  We report phenomenal yield strengths, up to one fourth of the theoretical
strength of silver, recorded in microcompression testing of initially
dislocation free silver micro and nanocubes synthesized from a multistep seed
growth process. These high strengths and the massive strain bursts that occur
upon yield are results of the initially dislocation free single crystal
structure of the pristine samples that yield through spontaneous nucleation of
dislocations. When the pristine samples are exposed to a focused ion beam to
fabricate pillars and then compressed, the dramatic strain burst does not
occur, and they yield at a quarter of the strength of their pristine
counterparts. Regardless of the defect state of the samples prior to testing, a
size effect is apparent, where the yield strength increases as the sample size
decreases. Since dislocation starvation and the single arm source mechanisms
cannot explain a size effect on yield strength in dislocation free samples, we
investigate the dislocation nucleation mechanisms controlling the size effect
through careful experimental observations and molecular statics simulations. We
find that intrinsic or extrinsic symmetry breakers such as surface defects,
edge roundness, external sample shape, or a high vacancy concentration can
influence dislocation nucleation, and thus contribute to the size effect on
yield strength in initially dislocation-free samples.
