The Evolution from Superatom- to Plasmon-Mediated Magnetic Circular Dichroism in Colloidal Metal Nanoparticles Spanning the Nonmetallic to Metallic Limits.

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 Au 25 (SC 8 H 9 ) 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 Au 25 (SC 8 H 9 ) 18 . Metallic and plasmon-supporting Au 459 (pMBA) 170 exhibited temperature-independent VTVH-MCD spectra dominated by Faraday A-terms. Au 144 (SC 8 H 9 ) 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 Au 144 (SC 8 H 9 ) 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.