Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.226
Publication date: 18th July 2019
Copper indium dichalcogenide, specifically CuInS2 (CIS), presents as a valuable material as a light absorber in quantum dot and thin film photovoltaics, due to its high absorption coefficient, and tunable optical properties from the visible spectrum to the near-infrared. CIS, with a band gap of 1.45 eV, could be a suitable alternative to more toxic, heavy metal based materials, such as CdSe.
Well established direct synthetic routes exist to produce small spherically shaped CIS NCs. However, achieving different morphologies with high control is still a challenge. Therefore, cation exchange reactions have proven to be an excellent alternative. This synthetic strategy is based on using one nanostructure as a template to generate the final structure with the desired composition via exchange of ions. This is usually achieved by partially or fully exchanging lattice cations, while retaining the initial anionic sublattice of the NCs.
We synthesized large copper sulfide nanorods in batch with aspect ratios of approximately 2:1, with reproducible and narrow size distributions. Copper sulfide exists in various crystal phases, ranging from stoichiometric low chalcocite Cu2S to various Cu-deficient phases, e.g. djurleite Cu1.94S and roxbyite Cu1.78S. Cu-deficient copper sulfide phases exhibiting vacancies are a very interesting starting material, since they provide pathways for cation diffusion.
Our synthesized copper sulfide nanorods exhibit the Cu-deficient phase of djurleite Cu1.94S with sizes of approximately 40 nm by 80 nm. CE reactions were carried out under different reaction conditions, i.e. by varying the temperature, and observing the reaction progress at different times. The reaction rate does not change, even with a large surplus of In3+ guest cations in the reaction solution, pointing towards zero order reaction kinetics. The progress of powder diffraction patterns not only exhibits the emergence of reflections, but in addition shifting of reflections. This points towards an alloying process, constantly changing lattice parameters, opposed to two distinct crystal phases in a core/shell mechanism, which would present in two sets of patterns changing in intensities, without much reflection shifting.
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