  Active control over the shape, composition and crystalline habit of
nanocrystals is a long sought-after goal. Various methods have been shown to
enable post-synthesis modification of nanoparticles, including the use of the
Kirkendall effect, galvanic replacement and cation or anion exchange, all
taking advantage of enhanced solid-state diffusion on the nanoscale. In all
these processes, however, alteration of the nanoparticles requires introduction
of new precursor materials. Here we show that for cesium lead halide perovskite
nanoparticles, a reversible structural and compositional change can be induced
at room temperature solely by modification of the ligand composition in
solution. Reversible transformation of cubic CsPbX3 nanocrystals to
rhombohedral Cs4PbX6 nanocrystals is achieved by controlling the ratio of
oleylamine to oleic acid capping molecules. This scheme does not only enable
fabrication of high purity monodisperse Cs4PbX6 nanoparticles with controlled
sizes. Rather, dependent on the Cs4PbX6 nanoparticles final size afforded by
reaction time, the back reaction yields CsPbX3 nanoplatelets with controlled
thickness. These results shed some light on the unique properties of this
family of materials, provide hints to the origin of their superb photovoltaic
performance and point at possible routes for their controlled fabrication and
stabilization.
