Surface characterization of a - Mo 2 C ( 0001 )

The (0001) surface of a-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 ‘(1×1)’ hexagonal LEED pattern on a diffuse background. This pattern is composed of at least three periodicities: a (1×1) hexagonal periodicity, a faint E3×E3 R30° periodicity, and spots of a third, unidentified periodicity. A bulk-terminated, unrelaxed surface is expected to exhibit a (1×1) 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 (4×4) hexagonal LEED periodicity. The decrease in Mo/C ratio is consistent with the change from an Mo to a C termination. The (4×4) hexagonal periodicity is equivalent to three domains of C termination with (2×1) orthorhombic symmetry rotationally misaligned by 120°. 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. © 1999 Elsevier Science B.V. All rights reserved.