  Colloidal cadmium chalcogenide nanoplates are two-dimensional semiconductors
that have shown great application prospect for light-emitting technologies.
Self-trapped state (STS), a special localized state originated from strong
electron-phonon coupling (EPC), has great potential in one-step white light
luminance owing to its broadband emission linewidth. However, achieving STS in
cadmium chalcogenide nanocrystals is extremely challenging due to their
intrinic weak EPC nature. By building hybrid superlattice (SL) structures via
self-assembly of colloidal CdSe nanoplates (NPLs), we demonstrated an emergence
of zone-folded longnitude acoustic phonons (ZFLAP) differ from monodispersed
NPLs, and observed a broadband STS emission in spectra range of 450-600 nm.
Through femtosecond transient absorption and impulsive vibrational
spectroscopy, we revealed that STS is generated in time scale of ~500 fs and is
driven by strong coupling of excitons and ZFLAPs with Huang-Rhys parameter as
large as ~22.7. Our findings provide a new avenue for generating and
manipulating STS emission by artificially designing and building hybrid
periodic structures superior to single material optimization.
