The effects of temperature profile on the growth of vertically-aligned multi-walled carbon nanotubes

Summary form only given. Vertically-aligned multi-walled carbon nanotubes (VAMWNTs) have received extensive attention recently. The chemical vapor deposition (CVD) has been regarded as a primary candidate for the large-scale production of vertically-aligned nanotubes [1-4]. It is well known that the structural characteristics of VAMWNTs grown by CVD depend on growth parameters such as reaction time, catalyst morphology, temperature, reaction gas flow rate and pressure. Although considerable amount of research has been focused on the growth parameters of CVD, the effects of temperature profile on the VAMWNT growth still need to be clarified further. In this paper, we investigated the effects of temperature profile on the growth of nanotubes using a 60 mm inner diameter quartz tube. Catalysts were deposited on a SiO₂ wafer following a previously published protocol [3,4], and identical catalysts were used in all studies. Fe (lnm)/ Al₂O₃ (30nm) was deposited on a SiO₂ wafer by an electron beam evaporator [3,4]. In order to provide different temperature conditions, several wafers were located at different locations inside the tube furnace and other parameters such as growth time and gas flow rates were identical. As-grown VAMWNTs were characterized using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The catalyst morphology was also investigated by atomic force microscopy (AFM). The length of VAMWNTs varied from 80μm to 3cm depending on positions as shown in Figs, 1a-b. The temperature profile was found to be slightly different at each location leading to a significant difference in the growth pattern. Fig. 1c demonstrates that I_D/I_G ratio of VAMWNTs is varied from 0.2 to 1.41. At position #1, radial breathing modes and a small D mode were observed. The tube length and D mode were increased as the wafer was moved to the directio- of position #5. Smaller diameter tubes with shorter length were found at position #1 whereas large diameter tubes with longer length were synthesized at position #5. The effects of the temperature profile on the catalyst formation and tube diameter and length are further discussed in the full manuscript and poster.