Single-Sized CdSe Nanocrystals with Bandgap Photoemission via a Noninjection One-Pot Approach

摘要

The manuscript addresses the synthesis and characterization of one colloidal CdSe single-sized nanocrystal ensemble exhibiting bandgap absorption and emission peaking at 463 and 465 nm, respectively, with full width at half-maximum of ca. 8 nm. This ensemble is called Family 463; also, these single-sized quantum dots (QDs) are termed as magic-sized quantum dots (MSQDs). A magic-sized quantum dot (MSQD) ensemble is similar to a regular quantum dot (RQD) ensemble but consisting of single-sized nanocrystals; meanwhile, MSQDs do not grow in size any more with longer reaction periods or higher temperature, after their well development during synthesis. The CdSe MSQDs were synthesized via a noninjection one-pot approach with long reaction periods at elevated temperature, with cadmium acetate dihydrate (Cd(OAc)2 · 2H2O) and elemental selenium as Cd and Se sources, respectively, one long-chain fatty acid as ligands, and 1-octadecene (ODE) as a reaction medium. Different carboxylic acids with the carbon-chain length ranging from C2 to C24, the number of which refers to the total number of carbons, were investigated. Deficient acid feed amount with respect to Cd is crucial for the formation of the CdSe MSQDs, leading to the resulting Cd precursor Cd(OAc)_x(OOC-(CH2)_n-CH3)_(2-x) (with tunable activity). It was also found that the fatty acids of C12-C18, with low acid-to-Cd and high Cd-to-Se feed molar ratios and the growth temperature of 200—240 °C, are optimum for MSQD Family 463, regarding the yield, purity, and their bandgap photoluminescent (PL) efficiency. Interestingly, in the initial stage of the formation at the growth temperature of 140—160 °C, there may be an "isomer competing" process. ~(13)C solid-state cross-polarization magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) indicates that the capping carboxylate groups are firmly attached to the nanocrystals, which allows cross-polarization from ~1H of the alkyl chains to ~(113)Cd. Thus ~(113)Cd NMR is able to distinguish surface (422 ppm) bonding to both —COO and Se and bulk (680 ppm) Cd species bonding to Se only. The CdSe MSQD Family 463 possesses long shelf-storage stability. The synthetic approach introduced here is ready and reproducible, featuring large-scale production. The present study on the growth kinetics of CdSe MSQD Family 463 brings insights for rational design strategies toward various MSQD ensembles with different sizes, via the choice of acids and their feed amount affecting the activity of the resulting Cd precursor.