ABSTRACT
The magneto-optical absorption properties of colloidal metal nanoclusters spanning nonmetallic to metallic regimes were examined using variable-temperature variable-field magnetic circular dichroism (VTVH-MCD) spectroscopy. Charge neutral Au25(SC8H9)18 exhibited MCD spectra dominated by Faraday C-terms, consistent with expectations for a nonmetallic paramagnetic nanocluster. This response is reconciled by the open-shell superatom configuration of Au25(SC8H9)18. Metallic and plasmon-supporting Au459(pMBA)170 exhibited temperature-independent VTVH-MCD spectra dominated by Faraday A-terms. Au144(SC8H9)60, which is intermediate to the metallic and nonmetallic limits, showed the most complex VTVH-MCD response of the three nanoclusters, consisting of 19 distinguishable peaks spanning the visible and near-infrared (3.0-1.4 eV). Variable-temperature analysis suggested that none of these transitions originated from plasmon excitation. However, evidence for both paramagnetic and mixed (i.e., nondiscrete) transitions of Au144(SC8H9)60 was observed. These results highlight the complexity of gold nanocluster electronic transitions that emerge as sizes approach metallic length scales. Nanoclusters in this regime may provide opportunities for tailoring the magneto-optical properties of colloidal nanostructures.
ABSTRACT
The magneto-optical signatures of colloidal noble metal nanostructures, spanning both discrete nanoclusters (<2 nm) and plasmonic nanoparticles (>2 nm), exhibit rich structure-property correlations, impacting applications including photonic integrated circuits, light modulation, applied spectroscopy, and more. For nanoclusters, electron doping and single-atom substitution modify both the intensity of the magneto-optical response and the degree of transient spin polarization. Nanoparticle size and morphology also modulate the magnitude and polarity of plasmon-mediated magneto-optical signals. This intimate interplay between nanostructure and magneto-optical properties becomes especially apparent in magnetic circular dichroism (MCD) and magnetic circular photoluminescence (MCPL) spectroscopic data. Whereas MCD spectroscopy informs on a metal nanostructure's steady-state extinction properties, its MCPL counterpart is sensitive to electronic spin and orbital angular momenta of transiently excited states. This review describes the size- and structure-dependent magneto-optical properties of nanoscale metals, emphasizing the increasingly important role of MCPL in understanding transient spin properties and dynamics.
