Surprisingly, the order of appearance of the different 2D phases does not strictly follow their formation energy in the N-3, N-5, and N-6 systems. In the N-3 system (Figs. and), the n=1 phase appeared earlier than the n=3 phase. Also, the n=3 phase appeared later than the n=2 phase in the N-3, N-5 and N-6 systems (Figs. 2e-2f, Figs.