Ultrafast Dynamics in Semiconductors

Motivation:

While we have concentrated mostly on the redox process at the interface, it is also necessary to understand electronic dynamics in semiconductor electrodes. Carrier dynamics, surface states, and band bending near the surface of semiconductors crucially influence electrochemistry. On this front, we have studied carrier dynamics in iron pyrite (FeS₂) and iron oxide (hematite Fe₂O₃) electrodes. Iron pyrite has electrocatalytic activity for hydrogen evolution, while iron oxide is studied extensively as a photoelectrocatalyst for water oxidation. Iron pyrite has been studied by the photovoltaics community as a possible absorber. However, its use is hampered due to surface states below the nominal bandgap of the material. The nature of these states is not well understood and is currently debated.

Carrier Dynamics in Iron Oxide

We have studied ultrafast relaxation of carriers in iron oxide (hematite) films, which are also studied for their electrocatalytic and photocatalytic properties in our lab. We have observed the intraband carrier relaxation time and have identified the transient signal due to absorption within the conductions bands of hematite. (Link to our work)

Sub-bandgap States in Iron Pyrite

We are the first group that has optically excited the low-energy surface states of iron pyrite using sub-bandgap pumps. This was made possible by our broadband transient reflection experiment with several pump wavelengths both above and below the nominal gap. We have shown the dynamics of carrier trapping by the surface states. Furthermore, we have demonstrated that the sub-bandgap states can be directly excited by low-energy near-IR light with long lifetime in the material. (Link to paper)