1-Octadecene (45 mL) was injected as a noncoordinating solvent into the flask at 150 °C and heated to 310 °C under N2 environment, yielding a clear solution of Cd(OA)2 and Zn(OA)2. At the elevated temperature of 300 °C, 0.6 mmol of Se and 12 mmol of S dissolved in 12 mL trioctylphosphine (TOP) was injected into the vessel containing Cd(OA)2 and Zn(OA)2. The reaction proceeded for 10 min for chemically graded CdSe@ZnS QDs. Benzenethiol solutions (5 to 20 mmol) were introduced in 20 mL vials for ligand exchange, and the solutions were stirred during 24 h, followed by injecting 20 mL hexane in each bezenethiol ligand exchanged CdSe@ZnS solutions. These solutions were centrifuged 12,000rpm for 10min to remove the by-product which are not soluble in hexane solvent. The resulting QDs solutions were purified thrice by precipitation and centrifugation with a solvent combination of hexane and anhydrous ethanol (1:4 volume ratio) by centrifugation (10,000 rpm, 10 min), Finally, the benzenethiol-capped QDs were dispersed in hexane at 10 mg/mL.
Synthesis of ZnO NPs
ZnO nanoparticles (NPs) were synthesized from zinc acetate dehydrate and tetramethyl-ammonium hydroxide (TMAH) for electron transfer layers. TMAH in ethanol was dropped at a rate of 1mL/min into 0.1 M zincacetate dihydrate in dimethyl sulfoxide (DMSO), and this solution was stirred for an hour at 4 °C. The ZnO nanoparticles were collected by centrifugation at 10,000 rpm for 10 min and dispersed in 25 mL ethanol.
Fabrication of QD-LEDs and Electron Only