Fig. 2. Synthetic strategy to produce chGNRs combining solution and on-surface synthesis.

a–d solution synthesis protocols for producing molecular precursors 1, 2, 3, for the synthesis of 3,1,w-chGNRs with different widths (w = 4, 6 and 8), and precursor 4 for 3,2,8-chGNRs. Synthesis details are described in the text and in the Supplementary Methods. e–h targeted chemical structures of chGNRs by using the four molecular precursors in a–d, respectively. Green circles highlight the carbon atoms across the GNRs, which measure the width of the GNRs. The GNRs are named following the sequence of z zigzag sites, a brominated site (see Supplementary Note 1), and w width, as z, a, w-chGNRs. Red(blue) bonds highlight the zigzag(armchair) segments of the chGNRs. Bonds in purple indicate the pathway for Ullmann coupling.  i–l STM overview images of the chGNRs formed on a Au(111) surface after stepwise annealing the sample covered with precursors 1–4 to temperature T₁ (10 min) and then to temperature T₂ for Ullmann coupling and CDH, respectively. (sample bias V_s = 1 V, scale bar as labeled in l).