A clear understanding of the detailed interactions between single molecules and surfaces is required in order to develop novel and functional molecular devices. Despite the fact that simulations have been performed in this connection, the understanding of the electronic interactions at play remains elusive. Normally, the magnitude of the systems investigate in this connection require that density-functional theory (DFT) based approaches be employed, as these methods are capable of simulating electronic structure at a computational cost that is lower than wavefunction-based methods. Nevertheless, there are numerous challenges associated with modeling surface-molecule interfaces using DFT. These include: 1) “Molecular” modeling techniques require the use of truncated (i.e. non-periodic) representation of bulk systems, which will not have a band structure, valence band or conduction band levels that correspond to those in bulk materials. 2) Many DFT methods are not capable of accurately predicting the energy levels associated with molecular orbitals. 3) The details of the molecular and surface structures are absolute requirements for obtaining representative electronic structure details. Periodic DFT computational approaches provide band structures and energy levels for bulk systems, but many DFT methods implemented in period DFT programs do not predict the energy level to be correct in an absolute or a relative sense. On the other hand, DFT-based methods implemented in “molecular” codes are capable of predicting molecular energy levels that are more consistent with experimental results, but these are subject to the limitations laid-out in point 1 above. Finally, many surfaces, including the conducting carbon substrates in which we have interest, have ill-defined structures making the prediction of representative electronic structure a “best guess”. We will present the results of a systematic investigation of molecules bound on conductive carbon substrates, modeled as regular graphene, using both periodic planewave DFT methods and molecular methods. The technical challenges associated with the application of DFT to problems of this type will be discussed.

Insights on molecular junctions through applied density-functional theory: Examining the changes in molecule and substrate properties upon junction formation

251st National Meeting (2016)

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