Invited
TOWARDS THE GROWTH OF 3D FORESTS OF CARBON NANOTUBES: SELECTIVE HEIGHT CONTROL USING RESERVOIRS AND OVERLAYERS

Despite massive progress achieved in the growth of carbon nanotube (CNT) forests on substrate, besides lithographic patterning of the catalyst, little has been done to selectively (locally) control CNT height. We will show how we can locally control CNT height, from no CNTs to up to 4X the nominal CNT height from iron catalyst on alumina underlayer by patterning reservoirs or by using overlayers during annealing or growth.

We will show how an iron thin film reservoir placed below the alumina underlayer can almost double CNT height while a copper/silver thin film reservoir placed under the alumina underlayer deactivates the iron catalyst. We will also show how a thin film reservoir of molybdenum enhances CNT growth by a factor of 4X. In short, by patterning the reservoir material, we can modulate four different CNT heights during the growth process: no growth (Cu/Ag reservoir), nominal growth (no reservoir), 2X growth (Fe reservoir), and 4X growth (Mo reservoir).

Additionally, we will show how a copper or nickel overlayer (stencil or bridge) placed above the catalyst surface during pre-annealing or during CNT growth deactivates the catalyst. With this technique, we can pattern regions with CNTs and without CNTs by simply annealing the sample with a patterned overlayer positioned above its surface to obtain patterned CNT forests using a simple process, without the need for lithography.

We will show examples of growth with patterned regions of CNTs with different heights. This modulation of the CNT height is a significant improvement compared to the "CNTs (one height) / no CNTs" patterning that has been achieved using lithography of the catalyst, and moves us closer to building 3D architectures of CNTs.