Identifying Clusters and/or Small-Size Quantum Dots in Colloidal CdSe Ensembles with Optical Spectroscopy

It is well-known that optical absorption and photoluminescence (PL) provide information that is sensitive to the size and size distribution of colloidal binary semiconductor quantum dots (QDs). To explore the nature of reaction products, clusters, and/or small-size QDs, we show that it is important to perform as well photoluminescence excitation (PLE) spectroscopy. For two non-hot-injection reactions of cadmium oleate (Cd(OA)₂) and selenium (Se) in 1-octadecene (ODE), we show that sequentially extracted products displayed a similar apparent red shift in both absorption and PL with a full width at half-maximum (fwhm) of ∼30 nm. We demonstrate that one reaction (with the presence of diphenyl phosphine (HPPh₂)) produced multiple types of clusters (with slightly different optical properties) in one ensemble, while the other reaction (without HPPh₂) yielded primarily small-size QDs. Our findings provide evidence for the probable existence of clusters within small-size CdSe QD products, the existence of which complicates the size determination of small-size CdSe QDs.