Probing ultrafast hot charge carrier migration in MoS₂ embedded CdS nanorods

Efficient utilization of hot charge carriers is of utmost benefit for a semiconductor-based optoelectronic device. Herein, a one-dimensional (1D)/two-dimensional (2D) heterojunction was fabricated in the form of CdS/MoS₂ nanorod/nanosheet composite and migration of hot charge carriers was being investigated with the help of transient absorption (TA) spectroscopy. The band alignment was such that both the electrons and holes in the CdS region tend to migrate into the MoS₂ region following photoexcitation. The composite system is composed of optical signatures of both CdS and MoS₂, with the dominance of CdS nanorods. In addition, the TA signal of MoS₂ is substantially enhanced in the heterosystem at the cost of the diminished CdS signal, confirming the migration of charge carrier population from CdS to MoS₂. This migration phenomenon was dominated by the hot carrier transfer. The hot carriers in the high energy states of CdS are preferentially migrated into the MoS₂ states rather than being cooled to the band edge. The hot carrier transfer time for a 400 nm pump excitation was calculated to be 0.21 ps. This is much faster than the band edge electron transfer process, occurring at 2.0 ps time scale. We found that these migration processes are very much dependent on the applied pump photon energy. Higher energy pump photons are more efficient in the hot carrier transfer process and place these hot carriers in the higher energy states of MoS₂, further extending charge carrier separation. This detailed spectroscopic investigation would help in the fabrication of better 1D/2D heterojunctions and advance the optoelectronic field.