Elemental contrast of local work function studied by scanning tunneling microscopy

We have obtained images of the local work function on Au/Cu(111) and Pd/Cu(111) surfaces using scanning tunneling microscopy (STM) and found that they form contrasts depending on the element on the surfaces. A work function image can be obtained simultaneously with a topographic STM image by measuring the response of tunneling current to variation in the tunneling gap distance during scanning. Using this technique, we successfully observed the difference in local work function between the Au/Pd overlayers and the Cu substrate. Our results show that the measured value of the work function on the Au overlayer is between those of Au(111) and Cu(111), consistent with the results obtained by another technique. On the other hand, the work function of the first layer Pd is already larger than that of the bulk Pd(111), and it increases further on the second Pd layer. We also observed a dark contrast along the step edge in work function images which is partly due to dipole moment formation by the Smoluchowski effect.