  Biexcitons have been considered as one of the fundamental building blocks for
quantum technology because of its overwhelming advantages in generating
entangled photon pairs. Although many-body complexes have been demonstrated
recently in mono-layer transition metal dichalcogenides (TMDs), the low
emission efficiency and scale up capability hinder their applications.
Colloidal nanomaterials, with high quantum efficiency and ease of
synthesis/processing, are regarded to be an appealing complement to TMDs for
biexciton sources. However, a progress towards biexciton emission in colloidal
nanomaterials has been challenging largely by small binding energy and
ultrafast non-radiative multiexciton recombination. Here, we demonstrate
room-temperature biexciton emission in Cu-doped CdSe colloidal quantum wells
(CQWs) under continuous-wave excitation with intensity as low as ~10 W/cm2. The
characteristics of radiative biexciton states are investigated by their super
linear emission with respect to excitation power, thermal stability and
transient photophysics. The interaction between the quantum confined host
carriers and the dopant ions increases biexciton binding energy by two folds
compared to the undoped CQWs. Such strong binding energy together with
suppressed Auger recombination and efficient, spectrally narrow
photoluminescence in a quasi-2D semiconductor enables sustained biexciton
emission at room temperature, providing a potential solution for efficient,
scalable and stand-alone quantum devices.
