Observation of long excitation lifetime in photoexcited Sb-doped Bi₂Se₃nanoplatelets

Bi₂Se₃ 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 Bi₂Se₃ and Bi_2-xSb_xSe₃ nanoplatelets which reveal starkly different carrier decay 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.