We study catalyst support interactions during chemical vapor deposition of carbon nanotubes by in situ X-ray photoelectron spectroscopy over a wide range of pressures. We observe Fe 2+ and 3+ interface states for metallic Fe on Al 2 O 3 in the absence of measurable Al reduction. This support interaction is much stronger than that on SiO 2 , and it restricts Fe surface mobility. The resulting much narrower Fe catalyst particle size distribution on Al 2 O 3 leads to a higher carbon nanotube nucleation density and a vertical nanotube alignment due to proximity effects. We record the growth kinetics of carbon nanotube forests by optical imaging to understand effects that contribute to growth termination.

Self-assembled nanowires offer the prospect of accurate and scalable device engineering at an atomistic scale for applications in electronics, photonics and biology. However, deterministic nanowire growth and the control of dopant profiles and heterostructures are limited by an incomplete understanding of the role of commonly used catalysts and specifically of their interface dynamics. Although catalytic chemical vapour deposition of nanowires below the eutectic temperature has been demonstrated in many semiconductor-catalyst systems, growth from solid catalysts is still disputed and the overall mechanism is largely unresolved. Here, we present a video-rate environmental transmission electron microscopy study of Si nanowire formation from Pd silicide crystals under disilane exposure. A Si crystal nucleus forms by phase separation, as observed for the liquid Au-Si system, which we use as a comparative benchmark. The dominant coherent Pd silicide/Si growth interface subsequently advances by lateral propagation of ledges, driven by catalytic dissociation of disilane and coupled Pd and Si diffusion. Our results establish an atomistic framework for nanowire assembly from solid catalysts, relevant also to their contact formation.

We study the Fe-catalyzed chemical vapor deposition of carbon nanotubes by complementary in-situ grazing-incidence X-ray diffraction, in-situ X-ray reflectivity and environmental transmission electron microscopy. We find that typical oxide supported Fe catalyst films form widely varying mixtures of bcc and fcc phased Fe nanoparticles upon reduction, which we ascribe to variations in minor commonly present carbon contamination levels. Depending on the as-formed phase composition, different growth modes occur upon hydrocarbon exposure: For γ-rich Fe nanoparticle distributions, metallic Fe is the active catalyst phase, implying that carbide formation is not a prerequisite for nanotube growth. For α-rich catalyst mixtures, Fe3C formation more readily occurs and constitutes part of the nanotube growth process. We propose that this behavior can be rationalized in terms of kinetically accessible pathways, which we discuss in the context of the bulk iron-carbon phase diagram with the inclusion of phase equilibrium lines for metastable Fe3C. Our results indicate that kinetic effects dominate the complex phase evolution during realistic CNT growth recipes.

How does your graphene grow? In situ X-ray photoelectron spectroscopy and X-ray diffraction measurements during chemical vapor deposition on Ni catalyst films show that graphene forms both isothermally and by precipitation on cooling (see picture). A coherent graphene growth model is devised and sub-surface dissolved carbon is shown to play an important role

We study catalyst-support and catalyst-carbon interactions during the chemical vapor deposition of singlewalled carbon nanotubes by combining environmental transmission microscopy and in situ, time-resolved X-ray photoelectron spectroscopy. We present direct evidence of what constitutes catalyst functionality by comparing the behavior of Ni, Fe, Pd, and Au model catalyst films on SiO 2 during preannealing in O 2 and NH 3 and during C 2 H 2 decomposition. The catalyst metal surface supplies sites to dissociate the hydrocarbon precursor and then guides the formation of a carbon lattice and the liftoff of a carbon cap. The catalysts are sharply distinguished by their reactivity toward activation of the hydrocarbon precursor, following trends known from heterogeneous catalysis. For Fe and Ni, the active state of the catalyst is a crystalline metallic nanoparticle. Graphitic networks do not form on oxidized Fe. Pd forms a silicide on SiO 2 under our reducing conditions. Pd (silicides) and Au nanocrystals are catalytically less efficient in terms of precursor dissociation, while the low adhesion of C on Au surfaces impedes nanotube nucleation.

We present a systematic study of the temperature and pressure dependence of the growth rate of vertically aligned small diameter (single- and few-walled) carbon nanotube forests grown by thermal chemical vapor deposition over the temperature range 560-800 degrees C and 10(-5) to 14 mbar partial pressure range, using acetylene as the feedstock and Al(2)O(3)-supported Fe nanoparticles as the catalyst. We observe a pressure dependence of P(0.6) and activation energies of <1 eV. We interpret this as a growth rate limited by carbon diffusion in the catalyst, preceded by a pre-equilibrium of acetylene dissociation on the catalyst surface. The carbon nanotube forest growth was recorded by high-resolution real-time optical imaging.

We present a detailed study of processes and interactions occurring during the Fe-catalyzed chemical vapor deposition of carbon nanotubes on metallic Ta supports. In situ X-ray photoemission spectroscopy and X-ray diffraction show that the Fe catalyst increases the reactivity of Ta toward oxidation and carbide formation, whereas Ta promotes the reduction of Fe. This causes an unusual temperature dependence of carbon nanotube growth, where at low temperatures (∼550 °C) vertically aligned forests of carbon nanotubes with ohmic contacts grow readily on metallic Ta, whereas at high temperatures (>600 °C) nanotube growth is sparse because of the diffusion of Fe away from the surface through grain boundaries of in situ formed polycrystalline Ta2O5. The Fe−Ta model system highlights general material selection criteria for nanotube applications that require a conductive support.

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