  Ultrathin semiconductor nanocrystals (NCs) with at least one dimension below
their exciton Bohr radius receive a rapidly increasing attention due to their
unique physicochemical properties such as strong quantum confinement, large
surface-to-volume ratio, and giant oscillator strength. These superior
properties highly depend on the shape and crystal phase of semiconductor NCs.
Slight changes in the shape and phase of NCs can cause significant changes in
their properties. Therefore, it is crucial to controllably synthesize
semiconductor NCs. Here, we demonstrate not only the synthesis of robust
well-defined ultrathin ZnS nanoplatelets (NPLs) with excitonic absorption and
emission, but also the precise shape and phase control of ZnS NCs based on a
soft template strategy. The key feature of our approach is the tuning of the
sulfur precursor amount, resulting in a simultaneous shape/phase transformation
between wurtzite (WZ) ZnS NPLs and zinc blende (ZB) ZnS nanorods (NRs) at
moderate temperatures (150 degree). UV-vis absorption and photoluminescence
(PL) spectra reveal very distinct optical properties between WZ-ZnS NPLs and
ZB-ZnS NRs. UV-vis absorption spectra of WZ-ZnS NPLs clearly exhibit a sharp
excitonic peak that is not observed in ZB-ZnS NRs. Besides, the PL
characterization shows that WZ-ZnS NPLs have a narrow excitonic emission peak
(292 nm), while the ZB-ZnS NRs exhibit a broad collective emission band
consisting of four emission peaks (335, 359, 395, and 468 nm). The appearance
of excitonic features in the absorption spectra of ZnS NPLs is explained by
interband electronic transitions, which is simulated in the framework of
density functional theory (DFT). The presented simple and effective synthetic
strategy opens a new path to synthesize further NCs with shape and phase
control for advanced applications in electronics and photonics.
