Excitonic recombination dynamics mediated by polymorph transformation in cadmium sulfide nanocrystals†
The excitonic recombination dynamics could be mediated by the direct/indirect-to-indirect/direct bandgap transition and the different overlap between electron and hole wave functions caused by polymorph transformation. Here, the direct bandgap zinc-blende semiconductor is unexpectedly observed as an intermediate phase in polymorph transformation from direct bandgap wurtzite to indirect bandgap rock-salt CdS semiconductor nanocrystals. The high-pressure phase of the rock-salt CdS structure has been recovered to the ambient conditions. The radiative recombination lifetimes at the band edge states are 6.167 ns, 8.027 ns and 19.325 ns for the wurtzite, zinc-blende and rock-salt CdS nanocrystals, respectively. The longest radiative recombination lifetime of the indirect bandgap rock-salt CdS semiconductor is probably attributed to the additional requirement for phonon assistance. The increased radiative recombination lifetime of the direct bandgap zinc-blende CdS semiconductor mainly results from the reduced electron–hole wave function overlap. The ability to tune the structure of a nanoscale semiconductor with exquisite precision opens up a new opportunity for mediating the excitonic recombination dynamics within only a single material.