Ultrathin semiconductor nanocrystals (NCs) with at least one dimension below their excitonBohr 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 welldefined 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 °C).
UV-vis absorption and photoluminescence (PL) spectra reveal very distinct optical properties betweenWZ-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.
We calculate the optical properties of InP and CdSe colloidal quantum dots (QDs) within the framework of the atomic effective pseudopotential approach and the screened configuration interaction theory. We obtain an excellent agreement with experiment with our microscopic and space-dependent screening function where the dielectric constant varies in real space with a sharp transition (width of ≈0.18 nm) from the QD material high-frequency bulk value inside the QD to the solvent or passivant high-frequency value outside. We obtain a reasonable agreement (with deviations less than 140 meV) for a computationally less demanding solvent-independent screening using the full high-frequency bulk screening, in contrast to the more commonly used reduced QD radius-dependent screening constant. We show theoretically that for QDs passivated with long-chained organic molecules, the influence of the solvent on the optical gap is in the range of 10 meV, while QDs passivated with short ligands can experience shifts in the order of 100 meV. Experiments on CdSe QDs passivated with octadecylphosphonic acid (ODPA, long-chained ligand) in two different solvents (toluene and chloroform) confirm the bandgap dependence. While the optical gap is weakly affected by the environment, the quasiparticle gap and the exciton binding energy show a strong environmental dependence. Finally, we show that the optical bandgap does not depend significantly on the crystal structure (wurtzite or zincblende) or the morphological details (faceted or “spherical” shape).
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