On-surface synthesis of graphene clusters from a Z-bar-linkage precursor with quaterphenyl branches

Abstract

The products of two-zone chemical vapor deposition on Au(111) from a Z-bar-linkage precursor with quaterphenyl-branches were investigated by scanning tunneling microscopy, Raman spectroscopy, and theoretical calculations. Annealing at 400 °C produced linear arrays of intramolecular-dehydrogenated precursors, and temperatures over 450 °C led to intermolecular dehydrogenation and to the formation of graphene clusters. This reaction pathway contrasts remarkably with the previous results reported for a Z-bar-linkage precursor with terphenyl branches, where homochiral polymerization proceeded and allowed the formation of acene-type graphene nanoribbons. The reason might originate from the conformation of the biradical form of the quaterphenyl-branched precursor, produced by debromination on Au(111) during 250 °C annealing. The quaterphenyl-branched precursor might favor a symmetric conformation with both the radicals pointing toward the Au(111) surface, whereas our previous results showed that the terphenyl-branched precursor might favor asymmetric (chiral) conformations, with one radical pointing toward and the other one away from the Au(111) surface. Steric hindrance of the symmetric conformations of the quaterphenyl-branched precursor is presumed to prohibit polymerization due to the strong affinity of the biradical to Au(111). Such symmetric precursor conformations lead to fused debrominated precursors via intramolecular dehydrogenation, and finally result in the conversion to graphene clusters via on-surface intermolecular fusion.