Some chalcogenide nanocrystals such as CuInS2 hold promise for photovoltaics, photocatalysis, thermoelectricity and photonics. Their optical and electronic properties can be adjusted by modifying their chemical composition or size through a quantum confinement effect.
After their synthesis in an organic solvent, these nanocrystals (NC) are "enveloped" by molecules (ligands) weakly bound to their "cores". This molecular "shell", which depends on the solvent and the chemical process utilized, plays a key role in the potential properties of the nanocrystal. For almost all applications, it is then necessary to replace the original ligands with active chemical groups: this is the "functionalization" step.
The Inac team studied the NCs of CuInS2 (CIS) for their exceptional luminescence properties. Their synthesis was conducted in dodecanethiol (DDT) using metal precursors (copper iodide and indium acetate) heated in two stages, at 100°C and then at 230°C. This process is very interesting because of its simplicity, even though the obtained NCs "resist" functionalization, which underscores the importance of studying their surface state in detail.
Using different liquid NMR techniques, the researchers were able to draw a complete image of the surface chemistry of the NCs. The ligand envelope is composed of a double layer of molecules (dodecanethiolate and didodecylsulphide) arranged head to toe. This result is in good agreement with the loss of mass observed by thermogravimetry.
Thanks to this detailed understanding of their surface, the chemists will now be able to develop a functionalization strategy for nanocrystals.