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Synthesis of armchair graphene nanoribbons from the 10,10′-dibromo-9,9′-bianthracene molecules on Ag(111): the role of organometallic intermediates

We investigate the bottom-up growth of N = 7 armchair graphene nanoribbons (7-AGNRs) from the 10,10′-dibromo-9,9′-bianthracene (DBBA) molecules on Ag(111) with the focus on the role of the organometallic (OM) intermediates. It is demonstrated that DBBA molecules on Ag(111) are partially debrominated at room temperature and lose all bromine atoms at elevated temperatures. Similar to DBBA on Cu(111), debrominated molecules form OM chains on Ag(111). Nevertheless, in contrast with the Cu(111) substrate, formation of polyanthracene chains from OM intermediates via an Ullmann-type reaction is feasible on Ag(111). Cleavage of C–Ag bonds occurs before the thermal threshold for the surface-catalyzed activation of C–H bonds on Ag(111) is reached, while on Cu(111) activation of C–H bonds occurs in parallel with the cleavage of the stronger C–Cu bonds. Consequently, while OM intermediates obstruct the Ullmann reaction between DBBA molecules on the Cu(111) substrate, they are required for the formation of polyanthracene chains on Ag(111). If the Ullmann-type reaction on Ag(111) is inhibited, heating of the OM chains produces nanographenes instead. Heating of the polyanthracene chains produces 7-AGNRs, while heating of nanographenes causes the formation of the disordered structures with the possible admixture of short GNRs.


Additional STM characterization of DBBA on Ag(111) at room temperature
Two co-existing self-assembled structures formed by DBBA molecules on Ag(111) at RT are illustrated in Fig. S1(a-c). The arrangement of the DBBA units in dense molecular islands shown in Fig. S1(a) prevails above 30 ˚C, while the domains composed of the "flower"-like hexamer features in Fig. S1(c) tend to form at slightly lower temperatures, hence indicating a low energy barrier for the structural rearrangement of the system. Self-assembly of DBBA molecules on the Ag(111) surface was investigated in the recent paper by Shen et al. 1 Nevertheless, partial debromination of DBBA molecules at RT was not taken into account by the authors. For the half-debrominated DBBA molecules the "one-legged" adsorption geometry, similar to the one for the half-debrominated DBBA on Cu(110) 2 , can be suggested.
This implies that molecular units tilted at various angles with respect to the surface can be imaged in the STM differently. Moreover, by analogy with previous studies of bromine-containing molecules on metal surfaces 2,3 , Br atoms detached from the DBBA units can appear in the STM images as additional features surrounding the molecules.
In agreement with work of Shen et al. 1 , dense molecular islands shown in Fig. S1(a) are most likely formed by the molecular rows (indicated by white lines), which lie side-by-side being regularly separated by the gaps due to the presence of dissociated bromine or due to relief of the strain in the molecular domain. Three types of domains in which DBBA rows align in three directions oriented at 120˚ to each other can be obtained for such molecular islands (Fig. S1c). The orientation of DBBA rows constituting each domain is governed by the interaction with close-packed Ag(111) surface and hence the domains are rotated by 120 ˚ with respect to each other. The ordered islands co-exist with some admixture of disordered phase, which is clearly seen close to the domain boundaries.    Fig.3 shows the typical STM images obtained from the sample prepared by deposition of DBBA on Ag(111) at RT (Fig. S3a) followed by the annealing at 100˚C (Fig. S3b). It can be seen that islands of OM chains appeared instead of the hexamer phase as a result of the complete debromination of DBBA molecules.

STM characterization of AGNRs on Ag(111) surface
The STM image of GNRs/Ag (111) Fig. 3 (e,f). For both OM chains the band gap is vanished as a result of their interaction with the metallic substrate. Moreover, interaction of molecular units with bridging metal atoms is responsible for the appearance of the additional sharp peak in the valence region (Fig. S5b,c). This characteristic feature can be used as an indication of the formation of organometallic bonds.

OM chains on Ag(111) and Cu(111)
As mentioned in the manuscript, when optimizing the structure of OM chains on Ag(111) and Cu (111) surfaces, the periodic boundary conditions were used. In this case, it is not individual isolated OM chain which is considered, but the periodic array of OM chains (see Fig. S6) Figure S6. First row -Unit cells used in DFT calculations of OM chains on Ag(111) and Cu(111) optimized using the periodic boundary conditions. Second row -3×3 and 2×3 supercells illustrating the array of unidirectional OM chains on Ag(111) and Cu(111) surfaces, respectively.