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Semiconductor quantum dots (QDs) have attracted great attention due to their unique optical and chemical properties. An increasing demand for size and shape uniformity of QDs, high quantum yield (QY) and photochemical stability can be resolved by synthetic methods. Here we report a study of the effect of the precursor ratios within the synthesis of a series of Cd–Zn–S QDs. Trisubstituted thiourea (N-phenylmorpholine-4-carbothioamide) as a new and environmentally friendly source of sulphur in the synthesis of Cd0.2Zn0.8S QDs (Cd, Zn : S molar ratios = 1 : 0.5; 1 : 1; 1 : 1.5; 1 : 2; 1 : 2.5) and CdxZn1−xS QDs (where x = 0.1; 0.25; 0.5; 0.75; 0.9) has been investigated. It was determined that an increase in the molar ratio of trisubstituted thiourea to metals leads to the growth of QDs and as a consequence, to a small emission red shift in the visible region of the spectrum. However, no significant changes in the elemental ratio in the material were detected. It was found that with an excess of trisubstituted thiourea taken to the reaction, oleylamine (OAm) as a co-ligand replaces linoleic acid from the protective shell of the QDs. In the case of an increase of Cd content in the CdxZn1−xS QDs, a significant shift in the emission to the red region of the spectrum was observed with an increase in the size of the QDs while their shape was unchanged. Based on the XRD data, a gradual transition from a cubic to a hexagonal crystal structure was detected. The high quantum yield (70%), the narrow photoluminescence (PL) signal (FWHM < 31 nm) and the size uniformity of the obtained nanomaterials are promising features for production of highly sensitive sensors and LEDs. [ABSTRACT FROM AUTHOR]

The surface of any binary or multi-component nanocrystal has imperfections and defects. The number of surface defects depends both on the nature of the nanomaterial and on the method of its preparation. One of the possibilities to confine the number of surface defects is the epitaxial growth of the shell, which leads to a change in the physical properties while maintaining the morphology of the core. To form a shell of the desired thickness, an accurate calculation of the amount of its precursors is substantial to avoid the appearance of individual crystals consisting of the shell material. This study aimed to develop an effective calculation method for the theoretical amount of precursors required for the formation of a ZnS shell on the surface of a Cd0.25Zn0.75Se core, followed by the practical implementation of theoretical calculations and characterization of the prepared nanomaterials. This method allows the complete control of the masses and volumes of the initial reagents, which will in turn prevent undesirable nucleation of nuclei consisting of the shell material. In the synthesis of Cd0.25Zn0.75Se/ZnS core/shell quantum dots (QDs), the sources of chalcogens were substituted seleno- and thioureas, which are capable of not only supplanting modern toxic sources of sulfur and selenium but also allowing one to perform the controlled synthesis of highly photoluminescent QDs with a low number of surface defects. The result of this shell overcoating method was an impetuous augmentation in the photoluminescence quantum yield (PL QY up to 83%), uniformity in size and shape, and a high yield of nanomaterials. The developed synthetic technique of core/shell QDs provides a controlled growth of the shell on the core surface, which makes it possible to transfer this method to an industrial scale. [ABSTRACT FROM AUTHOR]

Doping of semiconductor quantum dots (QDs) by transition metals (such as Mn, Cu, Ni, etc.) is a great technique to impact the optical and structural properties of the host materials. The introduction of a small amount of impurity into the host semiconductor matrix leads to the shift of PL emission due to the change in the recombination process. The new emission pathways created by dopant influence the photoluminescence intensity, Stokes shift, lifetime and stability of QDs. Mn-doped ZnS QDs (1–15 mol. %) have been synthesized by the effective low-cost and nontoxic hot-injection method. N-phenylmorpholine-4-carbothioamide has been successfully applied as a novel source of sulfur in the synthesis of Mn-doped ZnS QDs. X-ray diffraction (XRD), scanning transmission electron microscopy (STEM) and infrared (IR) spectroscopy were used for the characterization of morphology and structure of synthesized Mn-doped ZnS QDs. Our materials exhibit strong photoluminescence yellow-orange emission with a photoluminescence quantum yield (PL QY) of 41%. All Mn-doped ZnS QDs have a face-centered cubic structure with a mean crystalline size of 3.8–5.1 nm. Additionally, this synthetic approach enables the synthesis of Mn-doped ZnS QDs with tunable band gaps between ∼3.5 eV and 3.60 eV. • Mn-doped ZnS QDs were synthesized by an effective environment-friendly synthetic approach. • N-phenylmorpholine-4-carbothioamide was successfully employed as a novel sulfur source. • An optimum concentration of Mn doping for the highest photoluminescence quantum yield (PL QY) was determined. • Significant enhancement in PL QY up to 5% Mn doping was observed for Mn-doped ZnS QDs. [ABSTRACT FROM AUTHOR]