(Invited) Structures of Iron and Cobalt Bimetallic Clusters for CVD Growth of Single-Walled Carbon Nanotubes

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Metal catalysts play a critical role in the growth of single-walled carbon nanotubes (SWCNTs). Bimetallic catalysts have been often employed for efficient growth or chirality-selective growth. Iron (Fe) and cobalt (Co) alloys are often resulting in efficient growth. We reported a classical molecular dynamics (MD) result of Fe-Co clusters exhibited Fe 42 Co 13 Mackey structure with super-high melting point. In this symmetric structure, Co atoms are located at the center and 12 vertices of the outer icosahedron (v-Mackey) [1]. Later, we found this special cluster was found due to the peculiarity of the Fe-Co potential we employed. Hence, we are looking for other metal pairs for the possible v-Mackey clusters based on DFT-based machine learning potential. This is exactly the well-studied bulk synthesis CVD condition [2] when Fe:Co ratio is 50:50. In experiments, we started the exploration of Fe-Co system by extensively varying the Fe-Co ratio and a wide range of pressures and temperatures for the efficient growth through the chemical vapor deposition (CVD) process. Since we are interested both in chirality-specific growth and efficient growth, we used the zeolite-based alcohol CVD technique and employed absorption and photoluminescence spectroscopy for extracting the chirality-specific growth efficiency. We found 2 interesting experimental conditions through these experiments. At relatively low temperature of 600 °C, pure Co exhibited highest catalytic activity compared with Co-Fe alloy. This reminds us of the efficient growth of pure Co CVD in our previous report [3]. On the other hand, Fe 0.75 Co 0.25 was a highly efficient binary catalyst at 850 °C. We extended the absorption and photoluminescence spectroscopy by employing the 2D fitting of photoluminescence maps [4]. Furthermore, we found similar CVD growth conditions for SiO 2 substrate which resemble the zeolite system, through careful Raman analysis. Then, we performed careful transmission electron microscope (TEM) analysis of metal catalyst on SiO 2 TEM grid. At the same time, spin-polarized DFT based on-the-fly NEP machine learning potential was developed for the Fe-Co system, and cooling down MD simulations are extensively performed for stable metal cluster structures. The special Fe 42 Co 13 Mackey structure was not observed in TEM experiments nor MD simulations. Instead, TEM and Energy Dispersive X-ray Spectroscopy (EDS) analyses reveal that higher SWCNT yields correlate with the formation of uniform sized Fe-Co catalyst particles with surface segregated Co, consistent with MD simulations. Finally, extensive MD simulations results are summarized as a kind of phase-diagram shown in Fig. 1 [5]. Special behavior of Fe 0.75 Co 0.25 ratio observed for CVD of 850 °C was characterized as especially small and uniform cluster size by TEM and explained in the machine-learning MD based phase-diagram. References [1] S. Maruyama, "Super Mackay cluster Fe 42 Co 13 for chirality selective growth of single-walled carbon nanotubes," NT23 , Arcachon, (2023). [2] B. Hou, et al., Carbon , 119 , 502 (2017). [3] M. Liu, et al., Carbon , 146 , 413 (2019). [4] S. Cambré, et al., Nanoscale Horiz. , 10 , 3405 (2025). [5] Q. Hu, D. Hedman, Y. Feng, W. Dai, D. Asa, A. Fitό Parera, Y. Yao, Y. Zheng, K. Hisama, G. Auti, H. Daiguji, S. Chiashi, D. Levshov, W. Wenseleers, K. Otsuka, Y. Li, C. Bichara, S. Cambré, R. Xiang, S. Maruyama, (2025), to be submitted. Figure 1

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