Do Two-Dimensional “Noble Gas Atoms” Produce Molecular Honeycombs at a Metal Surface?

Abstract

Anthraquinone self-assembles on Cu(111) into a giant honeycomb network with exactly three molecules on each side. Here we propose that the exceptional degree of order achieved in this system can be explained as a consequence of the confinement of substrate electrons in the pores, with the pore size tailored so that the confined electrons can adopt a noble-gas-like two-dimensional quasi-atom configuration with two filled shells. Formation of identical pores in a related adsorption system (at different overall periodicity due to the different molecule size) corroborates this concept. A combination of photoemission spectroscopy with density functional theory computations (including van der Waals interactions) of adsorbate-substrate interactions allows quantum mechanical modeling of the spectra of the resultant quasi-atoms and their energetics.

Monte Carlo study of the honeycomb structure of anthraquinone molecules on Cu(111)

“…44 This size lattice is comparable to the size of defect-free regions in the experimental system. 45 In particular, the steps on this surface have a spacing on this order. This size also allows easy visual inspection without undue finite-size effects, as well as affording reasonable computational costs.…”

Patterns of Organics on Substrates with Metallic Surface States: Why?, So??

“…A greater problem was that subsequent experiments showed that pentaquinone (PQ) forms the same ordered structure (albeit less robustly) [36]. See Fig.…”

“…This renders suspect any explanation in terms of indirect interactions (based on the same Cu surface state). Hence, we proposed an alternative explanation: the structure was stabilized by electrons confined within the honeycomb cell [36]. We made the simplifying assumption that the barriers between cells were infinitely high, so that intercell coupling could be ignored in this zeroth approximation.…”

“…Wyrick et al's tabulations [36] considered pores with 3-6 AQ's per side. In each case, the regular (equal-sided) configuration had the lowest energy and of the four groupings, the case of 3 AQ's was the lowest.…”

“…Again, dispersion is observed, somewhat more for single-wall system. For AQ adsorbates, there is no evidence of dispersion in ARPES; this has been attributed to the great sensitivity of the giant honeycomb net to just the proper local coverage [36]. At low coverages, there are only strands of AQ's (see Fig.…”