Experimental and theoretical determination of the optical gap of the Au ₁₄₄(SC ₂H ₄Ph) ₆₀ Cluster and the (Au/Ag) ₁₄₄(SC ₂H ₄Ph) ₆₀ Nanoalloys

Au ₁₄₄PET ₆₀ and Au _144-xAg _xPET ₆₀ (PET = SC ₂H ₄Ph, phenylethylthiolate, and 30 ≤ x ≤ 53) clusters were studied by optical spectroscopy and linear response time-dependent density functional theory. Spectra of thin dry films were measured in order to reveal the onset for electronic absorption. The optical gap of the Au ₁₄₄PET ₆₀ cluster was determined at 0.19 ± 0.01 eV, which agrees well with the computed energy for the first optical transition at 0.32 eV for a model cluster Au ₁₄₄(SH) ₆₀ when the line width of individual transitions is taken into account. The optical gaps for the Au _144-xAg _xPET ₆₀ alloy clusters were observed in a range of 0.12-0.26 eV, in good agreement with the calculations giving 0.16-0.36 eV for the lowest-energy optical transitions for corresponding Au _144-xAg _x(SH) ₆₀ models. This indicates that the gap is only moderately affected by doping Au with Ag. This work constitutes the first accurate determination of the fundamental spectroscopic gap of these compounds.