Mechanistic insights into the optical limiting performance of carbonaceous nanomaterials embedded with core–shell type graphite encapsulated Co nanoparticles

Globular amorphous carbonaceous materials embedded with graphite encapsulated metallic Co-nanoparticles with a high degree of crystallinity are synthesized by pyrolysis and demonstrated as excellent candidates for optical limiters. The amount of metal precursor (Co-acetylacetonate) used with toluene for pyrolysis is chosen as a strategy to control the degree of graphitization of graphene-like shells around the embedded Co-nanoparticles and also the crystallinity of these Co nanoparticles in the samples. The graphitic shell with an optimum amount of defects tunes the electronic properties of these nanomaterials, providing the electronic states required for the enhancement of nonlinear optical absorption (NLA) through an excited state absorption (ESA) process. Simultaneously, the increase in the crystallinity of the Co nanoparticle enhances its metallic nature, which helps in increasing NLA performance through the free carrier absorption (FCA) process. The importance of highly metallic Co is to involve both the Co nanoparticle and its graphitic encapsulation in facilitating the FCA process, which substantially enhances NLA. In comparison with many similar samples (e.g., Fe3C@C at 100 μJ of laser energy), our present samples show superior NLA performance even at the much lower laser pulse energy of ∼15 μJ. This performance is much better than many of the present-day NLA materials too. The simple, low-cost and one-step pyrolysis synthesis process makes our materials even more attractive.