Origin of contrast in STM images of oxygen on Pd(111) and its dependence on tip structure and tunneling parameters

The dependence of the contrast and symmetry of scanning tunneling microscope images of O∕Pd(111)-2×2 on the structure of the tunneling tip and on tunneling parameters is explained using first-principles density functional theory. Experimentally, the contrast changes in different ways when a metal-terminated tip over hcp and top sites changes its bias and tip-sample distance. These changes are also reflected in the symmetry of the image. A detailed analysis of the tunneling contributions indicates that for the metallic tips, the Pd0.2em0exd orbitals are determining the image symmetry at close range and low bias, while at larger separations and high bias the Pd0.2em0expz orbitals are the ones that control the image contrast. For oxygen-terminated tips, we predict a positive image contrast, associated with the tip oxygen bonds, as opposed to the negative contrast images obtained with metallic tips.