  Excitons, quasi particles composed of an electron and a hole, play an
important role in optical responses in low-dimensional nanostructures. In this
work, we have investigated exciton diffusion in a monolayer MoSe2 encapsulated
between flakes of hexagonal boron nitrides (hBN/MoSe2/hBN). Through PL imaging
and numerical solving the 2D diffusion equation, we revealed that temperature
dependence of exciton mobility in the hBN/MoSe2/hBN shows non-saturating
increase at low temperature, which is qualitatively different from those of
quantum wells composed of compound semiconductors. Ultraflat structure of
monolayer MoSe2 in the hBN/MoSe2/hBN probably leads to the suppression of
charged-impurity scattering and surface-roughness scattering.
