Commensurability of Monolayer Silica on Cu(111)

We report a combined low-energy electron and X-ray photoemission electron microscopy (LEEM/XPEEM) study on the formation of monolayer silica by oxygen-induced segregation of bulk dissolved silicon on a (111)-textured Cu foil. The structure forms on Cu foils, on which graphene was previously grown by chemical vapor deposition (CVD) in a quartz tube reactor. The CVD reactor treatment transforms the initial polycrystalline into a (111)-textured Cu foil but also initiates the bulk dissolution of released silicon. After prolonged oxygen dosing, XPEEM proves the formation of micrometer-sized silica islands, which remain the exclusive surface layer after burning the covering graphene layer. LEEM data acquired from the silica islands delivers a diffraction pattern identical to the one reported for monolayer silica prepared by Si evaporation in the presence of oxygen on preoxidized Cu(111). The diffraction pattern analysis identifies the appearing k-space frequencies as the ones of a moire pattern and proves the commensurability of the silica layer on the Cu(111) support. The commensurate structure appears for a 30 degrees rotated silica lattice with the lattice constant of a[SiOx] = 5.15 & Aring; leading to a (7 x 7) unit cell of the Cu(111) support hosting a (2 root 3 x 2 root 3)R-30 degrees cell of the silica lattice. In our analysis, all possible moires at a rotation angle of 30 degrees are deduced for the case of SiOx on Cu(111) and it is explained why the observed moire structure appears. The performed moire analysis may be applied for many 2-dimensional materials and especially for silica on other hexagonally packed transition metals. The fact that the commensurate surface silica phase can be locally prepared without preoxidation of Cu(111) is discussed.