  In 2D materials, the quantum confinement and van der Waals-type interlayer
interactions largely govern the fundamental electronic and optical properties,
and the dielectric screening plays a dominant role in the excitonic properties.
This suggests strongly layer-dependent properties, and a central topic is to
characterize and control the interlayer interactions in 2D materials and
heterostructures. Black phosphorus is an emerging 2D semiconductor with
unusually strong interlayer interactions and widely tunable direct bandgaps
from the monolayer to the bulk, offering us an ideal platform to probe the
layer-dependent properties and the crossover from 2D to 3D (i.e., the scaling
effects). In this review, we present a comprehensive and thorough summary of
the fundamental physical properties of black phosphorus, with a special focus
on the layer-dependence characters, including the electronic band structures,
optical absorption and photoluminescence, and excitonic properties, as well as
the band structure engineering by means of electrical gating, strain, and
electrochemical intercalation. Finally, we give an outlook for the future
research.
