Electronic Dynamics in Natural Iron Pyrite Studied by Broadband Transient Reflection Spectroscopy

Iron pyrite (FeS2) is an abundant natural mineral with interesting physical and chemical properties, including its near IR bandgap and extremely high absorption coefficient throughout the visible range. The dynamics of photoinitiated carriers and their interactions with intrinsic and surface defects are still not fully understood, yet clearly are responsible for pyrite's underwhelming photovoltaic and photocatalytic performance. Here we report, to our knowledge for the first time, broadband ultrafast transient reflection from single-crystal natural iron pyrite with several excitation wavelengths both higher and lower than the accepted nominal bandgap of pyrite. We also demonstrate a method to transform transient reflection to transient absorption, without requiring any assumptions regarding the magnitude of either the absorption coefficient or the refractive index, allowing for a more direct interpretation of our results. An important finding from this work is the observation of a long-lived weak signal when pumping with 0.58 eV, an energy well below the accepted bandgap, which may be evidence for direct optical excitation of either intrinsic trailing edges of the bands or midgap defect states. We identify that after approximately 10 ps the transient spectra due to pumping at 2.59, 1.58, and 0.91 eV all appear qualitatively similar, suggesting relaxation to a common carrier distribution. This common distribution appears to decay on two time scales of about 30 and >>200 ps. Our results should play a role in understanding charge carrier dynamics within the intricate and complex band structure of pyrite and hopefully provide clarification and direction for future efforts in the development of iron pyrite based technologies.