  2D van der Waals materials have rich and unique functional properties, but
many are susceptible to corrosion under ambient conditions. Here we show that
linear alkylamines are highly effective in protecting the optoelectronic
properties of these materials such as black phosphorous (BP) and transitional
metal dichalcogenides. As a representative example, n-hexylamine can be applied
in the form of thin molecular monolayers on BP flakes with less-than-2nm
thickness and can prolong BP's lifetime from a few hours to several weeks and
even months in ambient environments. Characterizations combined with our
theoretical analysis show that the thin monolayers selectively sift out water
molecules, forming a drying layer to achieve the passivation of the protected
2D materials. The monolayer coating is also stable in air, hydrogen annealing,
and organic solvents, but can be removed by certain organic acids.
