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Characterization of Emissive States for Structurally Precise Au25(SC8H9)180 Monolayer-Protected Gold Nanoclusters Using Magnetophotoluminescence Spectroscopy

Green, Thomas D., Herbert, Patrick J., Yi, Chongyue, Zeng, Chenjie, McGill, Stephen, Jin, Rongchao, Knappenberger, Kenneth L.
The Journal of Physical Chemistry C 2016 v.120 no.31 pp. 17784-17790
energy, magnetism, mixing, nanogold, nanoparticles, optical properties, photoluminescence, physical chemistry, spectroscopy, temperature
Electronic relaxation dynamics and near-infrared emission of structurally precise Au₂₅(SC₈H₉)₁₈⁰ MPCs were studied using energy-resolved and time-resolved magneto-photoluminescence (MPL) spectroscopy. Measurements were carried out at sample temperatures spanning the range of 4.5 to 20 K following 3.1 eV laser excitation. These measurements revealed two main PL peaks detected at 1.78 and 1.98 eV. The emissive states giving rise to these peaks were characterized using magnetic circular polarized photoluminescence (MCPL) spectra, which were obtained from energy-resolved PL collected at positive and negative field polarities. Analysis of MCPL magnetization data yielded a Lande g-factor of g = 1.05 ± 0.04 for the 1.98 eV peak and g = 1.7 ± 0.1 for the 1.78 eV peak. The g-factor of the 1.78 eV peak suggested emission from a quartet state, which represents a high-spin configuration for this system. The time-resolved MPL data were fit to a biexpenoential decay function that included a stretch parameter. Arrhenius analysis of the 4.5 K field-dependent rate data identified an energy barrier of 0.66 ± 0.04 meV, which was interpreted as the energy gap separating dark and bright fine structure components of the manifold of nanocluster emissive states. The temperature dependence of this energy barrier was attributed to thermal population of the upper state, reducing the effect of field-induced mixing. These data provide new insight into the optical properties of structurally precise, condensed-phase metal clusters.