Multiscale Modeling of the Electronic, Optical, and Magnetic Properties of Low-dimensional Metal Systems

This thesis presents a study of the electronic, optical and magnetic properties of low-dimensional metal systems across multiple scales, using a variety of theoretical techniques including microscopic phenomenological model, ab initio density functional theory (DFT) and classical electromagnetics.

In the study of the interaction between a molecule and metal substrate, a new mechanism of the chemical enhancement for surface-enhanced Raman scattering (SERS) was discovered. Through a microscopic phenomenological model, it has been demonstrated that 102-104 chemical enhancement may originate from the coupling between an electric field parallel to the surface of a metal substrate and the perpendicular vibration mode of the Raman active molecule adsorbed on the substrate.

When extended to aggregates of metal nanostructures, the electronic coupling between two nanoparticles was studied using DFT with real atoms. It has been shown that when the two nanoparticles are separated from touching contact, the dimer undergoes a bond-breaking step, which establishes the striking existence of an optimal gap size defined by a maximal static polarizability in the linear response regime. For some dimers, the electronic coupling before the bond breaking can be strong enough to lift the spin degeneracy and induce a net magnetic moment in the dimer although each nanoparticle is nonmagnetic. The response of a dimer to a finite electric field may become nonlinear as the field energy is high enough. Interestingly, in the nonlinear regime, the magnetic property of a specific dimer can be easily tuned by the electric field magnitude.

Finally, using the classical electromagnetic (EM) theory, one-dimensional (1D) and 2D nanoparticle dimer arrays were studied within the context of SERS. It has been shown that the local EM enhancement in an array can reach 1014 at the resonant frequency and with the optimal geometry, due to the collective photonic effect constructively superposed onto the intrinsic enhancement associated with an isolated dimer. This photonic effect is also responsible for the oscillation of EM enhancement with the length of a 1D array or along an array with a fixed length.