Observation of long excitation lifetime in photoexcited Sb-doped $\text{Bi}_2\text{Se}_3$ nanoplatelets.

$\text{Bi}_2\text{Se}_3$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into the bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize the photoexcited carrier decay in $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets. We report a substantial slowing of the bulk carrier decay rate in bulk-insulating $\text{Bi}_{2-x}\text{Sb}_x\text{Se}_3$ nanoplatelets as compared to n-type bulk-metallic $\text{Bi}_2\text{Se}_3$ at low temperatures, which approaches $0.30 \text{ ns}^{-1}$ in the zero pump fluence limit. This long-lived decay is correlated across different fluences and Sb concentrations, revealing unique decay dynamics not present in n-type $\text{Bi}_2\text{Se}_3$, namely the slow bimolecular recombination of bulk carriers.