Horizontal alignment of chemical vapor-deposited SWNTs on single-crystal quartz surfaces: Further evidence for epitaxial alignment

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 (0111), 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.