Adsorption and binding of capping molecules for highly luminescent CdSe nanocrystals--DFT simulation studies

During CdSe nanocrystal growth, loss of surface capping molecules occurs leading to a decrease of photoluminescence (PL) quantum yield. In general, aliphatic capping molecules are applied to passivate the surface of CdSe nanocrystals to modulate the optical properties of the CdSe. In this work, two kinds of alkylamine (n-butylamine (n-BA) and n-hexylamine (n-HA)) and oleic acid (OA) were used to modify the surfaces of the CdSe nanocrystals. From the PL spectra and quantum yield analyses, we observed that the PL emission peak positions of the modified CdSe nanocrystals have blue shifted for all three capping molecules. However, the PL quantum yield of the CdSe nanocrystals increased after introduction of the alkylamine molecules, but decreased with oleic acid. The detailed mechanism was not clear until now. In this study, a density function theory (DFT) simulation was employed to demonstrate binding energy and charge analyses of CdSe with n-BA, n-HA and OA. By comparing the binding energy of the bare CdSe nanocrystals to that of the CdSe with the capping molecules, it was shown that n-BA and n-HA as capping molecules help to increase the charge on Se and decrease it on cadmium of the CdSe.