Effect of morphology of supporting alumina films on the synthesis of graphite-capped, vertically aligned carbon nanotube arrays

Abstract

We investigate the effects of thickness and roughness of supporting alumina film on the growth of graphite-capped, vertically aligned carbon nanotube (CNT) arrays (hereafter, referred to as “composite”). To prepare various thicknesses of cobalt catalyst or alumina film, we employed a combinatorial masked deposition (CMD) method. The growth was carried out by thermal chemical vapor deposition at 670 °C using ethanol vapor as a carbon precursor. With the increase in alumina thickness, CNT arrays and those with discontinuous (“intermediate”) and continuous (“composite”) graphite top layers are synthesized. In the case of thin alumina layers, catalytic particles are easily deactivated through penetration into the alumina film, leading to CNT arrays or intermediate structures. Rough alumina layers tend to give CNT or intermediate, because of the existence of very thin catalyst layers by the shading effect during the oblique deposition. The rough surface also provides recessed areas for the thick catalyst case, where protrusions are created in the catalyst layers, acting as nuclei of CNT growth under the top graphite films. The CNT alignment is analyzed using a Herman’s orientation factor to clarify the areal density of CNT arrays. On the basis of the above results, we propose a growth model for the composite.