Synthesis of Extremely Small CdSe and Highly Luminescent CdSe/CdS Core–Shell Nanocrystals via a Novel Two‐Phase Thermal Approach

Abstract

Some core±shell semiconductor nanocrystals, such as CdSe/ CdS and CdS/ZnS, show high luminescence properties and stability against photo-oxidation due to passivation of the nanocrystal surface. The surfaces of these nanocrystals are capped with an inorganic shell with wider bandgaps, e.g., CdS or ZnS, which have been widely used in light-emitting diodes (LEDs) [1] and biological labeling. [2] Because the inorganic shell provides a passivation of the nanocrystal surface, which effectively eliminates, to a great extent, the dangling bonds or coordination sites that form traps for photogenerated carriers, photogenerated carriers inside the nanocrystals will be mostly confined in the core, resulting in a higher quantum yield (QY). Among such core±shell systems, CdSe/CdS and CdSe/ZnS have been intensively studied over the past decade. As reported in the literature, the photoluminescence (PL) quantum yields for as-prepared core±shell nanocrystals at room temperature are 10±80 % in the spectral range of 470 to 630 nm; the nanocrystals were synthesized via an organometallic approach [3] and its alternative routes.[4] Until now, emission of pure blue color (PL peak~455 nm) has not been reported for CdSe/CdS or CdSe/ZnS core±shell nanocrystals because it is difficult to synthesize extremely small (< 1.6 nm diameter) nanocrystals of CdSe with narrow size distributions by the above-mentioned organometallic approaches and their alternative routes.[5] Generally, they are produced at relatively low temperatures (90±140 C) and a long reaction time.[6] However, lower temperatures will lead to a longer nucleation time, which results in polydisperse nanocrystals. Hence, it is a challenge to prepare extremely small nanocrystals of CdSe with narrow size distributions and CdSe/CdS core±shell nanocrystals whose emission is pure blue, through organometallic approaches and their alternative routes. Previously, we have prepared highly luminescent and nearly monodisperse CdS nanocrystals by a two-phase approach under mild conditions (< 100 C) [7] and found that a slow nucleation does not always lead to polydisperse colloids if the growth time of the nanocrystals is very long. Since decomposition rates of selenourea and thiourea are controllable by changing the reaction temperature, the size of the nanocrystals and their distribution can be controlled through tuning a balance between nucleation and growth. Herein, we took advantage of an autoclave to obtain a high temperature and pressure for the nucleation and growth of CdSe and CdSe/ CdS core±shell nanocrystals. The extremely small CdSe and highly luminescent CdSe/CdS core±shell quantum dots with CdSe cores ranging in diameter from 1.2 to 1.5 nm were synthesized in a two-phase system in the autoclave. Cadmium myristate (Cd-MA) and selenourea (thiourea) were used as a source of cadmium and selenium (sulfur), respectively. Oleic acid (OA) was used as the only capping agent. A solution of Cd-MA and OA in toluene and a solution of selenourea (or thiourea) in water were mixed and heated in ...

Growth of nanocrystals and thin films at the water–oil interface

“…Thus, the quality of the crystallites obtained herein, judged in terms of monodispersity and emission widths, is similar to those obtained by thermolysis of organometallic precursors. The gradual growth observed herein is in line with slow dynamics associated with interfacial transport processes (Birdi 1997;Binks 2002) and have been observed previously in semiconductor nanocrystals grown at the interface (Pan et al 2004(Pan et al , 2005(Pan et al , 2008(Pan et al , 2009. Significantly, throughout this long period of growth, the nanocrystals retain their monodisperse nature.…”

“…As alluded to earlier, CdS and CdSe nanocrystals have been synthesized in autoclaves at modest temperatures using Cd salts of a long-chain carboxylic acid and aqueous thio-or seleno urea (Pan et al 2004(Pan et al , 2005(Pan et al , 2008(Pan et al , 2009. In the present report, open vessels and air-stable precursors in common use have been employed for synthesizing high-quality CdS nanocrystals at a temperature a few degrees above the ambient.…”

Controlled Synthesis of CdSe Quantum Dots by a Microwave‐Enhanced Process: A Green Approach for Mass Production

“…However, the fluorescence of the CdSe products obtained by this approach cannot be controlled over a wide range. [14] Thus, it is still a challenging task to develop a method for preparing CdSe QDs with the desired quality under mild and environmentally friendly conditions using a low-cost and simple method. See Tables S4 and S5 in the Supporting Information for comparison of CdSe quantum dots obtained by various methods.…”

Highly efficient orange–red electroluminescence from a single layer MEH-PPV-POSS:CdS0.75Se0.25 hybrid PLED

“…Ternary-alloyed colloidal nanocrystals were synthesized in one step and one-pot based on the two -phase method introduced by Pan et al for core/shell QDs [15]. In their synthetic pathway, the colloidal CdSe nanocrystals was first prepared and purified, and then in the second-step, thiourea and cadmium myristate were added to the purified CdSe solution to form core-shell CdSe/CdS nanocrystal [15].…”

“…In their synthetic pathway, the colloidal CdSe nanocrystals was first prepared and purified, and then in the second-step, thiourea and cadmium myristate were added to the purified CdSe solution to form core-shell CdSe/CdS nanocrystal [15]. We modified the method to synthesize alloyed QDs.…”