Distortion of carbon nanotube array and its influence on carbon nanotube growth and termination

Effects of morphology on the micro-compression response of carbon nanotube forests

This study reports the mechanical response of distinct carbon nanotube (CNT) morphologies as revealed by flat punch in situ nanoindentation in a scanning electron microscope. We find that the location of incipient deformation varies significantly by changing the CNT growth parameters. The initial buckles formed close to the growth substrate in 70 and 190 μm tall CNT forests grown with low pressure chemical vapor deposition (LPCVD) and moved to ∼100 μm above the growth substrate when the height increased to 280 μm. Change of the recipe from LPCVD to CVD at pressures near atmospheric changed the location of the initial buckling event from the bottom half to the top half of the CNT forest. Plasma pretreatment of the catalyst also resulted in a unique CNT forest morphology in which deformation started by bending and buckling of the CNT tips. We find that the vertical gradients in CNT morphology dictate the location of incipient buckling. These new insights are critical in the design of CNT forests for a variety of applications where mechanical contact is important.

Fabrication of multi-level carbon nanotube arrays with adjustable patterns

Multi-level carbon nanotube (CNT) arrays with adjustable patterns were prepared by a combination of the breath figure (BF) process and chemical vapor deposition. Polystyrene-b-poly(acrylic acid)/ferrocene was dissolved in carbon disulfide and cast onto a Si substrate covered with a transmission electron microscope grid in saturated relative humidity. A two-level microporous hybrid film with a block copolymer skeleton formed on the substrate after evaporation of the organic solvent and water. One level of ordered surface features originates from the contour of the hard templates; while the other level originates from the condensation of water droplets (BF arrays). Ultraviolet irradiation effectively cross-linked the polymer matrix and endowed the hybrid film with improved thermal stability. In the subsequent pyrolysis, the incorporated ferrocene in the hybrid film was oxidized and turned the polymer skeleton into the ferrous inorganic micropatterns. Either the cross-linked hybrid film or the ferrous inorganic micropatterns could act as a template to grow the multi-level CNT patterns, e.g. isolated and honeycomb-structured CNT bundle arrays perpendicular to the substrate.

Population growth dynamics of carbon nanotubes

Understanding the population growth behavior of filamentary nanostructures, such as carbon nanotubes (CNTs), is hampered by the lack of characterization techniques capable of probing statistical variations with high spatial resolution. We present a comprehensive methodology for studying the population growth dynamics of vertically aligned CNT forests, utilizing high-resolution spatial mapping of synchrotron X-ray scattering and attenuation, along with real-time height kinetics. We map the CNT alignment and dimensions within CNT forests, revealing broadening and focusing of size distributions during different stages of the process. Then, we calculate the number density and mass density of the CNT population versus time, which are true measures of the reaction kinetics. We find that the mass-based kinetics of a CNT population is accurately represented by the S-shaped Gompertz model of population growth, although the forest height and CNT length kinetics are essentially linear. Competition between catalyst activation and deactivation govern the rapid initial acceleration and slow decay of the CNT number density. The maximum CNT density (i.e., the overall catalyst activity) is limited by gas-phase reactions and catalyst-surface interactions, which collectively exhibit autocatalytic behavior. Thus, we propose a comprehensive picture of CNT population growth which combines both chemical and mechanical cooperation. Our findings are relevant to both bulk and substrate-based CNT synthesis methods and provide general insights into the self-assembly and collective growth of filamentary nanostructures.

Measuring the lengthening kinetics of aligned nanostructures by spatiotemporal correlation of height and orientation

Owing to their inherent tortuosity, the collective height of vertically aligned nanostructures does not equal the average length of the individual constituent nanostructures, and therefore temporal height measurement is not an accurate measure of the genuine growth kinetics. We use high-resolution spatial mapping of alignment by small-angle X-ray scattering (SAXS) to transform real-time measurements of array height to the average length of the nanostructures. Applying this approach to carbon nanotube (CNT) forest growth transforms the kinetics from a sub-linear to a linear relationship with time, highlighting the potential for insights into the limiting growth mechanisms of CNTs and other one-dimensional nanostructures.