Title

Size-dependent evolution of the atomic vibrational density of states and thermodynamic properties of isolated Fe nanoparticles

Authors

Authors

B. R. Cuenya; L. K. Ono; J. R. Croy; K. Paredis; A. Kara; H. Heinrich; J. Zhao; E. E. Alp; A. T. DelaRiva; A. Datye; E. A. Stach;W. Keune

Comments

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Abbreviated Journal Title

Phys. Rev. B

Keywords

NUCLEAR RESONANT SCATTERING; ENHANCED SPECIFIC-HEAT; NANOCRYSTALLINE; NI3FE; SYNCHROTRON-RADIATION; THERMAL-CONDUCTIVITY; DYNAMICS; PHONONS; PD; NANOSTRUCTURES; PRESSURE; Physics, Condensed Matter

Abstract

We have gained insight into the internal degree of atomic disorder in isolated size-selected Fe nanoparticles (NPs) (similar to 2-6 nm in size) supported on SiO2/Si(111) and Al2O3(0001) from precise measurements of the low-energy (low-E) part of the phonon density of states [PDOS, g(E)] via Fe-57 nuclear resonant inelastic x-ray scattering (NRIXS) combined with transmission electron microscopy (TEM) measurements. An intriguing size-dependent trendwas observed, namely, an increase of the low-E excess density of phonon states (as compared to the PDOS of bulk bcc Fe) with increasing NP size. This is unexpected, since usually the enhancement of the density of low-E phonon modes is attributed to low-coordinated atoms at the NP surface, whose relative content increases with decreasing NP size due to the increase in the surface-to-volume ratio. Our NPs are covered by a Ti-coating layer, which essentially restores the local neighborhood of surface Fe atoms towards bulk-like coordination, reducing the surface effect. Our data can be qualitatively explained by the existence of low-coordinated Fe atoms located at grain boundaries or other defects with structural disorder in the interior of the large NPs (similar to 3-6 nm), while our small NPs (similar to 2 nm) are single grain and, therefore, characterized by a higher degree of structural order. This conclusion is corroborated by the observation of Debye behavior at low energy [g(E) similar to E-n with n similar to 2] for the small NPs, but non-Debye behavior (with n similar to 1.4) for the large NPs. The PDOS was used to determine thermodynamic properties of the Fe NPs. Finally, our results demonstrate that, in combination with TEM, NRIXS is a suitable technique to investigate atomic disorder/defects in NPs. We anticipate that our findings are universal for similar NPs with bcc structure.

Journal Title

Physical Review B

Volume

86

Issue/Number

16

Publication Date

1-1-2012

Document Type

Article

Language

English

First Page

11

WOS Identifier

WOS:000309385700006

ISSN

1098-0121

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