Thermodynamic Study of Ligand Coordination on Lead Halide Perovskite Quantum Dots

Inorganic lead halide perovskite (ILHP) has been extensively studied in thin films and bulk crystals as promising materials for their photovoltaic and optoelectronic properties. These are stable enough to be prepared as colloidal quantum dots that can be dispersed, deposited and used to achieve quantum confinement; however, selection of the suitable surface passivating ligand remains challenge for long term stability of the colloidal quantum dots. Several ligand coordination chemistries used to achieve greater stability or specific functions: these can be installed at time of synthesis in some cases they can also be installed to preexisting quantum dots by ligand exchange reactions, which allows NC growth on coordination to be separately optimized and permits quantitative thermodynamic investigations. Different ligands are exchanged with the in situ ligands, among them zwitterionic ligands and quaternary ammonium salts are especially effective to enhance the stability of lead halide perovskite quantum dots. Here we employed isothermal titration calorimetry techniques to study thermodynamics of ligand coordination, which quantify the enthalpy of ligand coordination, number of ligands coordinating, ligand exchange equilibrium constant and provides insight of on variation of these parameter with size of quantum dots. In this work, we analyzed a recently reported room temperature synthesis of CsPbBr3 perovskite quantum dots and their stability with native ligands. From our results, we conclude that if native ligands are not exchanged with the stronger surface passivating ligands, it leads to growth of intermediate dots but introduction of primary amine allowed a stable size series of QDs with chemically similar surfaces to be achieved. These primary amine stabilized perovskite QDs showed potential starting material that can be analyzed using isothermal titration calorimetry to comprehend thermodynamics of zwitterionic ligand exchange.