Fabrication of periodic nanoscale Ag-wire arrays on vicinal CaF2 surfaces

We report the fabrication of 20-50 nm period uniform arrays of 1.7-3.6 nm high, 5-7 nm wide, 500 nm long silver wires on ion-beam polished 4 degrees-miscut CaF2 (111) vicinal surfaces. The wire separation exceeds the substrate mean step separation of 4.5 nm by a factor of 4-10. The arrays are formed under ambient conditions by scanning the tip of an atomic force microscope across a dense (similar to 10(12) clusters/cm(2)) electron beam evaporated deposit of 3-9 nm diameter, similar to 3 nm high Ag-clusters. Such periodic array structures are not formed on cleaved (111) surfaces of CaF2. The wires are oriented along a preferential direction defined by the substrate. We assume this direction corresponds to the mean orientation of the 0.315 nm high surface steps defined by the Ca sublattice of the (111) CaF2 vicinal surface. We propose that the friction force experienced by cluster agglomerates, formed by scanning the tip, increases in a step-wise fashion as further clusters are added to the agglomerate and that this force increases markedly at crystal step edges. Transport of the agglomerate ceases when the friction force exerted by the substrate and other nearby adparticle structures exceeds that exerted on the agglomerate by the microscope lip. At our cluster densities, this is most likely to occur when the cluster agglomerate has grown sufficiently to intersect more than one substrate step.