  Although over the past number of years there have been many advances in the
materials aspects of topological insulators (TI), one of the ongoing challenges
with these materials is the protection of them against aging. In particular,
the recent development of low-carrier-density bulk-insulating Bi$_2$Se$_3$ thin
films and their sensitivity to air demands reliable capping layers to stabilize
their electronic properties. Here, we study the stability of the
low-carrier-density Bi$_2$Se$_3$ thin films in air with and without various
capping layers using DC and THz probes. Without any capping layers, the carrier
density increases by ~150% over a week and by ~280% over 9 months. In
situ-deposited Se and ex situ-deposited Poly(methyl methacrylate) (PMMA)
suppresses the aging effect to ~27% and ~88% respectively over 9 months. The
combination of effective capping layers and low-carrier-density TI films will
open up new opportunities in topological insulators.
