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 ${^{57}\text{F}\text{e}}_{1\ensuremath{-}\text{x}}{\text{Pt}}_{\text{x}}$ alloy nanoclusters (NCs) (height: $\ensuremath{\sim}2\text{ }\text{nm}$, diameter: $\ensuremath{\sim}8\text{ }\text{nm}$) supported on flat ${\text{SiO}}_{2}/\text{Si}(111)$ 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 ${^{57}\text{F}\text{e}}_{0.8}{\text{Pt}}_{0.2}$ (core)/PtSi(shell) NCs. By contrast, the PDOS of fcc ${^{57}\text{F}\text{e}}_{0.75}{\text{Pt}}_{0.25}$ (core)/PtSi(shell) NCs retains features of bulk ordered ${\text{Fe}}_{3}\text{Pt}$ Invar alloys (presumably due to a thicker PtSi shell), in particular, the transverse-acoustic [110] ${\text{TA}}_{1}$ mode near 9 meV. Apparently, this mode is not affected by size effects. The existence of the [110] ${\text{TA}}_{1}$ phonon mode is a prerequisite for the persistence of Invar-related effects in ${\text{Fe}}_{3}\text{Pt}$ NCs. Important thermodynamic properties of the NCs are derived, such as the vibrational entropy per atom.