  One of the main interests of 2D materials is their ability to be assembled
with many degrees of freedom for tuning and manipulating excitonic properties.
There is a need to understand how the structure of the interfaces between
atomic layers influences exciton properties. Here we use cathodoluminescence
(CL) and time-resolved CL experiments to study how excitons interact with the
interface between two twisted hexagonal boron nitride (hBN) crystals with
various angles. An efficient capture of free excitons by the interface is
demonstrated, which leads to a population of long lived and interface-localized
(2D) excitons. Temperature dependent experiments indicate that for high twist
angles, these excitons localized at the interface further undergo a
self-trapping. It consists in a distortion of the lattice around the exciton on
which the exciton traps itself. Our results suggest that this exciton-interface
interaction causes a broad optical emission of highly twisted hBN-hBN
structures around 300 nm (4 eV). Exciton self-trapping is finally discussed as
a common feature of sp2 hybridized boron nitride polytypes and nanostructures
due to the ionic nature of the B-N bond and their compact excitons.
