Anomalous specific heats of metallic nanocrystals induced by surface oxidation

Although thelow-temperaturespecific heat in metallicnanocrystals has been extensively demonstrated to be always enhanced due to thesurfacephonon softening, it is revealed here to be anomalously depressed by thesurface oxidation so that the excessspecific heat can be either positive or negative as a function of temperature with respect to the counterparts of thebulk 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-Alnanocrystals, both thesurface and volume VDOSs g( ω) of a-Al2O3nanoparticles are scaled as g( ω) ∼ ω1.5 at the low-frequency limit ( ω). The effectivespatial dimension for the a-Al2O3surfaces and volume as well as the fcc-Alsurfaces is thus assumed to be between 2D and 3D while it is 3D for the fcc-Al volume. Thespecific heat of a-Al2O3@Alnanoparticles is consequently revealed to show a differentT-dependent feature from those of both pure metallicnanocrystals andbulk crystals atlow temperatures.
Source: Applied Physics Letters - Category: Physics Authors: Source Type: research