A Theoretical Investigation of the Linear and Nonlinear Optical Responses of Scandium- and Yttrium-Doped (sub-nm) Ag and Au Clusters

TL;DR

This paper explores how doping silver and gold clusters with scandium and yttrium affects their optical properties, which could be useful for biomedical imaging.

Contribution

The study reveals novel optical behaviors in sub-nm Sc- and Y-doped Ag and Au clusters through theoretical modeling.

Findings

Sc and Y doping modifies optical responses by breaking symmetry and enhancing two-photon absorption.

Doped Ag clusters show stronger TPA in the biologically relevant 620–690 nm range.

YAu5 exhibits enhanced first hyperpolarizability at 1064 nm due to increased hybridization.

Abstract

We investigate the linear and nonlinear optical properties of (sub-nm) Ag6 and Au6 clusters doped with Sc and Y using time-dependent density functional theory. Both parent clusters have D3h ground-state geometries but exhibit noticeably different electronic structures; scalar-relativistic corrections in Au6 induce significant s-d hybridization, resulting in larger HOMO-LUMO gaps and reduced one-photon absorption (OPA) cross-sections compared to Ag6. Two-photon absorption (TPA) peaks in the UV region show resonance enhancement via coupling with OPA-active states, with Ag6 having larger cross-sections than Au6. Doping with Sc and Y modifies the optical responses by breaking configurational symmetry and lifting HOMO degeneracies. ScAg5 and YAg5 energetically prefer planar configurations with higher dopant orbital contributions, while ScAu5 and YAu5 prefer non-planar configurations. This leads to blue-shifted, intensified OPA transitions and larger TPA cross-sections in doped clusters than in parent clusters. Doped Ag clusters exhibit a significantly stronger TPA response in the biologically relevant 1.8–2.0 eV (620–690 nm) spectral region for in vivo imaging. Furthermore, a higher degree of Sc(Y)-Au hybridization generates additional TPA pathways and also facilitates electronic transitions at 1064 nm, enhancing the first hyperpolarizability (β (−2ω; ω, ω)) for YAu5. Overall, the results show that these (sub-nm) Sc/Y-doped noble metal clusters are promising candidates for photonic and biomedical imaging applications.

Figures