The characterization and control of atomic substitution process is crucial in fabricating high-quality two-dimensional layered compound materials and tuning their physical properties. With intensity-voltage low energy electron microscopy (IV-LEEM), we found that the concentration of copper in the topmost copper silicide monolayer on Si (111) substrates varies gradually from 1.7 to 1.0 ML while preserving it's unique '5 × 5' incommensurate phase in a transition region as large as 1000 nm. This gradual variation of the copper concentration is due to the incomplete substitution of the Si with Cu, as revealed by atomic-resolved scanning tunneling microscopy with a tip that nicely resolved the '5 × 5' periodicity. Our experiments indicate that besides the widely-accepted phase of CuSi with both substitutional and interstitial Cu atoms, another type of precursor copper silicide CuSi with only interstitial Cu atoms also plays important roles in the substitutional diffusion and reaction processes during the formation of the topmost copper silicide monolayer. This precursor phase might exist in the growth of other two-dimensional layered materials with potential applications in integrated optoelectronics, spintronics or low dissipative devices.
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