Plasmonic behavior of spherical core-shell structures in the tunneling regime

Many experimental studies have revealed that plasmonic behavior of metallic nanostructures with subnanometer gap distances cannot be predicted by conventional local response models due to nonlocal or quantum effects. Optical... [ view full abstract ]

Many experimental studies have revealed that plasmonic behavior of metallic nanostructures with subnanometer gap distances cannot be predicted by conventional local response models due to nonlocal or quantum effects. Optical interactions in these systems can be in principle correctly described by ab initio-based approaches, such as time-dependent density function theory (TD-DFT). However, TD-DFT methods can only handle very small systems due to its high computational costs. Quantum hydrodynamic theory (QHT), on the other hand, is a promising tool which is computationally much cheaper and can be applied to fairly large plasmonic particles to study both microscopic as well as macroscopic optical properties. QHT has proven its potential in efficiently and correctly describing the plasmon resonances, electron spill-out and retardation effects. In the present study, we apply state-of-the-art QHT to investigate the effect of electron tunneling on the optical properties of plasmonic nanostructures. In particular, we consider spherical core-shell structures, also known as nanomatryoshkas (NMs), with sub-nanometer core-shell gap distances, both for Au and Na metals. We compare the results obtained by using QHT with the reference TD-DFT computations, performed using an in-house developed code and we find an excellent agreement between the two theories. We also study optical properties of quite big systems, both for Au and Na, whose sizes make them inaccessible for DFT calculations and we examine the impact of core-shell spacing on near-field and far-field optical behavior of these systems. We find that the QHT method efficiently predicts the nonlocal and quantum behavior of plasmonic systems with different length scales. A systematic comparison between the local response approximation (LRA), Thomas-Fermi hydrodynamic theory (TF-HT) and QHT method has also been presented. The results show that as the core-shell distance decreases the nonlocal or quantum effects strongly influence the plasmonic properties of these systems which can be nicely described by the QHT. For numerical implementation of these structures, we fully exploit the symmetry of the geometry and use a 2.5D simulation technique which reduces the computational efforts to a great extent.