  Lead halide perovskite nanocrystals are attractive for light emitting devices
both as electroluminescent and color converting materials, since they combine
intense and narrow emissions with good charge injection and transport
properties. However, most perovskite nanocrystals shine at green and red
wavelengths, the observation of intense and stable blue emission still being a
challenging target. In this work, we report a method to attain intense and
enduring blue emission (470-480 nm), with a photoluminescence quantum yield
(PLQY) of 40%, originated from very small CsPbBr$_3$ nanocrystals
(diameter<3nm) formed by controllably exposing Cs$_4$PbBr$_6$ to humidity. This
process is mediated by the void network of a mesoporous transparent scaffold in
which the zero-dimensional (0D) Cs$_4$PbBr$_6$ lattice is embedded, which
allows the fine control over water adsorption and condensation that determines
the optimization of the synthetic procedure and, eventually, the nanocrystal
size. By temperature dependent photoemission analysis of samples with different
[CsPbBr$_3$]/[Cs$_4$PbBr$_6$] volume ratios, we show that the bright blue
emission observed results from the efficient charge transfer to the CsPbBr$_3$
inclusions from the Cs$_4$PbBr$_6$ host. Our approach provides a means to
attain highly efficient transparent blue light emitting films that complete the
palette offered by perovskite nanocrystals for lighting and display
applications.
