Surface characterization of alpha-Mo2C (0001)

The (0001) surface of alpha-Mo2C prepared by Ar ion bombardment and anneali ng was characterized using X-ray photoelectron spectroscopy and low energy electron diffraction (LEED). An ion-bombarded surface annealed to 1000 K ex hibits an Mo/C ratio of 2.2, characteristic of an Mo-terminated surface. Th e Mo-terminated surface exhibits a complex, but predominantly '(1x1)' hexag onal LEED pattern on a diffuse background. This pattern is composed of at l east three periodicities: a (1 x 1) hexagonal periodicity, a faint root 3 x root 3 R30 degrees periodicity, and spots of a third, unidentified periodi city. A bulk-terminated, unrelaxed surface is expected to exhibit a (1x1) o rthorhombic (i.e. rectangular) periodicity. The observed hexagonal periodic ity is explained by LEED simulations as arising from the extinction of cert ain orthorhombic spots. An ion-bombarded surface annealed to 1500 K exhibit s an Mo/C ratio of 1.8-1.9 and a (4 x 4) hexagonal LEED periodicity. The de crease in Mo/C ratio is consistent with the change from an Mo to a C termin ation. The (4 x 4) hexagonal periodicity is equivalent to three domains of C termination with (2 x 1) orthorhombic symmetry rotationally misaligned by 120 degrees. The presence of rotationally misaligned domains is confirmed by scanning tunneling microscopy. Whereas changes in both the composition a nd periodicity are associated with the transformation from an Mo to a C-ter minated surface, the details of the associated surface structures are not k nown. (C) 1999 Elsevier Science B.V. All rights reserved.