Scanning tunneling microscopy observation of CO on Pd(110) at cryogenic temperature; Imaging mechanism and novel one-dimensional array formation

Scanning tunneling microscopy (STM) is used to observe carbon monoxide (CO) on Pd(110) surface at cryogenic temperature. We observe the change of appearance of CO in the STM images with the variation of bias voltage and also the existence of CO on the tip apex. This is well explained by assuming a V-shaped density of states at O atom which is formed by the mixing of CO 2π* and Pd 4d. An intriguing one-dimensional ordering is observed along the [110] direction, when CO is dosed at RT and observed at 4.8 K even at coverages less than 0.1 ML. The CO spacing is twice the Pd lattice spacing. The diffusion in such arrays is observed above 110 K. It is found that the diffusion of CO molecules is anisotropic and practically restricted along a single Pd row; inter-row hopping is hardly observed. It is observed that CO molecules in longer chains are more stable than isolated CO molecules.