We had the opportunity to use the new toroidal analyzer at the Australian synchrotron to do ARPES of self-assembled monolayers of MgPc on Ag100.

Careful simultaneous fitting of different high-symmetry EDC measurements, in concert with the structural understanding gleaned from ncAFM & LEED characterization, allowed us to tease out a feature with bandwidth 20 meV, which was surprising to us given that we did the ARPES at room temperature.

Bruce Cowie & Anton Tadich made it possible to break into this kind of measurement with just a week of time; Anton has been instrumental in supporting the analysis that was required to get this one across the finish line.

Hellerstedt, J., et. al. (2022). Direct observation of narrow electronic energy band formation in 2D molecular self-assembly. Nanoscale Advances

Concerted Proton Transfer

We stumbled on a very curious observation in the summer of 2018 with DABQDI molecules provided by Olivier Siri‘s team.

ncAFM image of 26 molecule chain. Unfiltered data.
STM chain manipulation
Repeated manipulations with STM tip are capable of dragging a DABQDI chain around the Au111 surface.

While evaluating its experimental suitability for 1d coordination with metals, which has already proven to be fruitful, we noticed the molecules forming chain-like structures even before we introduced metal adatoms.

The low temperature SPM results are sublime: unusual mechanical stability, distinctive intermolecular bonding, and near-Fermi electronic states lighting up at the ends of the chains.

It took an extraordinary cast of theorists hailing from Pavel’s core group, FZU, Charles, Reykjavik, & Madrid Universities to unravel this puzzle and explain these observations as concerted proton tunneling causing a delocalization of electrons.

“Significance of Nuclear Quantum Effects in Hydrogen Bonded Molecular Chains”, ACS Nano, 2021. 10.1021/acsnano.1c02572

ArXiv link

MgPc-MgPc Hybridization

ncAFM atomic registration of MgPc molecule on Ag100
nc-AFM atomic registration of single MgPc molecule on Ag100 (surface atoms top and bottom stripes)

Marina Castelli studied the phthalocyanine containing magnesium (MgPc) via 5K scanned probe microscopies extensively during her PhD.

‘Routine’ STM characterisation showed that the molecules were interacting with one another on the Ag100 surface.

ncAFM showed identical contrast for all molecules, pointing to an electronic origin to the observed changes in appearance.

Our key observation was to track the shape of the occupied LUMO for different pairwise distances, an electronic feature that otherwise remained isoenergetic.

With multipass dI/dV mapping we were able to quantitatively track from four- to two-fold rotational symmetry, over distances out to ~3 nm. We found the spatial extent of this attractive hybridization quite surprising.

“Long-Range Surface-Assisted Molecule-Molecule Hybridization”, Small (2021). 10.1002/smll.202005974

ArXiv link

STM image of MgPc molecules on Ag100 surface
STM image showing the neighbor-induced symmetry reduction