The Si pyramids are generally clean and fairly uniform in size and density. The PECVD growth of the MWCNTs was performed on both pyramidally SHP099 cost structured and flat silicon substrates (Figure 1b,c). The MWCNTs were found to always grow perpendicularly to the substrate surface Wnt inhibitor either on the sides of the Si pyramids (as shown by the cross-section SEM view of Figure 1b) or on the untreated flat Si substrates (Figure 1c). This vertical alignment of the MWCNTs with respect to the substrate surface

is a consequence of appropriate electrical biasing of the substrate during the plasma growth process (Bower et al. [22]). The growth of MWCNTs was performed under the same PECVD conditions on all the silicon substrates (with various AR values) in order to obtain nearly identical density and morphology of emitters, facilitating thereby their comparison. The SEM images of Figure 1b,c confirm, to a certain extent, the similarity of the MWCNTs whether on Si pyramids or on flat Si substrates. One can nonetheless notice that a minority of

MWCNTs protrude from the main nanotube forest (Figure 1b,c). Those protruding emitters, due to their position above the CNT forest canopy, undergo higher electric fields during the FEE measurements. Figure 1 Typical SEM images. (a) Pyramidal texturing of the Si (100) substrates after their KOH chemical check details treatment; (b) illustration of the PECVD grown MWCNTs on a silicon pyramid; (c) vertically aligned MWCNTs grown by PECVD onto untreated, flat Si (100) substrate. Figure 2a

shows typical J-E curves of the developed hierarchal MWCNT cathodes as a function of the AR of the Si pyramids, while comparing them to that of the MWCNTs grown on flat silicon (AR = 0), used here as a non-KOH-treated reference cathode. It is clearly seen that the pyramidal structuring of the cathodes has a significant effect on their FEE performance. Firstly, the inset of Figure 2a shows that as the AR of the Si pyramids is increased, from 0 (flat Si) to 0.6, the J-E curves are seen to shift progressively towards lower electric field values, indicating a clear decrease of the TF. This TF reduction Phospholipase D1 is thought to be a consequence of the hierarchal structuring of the cathodes as the onset of electron emission occurs at the apex of the pyramids where higher fields are felt by the MWCNTs (Saito & Uemura [3]). Secondly, the J-E curves of Figure 2a show that the emitted current density significantly increases as the AR is increased from 0 to 0.6. Indeed, for an electric field of 4 V/μm for example, Figure 2b shows that the current density exponentially increases with the AR. This pyramidal texturing-induced enhancement of the current density is believed to be due to a higher number of MWCNT emitters because of the 3D structuring of the cathodes, which provides larger surface area and lesser screening effect on the pyramid sides.