Size-induced 2p3/2 and 3d5/2 energy levels shift of Ni nanostructures: Broken-bond-induced local strain and quantum trapping

Size dependence of the L(2p3/2) and M(3d5/2) energy levels shifts of Ni nanostructures deposited on TiO2 substrate has been measured using the Auger photoelectron coincidence spectroscopy (APECS) and analyzed on the base of the bond order-length-strength (BOLS) correlation mechanism [Sun, C.Q. Prog. Solid State Chem. 2007, 35, 1]. Correlation between the APECS involved L and M lines and the Auger kinetic energy EK has been established, clarifying that the energy shift of the Auger parameter, or the energy shift of the sum of EK and EL, is twice that of the M level. Using this approach, we have been able to estimate energies of these levels for an isolated Ni atom and their corresponding shifts upon bulk formation. Findings confirm that the size-induced electronic energy shifts of nanostructures originate from the broken-bond-induced local strain and the associated quantum trapping in the surface of skin depth.