Analysis, Manipulation and Simulation

Phase Stability Simulation of Copper Chalkogenide Nanoparticles From First Principles

Alkit Beqiraj1, Ahed Abouserie1, Benjamin Heyne2, Andreas Taubert1, Arnim Wedel2 and Thomas Körzdörfer1

1Institut für Chemie, Universität Potsdam, Karl-Liebknecht-Straße 24-25, 14476, Potsdam, GER

2Funktionale Polymersysteme, Frauenhofer Institut für Angewande Polymerforschung IAP, Geiselbergstraße 69, 14476, Potsdam-Golm, GER

Being p-type semiconductors, CuSx based nanoparticles demonstrate a large variety of potential applications in optoelectronic devices or energy technology. This is underlined by recent work of our collaborators, demonstrating that CuSx nanoparticles can be used as an efficient hole collection layer in an organic solar cell. The CuSx nanoparticles can be synthesized at room temperature using an ionic liquid precursor. This synthesis, however, is very challenging, since even minor variation of the synthesis conditions can change the bias of the reaction, leading to the unexpected precipitation and phase transformation of different oxidative species of copper sulfides. Especially one polymorph of covellite (CuS), i.e., the semiconductor digenit (Cu1.8S), is of particular interest, as it is a frequently encountered byproduct of the synthesis.
In order to understand the phase transformation, the chemical parameters, and the nature of the synthesis we carried out DFT calculations. To predict phase stability, vibrational contributions to the free energy were calculated within the harmonic supercell approach. From these calculations, we predict thermodynamic (ab initio thermodynamic) to alloy phase diagrams and T,p-gibbs free energy diagrams for solid/gaseous sulfur. From these results, it is possible to analyze which parameters influence the phase evolution of digenit and covellite. Overall, our theoretical results are in a good agreement with the experimental observations and, thus, can be used to intentionally manipulate the experimental conditions in favor of the desired polymorph.

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top: Phase stability digram of the corresponding polymorphs for the norm temperature and reaction temperature. bottom: dependence on the Gibbs free energy of formation for the phase transformation reaction from the aggregate of sulphur.