Theoretical and Experimental Investigations of Resonant Light-Matter Interactions in Ternary Metal Chalcogenide Nanocrystals

Abstract

The interaction of light with subwavelength nanomaterials has been extensively investigated for more than a century in both theoretical science and engineering. In particular, the resonant interaction of light with plasmonic metal nanocrystals (NCs) – including gold, silver, and platinum – has been of great interest in multiple fields of study due to their extraordinary optical effects in the near- and far-field enhancement via localized surface plasmon resonance (LSPR). Consequently, manipulation of light using plasmonic metal NCs has enabled the development of a broad range of applications from nonlinear optics to sensing. However, the nearly-fixed dielectric properties and high ohmic losses of metal NCs have been a central bottleneck for improving the performance of photonic and plasmonic devices.The maturation of nanophotonics has often necessitated the search for novel materials. Recently, new emphasis has been placed on ternary chalcopyrite-phase copper iron sulfide (CuFeS2) intermediate band semiconductor NCs as an alternative to plasmonic metal NCs due to their resonant light interactions that strongly absorb and scatter incident light in the visible region, analogous to the plasmonic response of metal NCs. Interestingly, however, the underlying physics of the optical properties of CuFeS2 NCs are distinctly different from plasmonic metal NCs, since CuFeS2 is an all-dielectric material having no free charges in their ground state. In order to fully realize its potential for technological applications, a detailed understanding of the unique optical properties of CuFeS2 NCs is a prerequisite. In this regard, this dissertation explores, both theoretically and experimentally, the resonant optical properties of ternary metal chalcogenide intermediate band semiconductor NCs. Herein, we present a set of experimental and computational results, allowing for a clear differentiation between two fundamentally different modes of resonant excitation present in ternary metal chalcogenide NC systems: (i) a quasi-static dielectric resonance (DR) and (ii) a LSPR. In the first part of the dissertation, we demonstrated that bornite-phase copper iron sulfide (Cu5FeS4) NCs exhibit tunable optical characteristics from visible to near-infrared that are strongly dependent on their iron content. Our experimental results on bornite-phase copper iron sulfide NCs showed that manipulating the iron content of the material modulates the intensity of the DR response, and moreover, that post-synthetic compositional manipulation via simple oxidative chemistry can be used to tune directly between the DR and LSPR mechanisms. We further confirmed an analogous DR-to-LSPR evolution in CuFeS2 NCs subjected to the influence of oxidizing agents and added ions. In addition, electronic band structure calculations by density functional theory demonstrated that the presence of an intermediate band of states formed by empty Fe d-orbitals plays an important role in the occurrence of a DR in the visible-frequency regime. In the second part of the dissertation, we expanded our knowledge on the DR properties of ternary metal chalcogenide NCs by developing a synthetic protocol for colloidal intermediate band silver iron sulfide (AgFeS2) NCs. Through synthetic studies of AgFeS2 NCs involving a two-step heterogeneous nucleation process, the synthesis of Ag seed NCs and their subsequent growth into AgFeS2 NCs, we clearly differentiate between the DR and LSPR. Moreover, we confirm that increasing the gap between the valence band and the intermediate band causes a blue-shift of the DR in ternary metal chalcogenide intermediate band NC systems based on a comparative study of the DR properties of AgFeS2 and CuFeS2 NCs. Taken together, this work represents a significant step forward in the understanding of the plasmonic-like optical response of intermediate-band-semiconductor ternary metal chalcogenide NCs, which could lead to important ramifications for future applications of these materials.