Small diameter single-walled carbon nano tube (SWNT) arrays with larger bandgap are more desirable as near-infrared optical absorbers for the fabrication of high performance photovoltaic and photo detector devices. We report herein a rational approach to selective growth of well-aligned subnanometer diameter (similar to 84% between 0.75 and 0.95 nm) SWNT arrays with a density of 0.3-0.5 tubes/mu m on quartz surfaces using solid Mo2C catalysts for short-time growth by low carbon feeding in hydrogen-free CVD. These subnanometer diameter SWNTs have a narrow chirality distribution (the ratio of (8,4), (8,5) and (7,6) is higher than 73%). During nanotube growth, only small size Mo nanoparticles are carbonized into stable Mo2C for catalyzing the growth of SWNTs through low carbon feeding rate over short time in the hydrogen-free environment, whereas larger catalysts are inactive due to underfeeding. Meanwhile, solid Mo2C catalysts are effective in reducing the chirality distributions of the as-grown SWNTs. Additionally, combining an annealing process after loading catalyst on the sapphire substrates, the average density is increased to similar to 15 tubes/mu m while maintaining small diameter and narrow chirality distribution. Our results offer more choices for structurally controlled growth of aligned-SWNTs, with potential applications in nanoelectronics.