Mie Resonance-Enhanced Light Absorption of FeS₂ Nanocubes in a Near-Infrared Region: Intraparticulate Synergy between Electronic Absorption and Mie Resonances

FeS2 nanocrystals with the diameter of more than several dozen nanometers exhibit well-defined optical extinction peaks in a near-infrared region. These peaks, which are not exhibited from bulk FeS2, have been ascribed to the localized surface plasmon resonance without solid evidence. However, this interpretation could be doubtful considering the low carrier density of semiconducting FeS2. In this study, we theoretically demonstrated that the near-infrared extinction peaks from the FeS2 nanocubes are ascribed to Mie resonances, which act as a dielectric optical nanoantenna. Also, we experimentally demonstrated that the Mie resonances of FeS2 nanocubes have a large optical absorption component. Consequently, these nanocubes caused a larger temperature increase with a higher photothermal conversion efficiency by a near-infrared laser irradiation, as compared with small FeS2 nanoclusters not exhibiting Mie resonances. Such results suggest that the optical absorption originating from the indirect interband transition of FeS2 was enhanced through the Mie resonance excitation of the FeS2 nanocrystal itself. Thus, the FeS2 nanocrystals are a bifunctional material playing the roles of light absorber and light-harvesting antenna. This is a useful paradigm in contrast to absorption-enhancing systems comprising individual light absorbers (e.g., organic chromophores) and plasmonic metal nanoparticles, in which the performance is compromised by undesirable quenching.