Which material can be used to protect a Si layer from Oxidation by a sub nanometer layer?
1nm layer is not much: let me remind you that this is roughly 10 atoms thickness...
But the good news is that we can easily create very thin films with a process of atomic layer deposition (ALD)
The isolating layer could be the classically grown $SiO_2$ (usually grown 0.5-2nm)
You could also use $TiO_2$ or $TiO_2/SiN_x$ stack which are also great for surface passivation but usually grown thicker 4-15nm
I would like to add that even if it will protect from a pure $O_2$ oxidation, at such dimensions the diffusivity of smaller particles is quite strong, the layer may also break from ions, and the layer will not be uniformly 1nm if the surface roughness is too important (will only work on properly polished Silicon wafer and not on the edges).
therefore in general we do not passivate (protection of aggressive elements) with such thin layer.