Engineering interfacial quantum states and electronic landscapes by molecular nanoarchitectures

Surfaces are at the frontier of every known solid. They provide versatile supports for functional nanostructures and mediate essential physicochemical processes. Being intimately related with 2D materials, interfaces and atomically thin films often feature distinct electronic states with respect to the bulk, which are key for many relevant properties, such as catalytic activity, interfacial charge-transfer, or crystal growth mechanisms. Of particular interest is reducing the surface electrons' dimensionality and spread with atomic precision, to induce novel quantum properties via lateral scattering and confinement. Both atomic manipulation and supramolecular principles provide access to custom-designed molecular superlattices, which tailor the surface electronic landscape and influence fundamental chemical and physical properties at the nanoscale. Herein, we review the confinement of surface state electrons focusing on their interaction with molecule-based scaffolds created by molecular manipulation and self-assembly protocols under ultrahigh vacuum conditions. Starting from the quasi-free 2D electron gas present at the (111)-terminated surface planes of noble metals, we illustrate the enhanced molecule-based structural complexity and versatility compared to simple atoms. We survey low-dimensional confining structures in the form of artificial lattices, molecular nanogratings or quantum dot arrays, which are constructed upon appropriate choice of their building constituents. Whenever the realized (metal-)organic networks exhibit long-range order, modified surface band structures with characteristic features emerge, revealing intriguing physical properties, such as discretization, quantum coupling or energy and effective mass renormalization. Such collective electronic states can be additionally modified by positioning guest species at the voids of open nanoarchitectures [...].