  We report a structural reconstruction-induced high photoluminescence quantum
yield of 25% in colloidal two-dimensional tin iodide nanosheets that are
synthesized by a hot-injection method. The as-synthesized red-colored
nanosheets of octylammonium tin iodide perovskites at room temperature
transform to white hexagonal nanosheets upon washing or exposure to light. This
structural change increases the bandgap from 2.0 eV to 3.0 eV, inducing a large
Stokes shift and a broadband emission. Further, a long photoluminescence
lifetime of about 1 microsecond is measured for the nanosheets. Such long-lived
broad and intense photoluminescence with large Stokes shift is anticipated to
originate from tin iodide clusters that are formed during the structural
reconstruction. Stereoactive 5s2 lone pair of tin (II) ions perturbs the
excited state geometry of the white hexagonal nanosheets and facilitates the
formation of self-trapped excitons. Such broadband and intensely emitting metal
halide nanosheets are promising for white light-emitting diodes.
