Direct proof of mesoscopic misfit in nanoscale islands by x-ray absorption spectroscopy

Abstract

Polarization-dependent extended x-ray absorption fine structure (EXAFS) measurements above the Co-K absorption edge were carried out to experimentally prove the theoretically predicted contraction of interatomic distances in nanoscale Co islands (⊘≈ 1nm) on Cu(001). Within the one monolayer (ML) thick Co islands which were deposited at 160 K substrate temperature, we find an in-plane Co-Co distance of 2.45±0.02Å , significantly shorter than in bulk Co (2.51Å) and in close agreement with theoretical predictions. The effective in plane coordination number (N * Co ( )) is equals to 3.2±0.5 and 5.0±0.7 (N*=6 for an infinite island) for samples covered by 0.3 and 0.7 monolayers (ML) of Co, respectively. The low value for the 0.3 ML sample is related to the finite island size and to about 20% of intermixing between adsorbate and substrate atoms. The experimental results are supported by ab-initio and molecular dynamics calculations.

Effect of mesoscopic misfit on growth, morphology, electronic properties and magnetism of nanostructures at metallic surfaces

“…35 schematically shows the idealized model of a cobalt nano-island on copper. Co-atoms shown as dark spheres reside in hollow sites of the Cu(001) surface atoms (light spheres) [129]. In this geometry the distances between Co-atoms ðR Co Þ and between Co and Cu atoms ðR Cu Þ are almost identical at 2.553 Å as was shown in a LEED study by Cerda et al [134].…”

“…geometry (see second panel). The increase of N n Cu is attributed to intermixing ð % 20%Þ in which cobalt atoms are embedded into the copper substrate thereby increasing the effective coordination number to 12 [129]. In the following the structure parameters R Cu ) and .…”

“…Under this condition the effective coordination number varies from N n Co ¼ 3 for an atom at the rim of the island to N n Co ¼ 6 for an atom inside the nano-island [129]. The EXAFS analysis yields an average effective coordination number N n Co .…”

“…In contrast to epitaxial atomic layers, where lattice misfit, epitaxial strain, vertical layer relaxation are well-defined properties [10,87,128], for individual nanostructures spatial variations of the atomic structures are expected [41,43] and experimentally observed [31,61,129]. Here, structural and electronic relaxations on the nanoscale are driven by uncompensated forces and (111) surface as a function of coverage at annealing temperatures of 12 K as measured experimentally (dark gray) and calculated with (red) and without (green) taking into account strain relaxations.…”

“…The modulation of the linear absorption coefficient μðEÞ from which the k 2 -weighted interference function is derived was measured in the fluorescence yield (FY) mode using a Ge(Li) solid state detector. The interference function of the reference bulk copper and cobalt samples were collected in the transmission mode [129]. a distinct shoulder is observed.…”