Atom- and cluster-assembled interfaces: Cr growth on ${\mathrm{Bi}}_2$${\mathrm{Sr}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{1+\mathit{x}}$${\mathrm{Cu}}_2$${\mathrm{O}}_{8+\mathit{y}}$

X-ray photoemission results for Cr interaction on single-crystal ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{1+\mathit{x}}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ demonstrate that interfaces nearly free of disruption can be formed by atom deposition at 20 K or by cluster assembly. For atom deposition at 20 K, a uniform metallic layer forms with only a slight reduction of ${\mathrm{Cu}}^{2+}$ bonding configurations, similar to that observed for nonreactive overlayers. This atom-assembled interface was stable when warmed to 300 K. Cluster assembly results in modification of the exposed Bi-O plane, but the Cu 2${\mathit{p}}_{3/2}$ emission shows no significant change because there was no change in Cu-O bonding. The irregular but complete layer of Cr on ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{1+\mathit{x}}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathit{y}}$ formed by cluster assembly exhibited limited reaction upon warming to 300 K. In contrast, interface formation at 300 K by atom deposition produced complete reduction of ${\mathrm{Cu}}^{2+}$-like bonding within 30–50 Å of the surface. These results demonstrate the competition between kinetic and thermodynamic processes.