Highly Efficient and Controllable Methodology of the Cd0.25Zn0.75Se/ZnS Core/Shell Quantum Dots Synthesis

Abstract

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.

The Versatile Synthesis of Polyhedron $\text{Core}/\text{shell}/\text{shellCd}_{0.1}\text{Zn}_{0.9}\text{Se}/\text{Cd}_{\mathrm{X}}\text{Zn}_{1-\mathrm{X}}\mathrm{S}/\text{ZnS}$ Quantum Dots

“…Selenium (Se, 99.9 %), phenyl isothiocyanate (PhNCS, 98 %), morpholine (99 %), aniline (PhNH2, 99 %), cadmium oxide (CdO, 99.9 %), zinc oxide (ZnO, 99 %), 1-octadecene (ODE, technical grade, 90 %), linoleic acid (LA, technical grade 60-74 %), oleylamine (OAm, technical grade, 70 %), chloroform-d (CDCl3, 99.8 atom % D) and silica gel (highpurity grade, pore size 60 Å, 230-400 mesh particle size) were purchased from Sigma-Aldrich and used without further purification. ((Z)-N-(octadec-9-enyl)morpholine-4carboselenoamide (SU) and (Z)-1-(octadec-9-enyl)-3phenylthiourea (TU) were synthesized in our laboratory previously [14]. Syntheses of QDs were carried out using standard Schlenk techniques under an inert atmosphere.…”

“…Synthesis of Cd0.1Zn0.9Se QDs was conducted similarly to that published earlier [14] with recalculation of the initial components amounts to obtain the corresponding composition. Briefly, a mixture of Cd and Zn linoleates was carried out from CdO (0.81 g, 0.00632 mol), ZnO (4.62 g, 0.0568 mol), LA (53.3 g, 0.19 mol) and 160 ml ODE at 150 °C for 1 hour.…”

“…The calculation of the materials required for the deposition of 2 MLs of the shell was conducted according to the scheme published earlier [14]. Cd0.1Zn0.9Se QDs (core, 1.0 g) were introduced into a freshly prepared solution of cadmium and zinc linoleates with a given ratio and a total volume of 25 ml.…”

“…And, despite the widespread use of the corresponding phosphines as sources of chalcogens [12], substituted thio-and selenoureas also proved to be highly reactive precursors [13], which make it possible to create homogeneous and highly photoluminescent nanomaterials. It should be emphasized that the product conversion, in the case of such sources of sulfur and selenium, is close to 100 % [14].…”

“…The deposition of a zinc sulfide shell onto the Cd 0.25Zn0.75Se core with the participation of disubstituted thiourea was for the first time performed by our group [14]. The development of this method has led to the creation of the polyhedron Cd0.1Zn0.9Se/CdxZn1-xS/ZnS (x = 0.1; 0.25; 0.4; 0.6; 0.75) core/shell/shell QDs series.…”