Noble metal nanocrystals have been long explored for their localized surface plasmon resonance (LSPR) properties, often subject to plasmon frequency (νp) tuning through changes in size and morphology. However, plasmonic semiconductor nanocrystals (PSNCs) have expanded electronic and spectral tunability potential for applications such as photothermal therapy, photocatalysis, and infrared neural stimulation. Through methods such as doping and stoichiometric modification, the free carrier density (n) and effective carrier mass (m*) for PSNCs can be modelled and related to νp through the simplified Drude model. While n can be directly measured through techniques such as chemical titration assuming constant m*, direct measurement of m* is non-trivial and has not been well studied for LSPR longitudinal (LO) and transverse (TO) modes that appear as a function of nanocrystal morphology. WO3-x is explored for its morphological tunability from sphere to rod shape and biologically transmissive LSPR between 1000 – 1700 nm. As such, WO3-x is an ideal candidate for applications in photothermal therapy and infrared neural stimulation. By varying the ratio of oleic acid and oleylamine in a microwave reactor synthetic scheme, various aspect ratios of WO3-x are obtained and compared through absorption spectroscopy. By changing the aspect ratio, it is hypothesized that differences in the m* should be observed between the LO and TO modes of the LSPR. Through the Drude model, a subsequent approximation of the local carrier density can be made along each observed mode with their respective measured m*, compared to direct measurement of global n through chemical titration. In order to directly measure m* for the observed LSPR modes, High field magnetic circular dichroism (MCD) is performed. As the aspect ratio of WO3-x increases, the observed LSPR modes νp increase in their energy difference, and MCD is used to quantify m* as a function of LSPR mode and changing aspect ratio. In supplement, femtosecond transient absorption and photothermal conversion efficiency experiments are used to explore carrier relaxation dynamics for all aspect ratios, relating observed lifetimes to measured m*. Subsequently, results are compared to the Drude model, and MCD spectra are deconvoluted by proposed electronic energy transitions observed in experimental absorption spectra for all aspect ratios, supplemented with electronic transition and band structure calculations performed in VASP.

Effect of morphology on WO3-x carrier effective mass investigated through magnetic circular dichroism

ACS Spring 2022

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