Steps on the Au/Cu(111) surface studied by local work function measurement with STM

The work function variation around step edges has been studied with a scanning tunneling microscope by taking local work function images on the Au/Cu(111) surface. While the local work function measured from large terraces is in good agreement with the results obtained by non-local techniques, the local work function around step edges is significantly lower than that on terraces. The lower work function is mainly due to the dipole moment formation at step edges. The measured local work function around steps can be reproduced nicely with a simple simulation which includes a potential formed by an array of dipole moments with a proper linear density as well as an effect of the topographic change at the steps. From the simulation, dipole moments formed at Au-Au and Cu-Cu steps were obtained. Our results show that the dipole moment at Cu-Cu step edges is about 2 times larger than that at Au-Au step edges. This study demonstrated that STM has the capability to reveal the details of how local work function varies around steps and other defects and that more information can be obtained by combining local work function measurements with STM topographic information.