  The study of two-dimensional (2D) van der Waals materials has been an active
field of research in the development of new optoelectronics and photonic
applications over the last decade. Organic-inorganic layered perovskites are
currently some of the most promising 2D van der Waals materials, due to their
exceptional optical brightness and enhanced excitonic effects. However, low
crystal quality and spectral diffusion usually broaden the exciton linewidth,
obscuring the fine structure of the exciton in conventional photoluminescence
experiments. Here, we propose a mechanical approach for reducing the effect of
spectral diffusion by means of hBN-capping on layered perovskites with
different thicknesses, revealing the exciton fine structure. We used a
stochastic model to link the reduction of the spectral linewidth with the
population of active charge fluctuation centres present in the organic spacer
taking part in the dynamical Stark shift. Active fluctuation centres are
reduced by a factor of 3.7 to 7.1 when we include hBN-capping according to our
direct spectral measurements. This rate is in good agreement with the analysis
of the overlap between the squared perovskite lattice and the hexagonal hBN
lattice. Van der Waals forces between both lattices cause the partial clamping
of the perovskite organic spacer molecules, and hence, the amplitude of the
dynamical Stark shift characteristic of the spectral diffusion effect is
reduced. Our work provides an easy and low-cost solution to the problem of
accessing important fine-structure excitonic state information, along with an
explanation of the important carrier dynamics present in the organic spacer
that affect the quality of the optical emission.
