Localized surface plasmon resonances (LSPRs) of metal nanoparticles (NPs) are commonly used for several applications including sensing, energy harvesting and cancer therapy [1]. LSPR in NPs are usually understood in terms of collective excitations of free carriers. Plasmonics in semiconductors are therefore generally limited to doped or degenerate semiconductors, which display metal-like properties [2]. Pure semiconductors, with no free carriers in their ground state are usually not considered suitable for applications in the field of plasmonics. It has however been recently demonstrated that undoped iron-based chalcopyrites NPs display a large light absorption in the visible range [3, 4]. The origin of this absorption is however controversial. In this talk I will discuss the mechanism of light absorption by iron-based chalcopyrites NCs, using a model which combines the classical Mie theory with ab-initio computed chalcopyrites dielectric functions. It will be shown that iron determines the presence of an empty intermediate band (IB), occurring between the valence and conduction band of chalcopyrites. This IB is responsible for the fulfillment of the Froehlich condition of light absorption in the visible range, i.e. IB semiconductors resonances can be understood within the same mathematical framework of metal LSPR. Furthermore, I will present a model for the relaxation of the electrons excited into the IB by optical pumping and compare it with the results obtained by transient spectroscopy pump-probe experiments. This represents a first attempt to quantitatively model the excellent photothermal efficiency of chalcopyrites NPs, which have been recently proved to be cytotoxic and thereby suitable for possible uses in photothermal therapy.

[1] Kovalenko MV, Manna L, Cabot A, Hens Z, Talapin DV, Kagan CR, et al. Prospects of Nanoscience with Nanocrystals. ACS Nano. 2015;9:1012-57.
[2] Luther JM, Jain PK, Ewers T, Alivisatos AP. Localized surface plasmon resonances arising from free carriers in doped quantum dots. Nat Mater. 2011;10:361-6.
[3] Ghosh S, Avellini T, Petrelli A, Kriegel I, Gaspari R, Almeida G, et al. Colloidal CuFeS2 Nanocrystals: Intermediate Fe d-Band Leads to High Photothermal Conversion Efficiency. Chemistry of Materials. 2016;28:4848-58.
[4] Gabka G, Bujak P, Ostrowski A, Tomaszewski W, Lisowski W, Sobczak JW, et al. Cu–Fe–S Nanocrystals Exhibiting Tunable Localized Surface Plasmon Resonance in the Visible to NIR Spectral Ranges. Inorganic Chemistry. 2016;55:6660-9.