A strong charge-transfer effect in surface-enhanced Raman scattering induced by valence electrons of actinide elements

Abstract

Surface-enhanced Raman scattering (SERS) is a powerful spectroscopic technique for highly sensitive molecular detection. As the great radial extent of 5f/6d orbitals is enough to support surface plasmons, SERS substrate materials containing actinide elements might bring a novel enhancement mechanism and even significantly enhanced magnitude. Here we study the SERS of a pyridine molecule on the typical actinide embedded structure Ac@Au₇ and on a similar structure of pure gold Au₈, respectively, using the time-dependent density functional theory (TDDFT) method. The calculated results show that the absorption spectrum of pyridine-Ac@Au₇ at 456 nm presents a strong peak belonging to charge-transfer (CT) transition from the metal to the pyridine molecule. Surprisingly, the corresponding CT-SERS enhancement can reach an order of magnitude of 10⁵, which could hardly be achieved in the pure gold systems (about 10³–10⁴). Furthermore, electronic structure analysis reveals that the initial orbitals of the CT transition contain a non-ignorable contribution from the 6d electrons of the Ac atom. Meanwhile, the contribution of the 6d electrons can be tuned by changing the conformation of the pyridine-Ac@Au₇ complex. The current findings provide a theoretical basis for exploring and synthesizing SERS materials based on actinide elements, which might subsequently facilitate applications of such structures in nanostructural characterization, single-molecule SERS signal detections and biological molecular recognitions etc.