On-surface azide-alkyne cycloaddition on Cu(111): does it "click" in ultrahigh vacuum?

Using scanning tunneling microscopy, we demonstrate that the 1,3-dipolar cycloaddition between a terminal alkyne and an azide can be performed under solvent-free ultrahigh vacuum conditions with reactants adsorbed on a Cu(111) surface. XPS shows significant degradation of the azide upon adsorption, which is found to be the limiting factor for the reaction.