LEEM/LEED investigation of reconstruction and initial oxidation of the W(001) surface

Structural modifications caused by oxidation of the hot W(001) surface have been investigated with complementary low energy electron diffraction (LEED) and microscopy (LEEM). The surface initially accommodates oxygen up to theta = 1.2 monolayer (ML) coverage by passing through a sequence of closely related missing-row reconstructed structures, including (2 x 1), (5 x 1) and incommensurate (5 x 1) arrangements. An unusual nucleation behaviour of a p(2 x 1) penetrated oxide at theta = 1.0 ML is also observed and demonstrated by dark-field LEEM to be due to preferential nucleation at domain walls of the underlying reconstructed surface. A relationship of the reconstructed domain morphology with percolation in two-dimensions is suggested by the domain fractal dimension, D = 1.875, determined from LEEM images. Growth of the p(2 x 1) and an additional p(2 x 2) oxide structure occurs rapidly on the reconstructed surface once it becomes oxygen-saturated at higher coverage.