(276g) Colloidal CuFeS2 Nanocrystals: Synthetic Design and Insights into the Unique Optoelectronic Properties Due to Intermediate Fe d-Band

In this presentation, we will describe the colloidal synthesis of phase-pure nanocrystals (NCs) of a highly abundant mineral, chalcopyrite (CuFeS₂). The steady state optical extinction spectrum of these NCs offers few surprises due to Fe which introduces deep energy levels in the electronic band structure, as supported by our density functional theory (DFT) calculations. The spectrum is characterized by absorption bands centered at around 480 and 950 nm, spanning almost the entire visible and near infrared regions. These are ascribable to electronic transitions from the valence band (VB) to the empty intermediate band (IB), located in the so-called fundamental band-gap and composed predominantly of Fe 3d orbitals. We further demonstrate, through spectroscopic measurements and ab initio calculations that these NCs actually sustain metal-like quasi-static optical resonances despite the absence of free carriers in the NC ground state owing to this unique band structure. Laser-irradiation (at 808 nm) of an aqueous suspension of these CuFeS₂ NCs exhibited significant heating, with a photothermal conversion efficiency of 49%. Such efficient heating is most likely due to the carrier relaxation within the broad IB band, as corroborated by transient absorption measurements and further described by DFT calculations. The intense absorption and high photothermal transduction efficiency (PTE) of these NCs in the so-called biological window (650−900 nm) make them suitable for photothermal therapy as demonstrated by tumor cell annihilation upon laser irradiation.

We intend to put forward the chemistry behind the synthesis of these low band-gap ternary semiconductor NCs and describe the optoelectronic properties using experimental and theoretical points of view, through this presentation. A suite of spectroscopic techniques, which include visible-infrared transient absorption, x-ray absorption and emission, resonant inelastic x-ray scattering (RIXS) were used in our analyses. The subsequent use as a photothermal agent is an offshoot of this understanding.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.

References:
1. Colloidal CuFeS₂ Nanocrystals: Intermediate Fe d-Band Leads to High Photothermal Conversion Efficiency. Sandeep Ghosh, Tommaso Avellini, Alessia Petrelli, Ilka Kreigel, Roberto Gaspari, Guilherme Almeida, Giovanni Bertoni, Andrea Cavalli, Francesco Scotognella, Teresa Pellegrino, and Liberato Manna Chem. Mater. 28(13), 4848- 4858 (2016).

2. Quasi-Static Resonances in the Visible Spectrum from All-Dielectric Intermediate Band Semiconductor Nanocrystals. Roberto Gaspari, Giuseppe Della Valle, Sandeep Ghosh, Ilka Kriegel, Francesco Scotognella, Andrea Cavalli, and Liberato Manna Nano Lett. 17(12), 7691–7695 (2017).