From wurtzite nanoplatelets to zinc blende nanorods: Simultaneous control of shape and phase in ultrathin ZnS nanocrystals

TL;DR

This paper presents a method to synthesize ultrathin ZnS nanocrystals with controlled shape and phase, enabling tunable optical properties for advanced electronic and photonic applications.

Contribution

It introduces a soft template synthesis strategy that allows simultaneous control of shape and phase in ZnS nanocrystals, which was not previously achievable.

Findings

Successful synthesis of ZnS nanoplatelets and nanorods with distinct optical properties.

UV-vis and PL spectra confirm shape and phase-dependent optical features.

Density functional theory explains the electronic transitions in ZnS nanocrystals.

Abstract

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.