Ni Nanoparticles Coated with Nitrogen-Doped Carbon for Optical Limiting Applications

In this work, we demonstrate the importance of the free carrier absorption (FCA) process in the nonlinear optical absorption (NLA) behavior of carbonaceous nanomaterials synthesized by pyrolysis at different heating rates (R-h = 1, 3, 10, and 20 degrees C/min). The chemical vapor deposition reaction through the pyrolysis of Ni(II) acetylacetonate, melamine, and toluene precursors was carried out at 600 degrees C, which leads to different carbonaceous nanostructures embedded with graphite-encapsulated Ni nanoparticles (Ni@C samples). A lower R-h (1 and 3 degrees C/min) value leads to the formation of globular carbon aggregates embedded with core-shell-type Ni-graphite nanoparticles, whereas a high R-h (10 and 20 degrees C/min) value leads to the formation of carbon nanotubes embedded with similar Ni-graphite core-shell nanoparticles. The samples prepared at moderate heating rates of 3 and 10 degrees C/min show prominently high NLA than those prepared at very slow or very fast (1 and 20 degrees C/min) heating rates. According to our results, the quality (and the amount) of graphitization and nitrogen defect centers enhance the NLA behavior of the Ni@C samples. This result is obtained through the enhancement of excited-state absorption (ESA) behavior of the samples due to the introduction of defect states within the band gap of the graphite layer. More importantly, the metallic Ni nanoparticles help in drastically enhancing the NLA through the FCA process. To prove this supposition, we demonstrated that the presence of any dielectric phase (e.g., NiO) within the metallic nanoparticles (or within the sample) acts as a barrier to FCA, thereby reducing the NLA. Our work highlights the importance of synthesis conditions (optimized heating rate), especially in controlling the quality of graphitization and the embedded metallic Ni particles in the Ni@C samples in enhancing the NLA. Furthermore, through the mechanistic insights, we emphasized the potential use of our Ni@C samples for optical limiting applications because of their excellent NLA behavior originating through the FCA and ESA processes.