A Functional 2D Carbon Allotrope Combining Nanoporous Graphene and Biphenylene Segments

The implementation of graphene in semiconductor technology requires bandgap tuning, which can be achieved by chemical doping, geometric confinement in nanoribbons or with the insertion of nanopores and non-hexagonal rings. The bottom-up on-surface synthesis approach in ultra-high vacuum allows for the synthesis of atomically well-defined graphene or biphenylene nanoribbons and nanoporous graphene (NPG) structures suitable for device applications. Here, a novel 2D carbon allotrope is synthesized in the form of a functional NPG with periodically spaced biphenylene segments. First, 12-armchair porous graphene nanoribbons (12-pGNRs) on Au(111) and Au(788) using 7,10-dibromo-1,4-diphenyl-triphenylene (DBDT) molecular precursor are grown. Low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS) and non-contact atomic force microscopy (nc-AFM) measurements reveal the presence of high-quality semiconducting 12-pGNRs. Thermal annealing of densely packed 12-pGNRs at 550 °C triggers their lateral fusion into diverse NPG structures featuring either graphene-type or biphenylene-type junctions. The structural and electronic properties are again characterized by LT-STM and nc-AFM in combination with density functional theory (DFT) calculations. DFT shows that while graphene-type NPGs are direct bandgap semiconductors, biphenylene-type NPGs manifest a smaller and indirect bandgap. The NPGs are stable upon oxygen and air exposure, and the nanopores feature a noticeable affinity to carbon monoxide, making them appealing systems for chemical sensors.