Carbon dioxide-assisted synthesis of single-walled carbon nanotubes and their thin film properties

Abstract

Owing to their unique chemical and physical properties, single-walled carbon nanotubes (SWCNTs) have attracted increasing attentation in various fields. The properties of SWCNTs strongly depend on their chirality and geometry. Thus, to realize the applications in desired fields, it is of significant importance to tune the chirality and geometry of SWCNTs. The floating catalyst chemical vapor deposition (FC-CVD) method, as a dry and continuous process, has been widely used in academic and industrial fields. However, tuning the growth of SWCNTs in FC-CVD is a challenge.

In this thesis, first of all, certain amounts of carbon dioxide (CO2) is introduced to tune the growth of SWCNTs in a FC-CVD reactor, where carbon monoxide (CO) is used as carbon source and ferrocene as catalyst precursor. We found that the SWCNT thin films display different colors with various CO2 concentration, specifically, a green and brown colors are observed . The optical absorption spectrum of the green film shows a distinct absorption peak in visible range. Further analyzing the chirality by electron diffraction reveals that the green film possesses a very narrow chirality distribution near armchair. Besides the chirality, we also studied the geometry, such as tube diameter, bundle length and diameter, modulated by CO2. The SWCNT diameter and bundle length are found to increase with CO2 concentration. In addition, the yield and bundle diameter are also affected by CO2.

Accordingly, SWCNT thin film prepared with certain CO2 concetration displays a remarkably reduced sheet resistance. Therefore, employing CO2 offers new strategy to tune the chirality and geometry of SWCNTs in FC-CVD. Furthermore, we also deposited our aerosol SWCNTs on wafer-scale substrates by large-scale thermophoretic precipitator (TP), such SWCNT thin films display ideal uniformity and conductivity. The scale-up deposition method is applicable in industrial productions of SWCNT transparent conductive films.

To futher reduce the sheet resistance of SWCNT thin films, we then deposited SWCNTs on graphene by TP. The SWCNTs/graphene hybrid film exhibits improved conductivity. By measuring the temperature dependent conductance, we found that due to the presence of graphene, the tunnelling barrier between tubes has been reduced, which enhances the carrier tunnelling efficiency and thus, improves the conductivity.