Near Fermi Superatom State Stabilized by Surface State Resonances in a Multiporous Molecular Network.

Shigeki Kawai,Jorge Lobo-Checa,Saika Izumi,Ali Sadeghi,Satoshi Minakata,Kewei Sun,Zakaria M Abd El-Fattah,Youhei Takeda,Mohammad A Kher-Elden

doi:10.1021/acs.nanolett.1c01200

Abstract

Two-dimensional honeycomb molecular networks confine a substrate’s surface electrons within their pores, providing an ideal playground to investigate the quantum electron scattering phenomena. Besides surface state confinement, laterally protruding organic states can collectively hybridize at the smallest pores into superatom molecular orbitals. Although both types of pore states could be simultaneously hosted within nanocavities, their coexistence and possible interaction are unexplored. Here, we show that these two types of pore states do coexist within the smallest nanocavities of a two-dimensional halogen-bonding multiporous network grown on Ag(111) studied using a combination of scanning tunneling microscopy and spectroscopy, density functional theory calculations, and electron plane wave expansion simulations. We find that superatom molecular orbitals undergo an important stabilization when hybridizing with the confined surface state, following the significant lowering of its free-standing energy. These findings provide further control over the surface electronic structure exerted by two-dimensional nanoporous systems.

Highlights

Two-dimensional honeycomb molecular networks confine a substrate’s surface electrons within their pores, providing an ideal playground to investigate the quantum electron scattering phenomena
Different in kind to surface state (SS) resonances, 2D organic networks can host molecular pore states at sufficiently small nanocavities, named as superatom molecular orbitals (SAMOs). These unoccuppied states feature high, site-dependent local density of states (LDOS) outprotruding from the carbon backbone into the pore that leads to molecular orbital (MO) overlapping with adjacent molecules.[25−28] Notably, SAMO undergo significant energy downshifts with respect to their calculated free-standing values.[25,27]
Using a combination of lowtemperature scanning tunneling microscopy and spectroscopy (STM/STS), density functional theory (DFT) calculations, and electron plane wave expansion (EPWE) simulations, we study in depth the pore states

Summary

■ DISCUSSION

The combination of experimental results and two theoretical methods allows us to unambiguously identify two different types of coexisting pore states and to confirm their correlation within tiny pores of organic networks. At the corner pores, the SAMO onset is in proximity to the ncorner = 4 SS resonance that features LDOS maxima at the pore rims In this way, three properties become important for this SAMO-SS resonance correlation: (i) the size of the nanocavity, since the smaller it is the easier for the MOs to overlap and fill the void; (ii) the LDOS shape of the SS resonance that preferentially promotes the stabilization of SAMOs at the rims of the nanocavities; and (iii) the SS resonance energies participating in the hybridization with the SAMO state. The presence of edge and corner pores contributes with a minor energy reduction of the central pore SS resonance energy

■ CONCLUSION

■ ACKNOWLEDGMENTS

■ REFERENCES