Observation of long carrier lifetime in photoexcited Sb-doped Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanoplates

Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ is a three-dimensional topological insulator (TI), characterized by a bulk band gap of approximately 0.3 eV and a Dirac-like protected surface state.~The material is usually n-type due to selenium vacancies, and chemical substitution, such as Sb-doping, is typically needed to bring the chemical potential into the bulk band gap.~We will present ultrafast transient reflectivity measurements on Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and (Bi,Sb)$_{\mathrm{2}}$Se$_{\mathrm{3}}$ nanoplates which reveal starkly different carrier dynamics in n-type vs. insulating samples.~This will be discussed in the context of optoelectronic applications of TIs, including as a material for exciton condensation.