  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 [...].
