Effect of adsorbates on single-molecule junction conductance

Electronic conduction through molecular junctions depends critically on the electronic state at the anchor site, suggesting that local reactions on the electrodes may play an important role in determining the transport properties. However, single-molecule junctions have never been studied with the chemical states of the electrodes controlled down to the atomic scale. Here, we study the effect of surface adsorbates on the molecular junction conductance by using a scanning tunneling microscope (STM) combined with density functional theory (DFT) and nonequilibrium Green's function (NEGF) calculations. By vertical control of a STM tip over a phenoxy (PhO) molecule on Cu(110), we can lift and release the molecule against the tip, and thus reproducibly control a molecular junction. Using this model system, we investigate how the conductance changes as the molecule is brought to the vicinity of oxygen atoms or a hydroxyl group chemisorbed on the surface. This proximity effect of surface adsorbates on the molecular conductance is simulated by DFT-NEGF calculations.