Single-walled carbon nanotubes (SWNTs) are promising building blocks for next-generation high-performance electronics, e.g. field-effect transistors (FET). For the application, many properties, such as chirality, orientation, density and length, have to be well controlled. Despite significant progresses in controlling the SWNT chirality and density in recent years, further breakthroughs are desirable to make the SWNT-based electronics reality. Detailed understanding on the growth process of individual SWNTs with a practical form (quality, length, and alignment) will accelerate the development of application-oriented growth. In this study, we developed an isotope labeling technique with digital coding for the ex situ monitoring of individual and long (>100 μm) SWNTs. Owing to the nature of ex situ measurement, an enormous number of SWNTs can be characterized on a single substrate. In addition, digital-coding of the isotope labels enables identification of time evolution of SWNT growth with high accuracy.

Horizontally aligned configuration were used for the tracing of individual SWNTs and for the breakdown of SWNT quantity into population and length [1]. Aligned SWNTs were grown on r-cut quartz substrates from iron catalysts patterned in stripes with 300-μm separation. Normal ¹²C ethanol was used for most parts of the growth, but pure ¹³C ethanol was periodically mixed in various ratios (33, 67, 100 %) to embed digital-coded isotope labels in SWNTs. For the standard growth, 5-sccm ethanol was introduced together with 50-sccm Ar/H₂ buffer gas into the reactor with total pressure of ~1.5 kPa. After the growth, position of each isotope label and the SWNT properties were identified by Raman mapping.

The sequence of the isotope labels clearly indicated that all the SWNTs that we observed grew in the root growth mode [2], where the catalysts stayed at the original position and SWNTs pushed forward. We obtained the time evolution of SWNT lengths, and found that the SWNTs started to grow after certain lengths of incubation time. For most cases, growth rate was constant until the abrupt termination of the growth. This suggests that the catalytic activity of metal particles did not gradually change with time under the current growth condition. Two different SWNTs with the same chirality (radial breathing mode (RBM) frequency of 190 cm^-1 with 532 nm excitation laser) had different growth rates (~8 and ~15 μm/min), indicating the dependence of the growth rate on catalyst sizes or some other properties. For the extraction of intrinsic chirality dependence of the growth rate, we focused on spontaneously formed intramolecular junctions. For example, an intramolecular junction changed the RBM frequencies of an SWNT from 139 to 144 cm^-1. Interestingly, the growth rate changed from ~10 to ~8 μm/min along with the chirality change.

Since the growth behavior can be traced along individual SWNTs, the dependence on growth conditions, such as temperature and pressure of carbon feedstocks, was also investigated by changing the conditions during the growth. First, ethanol flow rate, which is nearly proportional to the partial pressure, was increased from 3 to 9 sccm in 2 sccm steps every four minutes. Most SWNTs grew faster as the flow rate increased; however, some SWNTs stopped growing for one or two minutes when the flow rate was suddenly increased. We also observed temperature-dependent growth rate, which provides insights into the growth dynamics of individual SWNTs.

Part of this work was supported by JSPS KAKENHI Grant Numbers JP25107002, JP15H05760, JP17K06187 and JP17K14601. K.O. was financially supported by a JSPS Fellowship (JP15J07857).

References:

[1] T. Inoue, D. Hasegawa, S. Chiashi, and S. Maruyama, J. Mater. Chem. A 3, 15119 (2015).

[2] H. Ago et al., J. Phys. Chem. C 112, 1735 (2008).

[3] M. Ouyang et al., Science 291, 97 (2001).