Self-Assembled H2NC Molecular Lattices as a Platform for Tunable Quantum Superlattices

Abstract

Compared to van der Waals moir\'e systems, molecular assembly has emerged as an exciting alternative platform for superlattice engineering via heterointegration. The electronic properties of the self-assembled square lattice monolayer molecular crystal of metal-free naphthalocyanine (H$_2$Nc), in particular the electronic band dispersion and their tunability by metal substrates, remain less explored. Using density functional theory, supported by angle-resolved photoemission and scanning tunneling microscopy, we compare the electronic structure of a free-standing H$_2$Nc monolayer with that of H$_2$Nc lattice assembled on noble metal substrates. In the free-standing film, we identify both nearly flat, molecule-localized states and more dispersive bands, and we show that each can be compactly described by an anisotropic tight-binding Hamiltonian that yields band-resolved hopping anisotropies. We further show theoretically the wide tunability in the inter-site hopping and Coulomb interaction based on dielectric and screening environment, and show two experimental realizations using Ag(100) and Au(111) substrates. On Ag(100), orbital hybridization between molecular frontier states and the substrate drives finite spectral weight at the Fermi level and local interfacial polarization. This hybridization enhances the effective intermolecular hopping roughly thirty-fold, drastically reducing $U/t$. Complementary angle-resolved photoemission spectroscopy resolves substantial interfacial charge redistribution and enhanced bandwidth of the HOMO band, consistent with theoretical predictions. These results clarify how metal substrates and gates convert H$_2$Nc from isolated molecules into a tunable 2D lattice, and highlight molecular superlattices as a promising platform in which the effective interaction parameters can be engineered over a wide range.