Adsorption of alkanes on Au(111): Possible origin of STM contrast at the liquid/solid interface

The molecular structure and origin of contrast of monolayers formed at the interface between Au(111) surfaces and solutions containing n-alkanes $({\mathrm{C}}_n{\mathrm{H}}_{2n+2},$ $n=10\mathrm{}–50)$ has been studied by in situ scanning tunneling microscopy (STM) at room temperature. At the liquid/solid interface the monolayers consist of a close-packed arrangement of lamellae separated by troughs. It has been found that at the same experimental conditions the STM contrast of boundaries between lamellae can be different for different regions within the same close-packed monolayer island. Also this contrast strongly depends on the tunneling regime (bias voltage, tunneling current) and qualitatively differs for monolayers formed by alkanes with even and odd numbers. The dependence of STM contrast is being explained by the formation of misfit dislocations within the monolayer in association with a interference of different parallel current paths that connect the STM tip with the substrate through ends of the two nearest alkane molecules.