Although chemical modification of graphene creates a band gap, achieving thermal and chemical stability in fluorinated or hydrogenated graphene remains challenging. Band gap engineering through size confinement with graphene nanoribbons suffers from serious impediments to device fabrication [1]. Graphene antidot lattices are an elegant alternative to graphene nanoribbons that have the potential to be easily transferred onto device surfaces. Experimental work on graphene antidot lattices has been limited to top-down fabrication methods which do not reach size scales necessary for significant band gaps [2]. Bottom-up approaches have produced highly ordered polyphenylene networks, but not porous graphene [3].

We examine the formation of graphene antidot lattices through the on-surface polymerization of halogenated aromatic molecules. In addition to thermally mediated self-assembly, tip-induced assembly offers a potential route for directing nanostructure formation. We deposit 1,3,5-tris(2-bromophenyl) benzene (TBB) onto gold. We characterize the surfaces using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Samples prepared with the substrate held at room temperature during deposition appear streaky when imaged with STM, indicating that at room temperature the molecule is highly mobile on gold, and has not polymerized. With the substrate held above 200° C during deposition, TBB nucleates into disordered networks at step edges.

When areas imaged as streaky are exposed to high energy tunneling electrons, a disordered network appears underneath the path of the electron beam. Depositing the molecule onto a substrate heated at 400°C and above leads to extended networks that span entire terraces. High index facets show highly ordered regions which we attribute to close-packed bromine adatom islands. A disordered porous network assembles on low index facets. By examining the role of substrate temperature in the deposition of 1,3,5-tris(2-bromophenyl benzene) on gold, we have found conditions that lead to a porous structure. We also show evidence of tip-induced polymerization of halogenated aromatic molecules.

[1] J. Cai, et al., Nature 466, 7305 (2010)

[2] J. Eroms and D. Weiss, New J. Phys. 11(9), 095021 (2009)

[3] M. Bieri, et al., Chem. Commun. 6919-6921 (2009)

Adrian Radocea

University of Illinois, University of Illinois

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