Flash-Lamp and Thermal Annealing of Ternary Ag–In–S Quantum Dots Stabilized with Thioglycolic Acid

Photophysical/photochemical applications of colloidal quantum dots (QDs) raise the question of their stability under extreme conditions of elevated temperatures and intense or prolonged light irradiation. On the other hand, thermal or photonic influence can be used to obtain a particular phase composition on purpose. In this work, the effects of thermal annealing and flash-lamp annealing on the phase composition of Ag–In–S QDs were studied by Raman spectroscopy and X-ray diffraction. Thin films of Ag–In–S QDs purified from excess thioglycolic acid (TGA) ligands are found stable up to 250 °C and 15 J/cm², respectively. Under more extreme conditions, the initial mixed chalcopyrite/Cu–Au-cation ordered phase (CH/CA) is transformed into the spinel structure as the main phase with admixtures of the orthorhombic (OR) (350 °C) and OR/CH phases (above 24 J/cm²). In contrast to the purified QDs, Ag–In–S QDs stabilized with an excess of the TGA ligands revealed excellent stability, at least up to an energy density of 40 J/cm². The observed structural transformations take place at temperatures much lower than the melting or phase transition temperatures of bulk stoichiometric crystals. The transformation of the crystal lattice in the QDs is supposed to require less energy due to the small size (large surface) and is further facilitated by vacancies and nonstoichiometry.