Quantitative Electron Energy Spectroscopy Research On Under-coordinated Molybdenum

Solid skins and atomic clusters have attracted much attention both experimentally and theoretically due to their fascinating chemical and physical properties being different from their bulk counterparts. These abnormal properties include the dilute magnetism, catalytic ability, creation of Dirac-Fermions for topological insulators. All these properties are closely related to atomic undercoordination induced bond relaxation and electronic structure configuration. Therefore, exploring local electronic properties of systems induced by coordination imperfection would be crucial to improving the understanding of their electronic structure-related properties at the atomic level, such as cohesive segregation energies, heats of mixing, and charge transferring.In this work, we present our examination of the atomic under-coordination effect on the local bond relaxation, binding energy and the associated core-level energy shifts and valence band variation of Mo solid skins and atomic clusters based on tight-binding theory, bond order-length-strength correlation and nonbonding-electron polarization notion, photoelectron spectrometrics, and density functional theory calculations. Major progress is summarised as follows:(1) To better understand the effects of coordination imperfection and the physical origin of surface core level shift, we decomposed the Mo 3d5/2 spectra of the clean surface Mo(100) and Mo(110). From the decomposition, we obtained the 3d5/2 energy level of an isolated atom and the bulk shift upon bulk formation. It has been derived that the 3d5/2 BE of bulk and surface skin shift deeper from 2.707 to 3.100 eV with respect to that of the isolated Mo(224.862 ± 0.004 eV) atoms. It is found that the undercoordination-induced local bond strain contracted, relative binding energy density enhanced, relative atomic cohesive energy weakened and relative core-level binding energy shifted positively up to 12.67%, 71.92%, 62.31%, 14.51%, respectively, for the outermost three atomic layer of Mo surface.(2) To further confirm our predictions, we performed the bond contraction, binding energy and valence local DOS of Mo N clusters with different size and structure from the perspective of DFT calculations. This exercise has led to the following quantitative information:(i) the E4 s level shifts from- 61.229 eV for the Mo59 to- 61.620 eV for the Mo15 cluster;(ii) the valence band undergoes a 1.057 eV upward shift. The globally positive core-level shift arises from the local quantum entrapment due to bond contraction and strength gain. The densely entrapped core electrons polarize the valence electrons and hence raise the valence band energy.Consistency between quantum calculations and photoelectron spectroscopy measurements of Mo skins and nanoclusters confirms our expectations that atomic undercoordination shortens and strengthens the bond, localizes and entraps core electrons, and polarizes the valence charge, which modified the Hamiltonian, atomic cohesive energy, local binding energy density and finally resulted in the unusual properties of Mo skins and nanoclusters.