It has been observed by atomic force microscopy that amphiphilic but hydrophobic (perfluoroalkyl)alkyl diblocks self-assemble in quasi-circular domains when a monolayer of these compounds is transferred on solid substrate. Grazing incidence small angle X-ray scattering (GISAXS) experiments performed directly on the monolayer on the surface of water demonstrated that these domains pre-exist on this sub-phase and, when compressed, organize into perfect hexagonal two-dimensional crystals. Such 2D crystal has been recovered by spin-coating of one semifluorinated alkane onto wetted silicon wafers.