Films of multiwall carbon nanotubes (MWCNTs) grown by thermal chemical vapor deposition were studied using small-angle X-ray scattering (SAXS). We assessed the extent of alignment of carbon nanotubes (CNTs) by examining relative SAXS intensities as a function of azimuthal angle. We also identified features in the SAXS patterns that correspond well to CNT diameters measured through high-resolution transmission electron microscopy. For the case of thick films, corresponding to CNTs with lengths on the order of a millimeter, we were able to study the morphology of the films as a function of distance from the catalyst substrate. We examined two different films in which the morphologies of CNTs range from vertically aligned to entangled and tortuous. We determined that the alignment of CNTs as well as their average diameter can vary significantly throughout the film, demonstrating the utility of SAXS for quantitative structural analysis of CNT films, indicating the potential to reveal new information about the CNT growth process, and relating variations in morphology to evolution of the catalyst and reaction conditions.

In spite of much research progress in the science and synthesis of carbon nanotubes (CNTs), [1] control over the location and orientation of CNTs on substrates remains a major challenge. A key breakthrough was the synthesis of vertically aligned CNT (VA-CNT) arrays using thermal chemical vapor deposition (CVD) [2][3][4][5][6][7] and plasma-enhanced CVD, [8] where the CNTs self-orient perpendicular to the substrate surface due to initial crowding and continue to grow upward in this direction. These CNT arrays have wide-ranging applications, including membranes, heat dissipation, electrical interconnects, and nanoelectronics. [9][10][11][12][13] The catalysts for synthesis of VACNTs are commonly prepared by sputtering or evaporating a thin metal film onto a substrate, [14,15] which dewets to form catalyst nanoparticles at an elevated temperature prior to growth. [16][17][18] While these catalysts are easily prepared and patterned by shadow masking or lithography, [7,14] these approaches are not easily able to create nanocluster catalysts that have monodisperse diameters and quantifiable areal densities. In thin metal films, both the nanocluster size and areal density are coupled to the film thickness, and the annealing procedure affects the size, density, and the chemical state of the nanoclusters. Recently, Huh et al. [19] demonstrated a route for controlling the density of CNT growth using varying densities of colloidal cobalt nanoparticles; however, due to nanoparticle coalescence their route does not enable precise quantification of nanocluster areal density and leads to a broad distribution of CNT diameters. Zhang et al. [20] also recently demonstrated the control of CNT growth by varying the density of Co-Mo nanoparticles, although their route is unable to independently vary the diameter and the areal density of nanoparticles.We employ a methodology for synthesizing iron oxide nanoclusters that utilizes micelles formed by the amphiphilic block copolymer, polystyrene-b-poly(acrylic acid) (PS-b-PAA). [21,22] This catalyst system has significant value because it enables the creation of nanocluster arrays of a chosen metal species, with independent control of the nanocluster diameter and areal density. [22] In previous work, nanocluster diameters were varied between 5 and 16 nm and the areal density was varied from 6.0 × 10 10 to 1 × 10 9 cm -2 , although variation outside of these ranges is easily accessible. At higher areal densities the nanoclusters are hexagonally ordered. Further, as we have presented separately, the nanocluster arrays can be patterned on the micrometer length scale using microcontact printing.[23]Here, we utilize this system to create arrays of uniform-diameter iron oxide nanoclusters, with quantifiable areal densities that can be varied over more than an order of magnitude. We achieve vertical CNT growth from our catalyst system through appropriate selection of the substrate, catalyst preparation procedure, and reaction conditions. To the best of our knowledge, our work is the first exampl...

We report a novel approach that uses block copolymer micelles as a means to create large area arrays of iron-containing nanoclusters capable of catalyzing the growth of carbon nanotubes (CNTs). The amphiphilic block copolymer poly(styrene-block-acrylic acid) (PS-b-PAA) forms micelles in solution which are capable of self-organizing into ordered structures on surfaces. By spin-coating these solutions onto a variety of substrates, we can create quasihexagonal arrays of PAA spheres within a PS matrix. The carboxylic acids groups in the PAA domains can be utilized in an ion-exchange protocol to selectively sequester iron ions, which results in iron-containing nanoclusters that are nearly monodisperse in size (diameter ∼8 nm) and patterned at a density of approximately 10 11 particles per cm 2 . In principle, this route for synthesizing iron-containing nanoclusters offers the capability of controlling the cluster size and spacing by altering the molecular weight of the block copolymer. In this report, we demonstrate the ability of these block-copolymer-templated iron-containing nanocluster arrays to catalyze the growth of CNTs in a thermal chemical vapor deposition (CVD) process. We present transmission electron microscope (TEM) and scanning electron microscope (SEM) images of the as-grown CNTs still attached to their growth substrate, which allows us to characterize both the CNTs and the catalytic nanoclusters after CVD growth.

We report several strategies for varying the diameter, the center-to-center spacing, and the areal density of block copolymer micelles, or inorganic nanoclusters synthesized in the cores of the micelles, on planar substrates. The amphiphilic block copolymer, poly(styrene-b-acrylic acid) (PS/PAA), forms micelles in toluene solution that can be spin-coated onto a substrate to create quasi-hexagonal arrays of spherical PAA domains within a PS matrix. The carboxylic acid groups within the PAA domains can be utilized in a nanoreactor synthesis scheme to create inorganic nanocluster arrays, or the PAA domains can be cavitated to expose the carboxylic acid groups to the surface for possible covalent coupling reactions. The strategies we use to vary the planar arrangements include variation of the molecular weight of PS/PAA, variation of the amount of metal loaded into the micellar solution, addition of PS homopolymer into the micellar solution, and the mixing of different micellar solutions. Through these routes, we demonstrate varying the diameter of the inorganic nanoclusters from 4.7 to 16 nm and the areal density from 8 × 10 10 to 6.5 × 10 9 nanoclusters cm -2 . We are also able to create arrays of nanoclusters containing more than one inorganic species, with each nanocluster containing either one or all of the inorganic species, depending on the sequence of processing conditions employed. We characterize these arrays using energy-dispersive X-ray analysis on a scanning transmission electron microscope.