Direct Growth of Vertically Aligned Carbon Nanotube Arrays on Stainless Steel and Their Field Emission Properties*

We report the growth of VACNT arrays on stainless steel (SS) via dc plasma enhanced chemical vapor deposition without the use of a metal catalyst layer. TEM and AFM examinations revealed the occurrence of nano-hills formed on the SS surface during the heating process in an NH3 environment, which is a critical step for the uniform growth of VACNTs. The particles on the tips of most of the VACNTs were found to be single crystalline Fe metal, although a few other VACNTs were found to have alloy of Fe, Ni, Mn, and Cr at their tips. Field emission (FE) performance of a dense array of VACNTs was enhanced by changing the array morphology through a simple water treatment process. The FE performance was further enhanced by coating the exterior of CNTs with a layer of crystalline SnO2 nanoparticles of 4.18 nm in diameter. The enhancement of FE performance was ascribed to the morphological change and mechanical strength reinforcement of VACNTs by the SnO2 coating process. The characteristic features of the sample as the conductive substrate, ohmic contact between the VACNTs and the substrate, and bundled morphology resulted in a lower turn-on and threshold electric field, higher field enhancement, and excellent emission stability.