  Two-dimensional (2D) semiconductors are promising candidates for
optoelectronic application and quantum information processes due to their
inherent out-of-plane 2D confinement. In addition, they offer the possibility
of achieving low-dimensional in-plane exciton confinement, similar to
zero-dimensional quantum dots, with intriguing optical and electronic
properties via strain or composition engineering. However, realizing such
laterally confined 2D monolayers and systematically controlling size-dependent
optical properties remain significant challenges. Here, we report the
observation of lateral confinement of excitons in epitaxially grown in-plane
MoSe2 quantum dots (~15-60 nm wide) inside a continuous matrix of WSe2
monolayer film via a sequential epitaxial growth process. Various optical
spectroscopy techniques reveal the size-dependent exciton confinement in the
MoSe2 monolayer quantum dots with exciton blue shift (12-40 meV) at a low
temperature as compared to continuous monolayer MoSe2. Finally, single-photon
emission was also observed from the smallest dots at 1.6 K. Our study opens the
door to compositionally engineered, tunable, in-plane quantum light sources in
2D semiconductors.
