Semiempirical Modeling of Ag Nanoclusters: New Parameters for Optical Property Studies Enable Determination of Double Excitation Contributions to Plasmonic Excitation

Abstract

Quantum mechanical studies of Ag nanoclusters have shown that plasmonic behavior can be modeled in terms of excited states where collectivity among single excitations leads to strong absorption. However, new computational approaches are needed to provide understanding of plasmonic excitations beyond the single-excitation level. We show that semiempirical INDO/CI approaches with appropriately selected parameters reproduce the TD-DFT optical spectra of various closed-shell Ag clusters. The plasmon-like states with strong optical absorption comprise linear combinations of many singly excited configurations that contribute additively to the transition dipole moment, whereas all other excited states show significant cancellation among the contributions to the transition dipole moment. The computational efficiency of this approach allows us to investigate the role of double excitations at the INDO/SDCI level. The Ag cluster ground states are stabilized by slight mixing with doubly excited configurations, but the plasmonic states generally retain largely singly excited character. The consideration of double excitations in all cases improves the agreement of the INDO/CI absorption spectra with TD-DFT, suggesting that the SDCI calculation effectively captures some of the ground-state correlation implicit in DFT. These results provide the first evidence to support the commonly used assumption that single excitations are in many cases sufficient to describe the optical spectra of plasmonic excitations quantum mechanically.

Plasmon excitations in chemically heterogeneous nanoarrays

“…Molecular plasmons exhibit effects from finite-size quantum confinement, symmetry, and avoided crossings within nanosystems such as metallic clusters [18][19][20][21][22][23][24], arrays [16,25,26], chains [27][28][29], and conjugated and aromatic hydrocarbons [30][31][32][33][34]. As evidenced by previous computational studies [16,25,26], double-chain atomic arrays of metals, which we consider in this study, contain many of the physical and chemical phenomena which are central to any molecular plasmon.…”

Streptavidin-conjugated gold nanoclusters as ultrasensitive fluorescent sensors for early diagnosis of HIV infection

“…Second, we elucidated the role of the growing Au clusters in the interaction where the various sizes of gold clusters (Au x ) were built using Gaussian ranging from Au 1 to Au 57 . For this, we obtained the density functional theory–optimized structures ranging from Ag 1 to Ag 57 atoms to be treated as consequent ligands ( 33 ). The smallest being the neutral Ag atom, to the consequent icosahedral Ag 13 , tetrahedral Ag 20 , and the biggest being the icosahedral core–based Ag 66 cluster whose energetics were determined by MOE using MMFX94 force field ( 34 ).…”

A new release of MOPAC incorporating the INDO/S semiempirical model with CI excited states

“…Semiempirical quantum mechanical methods (SEQMs) have experienced a resurgence in recent years because of their capacity to address these challenges at accessible computational costs. 1,[12][13][14][15][16][17][18][19][20] SEQMs use low-cost Hamiltonians simplified from Hartree-Fock. The introduction of parameters fit to experimental and high-level computational results has enabled high enough accuracy to provide understanding of chemical properties including thermochemistry, 13,21,22 noncovalent interactions, 13,21,23 excited states, 24,25 pKa, 14,19 magnetism, 26 solvatochromism, 27 and electrochemical formal potentials.…”

“…[43][44][45][46] The INDO framework has also been extended to transition metals with d orbitals. [47][48][49][50] In recent years, we have parametrized INDO for the optical properties of Ag clusters; 17,51 these parameters have given insight into features such as the quantum mechanical nature of quadrupolar plasmons, 52 chemical effects in surface-enhance Raman spectroscopy (SERS), 51,53 and nonlinear optical properties of plasmons. 54 Several implicit solvation methods, including PCM 55 and COSMO, 27 have been implemented with INDO and yield solvatochromic shifts in reasonable agreement with experimental results.…”