  We perform a systematic study of the characteristics of shear transformation
zones (STZs) that nucleate at free surfaces of two-dimensional amorphous solids
subject to tensile loading using two different atomistic simulation methods,
the standard athermal, quasistatic (AQ) approach and our recently developed
self-learning metabasin escape (SLME) method to account for the finite
temperature and strain-rate effects. In the AQ, or strain-driven limit, the
nonaffine displacement fields of surface STZs decay exponentially away from
their centers at similar decay rates as their bulk counterparts, though the
direction of maximum nonaffine displacement is tilted away from the tensile
axis due to surface effects. Using the SLME method at room temperature and at
the high strain rates that are seen in classical molecular dynamics
simulations, the characteristics for both bulk and surface STZs are found to be
identical to those seen in the AQ simulations. However, using the SLME method
at room temperature and experimentally-relevant strain rates, we find a
transition in the surface STZ characteristics where a loss in the
characteristic angular tensile-compression symmetry is observed. Finally, the
thermally-activated surface STZs exhibit a slower decay rate in the nonaffine
displacement field than do strain-driven surface STZs, which is characterized
by a larger drop in potential energy resulting from STZ nucleation that is
enabled by the relative compliance of the surface as compared to the bulk.
