Conductive-probe measurements with nanodots of free-base and metallated porphyrins

The conductive properties of nanodots of model porphyrins were investigated using conductive-probe atomic force microscopy (CP-AFM). Porphyrins provide excellent models for preparing surface structures that can potentially be used as building blocks for devices. The conjugated, planar structure of porphyrins offers opportunities for tailoring the electronic properties. Two model porphyrins were selected for studies, 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II) (TPC) and its metal-free analog 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP). Nanodots of TPP and TPC were prepared within a dodecanethiol resist on gold using particle lithography. The nanopatterned surfaces exhibit millions of reproducible test structures of porphyrin nanodots. The porphyrin nanodots have slight differences in dimensions at the nanoscale, to enable size-dependent measurements of conductive properties. The size of the nanodots corresponds to ∼5-7 layers of porphyrin. The conductivity along the vertical direction of the nanodots was measured by applying a bias voltage between the gold surface and a metal-coated AFM cantilever. The TPP nanodots exhibited semi-conductive profiles while the TPC nanodots exhibited profiles that are typical of a conductive film or molecular wire. The engineered nanostructures of porphyrins provide an effective platform for investigation and measurement of conductive properties.