Origin of periodic rippling during chemical vapor deposition growth of carbon nanotube forests

Abstract

Origin of periodic rippling during chemical vapor deposition growth of carbon nanotube forestsgrowth conditions, these forests can show a growth instability that gives rise to periodic ripples that are coherent over a forest-sized scale. Previously, we showed that the uniformity and synchronization of the ripples is sufficient for them to behave as diffraction gratings for visible light. Here, we identify the conditions that reproducibly promote the formation of these ripples. We investigate the formation mechanism via ex situ scanning electron microscopy and in situ optical imaging. While the rippling amplitude varies appreciably, the rippling wavelength varies very little and can be estimated from simple mechanical considerations. We provide evidence that the rippling is a consequence of cohesive interactions between nanotubes and the build up of strain, driven by a non-uniform growth rate. The origin of the non-uniform growth rate is explained.Crown

Quantifying the Hierarchical Order in Self-Aligned Carbon Nanotubes from Atomic to Micrometer Scale

“…The corresponding fast Fourier transform (FFT) computed from each SEM image corroborates the ordered microstructure along the z direction (most evident in Figure a). Similar rippled morphologies (characteristic wavelength λ C ∼ 300–1000 nm) were previously observed in CNT forests grown in the presence of mechanical strain, either induced by an applied compressive force , or by the force exerted from an adjacent region of higher growth rate. , However, pronounced ripples have also been reported for as-grown CNT forests in the absence of external forces. ,, Despite the prevalence of this type of morphology reported in literature and its potential importance toward understanding forest growth, so far there has been a lack of robust, quantitative characterization of these ripples beyond the use of SEM or visible iridescence . Also, in the absence of external forces, it remains unclear whether the mismatch in local nanoscale growth rates and mechanical coupling generate such ordered microstructure, rather than simply a higher degree of uncorrelated tortuosity.…”

Room Temperature Resistive Volatile Organic Compound Sensing Materials Based on a Hybrid Structure of Vertically Aligned Carbon Nanotubes and Conformal oCVD/iCVD Polymer Coatings

“…The wavy and entangled structure is also shown in Figure . Previous researchers have also reported and studied the wavy structure. , This phenomenon is known to be due to the van der Waals interactions between nanotubes during the CNT growth. , The polymer coating penetrates into the vertically aligned structure of the VA-CNT matrix, and results in the increase of the overall diameters. The ability to penetrate into nanostructures and form a conformal coating is a unique advantage of the oCVD and iCVD technologies .…”

“…Especially, CNTs are very effective fillers with high electrical conductivity. The waviness of CNT arrays is known to be due to the van der Waals interactions between individual nanotubes during growth. , Although the average distance between VA-CNTs (80 nm) is much greater than the percolation threshold distance (1.8 nm − ), the waviness and entanglement may create short distance below the percolation threshold locally. Furthermore, additional evidence is that 1 vol % in a randomly oriented CNT–polymer composite already exceeds the percolation threshold as reported by Feng and Jiang .…”

Chemical Bath Deposition of Aluminum Oxide Buffer on Curved Surfaces for Growing Aligned Carbon Nanotube Arrays

“…62,63 Here, we propose a four-step mechanism based on this idea. As illustrated in Figure 7, the initial growth of a nanotube from a catalyst nanoparticle can be considered to be straight until the top of the nanotube touches a neighboring nanotube due to inter-CNT attraction or the tilt of underlying surface ( Figure 7a).…”