  The anisotropic nature of the new two-dimensional (2D) material phosphorene,
in contrast to other 2D materials such as graphene and transition metal
dichalcogenide (TMD) semiconductors, allows excitons to be confined in a
quasi-one-dimensional (1D) space predicted in theory, leading to remarkable
phenomena arising from the reduced dimensionality and screening. Here, we
report a trion (charged exciton) binding energy of 190 meV in few-layer
phosphorene at room temperature, which is nearly one to two orders of magnitude
larger than those in 2D TMD semiconductors (20-30 meV) and quasi-2D quantum
wells (1-5 meV). Such a large binding energy has only been observed in truly 1D
materials such as carbon nanotubes, whose optoelectronic applications have been
severely hurdled by their intrinsically small optical cross-sections.
Phosphorene offers an elegant way to overcome this hurdle by enabling quasi-1D
excitonic and trionic behaviors in a large 2D area, allowing optoelectronic
integration. We experimentally validated the quasi-1D nature of excitonic and
trionic dynamics in phospherene by demonstrating completely linearly polarized
light emission from excitons and trions. The implications of the
extraordinarily large trion binding energy in a higher-than-one-dimensional
material are far-reaching. It provides a room-temperature 2D platform to
observe the fundamental many-body interactions in the quasi-1D region. The
strong photoluminescence emission in phosphorene has been electrically tuned
over a large spectral range at room temperature, which opens a new route for
tunable light sources.
