Investigation of semiconductor quantum dot growth dynamics by in situ X-ray absorption spectroscopy

Copper indium sulphide (CuInS2, CIS) quantum dots are promising semiconductor nanomaterials for optoelectronic and photovoltaic applications owing to their low toxicity and tunable optical properties. Despite the extensive use of colloidal synthesis routes, the microscopic mechanisms governing their nucleation and growth are still not fully understood because of the transient nature of the intermediate species involved. This thesis investigates physical and chemical processes occurring on different timescales using synchrotron-radiation-based techniques. It combines a feasibility study of pump & probe experiments at the LISA beamline (ESRF) with an in situ XAS investigation of CuInS2 quantum-dot growth. While the available laser source proved insufficient for pump & probe measurements, the XAS campaign successfully yielded static and time-resolved Cu and In K-edge spectra throughout the synthesis. The EXAFS analysis allowed the evolution of the local coordination environment around both metal centres to be followed throughout the reaction. XANES analysis showed that indium remained as In(III) throughout the synthesis, whereas glutathione reduced Cu(II) to Cu(I). The most significant outcome of this work was provided by the time-resolved XAS measurements. The different kinetic responses observed at the Cu and In K-edges support a reaction pathway involving the formation of an indium-sulphide intermediate immediately after sulphide injection, followed by the slower incorporation of copper into the growing CuInS2 nanocrystals. To the best of our knowledge, this represents the first in situ XAS evidence supporting such an intermediate in this synthesis route. Overall, this work demonstrates the capability of synchrotron XAS to probe both the structural evolution and the reaction kinetics of colloidal nanocrystal synthesis, providing a general experimental strategy for investigating reaction mechanisms in colloidal nanocrystal synthesis under operando conditions.