Surface thermal stability of iron pyrite nanocrystals: Role of capping ligands

• Investigated the key role of capping ligands in protecting the FeS2 nanocrystals
• Demonstrated good surface stability in air and in N2
• Provided practical possibility of controllable oxygen doping of FeS2

Iron pyrite (FeS2) is a promising photovoltaic absorber material with a high natural abundance and low cost, but surface defects and low photoresponse inhibit sunlight energy conversion. The surface stability of pyrite FeS2 nanocrystals synthesized in oleylamine (OLA) with trioctylphosphine oxide (TOPO) as an additional capping ligand was investigated using Fourier transform infrared spectroscopy, Raman spectroscopy and X-ray diffraction. Tunable laser exposure during Raman spectroscopy measurement was developed for convenient and systematic evaluation of the stability of FeS2 nanocrystals. The surface stability of 100–200 nm diameter cubic nanocrystals with long-chain (OLA, TOPO) or small-molecule (pyridine) capping ligands was evaluated after high-intensity laser exposure as well as after thermal annealing in air and N2. While increasing surface coverage with OLA and TOPO capping ligands provided additional protection against oxidation, FeS2 nanocrystals capped with pyridine showed good stability at temperatures up to 200 °C in air and 400 °C in N2. These results provide greater understanding of the processing of nanocrystal-based iron pyrite thin films for photovoltaic applications.