SEM image of a nanowire array, illustration of geometry parameters, and nanowire morphology schematics. (a) SEM image of a typical SiNW nanowire array synthesized bottom-up by gold catalyzed VLS. (b) 2D illustration of the geometric parameters: diameter D, length L, and pitch P of cylindrical NWs. (c) Schematic illustration of cylindrical and (d) tapered NWs. All simulations were conducted three dimensionally without an additional substrate.