Non-linear optical properties of gold quantum clusters. The smaller the better

Abstract

By developing a new method for synthesizing atomically monodisperse Au15 nanoclusters stabilized with glutathione molecules and using the current state-of-the-art methods for synthesizing monodisperse protected Au25 nanoclusters, we investigated their nonlinear optical (NLO) properties after two-photon absorption. The two-photon emission spectra and the first hyperpolarizabilities of these particles were obtained using, in particular, a hyper-Rayleigh scattering technique. The influence on NLO of the excitation wavelength, the size as well as the nature of the ligands is also explored and discussed. Au15, the smallest stable thiolated gold nanocluster, presents remarkable nonlinear properties with respect to two-photon processes. The two-photon absorption cross-section at 780 nm for Au15 is ∼65,700 GM. This experimental cross-section value points to a quantum yield for two-photon emission of about 3 × 10(-7) at 475 nm for Au15. The first hyperpolarizability β for Au15 clusters (509 × 10(-30) esu), as compared to Au25 clusters (128 × 10(-30) esu), is larger considering the difference in the number of gold atoms. Also, 10(30) β per atom values reported for Au15 and Au25 clusters are more than two orders of magnitude larger than the values reported for Au NPs in the size range 10-50 nm, outlining the quantum cluster regime.

Importance of double-resonance effects in two-photon absorption properties of Au25(SR)18−

“…Previous work has demonstrated that such symmetry breaking leads to the observation of hyper-Rayleigh scattering of the Au 25 (SR) 18 – cluster which otherwise would be symmetry forbidden due the centrosymmetry. 19,22 To investigate the symmetry breaking effects on the TPA cross-section, we also considered the Au 25 (SH) 16 (SPh) 2 – cluster where two phenyl groups have been substituted. This structure is taken from previous work investigating the symmetry breaking effects on the first hyperpolarizabilities.…”

Bulky Counterions: Enhancing the Two‐Photon Excited Fluorescence of Gold Nanoclusters

“…However, the main conclusion so far is that liganded silver and gold quantum clusters are excellent two‐photon absorbers but are rather poor two‐photon excited emitters . For example, the two‐photon absorption cross‐section at 780 nm for Au 15 is ≈65700 GM, whereas its TPEF cross‐section at 475 nm is only ≈0.022 GM …”

“…Au 15 (SG) 13 was synthesized as reported by Russier‐Antoine et al . Au 18 (SG) 14 was synthesized as follow: 390 mg of glutathione (GSH) is dissolved in 45 mL of methanol, 8 mL of tributylamine and 4.5 mL of water.…”

Ligand-protected gold clusters: the structure, synthesis and applications

“…11 Recently, nonlinear optical properties of the Au 15 (SG) 20 cluster have been reported. 287 Understanding the origin of the optical properties of gold clusters requires taking account of the structure of the metal and ligand orbitals.…”