Tunable optical activities in chiral transition metal oxide nanoparticles

Chiral transition metal oxides (TMOs) are widely used in various optoelectronic devices. However, the currently poor understanding of how the optical activities of TMOs can be regulated considerably hinders their applications. We have synthesized a series of chiral TMO nanoparticles (NPs), i.e., MoO (x = 2, 2.4 and 2.5) and Co₃O₄. Compared with TMO NPs with L-/D-cysteine molecules as the capping ligand, L-/D-histidine-capped TMO NPs possess larger anisotropic factors (g_abs), which are as high as ∼0.01 and ∼0.02 for L-/D-histidine-capped MoO_2.5 and Co₃O₄ NPs, respectively. A nondegenerate coupled oscillator (NDCO) theoretical calculation confirms that L-/D-histidine molecules can generate a smaller electric dipole moment and thus induce higher optical activity than L-/D-cysteine molecules. Impressively, the chiral NPs exhibit broadband second harmonic generation. This work indicates that chiral TMO NPs have potential for application in nonlinear optical devices.