  Layered and two-dimensional (2D) materials such as graphene, boron nitride,
transition metal dichalcogenides(TMDCs), and black phosphorus (BP) have
intriguing fundamental physical properties and bear promise of numerous
important applications in electronics and optics. Of them, BP is a novel 2D
material that has been theoretically predicted to acquire plasmonic behavior
for frequencies below ~0.4 eV when highly doped. The electronic properties of
BP are unique due to an anisotropic structure, which could strongly influence
collective electronic excitations. Advantages of BP as a material for
nanoelectronics and nanooptics are due to the fact that, in contrast to metals,
the free carrier density in it can be dynamically controlled by electrostatic
gating, which has been demonstrated by its use in field-effect transistors.
Despite all the interest that BP attracts, near-field and plasmonic properties
of BP have not yet been investigated experimentally. Here we report the first
observation of nanoscopic near-field properties of BP. We have discovered near
field patterns of outside bright fringes and high surface polarizability of
nanofilm BP consistent with its surface-metallic, plasmonic behavior at
mid-infrared (mid-IR) frequencies. This behavior is highly
frequency-dispersive, disappearing above frequency, {\omega} =1070 cm-1, which
allowed us to estimate the plasma frequency and carrier density. We have also
observed similar behavior in other 2D semiconductors such as TMDCs but not in
2D insulators such as boron nitride. This new phenomenon is attributed to
surface charging of the semiconductor nanofilms. This discovery opens up a new
field of research and potential applications in nanoplasmonics and
optoelectronics.
