Scanning tunneling microscopy of bimetallic molecules

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 NixXo3-x (HITP)2 metal-organic frameworks (MOFs) are formed via an on-surface reaction of the organic molecules with the metals.  The MOFs are characterised by scanning tunnelling microscopy (STM) which reveals the hexagonal framework structures and growth processes.

The pure Ni3(HITP)2 frameworks have a similar isotropic island morphology, while the Co3(HITP)2 islands are ribbon shaped.  Individual Ni and Co metal centres 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 in topography.  The STM images further reveal that in the mixed-metal MOFs the Ni and Co centres are distributed randomly.

The experimental work, as published in Journal of Physical Chemistry C*, was carried out exclusively by this department: Dr Fabiao Gu and Dr Sisheng Shu in the Surface Nanoscience Group, led by Professor Martin Castell, in collaboration with DFT modelling carried out by Dr Chris Patrick.

*Scanning tunneling microscopy of bimetallic Ni/Co-HITP metal-organic framework monolayers'.