Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry

With the advent of novel nanoscale devices, fast and reliable thermal mapping with high spatiotemporal resolution is imperative for probing the characteristics of phonons and evaluating the local temperature at the nanoscale. In this work, Raman spectroscopy is employed as a rapid and noncontact optical thermometry technique to investigate phononic properties of macroscopic assemblies of monodisperse palladium oxide (PdO) nanocrystals. PdO has been extensively employed in high temperature catalytic devices; however, the phonon behavior which determines the thermal stability of PdO remains unexplored thus far. Our study focuses on homogeneous, large-scale assemblies of monodisperse 4 and 10 nm nanocrystals synthesized using colloidal chemistry to understand size-dependent effects on the measured thermal properties. By monitoring the Raman peak shifts, peak broadening, and alterations in peak intensities as a function of laser power and particle concentration, a size-dependent trend is observed attributable to confinement of optical phonons within nanocrystal grain boundaries and laser-induced heating, both influenced by nanocrystal size. This study correlates size-dependent single-particle heating effects with size-dependent interparticle heat transfer under laser irradiation and is enabled by controlled nanocrystal synthesis.