Natural circular dichroism in the surface plasmon resonance and interband transition of noble metal nanocrystals induced by surface magnetism

Abstract

Circular dichroism is typically used in the visible and ultraviolet wavelength range to study the structure of chiral substances which lacks inversion symmetry. When a magnetic dipole moment interferes the electric dipole transition, circular dichroism arises from achiral substances even in the absence of an applied magnetic field. Here, the experimental observation of natural circular dichroism in the surface plasmon resonances and interband transitions of spherical gold, silver, and copper nanocrystals in solution is presented. The anomalous circular dichroism is qualitatively explained by spin-orbit coupling and exchange interaction that occur through local magnetic moments from surface defects. The s-d and d-d exchange interactions occur between spins of adatoms and the conduction and d-band electrons. Dipolar and quadrupolar plasmon modes showed opposite sign of ellipticity (dipoles: −, quadrupoles: +), manifesting the conservation of orbital angular momentum and indicating the nature of an orbital excitation in the magnetoplasmon. The plasmon-mode dependent ellipticity was consistently observed at anisotropic nanocrystals including triangles, octahedra, decahedra, nanorods, and core-shell nanoparticles regardless of chirality and polarity of adsorbate molecules. Interband transitions in gold nanocrystals of small size showed a sign reversal to negative ellipticity, and thus the surface has magnetic moments of d-electrons opposite to the core.