Boosting the Performance of on-Chip Vertically-Aligned Carbon Nanotube Supercapacitors Via Solvent-Free Radiofrequency Nitrogen Plasma Treatment
Authors: Li Hung-Wei , Piwek Justyna , Ionescu Adrian Mihai 
Published: February 2024

Supercapacitors are essential components for energy storage devices in many fields, including renewable energy systems, electric vehicles, and portable electronics [1]. Carbon nanotube arrays have shown great promise for use in supercapacitors due to their high surface area and excellent electrical conductivity. However, the specific capacitance of these arrays needs to be increased to improve the overall energy storage capacitance of the supercapacitor.

This study presents an innovative approach to enhance the specific capacitance of on-chip vertically-aligned carbon nanotube (VACNT) structures for the development of high-performance supercapacitors. The proposed technique involves employing a solvent-free radiofrequency (RF) nitrogen plasma treatment to dope nitrogen onto VACNT surfaces. The plasma doping process utilizes reactive ion etching-induced coupled plasma (RIE-ICP) to optimize the parameters for nitrogen incorporation [2]. The nitrogen plasma treatment introduces nitrogen-containing functional groups, thereby improving their specific capacitance. The dry process is compatible with semiconductor fabrication and can be integrated into existing manufacturing techniques [1]. This study provides a promising avenue for tailoring the nitrogen doping process to achieve desired performance enhancements for on-chip VACNT supercapacitors.

The plasma doping process involves the generation of reactive nitrogen species in the plasma, which subsequently interact with the VACNT surfaces. Under the influence of the RF electric field, nitrogen gas molecules are ionized and can react with the carbon atoms of the VACNTs, resulting in the formation of nitrogen-containing functional groups and the substitution of carbon atoms with nitrogen atoms within the carbon lattice [3].

The improved capacitance can be attributed to the incorporation of nitrogen functional groups and to the modification of the electronic properties of the carbon nanotubes, including additional charge carriers, leading to an increase in electrical conductivity [4] and introduction of the pseudocapacitance effect [5]. There are various types of nitrogen species in the carbon lattice, such as pyridinic, pyrrolic, and graphitic nitrogen. Among these, pyridinic and pyrrolic nitrogen are known to be more electrochemically active [5], contributing to the enhanced specific capacitance. The increased electrical conductivity stems from the introduction of nitrogen dopants, which alter the electronic structure of the VACNTs, creating localized states near the Fermi level and thus facilitating charge transport [4].

The level of nitrogen doping depends on several process factors, including the plasma power, gas pressure, and treatment duration. Higher plasma power generates a larger number of reactive nitrogen, while lower gas pressure results in a higher ionization efficiency. Furthermore, a longer treatment duration increases the chance of nitrogen incorporation into the VACNT structures [3]. By carefully controlling these parameters during the nitrogen plasma treatment, it is possible to tailor the nitrogen doping process to achieve the desired performance enhancement for on-chip VACNT supercapacitors.

The treatment is performed in a chamber with a pressure of 10 Pa, a nitrogen gas flow rate of 10 sccm, an ICP power of 200W, and a bias voltage of -120 V. We report a specific capacitance of modified VACNTs of 18 mF/cm2(Figure 1), nearly twice that of pristine samples (Figure1(B)), for dense arrays of 1012 CNTs/ cm2. XPS result confirms 11.76% nitrogen content and 13% oxygen content in treated samples (N-VACNTs), aligning with previous studies on wet process nitrogen-doped carbon nanotubes [5]. Oxygen content also plays a crucial role in VACNT-based supercapacitor performance, improving surface wettability which will reduce the resistance between electrode and electrolyte [6, 7]. However, excessive oxygen content can negatively affect VACNT supercapacitor lifetime. By finding the trade-off between nitrogen and oxygen content, we optimized the doping process, enhancing specific capacitance, stability (Figure 2), and lifetime for on-chip VACNT structures.

In conclusion, our study demonstrates that the solvent-free RF nitrogen plasma treatment can effectively enhance the specific capacitance of on-chip VACNT structures by up to 100%, making them suitable for integration into high-performance on-chip supercapacitor.

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Figure 1