MD simulations of the catalyst-free, in situ, and fast formation of CNTs within the confined carbon structure. (a) Schematic showing the proposed mechanism of CNT growth in the confined carbon structure by Joule heating. The defect sites on C-wood have a higher temperature during Joule heating, which promotes the migration of the carbon atoms from high temperature regions (red) to low temperature regions (blue) and results in CNT growth. (b-g) MD simulation results verify the 1D CNT structure formation in the confined space via a substantial temperature gradient. The blue atoms belong to the low temperature region (300 K) while the red atoms belong to the high temperature region (3000 K).