Surface characterization of α-Mo2C (0001)

The (0001) surface of alpha-Mo2C prepared by Ar ion bombardment and annealing was characterized using X-ray photoelectron spectroscopy and low energy electron diffraction (LEED). An ion-bombarded surface annealed to 1000 K exhibits an Mo/C ratio of 2.2, characteristic of an Mo-terminated surface. The Mo-terminated surface exhibits a complex, but predominantly '(1x1)' hexagonal LEED pattern on a diffuse background. This pattern is composed of at least three periodicities: a (1 x 1) hexagonal periodicity, a faint root 3 x root 3 R30 degrees periodicity, and spots of a third, unidentified periodicity. A bulk-terminated, unrelaxed surface is expected to exhibit a (1x1) orthorhombic (i.e. rectangular) periodicity. The observed hexagonal periodicity is explained by LEED simulations as arising from the extinction of certain orthorhombic spots. An ion-bombarded surface annealed to 1500 K exhibits an Mo/C ratio of 1.8-1.9 and a (4 x 4) hexagonal LEED periodicity. The decrease in Mo/C ratio is consistent with the change from an Mo to a C termination. 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 and periodicity are associated with the transformation from an Mo to a C-terminated surface, the details of the associated surface structures are not known. (C) 1999 Elsevier Science B.V. All rights reserved.