Direct Observation of Photoexcited Hole Localization in CdSe Nanorods

Quantum-confined 1D semiconductor nanostructures are being investigated for hydrogen generation photocatalysts. In the photoreaction, after fast electron transfer, holes that remain in the nanostructure play an important role in the total quantum yield of hydrogen production. Unfortunately, knowledge of hole dynamics is limited due to lack of convenient spectroscopic signatures. Here, we directly probe hole localization dynamics within CdSe nanorods (NRs) by combining transient absorption (TA) and time-resolved terahertz (TRTS) spectroscopy. We show that when methylene blue is used as an electron acceptor, the resulting electron transfer occurs with a time constant of 3.5 ± 0.1 ps and leaves behind a delocalized hole. However, the hole quickly localizes in the Coulomb potential well generated by the reduced electron acceptor near the NR surface with time constant of 11.7 ± 0.2 ps. Our theoretical investigation suggests that the hole becomes confined to a ∼±0.8 nm region near the reduced electron acceptor and the activation energy to detrap the hole from the potential well can be as large as 235 meV.