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Understanding the size-dependent structures and properties of ligand-capped nanoclusters in solvent is of particular interest for the design, synthesis and application of II-VI colloidal QDs. Using DFT and TDDFT calculations, we studied the structure and optical property evolution of the cysteine-capped (CdSe)N clusters of N = 1-10, 13, 16 and 19 in gas, toluene, water and alkaline aqueous solution, and made a comparison with their corresponding bare clusters. The cysteine binds with (CdSe)Nvia several patterns depending on the medium they exist in, affecting the cluster structures and in consequence their optical absorption. In general, the absorption bands of (CdSe)N blueshift when cysteine is added, and the shift varies with the interaction strength between the cluster and the ligand, and the dielectric constant of the solvent. However, bare clusters retain their size sensitivity, in particular the redshift trend with increasing cluster size, and some similarity was noted for the optical absorption of the bare and ligated clusters regardless of the gas or solvent media. Population analysis reveals that the excitations are mainly from orbitals distributing on the (CdSe)N part, while the ligand is negligibly involved in the excitations. This is an important feature for the II-VI QDs as biosensors with which the information of biomolecules is detected from the size dependent optical absorption or emission of the QDs other than the biomolecules.

Although several physical models have been proposed to interpret the photoluminescence intermittency that hinders the applications of single semiconductor quantum dots, the structure dependence of radiative and nonradiative recombination of excitons remains unclear. In order to address the structure−property correlation of II−VI quantum dots, the variations of radiative and nonradiative decay rate constants, k r and k nr , of (CdSe) 13 clusters with structural descriptors are studied using first-principles calculations together with the semiclassical two-state model. Attributed to their small size and diverse structures, the four isomers of (CdSe) 13 exhibit distinct optical absorption and emission properties. Their k r and k nr values vary in a wide range, though they have the same cluster size. While a previously suggested correlation between k r and electron−hole spatial overlap is verified, dependence of k nr on reorganization energy, Huang−Rhys factor, transition energy, etc. is explored. Correlation of k nr with Huang−Rhys factor is further analyzed in terms of vibrational modes and cluster shapes. In addition, a descriptor based on chemical softness, Δs = s + − s − , is proposed, and a good correlation between k nr and Δs is revealed. The revealed structure-dependent radiative and nonradiative electron−hole recombination behaviors would be helpful for understanding and suppressing the photoluminescence intermittency in semiconductor quantum dots.

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