A study of optical absorption of cysteine-capped CdSe nanoclusters using first-principles calculations

Abstract

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.

Nature of electronic excitations in small non-stoichiometric quantum dots

“…Such computational methodology has been extensively tested and used across a broad variety of previous modeling studies. 31,35,36,47,69,70 Hirshfeld 61 charge analysis, as applied in Multiwfn 71 v3.7 software package, was used to obtain the charge distribution on all the atoms and calculate the Inverse Participation Ratios (IPRs). We obtained the partial density of states (PDOS) plots from Gaussian outputs using GaussSum software package 72 and visualized atomic structures and orbitals using the VESTA 73 v3.5.7 software.…”

Effect of Chiral Ligand Concentration and Binding Mode on Chiroptical Activity of CdSe/CdS Quantum Dots

“…Density functional theory (DFT) calculations 59 revealed that the interaction patterns between Cys and (CdSe) n vary with the cluster size and medium. Pattern S → Cd ← N is preferred in the gas phase, toluene, and alkaline solution, while pattern O → Cd and H → Se is preferred in water.…”

Capping Structure of Ligand–Cysteine on CdSe Magic-Sized Clusters

“…43 First-principles calculation on cysteine-coordinated (CdSe) N nanoclusters ( N = 1–10, 13, 16, and 19) shows that the sulfur and oxygen atoms of cysteine in alkaline solution cannot simultaneously coordinate to a cadmium atom at the surface of CdSe nanoclusters because of the structural rigidity of cysteine. 44 Hence, in cysteine-capped (CdSe) 34 , it is also unlikely for the carboxylate group to form bridging coordination to sodium and cadmium. To conclude, the carboxylate group of ligand–cysteine binds to the counter Na + ions in the solid-state CdSe-Cys and not to the CdSe core surface.…”