High-resolution transmission electron microscopy (HR-TEM) imaging was performed on a JEOL JEM-F200 operated in TEM mode at a beam energy of 200 kV. Samples were prepared by drop-casting diluted solutions of NPs onto ultra-thin carbon film-coated 400 mesh lacey carbon copper grids. TEM images were analyzed by ImageJ software. For the core size measurements, 138 particles were manually fitted with the "Elliptical Selections" tool, and for each particle, the lengths of the major and minor axes were derived.