CNMS RESEARCH

Atomistic Branching Mechanism for Carbon Nanotubes: Sulfur as the Triggering Agent

Jose M. Romo-Herrera [CNMS User, Institute for Scientific and Technological Research of San Luis Potosi (IPICYT)], Bobby G. Sumpter (CNMS Staff), David A. Cullen (Arizona State University), Humberto Terrones (CNMS User, IPICYT), Eduardo Cruz-Silva (CNMS User, IPICYT), David J. Smith (ASU), Vincent Meunier (CNMS Staff), Mauricio Terrones (CNMS User, IPICYT)

Achievement

Experimental realization of nanonetworks is challenging, and the most efficient and viable route appears to be by self-assembly. Although a number of chemical elements seem to play a key role in the formation of network nodes, little has been known about their role in microscopic processes governing branch formation. In this research, the role of sulfur during nanotube network growth was systematically studied, using a combination of theoretical and experimental techniques. Small concentrations of sulfur were shown to be sufficient to promote the formation of carbon heptagons during nanotube growth, and eventually to trigger the appearance of branching in nanotubes with stacked-cone morphologies. The network samples were characterized using high-resolution electron microscopy and detailed elemental analyses. The presence of minute amounts of sulfur was observed at the branching point between straight nanotube sections. These findings were rationalized and explained using molecular dynamics and other simulations based on density functional theory.

Significance

The controlled assembly of elongated nanostructures into ordered micronetworks constitutes a key challenge in nanotechnology. These types of carbon structures are of tremendous interest due to their fascinating mechanical and electronic properties that could be used in the fabrication of nanodevices. This research demonstrated that sulfur plays an important role in the formation of branched nanotube networks with stacked-cone morphologies. The growing branches possess minute amounts of sulfur that are sufficient to promote the formation of heptagons (negative curvature) and pentagons (positive curvature). This approach can be expanded to include other chemical species to further elucidate growth of carbon nanostructures.

Publications: Angewandte Chemie, 47(16), 2948-2953 (2008); and Nano Lett. 7, 57 (2007)

The theory/modeling for this research was conducted at the Center for Nanophase Materials Sciences, which is sponsored by the Division of Scientific User Facilities, U.S. Department of Energy. This work is part of Jose Romo-Herrera’s PhD thesis, completed in part under the supervision of Vincent Meunier at CNMS (ORNL).

Figure 1: Theoretical calculations explaining the role of sulfur at the atomic level.
a) Snapshots from quantum molecular dynamics simulations, showing the effect of widening the CNT diameter in the presence of sulfur. b) Stacked-cone structure promoted by sulfur widening the CNT. The angle caused by the heptagons ((a), at 120 ps) is very close to those observed in the stacked-cone morphologies (b). c) The position of a sulfur atom within a carbon nanotube lattice marked to indicate if it is in a pentagon (red), hexagon (blue), or heptagon (green), and d) the total energy for the resulting system.

Figure 2: a) Low-magnification TEM pictures showing that every time a branch opening occurs, there is a direction change of the main stem (indicated by arrows and white dots). b) The final Y-junction, along with high-resolution views of the structure at the developed arms (panel B), and right at the angle between them (panel A). c) Structure of the developing branch at the graphene layer level. Inset: Fast Fourier transform calculation from the image, allowing the angle between the stacking direction of the graphene layers to be measured (34.5 °: stacking change marked by left dashed line on HRTEM image, 37 °: stacking change at the right dashed line).

Atomistic Branching Mechanism for Carbon Nanotubes: Sulfur as the Triggering Agent**

José M. Romo-Herrera (CNMS User, IPICYT), Bobby G. Sumpter (CNMS Staff), David A. Cullen (ASU), Humberto Terrones (CNMS User, IPICYT), Eduardo Cruz-Silva (CNMS User, IPICYT), David J. Smith (ASU), Vincent Meunier (CNMS Staff), Mauricio Terrones (CNMS User, IPICYT)

Going out on a limb: A combination of theoretical techniques, high-resolution microscopy, and energy-dispersive X-ray spectroscopy shows the role sulfur plays in branching phenomena during carbon nanotube (CNT) network growth. A model is proposed in which small amounts of sulfur are enough to trigger the growth of a bud in a CNT, leading to kink formation and subsequent branch growth.

**Section Cover from Angewandte Chemie along with highlight provided by the journal.