Structure-property correlation of CdSe clusters using experimental results and first-principles DFT calculations

Structures and properties of CdSe quantum dots (clusters) up to a diameter of ∼2 nm were investigated by combining experimental absorption, photoluminescence (PL), and X-ray diffraction (XRD) spectroscopies as well as ab initio DFT calculations. These CdSe clusters were nucleated and grown from solutions containing respective cadmium and selenium precursors following the hot-injection technique that allows one to obtain size-controlled CdSe clusters having PL efficiency up to 0.5. The DFT calculations were performed at the B3LYP/Lanl2dz level and followed by time-dependent TDDFT calculations to estimate n energy singlet transitions. On the basis of the results of these experimental and theoretical studies, an approach to determine whether the proposed cluster with a mean diameter of ∼2 nm is more physically reasonable is discussed. It was shown that the minimum nucleus of a CdSe cluster consists of (CdSe)₃ with a six-membered ring and planar structure. No PL is observed for this structure. The formation of the next stable cluster depends on whether hexadecylamine (HDA) was used for the growth of the CdSe clusters. In the absence of HDA, the second cluster was found to be (CdSe)₆ characterized by a broad PL spectrum, while in the presence of HDA, it was found to be (CdSe)_n (where n ≥ 14) with a sharp PL spectrum.