In the quest for quantum dots (QDs) with “green” formulations, Cu-based ternary materials are emerging as promising alternatives to the well-known Cd- and Pb-based binary semiconductors, due to their promising photophysical properties - as the tunability of the absorption/photoluminescence spectra by tuning their size - combined with low toxicity.[1]

CuInS₂ QDs with selected size and morphology have been synthesized via high-temperature heat-up colloidal pathways.[2] A purification protocol was specially designed to enable the size selection of the QDs by exploiting their dispersibility in different solvents by fractional separation and centrifugation.[3] The process was closely monitored by a combination of laboratory X-ray diffraction and absorption/emission spectroscopies.

CuInS₂ is a I−III−VI₂ semiconductor with a direct band gap of 1.5 eV. It has tunable emission peaks ranging from 650 to 780 nm but has a low photoluminescence quantum yield. This can be improved using a core-shell CuInS₂/ZnS structure. CuInS₂ QDs have a broad absorption spectrum covering almost the entire visible light range and extending into the near-infrared region, making them suitable for many applications in energy harvesting, such as solar concentrators.

The first part of my work focused on studying the influence of the synthesis parameters (e.g. reaction temperature, growth time) on the optical properties. In the second part, I focused on characterising the structure and microstructure of samples of different sizes.

CuInS₂ bulk material crystallizes in the thermodynamically stable tetragonal chalcopyrite phase, in which Cu⁺ and In³⁺ atoms are arranged in ordered sites within the cation sublattice. The wurtzite phase, in which the two cations are randomly distributed within the cation sublattice, is stable at high temperature (> 1318K). At the nanoscale, chalcopyrite and wurtzite are the most identified phases [4-6] and few reports claim the identification of a zincblende-like structure.[7] However, an in-depth X-ray-based characterization of the phase stability, crystal structure and defectiveness of colloidal CuInS₂ QDs is presently missing.

In this contribution, we focus on these aspects by applying advanced, synchrotron-based high-resolution wide-angle X-ray Total scattering (WAXTS) techniques. X-ray data of CuInS₂ QDs in the size range 2-10 nm were collected at the Material Science beamline of the Swiss Light Source of the Paul Scherrer Institute, at the ID22 beamline of the European Synchrotron Radiation Facility, and at the Cristal beamline of the French national synchrotron facility. A thorough data reduction was performed to subtract the scattered intensity of extra-sample components (empty capillary, air, capillary filled with the solvent) and to account for absorption effects, according to a well-established protocol.

The structural and microstructural characterization of QDs was carried out by reciprocal space total scattering methods, combining WAXTS with small-angle X-ray scattering (SAXS) techniques, based on the Debye Scattering Equation (DSE).[8]

The development of detailed atomistic models to account for QDs preferential growth directions indicating anisotropic morphologies and the presence of stacking faults is presently ongoing. The results of these studies will be presented.