The 7 x 7 reconstruction on Si(111) was observed in real space by using scanning tunnelling microscopy. The results strongly favoured a modified adatom model, with 12 adatoms per unit cell and an inhomogeneously relaxed underlying top layer.

7 x 7 Reconstruction on Si(111) Resolved in Real Space. G.Binnig, H.Rohrer, C.Gerber, E.Weibel: Physical Review Letters, 1983, 50, 120-3