  A comprehensive picture of the aging and oxidation of the (0001) surface of
Bi2Se3 is critical to understanding the physical origin of changes in its
topologically protected surface states. We find that surface aging in ambient
conditions occurs in two major steps. Within two hours of exfoliation, a series
of ~ 3.6 A high islands are observed by atomic force microscopy over
approximate 10% of the surface. Subsequently, patch growth stops, and oxidation
begins after the two hours and continues until one quintuple layer has been
oxidized. X-ray photoelectron spectroscopy shows no sign of oxidation before ~
120 minutes of exposure to air, and the oxygen 1s peak is clearly present after
~ 190 minutes of ambient exposure. Variable angle spectroscopic ellipsometry
also indicates that the oxidation of a full quintuple layer occurs on the time
scale of days. These results are in good agreement with the time dependent
changes previously observed in the surface crystal structure by second harmonic
generation. In addition to providing the ability to non-destructively measure
oxide on the surface of Bi2Se3 crystals, ellipsometry can be used to identify
the thickness of Bi2Se3 flakes. These results are consistent with non-linear
optical methods including rotational anisotropy second harmonic generation,
which follow the time dependence of the changes to the top quintuple layer.
