Synergetic carbon nanotube growth Jason M. Parker * , H.-S. Philip Wong Department of Electrical Engineering and Center for Integrated Systems, Stanford University, Stanford, CA 94305, United States ARTICLE INFO Article history: Received 17 January 2013 Accepted 28 May 2013 Available online 5 June 2013 ABSTRACT We report an effect in vertically-aligned carbon nanotube growth in which small catalyst features in proximity to large features show an enhanced growth rate. We apply this so- called ‘‘synergetic growth’’ effect in micrometer-scale patterns to produce vertical-like growth horizontally. This approach enables carbon nanotube integration into device appli- cations requiring a fair degree of carbon nanotube alignment. The synergetic growth effect corroborates growth mechanisms describing carbon nanotube growth as a more complex process than elemental carbon supersaturating metal catalyst. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Two decades after the landmark paper by Iijima [1], the mech- anism for the growth of carbon nanotubes (CNTs) is still de- bated. This picture is complicated by the wide range of growth methods available, from arc discharge [2] to a high- pressure carbon monoxide process [3] to chemical vapor deposition (CVD) processes [4–10]. A CNT growth model sim- ilar to the vapor–liquid–solid (VLS) nanowire growth model, which describes growth as a diffusion of elemental material into metal catalysts followed by precipitation out, has had the earliest support [2,11–13]. However, evidence that the car- bon degradation and subsequent assembly into CNTs is a sur- face reaction and is intermediate-dominated refutes some claims of the VLS-based model [14–19]. Not only does the growth of vertically-aligned CNT (VA-CNT) forests in unique patterns provide insight into some of the questions about growth mechanism, but also these patterns provide a means to take advantage of high yield growths. Reports about VA- CNTs 10–1000 s of microns tall suggest that van der Waals forces [6,20] and/or an interlaced top ‘‘crust’’ layer providing mechanical support [20,21] promote vertical growth. Demon- strations of patterned VA-CNTs have shown single-walled CNT (SWCNT) sheets of 5–10 lm · 1+ cm and pillars 300 lm in diameter [4], SWCNT pillars 600 lm in diameter and honey- combs with 700 lm holes [5], multi-walled CNT (MWCNT) pil- lars 3–20 lm in diameter and complex patterns [6], MWCNT cylindrical pillars 10 lm in diameter [7], and MWCNT pillars, strips, and intertwined shapes of feature size 2–500 lm and their resultant length dependence [10]. The work done by Jeong et al. [10], in particular, provides a thorough and insight- ful look at the CNT length dependence on pattern size and pitch and is complementary to some of the catalyst pattern- ing experiments performed in this work. In CNT synthesis, if vertically-aligned growth is not nucleated, CNT lengths are severely stunted [20,22] and the result is attributed to ste- ric hindrance [6]. An areal density of up to 14,000 lm 2 is seen in these types of VA-CNT forests [23]. Processing this vertical- type growth into lateral designs can provide an avenue for their integration into device applications requiring a fair de- gree of CNT alignment, as has been demonstrated in exam- ples of transparent films [24] and crossbar arrays [25]. This paper presents a synergetic effect in VA-CNT growth much like that in the previously-reported synergetic nano- wire growth [26]. This effect can result in long, vertical-like CNT growth in features lacking the usual structural stiffness required for a vertically-aligned forest. Although this pro- cess for CNTs is named ‘‘synergetic growth’’ in order to par- allel the similar effect in nanowire growth, it is noted that this naming convention is not meant to draw an exact par- allel between semiconductor nanowire and carbon nanotube growth mechanisms. In fact, a discussion of the increasing evidence of the strong role for polydisperse, gas-phase pre- cursor diffusion in CNT growth is carried out in this work. 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.05.053 * Corresponding author. E-mail address: jaypark@stanford.edu (J.M. Parker). CARBON 62 (2013) 61 – 68 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon