CdS Magic-Sized Nanocrystals Exhibiting Bright Band Gap Photoemission via Thermodynamically Driven Formation

Abstract

CdS magic-sized nanocrystals (MSNs) exhibiting both band gap absorption and emission at 378 nm with a narrow bandwidth of approximately 9 nm and quantum yield (QY) of approximately 10% (total QY approximately 28%, in hexane) were synthesized via a one-pot noninjection approach. This CdS MSN ensemble is termed as Family 378. It has been acknowledged that magic-sized quantum dots (MSQDs) are single-sized, and only homogeneous broadening contributes to their bandwidth. The synthetic approach developed is ready and highly reproducible. The formation of the CdS MSQDs was carried out at elevated temperatures (such as 90-140 degrees C) for a few hours in a reaction flask containing bis(trimethylsilyl)sulfide ((TMS)(2)S) and Cd(OAc)(OA) in situ made from cadmium acetate dihydrate (Cd(OAc)(2).2H(2)O) and oleic acid (OA) in 1-octadecene (ODE). Low OA/Cd and high Cd/S feed molar ratios favor this formation, whose mechanism is proposed to be thermodynamically driven. (13)C solid-state cross-polarization magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) demonstrates that the capping ligands are firmly attached to the nanocrystal surface via carboxylate groups. With the cross-polarization from (1)H of the alkyl chains to surface (113)Cd, (113)Cd NMR is able to distinguish the surface Cd (471 ppm) bonding to both -COO(-) and S and the bulk Cd (792 ppm) bonding to S only. DOSY-NMR was used to determine the size of Family 378 ( approximately 1.9 nm). The present study provides strategies for the rational design of various MSNs.