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

The Fe-projected phonon density of states (PDOS) of monolayer-thick films of self-assembled, size-selected, isolated (57)Fe(1-x)Pt(x) alloy nanoclusters (NCs) (height: similar to 2 nm, diameter: similar to 8 nm) supported on flat SiO(2)/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)Fe(0.8)Pt(0.2) (core)/PtSi(shell) NCs. By contrast, the PDOS of fcc (57)Fe(0.75)Pt(0.25) (core)/PtSi(shell) NCs retains features of bulk ordered Fe(3)Pt Invar alloys (presumably due to a thicker PtSi shell), in particular, the transverse-acoustic [110]TA(1) mode near 9 meV. Apparently, this mode is not affected by size effects. The existence of the [110]TA(1) phonon mode is a prerequisite for the persistence of Invar-related effects in Fe(3)Pt NCs. Important thermodynamic properties of the NCs are derived, such as the vibrational entropy per atom