Phonon density of states of self-assembled isolated Fe-rich Fe-Pt alloy nanoclusters

Abstract

The Fe-projected phonon density of states (PDOS) of monolayer-thick films of self-assembled, size-selected, isolated F 57 e1-x Ptx alloy nanoclusters (NCs) (height: □2nm, diameter: □8nm) 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 bcc F 57 e0.8 Pt0.2 (core)/PtSi(shell) NCs. By contrast, the PDOS of fcc F 57 e0.75 Pt0.25 (core)/PtSi(shell) NCs retains features of bulk ordered Fe3 Pt 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 Fe3 Pt NCs. Important thermodynamic properties of the NCs are derived, such as the vibrational entropy per atom. © 2009 The American Physical Society.