Role of functionalized transition-metal coated W tips in STM imaging: Application to epitaxial graphene on SiC(0001)

Scanning tunneling microscopy using W tips functionalized with transition-metal (Cr, Fe) coatings is demonstrated to facilitate the imaging of states within a few millielectron volts (meV) of the Fermi level for epitaxial graphene on SiC(0001), which are not accessible with bare W tips. First-principles modeling of these probe tips as pyramidal structures on W(110) indicates that an apex atom is stable for the Cr/W(110) tip but not for the Fe/W(110) or W/W(110) tips. This difference in their atomic structures, together with the variation in the extent of the 3d and 5d orbitals, is found to be responsible for the capability of Cr and Fe functionalized tips to selectively image the complex electronic properties of epitaxial graphene on SiC(0001).