Collective Mechanism for the Evolution and Self-Termination of Vertically Aligned Carbon Nanotube Growth

Bedewy, Mostafa; Meshot, Eric R.; Guo, Hongjun; Verploegen, Eric; Lu, Wenfei; Hart, A. John

doi:10.1021/jp904152v

Cited by 215 publications

(253 citation statements)

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“…The measured, single-walled CNT films have a disordered and dense crust layer on top of an aligned middle region. This is similar to observations of multiwalled CNT films in past work (21,27,34). Our single-walled CNTs have been observed to be denser and stiffer due to a smaller diameter as well as better quality.…”

supporting

confidence: 91%

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Won

Gao

Panzer

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Reliably routing heat to and from conversion materials is a daunting challenge for a variety of innovative energy technologies--from thermal solar to automotive waste heat recovery systems--whose efficiencies degrade due to massive thermomechanical stresses at interfaces. This problem may soon be addressed by adhesives based on vertically aligned carbon nanotubes, which promise the revolutionary combination of high through-plane thermal conductivity and vanishing in-plane mechanical stiffness. Here, we report the data for the in-plane modulus of aligned single-walled carbon nanotube films using a microfabricated resonator method. Molecular simulations and electron microscopy identify the nanoscale mechanisms responsible for this property. The zipping and unzipping of adjacent nanotubes and the degree of alignment and entanglement are shown to govern the spatially varying local modulus, thereby providing the route to engineered materials with outstanding combinations of mechanical and thermal properties.nanostructured materials | van der Waals | thermal interface materials | thermoelectrics | energy conversion N anostructured materials provide unique combinations of properties that promise performance breakthroughs for applications ranging from energy conversion to data storage and computation (1-4). In many cases it is the very unusual combination of two properties (2), neither of which is an extreme value when considered alone, that leads to adoption and major performance benefits. An example is the search for a mechanically compliant thermal conductor that can, for example, link semiconducting materials with the metals used for heat spreading and exchange. A particularly compelling case is thermoelectric waste heat recovery systems (3), for which the interfaces between the thermoelectric materials, electrodes, insulators, and heat exchangers must accommodate enormous, repetitive thermomechanical stress. However, nature does not provide a material combining the necessary high thermal conductivity with the required low elastic modulus (5, 6).Vertically aligned carbon nanotube (CNT) films may combine mechanical compliance with high thermal conductivity (7-18), but there have been few reports of the in-plane modulus of these films and little physical explanation for the wide range for the data (2-300 MPa) (19-25). Our previous data for the thickness-dependent in-plane modulus of multiwalled CNT films indicated a strong dependence on the nanotube density and alignment (21), which are linked to the detailed growth details (26-28). Other approaches, such as mesoscopic simulations or atomistic models, found that CNT networks exhibit unique self-organization, including bending and bundling (29-31). Therefore, relating the nanoscale morphological details to the mechanical properties is critical.Combined experimental, theoretical, and computational techniques applied to the more complex and fundamentally challenging single-walled CNT system are presented in this paper, along with the in-plane data for the modulus for sin...

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supporting

confidence: 91%

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Won

Gao

Panzer

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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“…Such a periodic rippling pattern can form spontaneously during growth. [2][3][4][5][6] Ripples can also be obtained by compressing a forest along the axis of the nanotubes, either during growth, 7 or postgrowth. [8][9][10] Iridescence from periodic arrangements of individual vertically aligned multiwalled nanotubes on a substrate [11][12][13] and periodic arrangements forests on a substrate 14 has previously been observed.…”

mentioning

confidence: 99%

“…1͑a͒, which appear similar to previous reports. [2][3][4][5][6][7][8][9][10] Here, ripples form spontaneously during growth, likely due to a growth rate difference between different nanotubes in the forest, 3,4 which in turn causes a compressive strain that bends the nanotubes. It is known that growing nanotubes exert a mechanical force, 7 which likely causes the compressive strain.…”

mentioning

confidence: 99%

Visible iridescence from self-assembled periodic rippling in vertically aligned carbon nanotube forests

Vinten

Jacques

Finnie

2010

Applied Physics Letters

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We observe iridescence in the form of spectrally dispersed white light reflected from the structured sidewalls of vertically aligned carbon nanotube forests. The iridescence is a result of diffraction from a self-assembled periodic rippling pattern on the forest sidewalls that acts as a reflection grating. We measure the grating spacing via white light and laser diffraction experiments and see good agreement with the spacing of the rippling pattern as measured via scanning electron microscopy. The periodic rippling pattern self-assembles during chemical vapor deposition growth of the forests.

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“…We previously used small angle x-ray scattering (SAXS) to nondestructively measure the average diameter and the orientation of the CNTs as a function of position within vertically aligned forests, 35,36 and to develop collective models of growth kinetics; 53,54 however conventional SAXS techniques typically cannot probe the q range below 0.1 nm À1 . 37 USAXS can probe q ¼ 0.001-0.4 nm À1 , enabling features larger than one micron to be resolved.…”

Section: Methodsmentioning

confidence: 99%

Non-destructive characterization of structural hierarchy within aligned carbon nanotube assemblies

Verploegen

Hart

Volder

et al. 2011

Journal of Applied Physics

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Understanding and controlling the hierarchical self-assembly of carbon nanotubes (CNTs) is vital for designing materials such as transparent conductors, chemical sensors, high-performance composites, and microelectronic interconnects. In particular, many applications require highdensity CNT assemblies that cannot currently be made directly by low-density CNT growth, and therefore require post-processing by methods such as elastocapillary densification. We characterize the hierarchical structure of pristine and densified vertically aligned multi-wall CNT forests, by combining small-angle and ultra-small-angle x-ray scattering (USAXS) techniques. This enables the nondestructive measurement of both the individual CNT diameter and CNT bundle diameter within CNT forests, which are otherwise quantified only by delicate and often destructive microscopy techniques. Our measurements show that multi-wall CNT forests grown by chemical vapor deposition consist of isolated and bundled CNTs, with an average bundle diameter of 16 nm. After capillary densification of the CNT forest, USAXS reveals bundles with a diameter >4 lm, in addition to the small bundles observed in the as-grown forests. Combining these characterization methods with new CNT processing methods could enable the engineering of macro-scale CNT assemblies that exhibit significantly improved bulk properties.

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Collective Mechanism for the Evolution and Self-Termination of Vertically Aligned Carbon Nanotube Growth

Cited by 215 publications

References 50 publications

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Zipping, entanglement, and the elastic modulus of aligned single-walled carbon nanotube films

Visible iridescence from self-assembled periodic rippling in vertically aligned carbon nanotube forests

Non-destructive characterization of structural hierarchy within aligned carbon nanotube assemblies

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