Enhanced nonlinear optical properties of Au25 nanocluster oligomers linked by bidentate dithiol

Abstract

Atomically-precise gold nanoclusters covered with thiol ligands are attractive candidates for advanced photonics, sensing and bioimaging applications. We show that covalent linking of Au₂₅(PET)₁₈ (PET – phenylethylmercaptan) with (1R,1′R)-6,6′-(1,4-phenylene)di-1,1′-binaphthyl-2,2′-dithiol (diBINAS) may offer a pathway towards atomically-precise materials with enhanced nonlinear optical properties. Interestingly, nanocluster oligomerization leads to moderate modification of the linear optical properties, but has a significant impact on their two-photon absorption performance. Two photon absorption cross-sections σ₂ of the monomers, dimers and trimers were measured via the z-scan technique in a wide range of wavelengths. The relative values of σ₂ are different from a simple additive scheme, i.e. in the resonance region the σ₂ enhancement is equal to factors of ∼4 and ∼8 for the dimer and trimer, respectively. In the off-resonance spectral region, the trimers exhibit 4 times higher two-photon absorption cross-section values than the monomers, while the dimers are characterized by enhancement factors slightly larger than 2.