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 moiré pattern and proves the commensurability of the silica layer on the Cu(111) support. The commensurate structure appears for a 30° rotated silica lattice with the lattice constant of a[SiOx] = 5.15 Å leading to a (7 × 7) unit cell of the Cu(111) support hosting a (2√3 × 2√3)R30° cell of the silica lattice. In our analysis, all possible moirés at a rotation angle of 30° are deduced for the case of SiOx on Cu(111) and it is explained why the observed moiré structure appears. The performed moiré 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.