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[en] Highlights: • Aggregated nanoparticle spheres (AN-spheres) are self-assembled spherical or hemispherical structures formed during femtosecond laser surface processing that are composed entirely of nanoparticles. • AN-spheres have an onion-like layered morphology with different nanoparticle densities and compositions between concentric layers. • Low density layers are a result of the redeposition of ablated nanoparticles while high density layers are attributed to femtosecond laser sintering of the low density layers. • For the first time, AN-spheres have been produced on silicon, silicon carbide, and aluminum. - Abstract: A detailed structural and chemical analysis of a class of self-organized surface structures, termed aggregated nanoparticle spheres (AN-spheres), created using femtosecond laser surface processing (FLSP) on silicon, silicon carbide, and aluminum is reported in this paper. AN-spheres are spherical microstructures that are 20–100 μm in diameter and are composed entirely of nanoparticles produced during femtosecond laser ablation of material. AN-spheres have an onion-like layered morphology resulting from the build-up of nanoparticle layers over multiple passes of the laser beam. The material properties and chemical composition of the AN-spheres are presented in this paper based on scanning electron microscopy (SEM), focused ion beam (FIB) milling, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX) analysis. There is a distinct difference in the density of nanoparticles between concentric rings of the onion-like morphology of the AN-sphere. Layers of high-density form when the laser sinters nanoparticles together and low-density layers form when nanoparticles redeposit while the laser ablates areas surrounding the AN-sphere. The dynamic nature of femtosecond laser ablation creates a variety of nanoparticles that make-up the AN-spheres including Si/C core-shell, nanoparticles that directly fragmented from the base material, nanoparticles with carbon shells that retarded oxidation, and amorphous, fully oxidized nanoparticles.