,$^{ 1 }$LEM, CNRS-ONERA, Ch\^{a}tillon, France, $^{2 }$GDPC, Universit\'{e} Montpellier II, France, $^{3 }$LPS, Universit\'{e} Paris-Sud, Orsay, France, $^{4}$GHMFL, MPI-CNRS, Grenoble, France.   

Growth Kinetics of Water-Assisted Single-Walled Carbon Nanotube Synthesis-``Super-Growth''

Recently, we have reported the highly efficient synthesis of vertically aligned, highly dense and pure single-walled carbon nanotubes (SWNT) by chemical vapor deposition through the introduction of a small, controlled level of water in the growth ambient [1]. The dramatic increase in catalyst efficiency resulted in the growth of SWNT forests as tall as 2.5 millimeters in a 10-minute growth time. The need to fully utilize the catalytic enhancement by water requires the optimization of the growth conditions and the understanding of the growth mechanism. Here we report the growth kinetics describing water-assisted SWNT synthesis, which we attained through a systematic investigation of the yield [2]. Our extensive analysis of water-assisted growth revealed an unexpected simplicity, in that the growth evolution could be completely described by two characteristic quantities: the initial growth rate and the catalytic lifetime. Furthermore, our studies revealed how these quantities reflected changes in the relative water and ethylene levels. [1] K. Hata \textit{et al}, Science, \textbf{306,} 1362 (2004). [2] D.N. Futaba\textit{ et al}, Nature Materials (\textit{submitted}).   

Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes-``Super-Growth''

We demonstrate an extremely efficient chemical vapour deposition synthesis of single-walled carbon nanotubes where the activity and lifetime of the catalysts are enhanced by water [1]. Water-stimulated enhanced catalytic activity results in massive growth of super-dense and vertically-aligned nanotube forests with heights up to 2.5 millimeters that can be easily separated from the catalysts, providing nanotube material with carbon purity above 99.98{\%}. Moreover, patterned highly organized intrinsic nanotube structures were successfully fabricated. The water-assisted synthesis method addresses many critical problems that currently plague carbon nanotube synthesis. [1] K. Hata, et al., Science, 306, 1362 (2004).   

Atomic Step-Templated Formation of Single-Wall Carbon Nanotube Patterns

\textbf{Single-wall carbon nanotubes catalytically produced on miscut C-plane sapphire wafers grow along the 0.2nm-high atomic steps of the vicinal $\alpha ${\-}Al}$_{2}$\textbf{O}$_{3}$\textbf{ (0001)~surfaces, yielding highly aligned, dense arrays of discrete nanotubes on a dielectric material [1]. The nanotubes reproduce the atomic features of the surface, including steps, facets and kinks. Microscopy, X-ray diffraction and single-nanotube Raman spectroscopy [2] shed light into the possible structure and mechanism of the step-templated carbon nanotube growth. The orientation, density and morphology of the atomic steps can be macroscopically controlled by the crystal cutting process. Hence, these findings open up the possibility of assembling nanotube architectures by atomic-scale surface engineering.} [1] A. Ismach, et al.,\textit{ Angew. Chem. Int. Ed.} \textbf{2004}, 43, 6140. [2] M. Souza, et al., \textit{Phys. Rev. B} \textbf{2004}, 241403R.   

Ultra-thin films of Single-walled Carbon Nanotubes

Recently the fabrication and optical properties of homogeneous, uniform-thickness films of pure, single-walled carbon nanotubes sufficiently thin to be optically transparent were described. We will discuss progress in the fabrication of transparent films in which the nanotubes are aligned along an axis within the plane of the film. The uniformity of these films also provide additional opportunities, both for study of the nanotubes and for their application. We will describe results of electronic transport measurements in the pure and modified films that further distinguish their properties from those of common 3-D and 2-D systems.   

Thermal Conversion of Bundled Carbon Nanotubes into Graphitic Ribbons

The morphological evolution of purified bundled single-walled carbon nanotubes (SWNTs) heat-treated in a dynamic vacuum from T = 200-2200 \r{ }C is investigated by transmission electron microscopy, UV-Vis and resonant Raman spectroscopies. The coalescence of neighboring tubes was observed to begin at $\sim $1400 \r{ }C in both materials. HiPCO (ARC) tubes exhibited $\sim $ 100{\%} (70{\%}) coalescence of the tubes that survive 1600 \r{ }C. At $\sim $1800 \r{ }C, the ARC material exhibits a much stronger conversion to multiwall nanotubes (MWNTs) with $\sim $3-5 shells, and, in contrast to the HiPCO material, these MWNTs are often bundled and collapse into graphitic ribbons. To our knowledge, this is the first report of these multilayer nanofilaments. With increasing temperature, Raman scattering and TEM indicate a preferential early loss of small diameter tubes. The small d tubes in HiPCO material appears to produce fragments that coat the walls of the MWNTs and lead to a more structurally disorganized material at 2200 \r{ }C. Raman scattering spectra indicate that some coalesced SWNTs of d $\sim $ 2.5nm survive vacuum annealing for $\sim $6 h at 2000 \r{ }C.   

Rings of Functionalized Carbon Nanotubes: Characterization and Properties of a Novel Magnetic Material

Carbon nanotubes are a promising material for electronic, optical and biological applications. However, applications that require large nanotube quantities are hindered by difficulties in processability, due to the perennial presence of amorphous carbon impurities, and to the tubes' insolubility. Side wall covalent functionalization is a possible solution to generate soluble nanotubes but large scale purification remains a challenge. Here we show that single walled carbon nanotubes, appropriately functionalized, spontaneously assemble into rings of varying size. A thorough scanning probe microscopy analysis shows that each ring is formed by bundles of nanotubes and is composed by the assembly of shorter building blocks. Models explaining the ring formation will be presented. Also, magnetic measurements show that eddy currents flow through the rings when an alternating magnetic field is applied. Thus these rings exhibit a strong magnetic response both in solution and when assembled onto a substrate. We show that this property can be used to separate nanotubes from non-magnetic impurities; opening up new possibilities for large scale carbon nanotube purification.   

Growth of Vertically Aligned Carbon Nanotube Films: Single- versus Multi-walled

Vertically aligned high density small diameter carbon nanotube films were deposited by microwave CVD technique. The iron catalyst was prepared by E-beam evaporation on thermally grown silicon dioxide n-type Si(100) substrates. Experiments show that by continuous reduction in the thickness of Fe ($\sim $ 3-5), smaller diameter carbon nanotube can be achieved. Scanning electron and high-resolution transmission electron microscopy show that the diameter of carbon nanotubes ranged $\sim $ 1 - 5 nm and the films are comprised of both the single- and double-wall carbon nanotubes. Visible Raman spectroscopy was used to further verify the diameter of nanotubes. A thick iron film (80 nm) was also used to grow nanotubes for comparison. The results show that the catalyst islands become greater than hundred nanometers with increasing thickness and induce multi-wall and bamboo-like microstructures. While for thinner layer of iron films smaller sizes of catalyst particles/droplets produce hollow concentric tubes without bamboo structure and with less number of walls (single-wall and double-wall carbon nanotubes). The base growth was the most appropriate model to describe the growth mechanism for our films. The electron field emission properties such as field electron emission microscopy (FEEM) in conjunction with the temperature dependence (T-FEEM) were measured to investigate the emission site density and their intensity variation. These findings in terms of the role of adsorption will be briefly discussed.