Noble metal nanocrystals with localized surface plasmon resonance (LSPR) extinction features have undergone extensive research for applications such as refractometric sensing, photothermal therapy, and photocatalysis. However, plasmonic semiconductor nanocrystals (PSNCs) have expanded the spectral range of relevant material applications in the NIR through doping and compositional modification to directly alter the optoelectronic properties of the observed LSPR. The simplified Drude model highlights the free carrier density (n) and effective carrier mass (m*) as fundamental parameters which dictate the observed LSPR frequency. Prior literature has often neglected direct measurement of m* in favor of n acquisition through techniques such as chemical titration while assuming constant m*. This assumption is challenged in order to obtain a complete view of the optoelectronic properties of next generation PSNCs. For this experiment, CuxFeS4 (x=7,5,3) PSNCs are selected due to their high biocompatibility, low elemental cost, and biologically transmissive plasmon frequency at 1100 nm. Subsequently, CuxFeS4 is an ideal candidate for applications such as infrared neural stimulation and photothermal therapy. Adjusting the Cu:Fe ratio is hypothesized to directly impact the resulting defect density and structural composition of the NCs, however the observed LSPR does not significantly change in either frequency or spectral linewidth. To understand how free carriers within the NC are changing in such a way to produce near identical LSPR extinction features, a full elucidation of m* and n is required. High field magnetic circular dichroism (MCD) is performed to obtain direct measurements of m* by inducing cyclotron resonance via an applied magnetic field, while chemical titration with a one electron reductant, cobaltocene, is performed to directly measure n. These results are compared to Drude model fittings that would predict m* from n as is often done in literature, and adjustments to the Drude model are suggested. Additionally, MCD spectra were deconvoluted by energy level splitting observed and subsequently fit according to experimental absorption spectra for all three ratios. From these fits, an “ab initio” electronic band structure diagram is proposed for CuxFeS4 and compared with current density functional theory calculations.

Comparison of the Drude model to direct measurement of carrier density and effective mass of CuxFeS4

ACS Fall 2021

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