The systematic study of the precursor ratios effect in the Cd-Zn-S quantum dots synthesis

Semiconductor Quantum Dots (QDs) have attracted a 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 precursors' ratios within the synthesis of the Cd-Zn-S QDs series. The 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‘s 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 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 and 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 %), narrow photoluminescence (PL) signal (FWHM < 31 nm) and the size uniformity of the obtained nanomaterials are promising features for high-sensitive sensors and LEDs production.