Modelling electron tunnelling in the presence of adsorbed materials
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The transport characteristics of single-molecule Au(lll) junctions are investigated using density functional theory (DFT) together with the non-equilibrium Green’s functions formalism (NEGF). DFT optimisations of the adsorption of various molecules on a Au(lll) surface are used as starting points for the equilibrium junction geometries. Test calculations are performed to find a recommended set of parameters for the final DFT results. The interaction energies of several molecules with the Au(lll) surface obtained within the same level of theory are compared. Amine compounds bind preferentially in an adatom geometry and weakly in the out op site. A Z-matrix optimiser is implemented in the SIESTA code as a useful tool for future surface and molecular junction optimisations. Transport properties are calculated for molecular junctions in their equilibrium geometry. While the conductances are orders of magnitude larger than experimental data, the sizes are in line with expectation. The junction geometries are altered in various ways. Changing the binding site or altering the nature of the sulphur-gold interaction in a phenylenedimethanethiol junction, reduces the conductance by a factor of two. Orders of magnitude reduction of conductance is only observed when increasing the distance between a physisorbed molecule and the surface. Increasing this distance for a chemisorbed molecule, results in a surprising increase in conductance. This is attributed to an interplay between the coupling strength of the molecule with the surface and the location of the molecular energy levels relative to the Fermi level. When the chemical bond is broken, the system is spin-polarised and the conductances for electrons of opposite spin types are different by a factor of 250 - the junction acts as a spin-filter. When stretching a diethynylbenzene junction, the strong gold-carbon bond does not break, but rather extracts a gold atom from the surface. In this case the conductance decreases rapidly with stretching. A WKB tunnel barrier model is used as an alternate much faster method for calculating I(V) characteristics. With the surface work functions acting as barrier heights, the relative junction conductances are in good agreement with the DFT results. However, the direction of asymmetry in the I(V) characteristics predicted by the two levels of theory are opposite. More sophisticated barrier shapes may be needed to correctly predict the asymmetries. The tunnelling model is used in conjunction with the DFT results to quantify the effect a gap between an STM tip and monolayer may have on STS measurements.
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