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

Abstract

Electronic relaxation dynamics and near-infrared emission of structurally precise Au 25 (SC 8 H 9 ) 18 0 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.

Spin‐Polarized Photoluminescence in Au25(SC8H9)18 Monolayer‐Protected Clusters

“…Green et al. quantified relaxation energy barriers of 610 ± 130 μeV and 660 ± 40 μeV from field‐dependent Arrhenius analysis of PL lifetimes, [ 24 ] which are in good agreement with those reported here for the 1.78 and 1.98‐eV peaks. The assignment of these activation barriers determined from field‐dependent Arrhenius analysis to VTV‐MCPL is further substantiated by consideration of the quantified Faraday term contributions.…”

“…[ 31,32 ] Previously, the observation of field‐dependent PL lifetime acceleration was correlated to magnetic‐field‐induced state mixing. [ 24 ] Here, the 1.78 and 1.94 eV components exhibited the highest relative B‐term contributions. The agreement between activation energies and contributions from field‐induced mixing suggests that the 1.78 and 1.94 eV components are associated with the two previously reported energy barriers.…”

“…This distinct temperature‐dependent PL yield has previously been observed in multinuclear Au‐centered transition metal complexes and semiconductor materials. [ 16–18 ] The increase in PL yield as a result of increasing temperature can arise from the presence of dark‐ or trap‐states that can thermally populate nearly‐degenerate (≈ k B T ) bright states. Previously, analysis of temperature‐dependent integrated PL intensities from semiconductor‐like carbon nanotubes was used to quantify the energy separation between bright and dark excitonic states [ 19–21 ] …”

“…[ 22,23 ] The broad spectral profile results from a combination of individual, spectrally convoluted PL components. [ 24,25 ] Figure 2a also shows the DOCP spectrum of the Au 25 (SC 8 H 9 ) 18 cluster acquired at 4.5 K and 17.5 T. The DOCP spectrum was determined using Equation () …”

“…The agreement between activation energies and contributions from field‐induced mixing suggests that the 1.78 and 1.94 eV components are associated with the two previously reported energy barriers. [ 24 ] The authors note that the PL lifetimes acquired at 4.5 K were fit to two exponential decays with time constants of 51.3 ± 0.7 and 158 ± 8 μsec. [ 24 ] The notable absence of a third emissive component, as observed here for the spectrally resolved measurements, may indicate that the radiative lifetime of the additional component is shorter than the 30 ns instrument response of the TCSPC instrument used previously.…”