  Ultrathin two-dimensional nanosheets raise a rapidly increasing interest due
to their unique dimensionality-dependent properties. Most of the
two-dimensional materials are obtained by exfoliation of layered bulk materials
or are grown on substrates by vapor deposition methods. To produce
free-standing nanosheets, solution-based colloidal methods are emerging as
promising routes. In this work, we demonstrate ultrathin CdSe nanosheets with
controllable size, shape and phase. The key of our approach is the use of
halogenated alkanes as additives in a hot-injection synthesis. Increasing
concentrations of bromoalkanes can tune the shape from sexangular to
quadrangular to triangular and the phase from zinc blende to wurtzite. Geometry
and crystal structure evolution of the nanosheets take place in the presence of
halide ions, acting as cadmium complexing agents and as surface X-type ligands,
according to mass spectrometry and X-ray photoelectron spectroscopies. Our
experimental findings show that the degree of these changes depends on the
molecular structure of the halogen alkanes and the type of halogen atom.
