Effect of laser intensity on yield and physical characteristics of single wall carbon nanotubes produced by the Nd:YAG laser vaporization method

We report on the effect of the Nd:YAG laser intensity on diameter distribution, yield and physical characteristics of single-wall carbon nanotubes (SWNTs) while comparing three different laser configurations (namely: (i) single 532 nm pulse; (ii) single 1064 nm pulse; and (iii) 532 nm followed by the 1064 nm double pulse). The carbon SWNTs were synthesized at a furnace temperature of 1150°C and characterized by means of laser micro-Raman spectroscopy and high resolution transmission electron microscopy (HRTEM). Regardless of the laser configuration used, it is found that both the yield and the structural characteristics of the SWNTs are highly sensitive to the laser intensity. Indeed, by combining Raman analyses together with HRTEM observations we were able to point out the existence of an optimal laser intensity which leads not only to the highest yield of SWNTs and the largest bundles but also to the lowest level of amorphous and, or disordered sp2 carbon in the deposits. While the optimal laser intensity was found to increase from 1.7 to 2.9×109 W/cm2 when the laser wavelength is changed from 1064 to 532 nm, the double pulse configuration offered a larger process latitude since high yield of SWNTs was obtained over the (0.8–3.5)×109 W/cm2 laser intensity range centered around the optimal value of 2.3×109 W/cm2. Moreover, it is shown that the increase of the laser intensity (from ∼0.5 to 5.6×109 W/cm2) favors the growth of large nanotubes (1.4 nm-diam.) to the detriment of smaller ones (1.1 nm-diam.). A tendency to form larger nanotubes was also observed when increasing the furnace temperature from 1000 to 1150°C. Finally, the laser intensity effect is interpreted in terms of near-surface or deep laser energy absorption in the graphite target.