Nanosecond dynamics in intrinsic topological insulator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi>Bi</mml:mi><mml:mrow><mml:mn>2</mml:mn><mml:mo>−</mml:mo><mml:mi>x</mml:mi></mml:mrow></mml:msub><mml:msub><mml:mi>Sb</mml:mi><mml:mi>x</mml:mi></mml:msub><mml:msub><mml:mi>Se</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math> revealed by time-resolved optical reflectivity

${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is an ideal three-dimensional topological insulator in which the chemical potential can be brought into bulk band gap with antimony doping. Here, we utilize ultrafast time-resolved transient reflectivity to characterize photoexcited carrier decay ${\mathrm{Bi}}_{2\ensuremath{-}x}{\mathrm{Sb}}_{x}{\mathrm{Se}}_{3}$ nanoplatelets. We report a substantial slowing of relaxation time bulk-insulating as compared $n$-type bulk-metallic at low temperatures, approaches $3.3\phantom{\rule{0.16em}{0ex}}\mathrm{ns}$ zero pump fluence limit. This long-lived correlated across different fluences and concentrations, revealing unique dynamics not present ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, namely slow bimolecular recombination carriers.