The molecule-metal interface plays a key role in many fields varying from molecular electronics to plasmon-induced catalysis. In particular, the frontier molecular orbital (FMO) energy, i.e., HOMO or LUMO, is critical to quantify the energy barrier for direct charge injection into the molecule. In terms of the first-principle quantum mechanical modeling, Kohn-Sham density functional theory (KS DFT) has been widely used despite its single particle states cannot be directly interpreted as true quasiparticle (QP) states representing FMO. Many body perturbation theory (MBPT) inherently invokes the quasiparticle picture in which all many-body interactions, including couplings to surface plasmons, are downfolded into the electron/hole self-energy. We use stochastic MBPT for the first time to address nano-scaled molecule-metal interface simulation. The interface systems of CO₂ on the infinite Au surface and Au nanoparticle (NP) facets are used to demonstrate the large discrepancy between DFT orbital eigenvalues and QP excitation energies. Namely, there is a wide leap, more than 0.7eV for LUMO QP energy and 1.8eV for HOMO QP energy between the periodic boundary calculation and the confined system calculation not found by DFT. The adsorption energy computed by reconstructed molecular LUMO(HOMO) QP at Au (111) facet center is 0.6eV higher(1.4eV lower) than NP edge, indicating CO₂ LUMO(HOMO) is less electron accepting(donating) at Au (111) facet center. However, hybridized orbital results imply CO₂to be more hybridized with Au states at Au (111) facet center and LUMO(HOMO) QP energy shows a decline(incline) trend with increasing hybridization level despite the facet type or molecule-surface separation. Specifically this result tells us CO₂is more apt to hybridize with Au states at Au (111) facet center and accordingly the LUMO(HOMO) becomes more electrophilic(nucleophilic). For the first time, we report such signals of reactive spots on NP facets from the perspective of molecular FMO favoring single-electron/hole transfer.

Vojtech Vlcek

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