Simple use of SiO₂ film thickness for the control of carbon nano-tube diameter during ferrocene catalyzed CVD growth

Selective growth of carbon nano-tubes (CNT) on micron scale patterned substrates has been accomplished by taking advantage of the non-reactivity of ferrocene catalyst on H-terminated Si surfaces in a CVD process. Demonstrated here is that this phenomenon can be used to control the diameter of CNTs when sufficiently narrow lines of SiO₂ surrounded by H-terminated Si are used. Narrow lines of SiO₂ (12-60nm) are formed at the etched face of a Si/SiO₂/Si multilayer structure. This allows the precisely controllable thickness of an SiO₂ film to determine an exposed SiO₂ line width. There is no need for e-beam lithography since film thickness determines nm-scale line dimensions. CNTs are then formed by CVD with a ferrocene/H₂/Ar mixture at 700°C. CNTs are observed to grow only on the exposed SiO₂ surface at the edge of the 'mesa' structure. CNT diameters of 13.2, 20.5, 34.2, 64.3nm are observed for SiO₂ film thickness of 12, 19, 35, and 65 nm. The larger distribution of CNT diameter with increased line width is consistent with wider SiO₂ linewidths not being able to affect smaller nucleation centers. These results are consistent with the use of self-assembly chemistry of iron catalyst onto nanoparticles of catalyst support.