Cumulative and continuous laser vaporization synthesis of single wall carbon nanotubes and nanohorns

The conditions for the scaled synthesis of single wall carbon nanotubes (SWNTs) and single wall carbon nanohorns (SWNHs) by laser vaporization at high temperatures are investigated and compared using in situ diagnostics. An industrial Nd:YAG laser (600 W, 1–500 Hz repetition rate) with tunable pulse widths (0.5–50 ms) is utilized to explore conditions for high-yield production. High-speed videography (50000 frames/s) of the laser plume and pyrometry of the target surface are correlated with ex situ high resolution transmission electron microscopy analysis of the products for pure carbon targets and carbon/catalyst targets to understand the effects of the processing conditions on the resulting nanostructures. Carbon is shown to self-assemble into single-wall nanohorn structures at rates of ∼1 nm/ms, which is comparable to the catalyst-assisted SWNT growth rates. Two regimes of laser ablation, cumulative ablation by multiple pulses and continuous ablation by individual pulses, were explored. Cumulative ablation with spatially overlapping 0.5-ms pulses is favorable for the high yield and production rate of SWNTs at ∼6 g/h while continuous ablation by individual long laser pulses (∼20 ms) at high temperatures results in the highest yield of SWNHs at ∼10 g/h. Adjustment of the laser pulse width is shown to control SWNH morphology.     

Altering the catalytic activity of thin metal catalyst films for controlled growth of chemical vapor deposited vertically aligned carbon nanotube arrays

The growth rate and terminal length of vertically aligned carbon nanotube arrays (VANTAs) grown by chemical vapor deposition have been dramatically improved through pulsed KrF excimer laser pretreatments of multilayer metal catalyst films. Silicon wafers coated with Al, Mo, and Fe layers were laser processed in air with single laser shots of varying fluence through circular apertures, then heated to ∼750°C and exposed to acetylene and ferrocene containing gas mixtures typically used to grow vertically aligned nanotube arrays. In situ videography was used to record the growth kinetics of the nanotube arrays in both patterned and unpatterned regions to understand the changes in catalytic activity, growth rates, and termination of growth. The height of the patterned regions varied with fluence, with the most successful treatment resulting in 1.4 cm tall posts of nanotubes embedded in a 0.4 cm tall nanotube carpet. High-resolution transmission electron microscopy images from the nanotubes in the posts revealed fewer walls, smaller diameters, and a much narrower distribution of diameters compared to the nanotubes grown in the carpet. This information, along with data obtained from weighing the material from each region, suggests that pulsed laser processing can also significantly increase the areal density of VANTAs. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy.     

Molecular beam-controlled nucleation and growth of vertically aligned single-wall carbon nanotube arrays

The main obstacle to widespread application of single-wall carbon nanotubes is the lack of reproducible synthesis methods of pure material. We describe a new growth method for single-wall carbon nanotubes that uses molecular beams of precursor gases that impinge on a heated substrate coated with a catalyst thin film. In this growth environment the gas and the substrate temperature are decoupled and carbon nanotube growth occurs by surface reactions without contribution from homogeneous gas-phase reactions. This controlled reaction environment revealed that SWCNT growth is a complex multicomponent reaction in which not just C, but also H, and O play a critical role. These experiments identified acetylene as a prolific direct building block for carbon network formation that is an order of magnitude more efficient than other small-molecule precursors. The molecular jet experiments show that with optimal catalyst particle size the incidence rate of acetylene molecules plays a critical role in the formation of single-wall carbon nanotubes and dense vertically aligned arrays in which they are the dominant component. The threshold for vertically aligned growth, the growth rate, the diameter, and the number of walls of the carbon nanotubes are systematically correlated with the acetylene incidence rate and the substrate temperature.     

Pulsed laser CVD investigations of single-wall carbon nanotube growth dynamics

The nucleation and rapid growth of single-wall carbon nanotubes (SWNTs) were explored by pulsed-laser assisted chemical vapor deposition (PLA-CVD). A special high-power, Nd:YAG laser system with tunable pulse width (>0.5 ms) was implemented to rapidly heat (>3×10⁴°C/s) metal catalyst-covered substrates to different growth temperatures for very brief (sub-second) and controlled time periods as measured by in situ optical pyrometry. Utilizing growth directly on transmission electron microscopy grids, exclusively SWNTs were found to grow under rapid heating conditions, with a minimum nucleation time of >0.1 s. By measuring the length of nanotubes grown by single laser pulses, extremely fast growth rates (up to 100 microns/s) were found to result from the rapid heating and cooling induced by the laser treatment. Subsequent laser pulses were found not to incrementally continue the growth of these nanotubes, but instead activate previously inactive catalyst nanoparticles to grow new nanotubes. Localized growth of nanotubes with variable density was demonstrated through this process and was applied for the reliable direct-write synthesis of SWNTs onto pre-patterned, catalyst-covered metal electrodes for the synthesis of SWNT field-effect transistors.     

Calcium as the superior coating metal in functionalization of carbon fullerenes for high-capacity hydrogen storage

We explore theoretically the feasibility of functionalizing carbon nanostructures for hydrogen storage, focusing on the coating of C60 fullerenes with light alkaline-earth metals. Our first-principles density functional theory studies show that both Ca and Sr can bind strongly to the C60 surface, and highly prefer monolayer coating, thereby explaining existing experimental observations. The strong binding is attributed to an intriguing charge transfer mechanism involving the empty d levels of the metal elements. The charge redistribution, in turn, gives rise to electric fields surrounding the coated fullerenes, which can now function as ideal molecular hydrogen attractors. With a hydrogen uptake of >8.4 wt % on Ca32C60, Ca is superior to all the recently suggested metal coating elements.     

Understanding the Metal-Directed Growth of Single-Crystal M-TCNQF(4) Organic Nanowires with Time-Resolved, in Situ X-ray Diffraction and First-Principles Theoretical Studies

The deterministic growth of oriented crystalline organic nanowires (CONs) from the vapor-solid chemical reaction (VSCR) between small-molecule reactants and metal nanoparticles has been demonstrated in several studies to date; however, the growth mechanism has not yet been conclusively understood. Here, the VSCR growth of M-TCNQF(4) (where M is Cu- or Ag-) nanowires is investigated both experimentally and theoretically with time-resolved, in situ X-ray diffraction (XRD) and first-principles atomistic calculations, respectively, to understand how metals (M) direct the assembly of small molecules into CONs, and what determines the selectivity of a metal for an organic vapor reactant in the growth process. Analysis of the real-time growth kinetics data using a modified Avrami model indicates that the formation of CONs from VSCR follows a one-dimensional ion diffusion-controlled tip growth mechanism wherein metal ions diffuse from a metal film through the nanowire to its tip where they react with small molecules to continue growth. The experimental data and theoretical calculations indicate that the selectivity of different metals to induce nanowire growth depends strongly upon effective charge transfer between the organic molecules and the metal. Specifically, the experimental finding that Cu ions can exchange and replace Ag ions in Ag-TCNQF(4) to form Cu-TCNQF(4) nanowires is explained by the significantly stronger chemical bond between Cu and TCNQF(4) molecules than for Ag, due to the strong electronic contribution of Cu d-orbitals near the Fermi level. Understanding how to control the VSCR growth process may enable the synthesis of novel organic nanowires with axial or coaxial p/n junctions for organic nanoelectronics and solar energy harvesting.