  Two-dimensional electron systems (2DESs) in functional oxides are promising
for applications, but their fabrication and use, essentially limited to
SrTiO$_3$-based heterostructures, are hampered by the need of growing complex
oxide over-layers thicker than 2~nm using evolved techniques. This work shows
that thermal deposition of a monolayer of an elementary reducing agent suffices
to create 2DESs in numerous oxides.
