Tuning Ag29 nanocluster light emission from red to blue with one and two-photon excitation

Abstract

We demonstrate that the tuning of the light emission from red to blue in dihydrolipoic acid (DHLA) capped Ag₂₉ nanoclusters can be trigged with one and two photon excitations. The cluster stoichiometry was determined with mass spectrometry and found to be Ag₂₉(DHLA)₁₂. In a detailed optical investigation, we show that these silver nanoclusters exhibit a strong red photoluminescence visible to the naked eye and characterized by a quantum yield of nearly ∼2% upon one-photon excitation. In the nonlinear optical (NLO) study of the properties of the clusters, the two-photon excited fluorescence spectra were recorded and their first hyperpolarizability obtained. The two-photon absorption cross-section at ∼800 nm for Ag₂₉(DHLA)₁₂ is higher than 10⁴ GM and the hyperpolarizability is 106 × 10⁻³⁰ esu at the same excitation wavelength. The two-photon excited fluorescence spectrum appears strongly blue-shifted as compared to the one-photon excited spectrum, displaying a broad band between 400 and 700 nm. The density functional theory (DFT) provides insight into the structural and electronic properties of Ag₂₉(DHLA)₁₂ as well as into interplay between metallic subunit or core and ligands which is responsible for unique optical properties.