Core-Shell versus Other Structures in Binary Cu38-n Mn Nanoclusters (M = Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au; n=1, 2, and 6): Theoretical Insight into Determining Factors

DFT calculations of binary transition-metal nanoclusters Cu38-nMn (M = Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au; n = 1, 2, and 6) clearly show that a core shell structure Cu32M6(core) with M in the core is stable for M = Ru, Rh, Os, and Ir but unstable for M = Pd, Ag, Pt, and Au. These results are consistent with the segregation energies evaluated for Cu37M. Electron population is more accumulated on the core M atoms in Cu38-nMn (core) (M = Ru, Rh, Os, and Ir) than on the core Cu atoms in Cu-38. Such electron accumulation substantially occurs for M = Ru, Rh, Os,. and Ir because the d orbitals of these transition metals are not fully occupied. A linear relationship was first found between the segregation energy and the increase in the d-orbital population of the core atom, indicating that the electron accumulation at the M-n core is one of the important factors for the segregation energy and the stabilization of the core shell structure; in other words, a core shell structure with M atom(s) in the core is stable when the d orbitals of M are not fully occupied. For M = Pd, Pt, and Au, the fused-alloy structure is more stable than the core shell and phase-separated structures. For M = Ag, the fused-alloy structure is as stable as the phase-separated one but the core shell structure is less stable. In these metals, the d orbitals are either nearly or fully occupied, and as a result, electron accumulation at the M core does not occur as much. For Cu32M6(core), the deformation energy of the Cu-32 shell increases in the order Ru < Rh << Pd < Ag and Os < Ir << Pt < Au, because the size of the M-6 core is substantially large for M = Pd, Ag, Pt, and Au. These results suggest, that a large atom tends not to take the core position. The cohesive energies of Ru, Rh, Os, and Ir are larger than those of Pd, Ag, Pt, and Au, indicating that the cohesive energy is also an important property for understanding and discussing the structures of binary metal clusters/particles.