Atomic-Layer- and Crystal-Orientation-Resolved 3d_5/2 Binding Energy Shift of Ru(0001) and Ru(1010) Surfaces

Abstract

Theoretical reproduction of the Ru(0001) and Ru(101̅0) surface XPS profiles has led to the following information: (i) the 3d5/2 energy level of an isolated Ru atom (E3d(z = 0) = 275.883 ± 0.002 eV); (ii) the energy shift of the 3d5/2 energy (ΔE3d(z = 12) = 3.661 eV) from the E3d(0) upon bulk formation; (iii) the effective z values of the surface sublayers; (iv) the coordination (z) dependence of the energy shift: E3d(z) = ⟨E3d(0)⟩ + ΔE3d(12)cz−1 = 275.883 ± 0.002 + 3.661/cz (eV), with cz = 2/{1 + exp[(12 − z)/(8z)]} being the z-dependent bond contraction coefficient; (v) the z-dependent lattice strain, binding energy density gain, and atomic cohesive energy remnant in the surface layers; and importantly, (vi) the physical origin for the surface-induced positive core-level shift. It has been clarified that the binding energy shift arises from the perturbation in the Hamiltonian by the shorter and stronger bonds between undercoordinated atoms. The developed approach should enhance the power of XPS for gaining information as such that is beyond the scope of conventional approaches.