Scanning Tunneling Microscopy of Bimetallic Ni/Co-HITP Metal–Organic Framework Monolayers

Abstract

Metals (Ni, Co) and hexaaminotriphenylene (HATP) molecules are evaporated separately in an ultrahigh vacuum onto the Au(111) surface. During an annealing step, monolayer single metal and bimetallic Ni_xCo_3–x(HITP)₂ metal–organic frameworks (MOFs) are formed via an on-surface reaction of the organic molecules with the metals. The MOFs are characterized by scanning tunneling microscopy (STM), which reveals the hexagonal framework structures and growth processes. The pure Ni₃(HITP)₂ and bimetallic Ni_xCo_3–x(HITP)₂ frameworks have a similar isotropic island morphology, while the Co₃(HITP)₂ islands are ribbon shaped. Individual Ni and Co metal centers can be atomically identified by their different apparent heights in the STM images of the bimetallic framework. Density functional theory (DFT) calculations indicate that the apparent height difference is caused by electronic effects rather than the difference of topography. The STM images further reveal that in the mixed-metal MOFs the Ni and Co centers are distributed randomly.