Fei Wei’s team from the Department of Chemical Engineering achieve the synthesis of 99.9999% pure semiconducting carbon nanotube arrays in one step
On October 2nd, Professor Fei Wei's team from the Department of Chemical and Engineering in Tsinghua University published an article "Rate selected growth of the ultrapure semiconducting carbon nanotube arrays" in Nature Communications. The paper points out that the atomic assembly rate of carbon nanotubes (CNTs) in the growth process is locked to their bandgap. The quantity of metallic CNTs decreases much faster than that of semiconducting CNTs during the natural CNT growth process. It facilitates a spontaneous separation of the ultrapure semiconducting CNT arrays in one step when the CNT length extends to 154 mm. This method has provided a new technical route for the world-wide challenge in synthesizing perfectly assembled, highly pure semiconducting CNT arrays, which is of great value to the novel carbon-based electronic materials.
With the rapid development of information technology, semiconductor chips have become an important foundation of digital economy and national security. In recent years, Moore's Law based on silicon is approaching its end. Among many alternative materials, CNTs have become the ideal candidate for the new-generation chip electronics due to their nanoscale size and high carrier mobility.
China has significant advantages in the engineering application field of CNT electronics and material production, and has made many original contributions in this field, especially in non-doping electronics in single CNT transistors and minimum CNT devices. In the field of macroscale synthesis of CNTs, it also has the world's highest output with kiloton agglomerated and vertically aligned CNTs, which have been applied in batteries on an industrial scale. However, the structural defects and band structure control of CNTs are still key problems restricting the application of high-performance carbon-based chips.
Regarding the above issues, Professor Fei Wei's team has focused on developing perfectly-assembled ultralong CNTs for more than 10 years. They firstly produced chiral-consistent ultralong CNTs with meter scale length and achieved the world's longest 550 mm CNT. Furthermore, they verified that the quantity of aligned CNTs exponentially decreases with CNT length, following the Schulz-Flory distribution. This work deeply analyzed the subparts of all the CNTs, namely metallic and semiconducting CNTs. They fit with the Schulz-Flory distribution, but the half-attenuation length (inverse to the decay rate of quantity with CNT length) of semiconducting CNTs is 10 times longer than that of metallic CNTs. Characterizations involving Raman scattering, Rayleigh spectra and isotope labeling all demonstrate that the difference in half-attenuation length is caused by the bandgap-determined growth rate.
Theoretically, it is important to narrow the activation energy difference between the diffusion and poisoning processes in non-homogenous catalysis. This is because it is not only the key point to improve the CNT length, but also a critical step in producing semiconducting CNT arrays with a narrower bandgap distribution. Accordingly, the team have designed a layered rectangular reactor in order to precisely control the gas flow and temperature field. By optimizing the structure of the temperature-constant zone, they have lowered the catalyst inactivity probability to one billionth and successfully produced horizontally aligned CNTs on seven 4-inch silicon wafers. The maximum length of the CNTs can be up to 650 mm and the turnover frequency per unit active site reaches 1.53 x106 s-1.
The device fabricated with 154 mm CNT arrays delivered a high on/off ratio of 108, mobility of 4000 cm2/Vs and a current density of 14 mA/mm, demonstrating the excellent electrical performances of ultralong CNT arrays for the first time. This strategy of bandgap mediated growth rates has provided a novel route for the spontaneous separation of highly pure semiconducting CNTs. It also lays a solid foundation in channel materials for the development of high-end carbon-based electronics.
This work is another creative piece from Professor Fei Wei's team, followed by the synthesis of half-meter long CNTs and self-assembled CNT tangles with consistent chirality. All these pieces of work have revealed a feasible route to achieve the applications of CNTs in high-end electronics, which will contribute remarkably to the national microelectronic industry.
The corresponding author of this article is Professor Fei Wei. The first author is Zhenxing Zhu, PhD candidate in the Department of Chemical Engineering from Tsinghua University. Other coordinators include Nan Wei, postdoc in the Department of Applied Physic from Alto University, Professor Jun Xu and Dr. Weijun Cheng from the Department of Microelectronics in Tsinghua University, Associate Professor Yao Wang, Assistant Professor Rufan Zhang, and Ph.D. students Boyuan Shen, Silei Sun and Gao Jun from the Department of Chemical Engineering. The research was funded by the National Key Basic Research and Development Program, the National Natural Science Foundation Commission and the Beijing Science and Technology Commission.
