Surface-Induced Deprotonation of Thiol Ligands Impacts the Optical Response of CdS Quantum Dots

Abstract

Cadmium chalcogenide quantum dots (QDs) passivated by thiol-based ligands exhibit several advantages in their applications in lighting, sensing, and imaging technologies. However, their emission is sensitive to thiol concentrations, pH conditions, and temperatures. Using calculations based on the density functional theory, we identify conditions for thiol/thiolate equilibrium at the CdS QD surface that either eliminate or introduce optically inactive hole trap states favoring or disfavoring the emission. Our calculations indicate much weaker interactions between the QD and protonated species (thiols), compared to their deprotonated counterparts (thiolates). Additionally, the surface of CdS QD facilitates the partial deprotonation of thiols, leading to the formation of an additional stable networking conformation where the proton is shared between the ligand and the QD surface. Thiolates strongly reduce the optical intensity of low-energy transitions in CdS QDs, contributing thiolate-localized hole trap states at the QD band gap. However, networking between the thiols and the surface, as well as the presence of native ligands such as primary amines, stabilize such trap states brightening the lowest optical transitions. This explains the increased emission of thiol-passivated QDs at lower concentrations in neutral or acidic solutions. Surface-mediated bias toward deprotonated species and their contribution to optically inactive states also rationalizes irreversible emission quenching and bleaching in the CdSe/CdS QDs exposed to high temperatures or intensive laser pulse.

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.…”

“…Several simulation techniques, ranging from the adiabatic ground state to non-adiabatic excited-state calculations, have been applied to understand the photo-chemo-physical properties of QDs in detail. 25–28 However, most of these attempts 29–37 have focused on stoichiometric QDs (with an equal number of cation and anion atoms) that are less frequently obtained from the conventional synthesis. The more common non-stoichiometric QDs, with cation-to-anion ratios deviating from one, that are critical to many optical and photocatalytic applications, remain largely unexplored.…”

Quaternary Alloy Quantum Dots as Fluorescence Probes for Total Acidity Detection of Paper-Based Relics

“…The mechanism of the fluorescence response of CdZnSeS-pATP QDs to acidity is inferred as follows. It is reported that the low fluorescence intensity under acidic conditions results from the dissociation of the thiol ligand on QDs due to protonation of the surface-binding thiolate [ 43 , 44 , 45 ]. As the thiol ligand, pATP (pK a = 6.86) [ 46 ] can also dissociate from CdZnSeS QDs, which causes the direct exposure of CdZnSeS QD surfaces to solvent molecules.…”

The defective nature of CdSe quantum dots embedded in inorganic matrices

“…Instead, molecular simulations have proven to be a reliable approach to investigate the atomic and electronic structure of colloidal QDs. Atomistic modelling methods were used for the characterization of the ground state [22][23][24][25] as well as excited states [26][27][28][29][30][31][32][33][34][35][36] of CdSe QDs. Those theoretical studies investigated the origin of defect emission of colloidal QDs and provide a better understanding of QDs photophysics, giving guidance to the experimental design of novel materials, e.g., by the passivation method.…”