Colloidal CuFeS2 Nanocrystals: Intermediate Fe d-Band Leads to High Photothermal Conversion Efficiency

TL;DR

This study reports the synthesis of CuFeS2 nanocrystals with an intermediate Fe d-band that exhibit high photothermal conversion efficiency, making them promising for biomedical applications like tumor therapy.

Contribution

The paper introduces a colloidal synthesis method for phase-pure CuFeS2 nanocrystals with unique optical properties due to an intermediate Fe d-band, leading to enhanced photothermal performance.

Findings

Photothermal conversion efficiency of 49% at 808 nm.

Strong absorption in the biological window (650-900 nm).

Effective tumor cell destruction upon laser irradiation.

Abstract

We describe the colloidal hot-injection synthesis of phase-pure nanocrystals (NCs) of a highly abundant mineral, chalcopyrite (CuFeS2). Absorption bands centered at around 480 and 950 nm, spanning almost the entire visible and near infrared regions, encompass their optical extinction characteristics. These peaks are ascribable to electronic transitions from the valence band (VB) to the empty intermediate band (IB), located in the fundamental gap and mainly composed of Fe 3d orbitals. Laser-irradiation (at 808 nm) of an aqueous suspension of CuFeS2 NCs exhibited significant heating, with a photothermal conversion efficiency of 49%. Such efficient heating is ascribable to the carrier relaxation within the broad IB band (owing to the indirect VB-IB gap), as corroborated by transient absorption measurements. The intense absorption and high photothermal transduction efficiency (PTE) of these NCs in the so-called biological window (650-900 nm) makes them suitable for photothermal therapy as demonstrated by tumor cell annihilation upon laser irradiation. The otherwise harmless nature of these NCs in dark conditions was confirmed by in vitro toxicity tests on two different cell lines. The presence of the deep Fe levels constituting the IB is the origin of such enhanced PTE, which can be used to design other high performing NC photothermal agents.