pH and concentration dependence of the optical properties of thiol-capped CdTe nanocrystals in water and D₂O

Abstract

The optical properties of semiconductor nanocrystals (SC NCs) are largely controlled by their size and surface chemistry, i.e., the chemical composition and thickness of inorganic passivation shells and the chemical nature and number of surface ligands as well as the strength of their bonds to surface atoms. The latter is particularly important for CdTe NCs, which - together with alloyed CdxHg1-xTe - are the only SC NCs that can be prepared in water in high quality without the need for an additional inorganic passivation shell. Aiming at a better understanding of the role of stabilizing ligands for the control of the application-relevant fluorescence features of SC NCs, we assessed the influence of two of the most commonly used monodentate thiol ligands, thioglycolic acid (TGA) and mercaptopropionic acid (MPA), on the colloidal stability, photoluminescence (PL) quantum yield (QY), and PL decay behavior of a set of CdTe NC colloids. As an indirect measure for the strength of the coordinative bond of the ligands to SC NC surface atoms, the influence of the pH (pD) and the concentration on the PL properties of these colloids was examined in water and D2O and compared to the results from previous dilution studies with a set of thiol-capped Cd1-xHgxTe SC NCs in D2O. As a prerequisite for these studies, the number of surface ligands was determined photometrically at different steps of purification after SC NC synthesis with Ellman's test. Our results demonstrate ligand control of the pH-dependent PL of these SC NCs, with MPA-stabilized CdTe NCs being less prone to luminescence quenching than TGA-capped ones. For both types of CdTe colloids, ligand desorption is more pronounced in H2O compared to D2O, underlining also the role of hydrogen bonding and solvent molecules.

Effect of the nature of dispersion medium on the CdTe/TGA nanocrystal formation in colloidal solutions and polymeric membranes

“…The method of colloidal synthesis allows obtaining CdTe NCs with hydrodynamic radius within the range 1…1000 nm and pronounced PL properties. However, the environment and the chemical reaction conditions have a significant effect on the kinetics of nanocrystal growth [12,17,18]. The analysis of data obtained in the previous section suggests that methanol, ethanol, and propanol are the best used as the dispersion medium during colloidal synthesis of CdTe nanocrystals.…”

“…The use of thioglycolic acid (TGA) as a stabilizer of the surface of nanosized CdTe particles in the course of their preparation has been already described in the literature [11,[13][14][15][16][17][18]. In this work, to understand the mechanisms of surface stabilization acting in CdTe micro-and nanocrystals, the influence of introducing microquantities of this stabilizer not only into the reaction medium, but also into the already formed suspensions of cadmium telluride was investigated.…”

Управление Синтезом И Составом Полупроводников. Малодефектный Карбид Кремния И Низкоразмерный Селенид Кадмия

“…С помощью ПАВ можно регулировать форму растущих кристалликов и получать анизотропные структуры -палочки, тетраподы (тетраэдроподобные), кубы и др. [21,22].…”

Photoluminescence properties of self-assembled chitosan-based composites containing semiconductor nanocrystals

“…It is well known that the acidity of the solution tremendously influences the PL properties of thiol-capped NCs. [23][24][25][26][27][28] Thus, the increasing of acid concentration results in the irreversible loss of NCs PL properties. HCl concentration up to 3 mM.…”

“…The maximal content of NCs (on example of CdTe NCs with A=0.058) was estimated as 1.2 µM. 17 Assuming the number of thiol groups on the surface of a single NC to vary from 50 to 200, 23 the calculated variation range of their concentration lies between 3.5×10 -5 M and 1.4×10 -4 M. The pKa of TGA is 3.7 and MPA is 4.3, 23 therefore the dissociation degree of carboxyl groups can be assumed as almost complete (95-99%). Thus, the addition of NCs with available carboxyl groups results in their electrostatic linking to chitosan.…”