Anomalous specific heats of metallic nanocrystals induced by surface oxidation.

Although the low-temperature specific heat in metallic nanocrystals has been extensively demonstrated to be always enhanced due to the surface phonon softening, it is revealed here to be anomalously depressed by the surface oxidation so that the excess specific heat can be either positive or negative as a function of temperature with respect to the counterparts of the bulk crystals. The background mechanism has been theoretically deduced to well explain these experimental phenomena by calculating and comparing the vibrational densities of states (VDOS) of face-centered cubic-aluminum (fcc-Al) and amorphous-alumina (a-Al2O3) nanoparticles. Different from fcc-Al nanocrystals, both the surface and volume VDOSs g(ω) of a-Al2O3 nanoparticles are scaled as g(ω) ~ ω1.5 at the low-frequency limit (ω). The effective spatial dimension for the a-Al2O3 surfaces and volume as well as the fcc-Al surfaces is thus assumed to be between 2D and 3D while it is 3D for the fcc-Al volume. The specific heat of a-Al2O3@Al nanoparticles is consequently revealed to show a different T-dependent feature from those of both pure metallic nanocrystals and bulk crystals at low temperatures. [ABSTRACT FROM AUTHOR]