Asymmetric structure of germanene on an Al(111) surface studied by total-reflection high-energy positron diffraction

The structure of germanene on an Al(111) surface has been experimentally investigated using the total-reflection high-energy positron diffraction (TRHEPD) method. The observed spot intensities are asymmetric, revealing no mirror symmetry in the atomic coordinates of germenene with respect to the 〈110〉 direction. Quantitative TRHEPD rocking curve analysis, based on dynamical diffraction theory, has revealed that the germanene layer has a 3×3 structure with asymmetrical buckling due to the protrusion of one of the Ge atoms in the unit cell, which is unlike the structural model proposed in previous studies. The magnitude of the buckling was found to be 0.94 A, and the spacing between the germanene and the Al(111) substrate to be 2.51 A. The new structure proposed in the present investigations, though different from that reported in studies before, does not contradict the other characteristics which were found experimentally in the previous studies.