Multi-stage array
SEM images of (a) vertically orientated multiwalled carbon nanotube (MWCNT) arrays grown on porous silicon; (b) MWCNTs; (c) CNT multistage structure showing thin MWCNTs and single walled CNTs on top of MWCNTs; (d) schematic of the multistage structure and field emission measurement scheme. (Image credit: FIU)