Number-controlled growth of CNTs by using apertures with different diameters. (a, b) SIM images of 21 × 21 nanoapertures in a stencil mask consisting of 140-nm-diameter apertures and 10-μm spacing between each aperture. The inset in (b) shows an enlarged view of the aperture shown in (a). (c, d, e) SEM images of CNTs synthesized using apertures of various diameters. Some CNTs (c), mainly double CNTs (d), and individual CNT (e) were synthesized through apertures whose diameters were 140, 80, and 40 nm, respectively. The insets show the apertures used to synthesize CNTs. (f) The number of CNTs synthesized was strongly dependent on the diameter of the nanostencil aperture. Yield was 39.6% for individual CNTs synthesized using 40-nm-diameter aperture.