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Abstract:
The Fe-projected phonon density of states (PDOS) of monolayer-thick films of self-assembled, size-selected, isolated 57Fe1-xPtx alloy nanoclusters (NCs) (height: ∼2 nm, diameter: ∼8 nm) supported on flat SiO2/Si(111) substrates has been measured by nuclear resonant inelastic x-ray scattering. The samples were characterized by atomic force microscopy (AFM), transmission electron microscopy, and x-ray photoelectron spectroscopy (XPS). Surface segregation of Pt and PtSi formation at the NC surface due to the sample-preparation process is inferred from the XPS data. As compared to the bulk, pronounced modifications of the PDOS beyond the bulk cut-off energy are observed in 57Fe0.8Pt0.2 (core)/PtSi(shell) NCs. By contrast, the PDOS of fcc 57Fe0.75Pt0.25 (core)/PtSi(shell) NCs retains features of bulk ordered Fe3Pt Invar alloys (presumably due to a thicker PtSi shell), in particular, the transverse-acoustic [110] TA1 mode near 9 meV. Apparently, this mode is not affected by size effects. The existence of the [110] TA1 phonon mode is a prerequisite for the persistence of Invar-related effects in Fe3Pt NCs. Important thermodynamic properties of the NCs are derived, such as the vibrational entropy per atom.
