Influence of end group and surface structure on the current-voltage characteristics of alkanethiol monolayers on Au(11)

We present density functional theory calculations of the èlectronic structure and tunneling characteristics of alkanethiolate monolayers on Au(111). We systematically analyze √3 × √3 full coverage monolayers of SC₆H₁₂X molecules with different terminal groups, X = CH₃, NH₂, SH, OH, COOH, OCH₃, on defect-free ("perfect") Au(111). We also study the influence of the surface-molecule bonding structure by comparing the properties of monolayers of SC₆H₁₂CH₃ molecules on the perfect surface and on Au(111) surfaces with vacancies or adatoms. The tunneling currents (I) through the adsorbed monolayers with a single chemical contact have been calculated within the Tersoff-Hamann approach for voltages between -1 and +1 V. Computed currents are found to depend linearly on V at low voltage, with typical values of ∼60 and 150 pA/molecule at 0.2 and 0.5 V, respectively, in good agreement with several experimental data. Computed tunneling currents show also a significant dependence on both the terminal group X and the surface structure. In particular, in order of decreasing intensities, currents for the different end groups are NH₂ ≈ SH > CH₃ > OH > OCH ₃ > COOH. The relationships between the tunneling current, the work function of the surface + SAM, and the lineup of the HOMO with respect to the Fermi energy of the metal surface are examined.