Horizontal Alignment of Chemical Vapor-Deposited SWNTs on Single-Crystal Quartz Surfaces: Further Evidence for Epitaxial Alignment

Abstract

It has been well documented that single-crystal stable-temperature (ST)-cut quartz substrates can horizontally align single-walled carbon nanotubes (SWNTs) along the x direction ([100]) during catalyzed chemical vapor deposition. It has been suggested that alignment is due to either surface-guided growth along step edges and/or lattice directions, although recent atomic force microscopy (AFM) evidence suggests the latter is more appropriate [Liu, et al. J. Am. Chem. Soc. 2008, 130, 5428]. In light of this we explore the growth mechanism on single-crystal quartz in more detail, focusing particularly on the crystallography of the alignment surface and the effects of annealing treatments. X-ray diffraction (XRD) studies enable us to identify, for the first time, the crystallographic plane of ST-cut quartz as (011 j 1), with a surface normal lying close to [121], which is 42.5°from the y axis ([010]). For this plane, formulation of both crystallographic orientation and polar diagrams, the latter which reveals the magnitude of the longitudinal piezoelectric effect, indicate a high degree of correlation between the preferential SWNT growth direction and both the atomic arrangement and piezoelectric effect, strongly supporting a crystal plane driven alignment process. This is further supported by the absence of steps on the surface, observed topographically using AFM. Further evidence for crystal plane controlled alignment comes from field emission scanning electron microscopy (FE-SEM) and AFM observations of bending of the SWNTs away from the preferred alignment direction at defined angles. These angles are in agreement with the Si-O-Si atomic bond angles in quartz. Additionally, surfaces which are shown by XRD to have either minimal distortion in the lattice or surface misorientation caused by, e.g., tilts, cracks, etc., are those that are found to align the SWNTs best. Improvement in surface crystallinity can be achieved by annealing in air at high temperature; optimal annealing conditions of 950°C in air for 30 mins were determined, in agreement with XRD data and FE-SEM images.

A Comparative Study of Cobalt Catalyst Behavior between Silica and Quartz Substrates for Single-Walled Carbon Nanotube Growth

“…In our case, even under optimal growth conditions it was not possible to obtain 100% perfect alignment of SWNTs. The obvious disruption in alignment significantly and typically occurs at the early growth stage before SWNTs find the preferable direction or when they grow to such a length that they encounter other catalyst particles, nanotubes or defects on the surface in growth area [23]. The shorter nanotubes shown in Fig.…”

“…By contrast, on quartz, straightly aligned SWNTs with a lower density were successfully produced. The strong preference for the SWNT alignment was in the x direction [211 0] due to anisotropic van der Waals interactions between SWNTs and the ordering arrangement of atoms (either Si or O atoms or SiO 4 reactive sites) on the crystal surface [18,19,23]. Besides, on quartz substrates, there was a certain portion of SWNTs showing some deviation from the preferential growth direction.…”

Recent Advances in Directed Assembly of Nanowires or Nanotubes

“…Till now, researchers have developed a variety of NW/NT assembly methods on various substrates, which can generally be categorized as two strategies [1][2][3][4][5][6][7][8][9]: (1) Alignment-after-Growth, which means, the NWs are assembled on substrates after synthesis; (2) Alignment-during-growth, which means, the NWs are assembled and aligned directly during the synthesis process. For example, by pre-defining the locations of catalyst and growth space of the NWs [10][11][12][13], applying electric field in the seeded growth system [14][15], applying directional air flow in the growth system [16][17], or directly on a surface based on physical atomic structures [18][19][20][21][22][23][24][25][26][27][28], by electrospinning [29][30][31], or superaligning [32][33], NWs can be aligned as grown simultaneously. However, these approaches are limited to a few types of materials and relatively simple structures, and the uniformity of the grown NWs is hard to control.…”

Improvement in alignment of single‐walled carbon nanotubes grown on quartz substrate

“…In order to obtain SWNTs with minimal misalignment, annealing of the substrate at about 900°C is required before the growth of SWNTs [5,9,10]. However, misalignment and kinks of SWNTs has been observed in magnified images even when a pre-annealing process was carried out [11,12].…”