## 3 Results and Discussion
### Film Morphology  
Figure 3 shows the microscale morphology of SWCNT films fabricated with both slow and fast filtration at different volumes. The only change visible with increasing SWCNT suspension volume (and thus SWCNT concentration) is an improvement in surface coverage. It is clear that directional SWCNT alignment was not achieved in any of the samples with the SWCNTs exhibiting random orientations in all of the slow and fast-filtered samples. However, a stark difference is observed in film roughness with slow filtration when compared to fast filtration. Large buckled SWCNT regions are seen in the fast-filtered films whereas these are not present in the slow-filtered films. From this alone, it is likely that slow-filtered films are preferable for use in CNT/Si devices, as smoother films will provide a better contact between the SWCNT film and the silicon substrate. In addition, polarised Raman spectroscopy was performed on slow-filtered films at two different orientations (see Appendix A). This showed no difference in peak intensity when the orientation angle was changed, further confirming the lack of CNT alignment in slow-filtered films. Figure 2: (**a**) Volume loss with time for gravity filtration of 100 \(\upmu\)L aliquot of an arc-discharge SWCNT suspension diluted up to 10 mL with water following brief water filtration under vacuum. (**b**) Flow rate for gravity filtration of 100 \(\upmu\)L aliquot of an arc-discharge SWCNT suspension diluted up to 10 mL with water following brief water filtration under vacuum, horizontal lines indicate the optimum flow rate as published by [PERSON] et al. [34]. (**c**) Optical image of slow-filtered arc-discharge SWCNT film. The grey areas on the white membrane show the presence of SWCNTs. The darker the shade, the denser the nanotubes are on the surface. The void in the film is where a section was removed for testing. Figure 3: SEM images of arc-discharge SWCNTs filtered quickly and slowly at different volumes. As observed from SEM images (Figure 3) the fast-filtered films appeared much rougher on the microscale than slow-filtered films. The smoother a SWCNT film, the better contact it will make with the silicon surface and will thus lead to an increase in photocurrent production. The roughness of the film was analysed using AFM. Two areas on a film of each filtration type were imaged by AFM and the images are shown in Figure 4. Additionally, the root mean square average height deviation (Rq) and the arithmetic average of the surface height deviations (Ra) were taken from the image and are shown in Table 1, together these are quantitative measurements of the roughness of the film. Both the AFM images themselves and the quantitative roughness data and Z-Range demonstrate clearly that the slow-filtered films are significantly flatter than fast-filtered films. The Rq values and Ra values for fast-filtered film were both 3-4 times higher than the slow-filtered film. Additionally, the Z-Range was also roughly 3 times higher. By observing the AFM images, large ridges can be seen in the fast-filtered film which dwarf the highest of'mounds' visible in the slow-filtered film. These ridges are a product of the turbulent water flow caused by the vacuum filtration method and are the cause of the higher Z-range, Rq, and Ra values. Slow and fast-filtered films were produced for use in CNT/Si devices. Both sets of films were produced with the same volume from the same SWCNT suspension. However, as slow filtration is \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Image Z Range (nm)** & **Image Rq (nm)** & **Image Ra (nm)** \\ \hline Fast Film Spot 1 & 248.9 & 31.6 & 23.3 \\ Fast Film Spot 2 & 307.7 & 40.1 & 28.9 \\
**Average** & **278.3** & **35.85** & **26.1** \\ Slow Film Spot 1 & 84.7 & 10.7 & 7.54 \\ Slow Film Spot 2 & 94.6 & 11.6 & 8.25 \\
**Average** & **89.65** & **11.15** & **7.90** \\ \hline \hline \end{tabular}
\end{table}