Vertically-aligned multiwalled carbon nanotubes (VA-MWNTs) are of interest to many fields due to their possession of both the intrinsic properties of carbon nanotubes (CNT) and the ordered vertical architecture of the nanostructure. In particular, the presences of vertical alignment within the nanostructure has been shown to enhance the material’s performance. Current techniques for covalent modification of CNTs are dominated by chemical approaches. These approaches are ill-suited for modifying VA-MWNTs, as the need for dispersion disrupts the nanostructure by destroying the vertical alignment. Thus, despite the many advantages of VA-MWNTs, the lack of non-disruptive modification techniques have impeded their application.

We present a novel approach for modifying VA-MWNTs, which uses a cold atmospheric pressure plasma jet (APPJ). Apart from being able to modify the VA-MWNTs, the confined nature of the plasma allows the modification to be spatially defined. The usefulness of the described plasma treatment was demonstrated when the modified CNTs were decorated with SERS-active gold nanoparticles (Au NPs). In particular, we showed that the decoration transforms the VA-MWNTs into a 3-D substrate capable of SERS.

VA-MWNTs were grown on a Si substrate with a layer of Fe catalyst by thermal CVD with C₂H₂ as the gaseous carbon source. The APPJ was scanned across the synthesised VA-MWNTs for 30 seconds (at 0.33 mm/s). An aqueous solution containing Au NPs was dropped onto the modified region and dried for 12 hours. After being decorated, the VA-MWNTs were immersed in a solution of 4-aminothiophenol (4-ATP) for 4 days.

After the sample was treated by the APPJ, a localised change in wettability was observed. This change in wettability allowed a solution of Au NPs to be dispersed within the treated region. SEM and optical microscopy revealed that within the VA-MWNTs, Au NPs decorated the MWNTs individually and the vertically-aligned architecture was preserved. Furthermore, the Au NPs were confined to the modified region and were not observed at locations greater than 4 μm from the boundary of the modified region.

The decorated VA-MWNTs were exposed to 4-ATP to determine their SERS response. The Raman spectra revealed the decorated VA-MWNTs were capable of supporting surface plasmons and responding to the 4-ATP. SERS response was absent in the untreated regions.

In summary, we have demonstrated the effectiveness of using APPJ to modify VA-MWNTs. The advantages of our technique lie in the preservation of the nanostructure of the VA-MWNTs and the spatial confinement of the modification despite the absences of any stencils or masks. Furthermore, our approach is significantly faster and simpler compared to conventional chemical approaches. What once required several hours and multiple steps could be achieved in a single 30 second step. The APPJ was able to define the region of modification to a line width of 2.000 ± 0.004 mm. This spatially-defined modification enables patterning by decoration of particular regions with Au NPs. The presence of the Au NPs transformed the VA-MWNTs into a 3-D SERS substrate. Thus, the work presented opens up a new avenue in creating functional materials based on CNT arrays.