Abstract
We report a computational study via time-dependent density-functional theory (TDDFT) methods of the photo-absorption spectrum of an atomically precise monolayer-protected cluster (MPC), the Ag24Au(DMBT)18 single negative anion, where DMBT is the 2,4-dimethylbenzenethiolate ligand. The use of efficient simulation algorithms, i.e., the complex polarizability polTDDFT approach and the hybrid-diagonal approximation, allows us to employ a variety of exchange-correlation (xc-) functionals at an affordable computational cost. We are thus able to show, first, how the optical response of this prototypical compound, especially but not exclusively in the absorption threshold (low-energy) region, is sensitive to (1) the choice of the xc-functionals employed in the Kohn-Sham equations and the TDDFT kernel and (2) the choice of the MPC geometry. By comparing simulated spectra with precise experimental photoabsorption data obtained from room temperature down to low temperatures, we then demonstrate how a hybrid xc-functional in both the Kohn-Sham equations and the diagonal TDDFT kernel at the crystallographically determined experimental geometry is able to provide a consistent agreement between simulated and measured spectra across the entire optical region. Single-particle decomposition analysis tools finally allow us to understand the physical reason for the failure of non-hybrid approaches.
Highlights
Precise nanoclusters, called nanomolecules or monolayer-protected clusters (MPCs), i.e., metal nanoclusters protected by a layer of coating ligands with a well-defined stoichiometry and chemical structure, constitute a class of materials of great interest in fundamental science and applications[1–27] and are ideal systems to test and validate our physicochemical understanding of the phenomena exhibited by metal nanostructures at work.[28–31] Knowledge of the MPC precise chemical entity allows researchers to apply multiple characterization schemes, both experimental and theoretical, to these systems, cross-validating methods and results in a rigorous framework
Precise nanoclusters, or monolayer-protected clusters (MPCs), represent an ideal playground to assess and validate the performance of computational methods via a stringent comparison with chemical–physical experimental data, enabling singling out accurate simulation protocols that can be applied with predictive confidence in the rational design of the optical properties of new metal-cluster-based materials.[1–39]
We focus on the prediction of the optical response of these systems, we consider a prototypical compound, the Ag24Au(DMBT)[18] single negative anion (DMBT = 2,4-dimethylbenzenethiolate ligand) as an alloyed and ligand-conjugated MPC, and we conduct a validation study to investigate which density-functional theory (DFT)/time-dependent density-functional theory (TDDFT) simulation tools provide the best agreement between predicted and measured spectra
Summary
Precise nanoclusters, called nanomolecules or monolayer-protected clusters (MPCs), i.e., metal nanoclusters protected by a layer of coating ligands with a well-defined stoichiometry and chemical structure, constitute a class of materials of great interest in fundamental science and applications[1–27] and are ideal systems to test and validate our physicochemical understanding of the phenomena exhibited by metal nanostructures at work.[28–31] Knowledge of the MPC precise chemical entity allows researchers to apply multiple characterization schemes, both experimental and theoretical, to these systems, cross-validating methods and results in a rigorous framework. Knowledge of the MPC precise chemical entity allows researchers to apply multiple characterization schemes, both experimental and theoretical, to these systems, cross-validating methods and results in a rigorous framework This possibility is especially important in the field of the computational prediction of the structure and properties of materials. Given the availability of widely different computational methods and schemes employing different approximations and techniques, it is crucial to assess precisely the performances and the pitfalls of these different choices, so as to identify the computational protocols best reproducing experimental results and properties. These protocols can be employed to obtain accurate information in those cases in which theory is called to provide missing information that would be difficult or even impossible to obtain experimentally. Once validation is successfully achieved, the computational protocol can be trusted within scitation.org/journal/jcp well-understood limits and can be applied with grounded confidence in a predictive way to design new materials with desired optical properties
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