1 research outputs found
Site-Dependent Oxidation States of Single Cobalt Atoms in a Porphyrin-Based Monolayer on Graphene
We investigate a
layer of cobalt tetrapyridyl porphyrins (CoTPyPs)
self-assembled on an almost freestanding graphene (GR) sheet supported
by Ir(111) with complementary experimental techniques and density
functional theory (DFT) ab initio simulations. Beside the metal atoms
enclosed within the porphyrin macrocycles, additional Co atoms can
be accommodated at the molecular network’s interstice via physical
vapor deposition and can bind up to four adjacent molecules. Therefore,
such a system presents two metallic sites, both tetra-coordinated
to nitrogen atoms. At the same time, a rearrangement of the network
occurs depending on the coverage of such additional atoms. The bare
CoTPyPs arrange themselves on GR in an almost hexagonal close-packed
pattern with alternating orientations. The addition of extra Co atoms
causes a dramatic transformation in the network. At full peripheral
metal coverage (i.e., one additional Co per CoTPyP), the network drastically
changes becoming almost square. Intermediate coverages display different
peculiar patterns characterized by unique chiral structures. Importantly,
our DFT calculations reveal a remarkable effect on the system’s
work function attributed to the presence of these additional metal
atoms, despite their extremely small amount even at full coverage
(less than 2% of a monolayer with respect to the number of carbon
atoms in the GR sheet). Furthermore, we report a different behavior
of the two Co sites showing different oxidation states and molecular
orbital occupations