Tuning the Diameter of Single-Walled Carbon Nanotubes by Temperature-Mediated Chemical Vapor Deposition

In the present study, we systematically described the tuning of the diameter of single-walled carbon nanotubes (SWNTs) from individual catalyst particle by temperature-mediated chemical vapor deposition (CVD). The basic idea is to change the temperature quickly during the growth of ultralong SWNTs via a "kite mechanism". It has been found that the diameter of a SWNT varied with temperature oscillation; higher temperature led to thinner SWNTs with the same catalyst particle and lower temperature led to thicker ones. These diameter variations of SWNTs were measured by resonance Raman spectroscopy and atomic force microscopy (AFM). It has also been found that the diameter changes were highly related to their initial diameters. For each initial diameter, there are various probabilities of different diameter changes, but all of them are below a certain maximum, which depends on the initial diameter. The maximum diameter change reaches an extremum of 0.40 nm, while the initial diameter is around 1.6 nm, and the maximum diameter changes are much lower for nanotubes with a larger or smaller diameter, which implies that the diameter mediation by temperature is limited. Based on the results, a well-aligned individual ultralong SWNT array with a narrow diameter distribution of small diameter (around 1.0 nm) or larger diameter (around 2.0 nm) could be grown by ascending or descending temperature-mediated CVD, respectively. At the same time, a simple growth model supposing there is a graphene transition state when the tube formation is initiated is proposed to explain these phenomena. Under this model, the relationship between the growth temperature and the diameter of SWNTs was calculated. It has been found that, a catalyst particle with certain mole fraction of carbon will produce SWNTs with smaller diameter under higher growth temperature and vice versa. This trend accords well with our experimental data. Such diameter modulation of SWNTs by temperature would also help us to understand the CVD growth mechanism of SWNTs.