  Optical properties of atomically thin transition metal dichalcogenides are
controlled by robust excitons characterized by a very large oscillator
strength. Encapsulation of monolayers such as MoSe$_2$ in hexagonal boron
nitride (hBN) yields narrow optical transitions approaching the homogenous
exciton linewidth. We demonstrate that the exciton radiative rate in these van
der Waals heterostructures can be tailored by a simple change of the hBN
encapsulation layer thickness as a consequence of the Purcell effect. The
time-resolved photoluminescence measurements together with cw reflectivity and
photoluminescence experiments show that the neutral exciton spontaneous
emission time can be tuned by one order of magnitude depending on the thickness
of the surrounding hBN layers. The inhibition of the radiative recombination
can yield spontaneous emission time up to $10$~ps. These results are in very
good agreement with the calculated recombination rate in the weak
exciton-photon coupling regime. The analysis shows that we are also able to
observe a sizeable enhancement of the exciton radiative decay rate.
Understanding the role of these electrodynamical effects allow us to elucidate
the complex dynamics of relaxation and recombination for both neutral and
charged excitons.
